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615 Cards in this Set
- Front
- Back
wood structural system
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oldest and most common systems
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one-way system
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load xmited through structural members in one direction at a time
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wood joists
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typicaly 2x members
spaced at 12-16-24 in span 20-25ft. |
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bridging
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used to stabalize bottom chord of joist
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plank and beam framing
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uses solid wood beams, 4-6in nominal widths betw. girders of bearing walls, spaced at 4-6-8ft.
span 10-20 ft. |
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glu-lam
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3/4-1 1/2" pieces flued together
span 15-60ft. better appearance, less defects |
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wood truss
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depth of 12-36in.
span 24-40t space @ 24in. oc. |
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steel ductility
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can tolerate some deformation and return to its orifinal shape, will bend before it breaks
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beam and girder
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large members (girders) spanning between vertical supports with smaller beams betweeen
spans 25-40ft beams spaced @ 8-10ft |
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open web steel joist
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joist spanning betw bearing wall or beam
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standard joist
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standard span- 60ft.
depth- 8-30in |
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long span joist
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long span- 96 ft.
depth- 18-72in |
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deep long span joist
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deep long span- 144ft.
depth 18-72in |
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floor joist
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spaced @ 2-4 ft. oc.
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roof joist
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spaced at 4-6 ft oc.
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lift slab construction
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precast on site, one slab on another and jacked into place and attached to columns
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tilt up panel construction
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precast on site and tilted into position
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concrete one way system
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CIP, slab and beams xfer load in one direction, usually to an interm. beam and then to a girder
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concrete two way system
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CIP, most are designed for use in a rect. bay where distance btw columns is approx. the same
flat plate, flat slab, and waffle slab |
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conc. beam and girder
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spans 15-30 ft.
simple to form and allow penetrations |
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flat plate
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two-way system, slab designed and reinforced to xmit loads in both directions to the columns
spans up to 25 ft. slab depth from 6-12 in. |
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flat slab
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two way system, slab is thickened at columns to resist shear failure. column capital of cone or pyramid used to dissipate shear and bending moments
spans up to 30 ft. |
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waffle slab
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two way system, forms of prefab. mtl or fiberglass that allow quick construction
span up to 40 ft. |
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precast unit connections
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made in field by using cast weld plates
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precast production
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prestressed, w/ high strength stl cables stretched in the form. after conc reaches min. strength cables released providing built in compressive stress that resists forces of dead and live loads
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camber
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built in curvature of beams that is reduced when loaded
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post tensioned concrete
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on site fabrication, tendons in form are stressed after the concrete has been poured. use hydraulic jacks to stress;
100-250 psi for slabs 200-500 psi for beams |
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masonry reinforcing
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horizontal and vertical, strengthens wall and controls shrinkage cracks
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thickness of masonry wall
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determines slenderness ratio, flexural strength and fire resistance
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slenderness ratio
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ratio of walls unsupported height to its thickness, indication of the ability of the wall to resist buckling
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flexural strength
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resistance to lateral forces (wind)
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composite construction
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consits of two or more materials desined to act together to resist loads
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headed stud anchors
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xfer load btw. concrete and steel making the material act as one unit
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considerations for non-structural envelope
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weight of the envelope and ow the loads will be xfered to the frame
must allow for expansion and contraction due to temperture and movement of frame |
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structural frame movement
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steel frame does not present much problem, concrete and wood move
concrete will creep, deform to loads wood will shrink, but not as big an issue as dealing with smaller structure |
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arches
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fixed or hinged supports, primarily subjected to compressive forces
functional shape is a parabola spans: wood- 50-240 ft. concrete- 20-320 ft. steel- 50-500 ft. |
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funicular shape
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shape of arch to resist loads only in compression
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thrust
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horizontal forces acting on arch supports attempting to spread the arch outward. must be resisted by either tie rods or foundations that prevent the spread
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thrust is inversely proportion to...
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the rise, or height of the arch
if height reduce by 1/2, the thrust increase by 2 |
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hinge supports
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allow arch to remain flexible and avoid ending stress under live loading
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three hinge arch
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at apex, allows the structure to be statically determinate
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rigid frame
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beam is partially restrained by columns and is more rigid to vertical bending forces and both columns resist lateral forces because they are tied together. columns then must resist thrust
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gabled frame
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does not req. horiz. beam. develop high moment and often increase the depth at connections
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space frame
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efficient at enclosing large horizontal space.
span up to 350 ft. span to depth ratio from 20:1 to 30:1` |
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folded plate
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act as beams btw support
depth 3-6 in. span 30-100 ft (more w/ reinforced conc.) |
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thin shelled structure
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curved surface that resists loads through tension, compression and shear
depth 3-6" span: dome- 40-200' hyperbolic paraboloid- 30-160' |
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suspension cable
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similar and opposite to arches. reaction forces are verticle and outward. can only resist in tension, and a secondary system must be used for lateral forces
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unanticipated loads
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changes in building use
extra people or equipment unusual snow or rain loads degradation of the structure |
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fire resistance considerations
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combustibility of the framing
loss of strength when subjected to intense heat |
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wish to create a building with highly irregular form...
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simple floor and roof framing, fab on-site:
sitecast concrete w/o beams or ribs LGMF platform frame masonry construction w/ concrete slab or wood floor framing |
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wish to leave structure exposed while retaining high fire resistence...
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inhearently fire resistance:
all concrete systems heavy timber frame mill construction |
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wish to allow column placements that deviate from a regular grid...
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systems w/o beams or joists in the floor or roof:
sitecast concrete two way flat plate or slab space frame |
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wish to minimize floor thickness to reduce total building height or reduce floor spandrel depth on the building facade...
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use thin system, concrete slab w/o ribs:
sitecast concrete two way precast hollow core or soild slab posttension one way slab |
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wish to minimize area occupied by columns or bearing walls...
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long span systems:
wood truss glulam beam and arch steel frame stl joists rigid stl frame stl. trusses sitecast concrete waffle slab, when posttensioned precast concrete single or double tees |
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wisht to allow for changes to building over time...
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short span one way systems, easy to modify:
LGMF wood system masonry construction sitecast oneway slab or one way joist construction (not posttension) precast slab |
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wish to permit construction under adverse weather conditions...
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not dependant upon on site chemical processes such as concrete curing:
steel system wood system precast systems that minimize sitecast concrete toppings |
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wish to minimize on site erection time...
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highly preprocessed, prefab or modular components:
single story rigid stl frame conventional steel frame with hinge connections precast concrete heavy timber |
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wish to minimize construction time for a one or two story building...
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lightweight and easy to form or prefab and easy to assemble:
steel system heavy timber platform frame |
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wish to minimize construction time for a 4-20 story building...
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precast concrete
convential steel frame if no lead time, site cast |
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wist to minimize construction time for a building 30 story or more...
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strong, lightweight, prefab and easy to assemble:
steel fame precast or sitecast in some regions |
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wish to minimize need for diagonal bracing or shear walls
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system of forming rigid joints:
sitecast concrete steel frame with welded connections single story rigid frame |
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wish to minimize the dead load on the building foundation...
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lightweight or short span system:
steel system wood system |
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wish to mimimize structural distess due to unstable foundation conditions...
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frame systems w/o rigid joints:
steel frame w/ bolted connections heavy timber frame precast concrete systems platform framing |
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wish to minimize the number of seperate trades and contracts req. to complete the building...
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systems that incorporate many function in a complete wall system:
masonry construction, incl. mill or ordinary constructions precast concrete loadbearing wall panel systems |
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wish to provide concealed space w/i the structure itself for ducts, pipes, wires and other mechanical systems...
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systems w/ natrual hollow spaces:
truss and open web joist LGMF platform framing |
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dynamic loads
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loads such as wind that vary. in calculations these are assumed to be static loads
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reduction in live load
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IBC allows live load to be reduced in most cases. may not be reduced for any public assembly occupancy with load less than or equal to 100psf or for any memeber supporting one floor of a parking garage
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adjusted load (L) limitations
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L must not be less than:
0.5Lo for members supporting one floor 0.4Lo for members supporting more than one floor 0.8Lo for members supporting more than one floor with a live load exceeding 100psf |
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live load reduction formula
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R=r(A-150)
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live load reduction limitations
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reduction cannot exceed 40% for horiz. members, 60% for vertical members
cannot exceed percentage of formula: R=23.1(1 + [D/Lo]) |
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load combinations
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IBC requires several combination of load factors be analyzed to determine the most critical case
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A
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area of floor or roof, sq. ft.
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At
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tributary roof area supported by a structural member, sq ft
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AsubT
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tributary floor area supported by a structural area, sq ft
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D
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dead load, lbf/ sq ft
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E
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earthquake or seismic load, lbf/sq ft
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f sub1
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floor live load occupancy combination factor
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f sub2
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snow load roof shape combination factor
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F
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roof slope, in/ft
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h
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depth of retaining wall, ft
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K subLL
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live load element factor
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L
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floor live load, lbf/sq ft
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Lo
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unreduced floor live load, lbf/sq ft
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Lr
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roof live load
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p (wind)
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direct wind pressure, lbf/sq ft
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p (soil)
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max soil pressure o retaining wall, lbf/sq ft
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P
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lateral soil force, lbf/sq ft
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q
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lateral soil pressure, lbf/sq ft
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r
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rate of reduction of live load
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R (floor load)
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allowable reduction of floor live load, %
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R (rain)
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rain load, lbf/sq ft
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R sub1
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roof area reduction factor
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R sub2
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roof slope reduction factor
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S
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snow load, lbf/sq ft
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v
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wind velocity, lbf/sq ft
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w
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uniform total load, lbf/sq ft
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W
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wind load, lbf/sq ft
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boring depth
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Borings must extend to firm Strata (go through unsuitable foundation soil) and then extend at least 20 feet more into bearable soil.
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Borings are not taken directly under proposed columns.
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true
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soil report includes
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1.Bearing capacity of soil.
2.Foundation design recommendations. 3.Paving design recommendations. 4.Compaction of soil. 5.Lateral strength (active, passive, and coefficient of friction). 6.Permeability. 7.Frost depth. |
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Surface investigations: (Danger Flags)
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High Water Table.
Presence of trouble soils: Peat, soft clay, loose silt, or fine water bearing sands. Rock close to the surface (require blasting for excavations). Dumps or Fills. Evidence of slides or subsidence. |
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soil condition- nearby buildings
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require shoring or earth and existing foundations.
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soil conditions- rock outcropping
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indicate bedrock, good for bearing and frost resistance, bad for excavations.
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soil conditions- water (lake)
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Water (lake) – indicate high water table, some waterproofing of foundations is required.
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soil conditions- level terrain
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Level Terrain – easy site work, fair bearing, but poor drainage.
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soil conditions- gentle slopes
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Gentle Slopes – easy site work, and excellent drainage.
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soil conditions- convex terrain (ridges)
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Convex Terrain (Ridge) – dry solid place to build.
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soil conditions- concave terrain (valley)
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Concave Terrain (Valley) – wet soft place to build.
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soil conditions- steep terrain
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Steep Terrain – costly excavations, potential erosion, and sliding soils.
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soil conditions- foliage
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Foliage – some trees indicate moist soil. Large trees indicate solid ground.
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Soil is classified on the basis of
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1.Percentage of gravel, sand, and fines.
2.Shape of grain. 3.Plasticity and compressibility characteristics. |
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Spread Footings:
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Used for most buildings where the loads are light and / or there are strong shallow soils.
At columns there are single spot square pads where bearing walls have an elongation form. These are almost always reinforced. These footing deliver the load directly to the supporting soils. Area of spread footing is obtained by dividing the applied force by the soils safe bearing capacity (f=P/A). Generally suitable for low rise buildings (1-4 Stories). Requires firm soil conditions that are capable of supporting the building on the area of the spread footings. When needed footings at columns can be connected together with grade beams to provide more lateral stability in earthquakes. These are most widely used because they are most economical. Depth of footings should be below the top soil, and frost line, on compacted fill or firm native soil. Spread footings should be above the water table. Concrete spread footings are at least as thick as the width of the stem. |
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Drilled Piers or Caissons:
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For expansive soils with low to medium loads, or high loads with rock not too far down, drilled caissons (piers) and grade beams can be used.
The caissons might be straight or belled out at bottom to spread the load. The grade beam is designed to span across the piers and transfer the loads over to a column foundation. Caissons deliver the load to soil of stronger capacity which is located not too far down. |
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Piles:
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for expansive soils or soils that are compressive with heavy loads where deep soils can not take the building load and where soil of better capacity if found deep below.
There are two types of piles. 1.Friction piles – used where there is no reasonable bearing stratum and they rely on resistance from skin of pile against the soil. 2.End bearing – which transfer directly to soil of good bearing capacity. |
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Mat Foundations:
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Reinforced concrete raft or mats can be used for small light load buildings on very weak or expansive soils such as clays.
They are often post tensioned concrete. They allow the building to float on or in the soil like a raft. Can be used for buildings that are 10-20 stories tall where it provides resistance against overturning. Can be used where soil requires such a large bearing area and the footing might be spread to the extant that it becomes more economical to pour one large slab (thick), more economical – less forms. It is used in lieu of driving piles because can be less expensive and less obtrusive (i.e. less impact on surrounding areas). Usually used over expansive clays, silts to let foundation settle without great differences. |
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ALLOWABLE STRESS DESIGN.
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A method of proportioning structural members, such that elastically computed stresses produced in the members by nominal loads do not exceed specified allowable stresses (also called working stress design).
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BALCONY, EXTERIOR.
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An exterior floor projecting from and supported by a structure without additional independent supports.
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BASE SHEAR.
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Total design lateral force or shear at the base.
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BOUNDARY MEMBERS.
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Portions along wall and diaphragm edges strengthened by longitudinal and transverse reinforcement and/or structural steel members.
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CANTILEVERED COLUMN SYSTEM.
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A structural system relying on column elements that cantilever from a fixed base and have minimal rotational resistance capacity at the top with lateral forces applied essentially at the top and are used for lateral resistance.
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COLLECTOR ELEMENTS.
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Members that serve to transfer forces between floor diaphragms and members of the lateral-force-resisting system.
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CONFINED REGION.
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The portion of a reinforced concrete component in which the concrete is confined by closely spaced special transverse reinforcement restraining the concrete in directions perpendicular to the applied stress.
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COUPLING BEAM.
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A beam that is used to connect adjacent concrete wall piers to make them act together as a unit to resist lateral loads.
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DEAD LOADS.
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The weight of materials of construction incorporated into the building, including but not limited to walls, floors, roofs, ceilings, stairways, built-in partitions, finishes, cladding, and other similarly incorporated architectural and structural items, and fixed service equipment, including the weight of cranes.
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DECK
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An exterior floor supported on at least two opposing sides by an adjacent structure, and/or posts, piers or other independent supports.
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DEFORMABILITY
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The ratio of the ultimate deformation to the limit deformation.
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DESIGN STRENGTH.
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The product of the nominal strength and a resistance factor (or strength reduction factor).
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DIAPHRAGM, FLEXIBLE.
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A diaphragm is flexible for the purpose of distribution of story shear and torsional moment when the lateral deformation of the diaphragm is more than two times the average story drift of the associated story, determined by comparing the computed maximum in-plane deflection of the diaphragm itself under lateral load with the story drift of adjoining vertical-resisting elements under equivalent tributary lateral load.
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DIAPHRAGM, RIGID.
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A diaphragm that does not conform to the definition of flexible diaphragm.
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DURATION OF LOAD.
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The period of continuous application of a given load, or the aggregate of periods of intermittent applications of the same load.
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Ductile element.
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An element capable of sustaining large cyclic deformations beyond the attainment of its nominal strength without any significant loss of strength.
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Limited ductile element.
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An element that is capable of sustaining moderate cyclic deformations beyond the attainment of nominal strength without significant loss of strength.
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Nonductile element.
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An element having a mode of failure that results in an abrupt loss of resistance when the element is deformed beyond the deformation corresponding to the development of its nominal strength. Nonductile elements cannot reliably sustain significant deformation beyond that attained at their nominal strength
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EQUIPMENT SUPPORT.
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Those structural members or assemblies of members or manufactured elements, including braces, frames, lugs, snuggers, hangers or saddles, that transmit gravity load and operating load between the equipment and the structure.
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ESSENTIAL FACILITIES.
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Buildings and other structures that are intended to remain operational in the event of extreme environmental loading from flood, wind, snow or earthquakes.
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FACTORED LOAD.
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The product of a nominal load and a load factor.
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FLEXIBLE EQUIPMENT CONNECTIONS.
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Those connections between equipment components that permit rotational and/or translational movement without degradation of performance.
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Braced frame.
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An essentially vertical truss, or its equivalent, of the concentric or eccentric type that is provided in a building frame system or dual frame system to resist shear.
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Concentrically braced frame (CBF).
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A braced frame in which the members are subjected primarily to axial forces.
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Eccentrically braced frame (EBF).
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A diagonally braced frame in which at least one end of each brace frames into a beam a short distance from a beam-column or from another diagonal brace.
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Ordinary concentrically braced frame (OCBF).
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A steel concentrically braced frame in which members and connections are designed in accordance with the provisions of AISC Seismic without modification.
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Special concentrically braced frame (SCBF).
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A steel or composite steel and concrete concentrically braced frame in which members and connections are designed for ductile behavior.
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Intermediate moment frame (IMF).
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A moment frame in which members and joints are capable of resisting forces by flexure as well as along the axis of the members.
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Ordinary moment frame (OMF).
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A moment frame in which members and joints are capable of resisting forces by flexure as well as along the axis of the members.
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Special moment frame (SMF).
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A moment frame in which members and joints are capable of resisting forces by flexure as well as along the axis of the members.
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Building frame system.
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A structural system with an essentially complete space frame system providing support for vertical loads. Seismic force resistance is provided by shear walls or braced frames.
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Dual frame system.
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A structural system with an essentially complete space frame system providing support for vertical loads. Seismic force resistance is provided by a moment-resisting frame and shear walls or braced frames.
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Space frame system.
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A structural system composed of interconnected members, other than bearing walls, that is capable of supporting vertical loads and that also may provide resistance to seismic forces.
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IMPACT LOAD.
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The load resulting from moving machinery, elevators, craneways, vehicles, and other similar forces and kinetic loads, pressure and possible surcharge from fixed or moving loads.
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JOINT
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A portion of a column bounded by the highest and lowest surfaces of the other members framing into it.
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LIMIT STATE.
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A condition beyond which a structure or member becomes unfit for service and is judged to be no longer useful for its intended function (serviceability limit state) or to be unsafe (strength limit state).
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LIVE LOADS.
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Those loads produced by the use and occupancy of the building or other structure and do not include construction or environmental loads such as wind load, snow load, rain load, earthquake load, flood load or dead load.
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LIVE LOADS (ROOF).
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Those loads produced (1) during maintenance by workers, equipment and materials; and (2) during the life of the structure by movable objects such as planters and by people.
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LOAD AND RESISTANCE FACTOR DESIGN (LRFD).
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A method of proportioning structural members and their connections using load and resistance factors such that no applicable limit state is reached when the structure is subjected to appropriate load combinations. The term "LRFD" is used in the design of steel and wood structures.
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LOAD FACTOR.
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A factor that accounts for deviations of the actual load from the nominal load, for uncertainties in the analysis that transforms the load into a load effect, and for the probability that more than one extreme load will occur simultaneously.
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LOADS
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Forces or other actions that result from the weight of building materials, occupants and their possessions, environmental effects, differential movement, and restrained dimensional changes. Permanent loads are those loads in which variations over time are rare or of small magnitude. Other loads are variable loads (see also "Nominal loads").
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LOADS EFFECTS.
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Forces and deformations produced in structural members by the applied loads.
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NOMINAL LOADS.
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The magnitudes of the loads specified in this chapter (dead, live, soil, wind, snow, rain, flood and earthquake).
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P-DELTA EFFECT.
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The second order effect on shears, axial forces and moments of frame members induced by axial loads on a laterally displaced building frame.
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PANEL (PART OF A STRUCTURE).
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The section of a floor, wall or roof comprised between the supporting frame of two adjacent rows of columns and girders or column bands of floor or roof construction.
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RESISTANCE FACTOR.
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A factor that accounts for deviations of the actual strength from the nominal strength and the manner and consequences of failure (also called strength reduction factor).
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SHALLOW ANCHORS.
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Shallow anchors are those with embedment length-to-diameter ratios of less than 8.
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SHEAR PANEL.
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A floor, roof or wall component sheathed to act as a shear wall or diaphragm.
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SHEAR WALL.
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A wall designed to resist lateral forces parallel to the plane of the wall.
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SPECIAL TRANSVERSE REINFORCEMENT.
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Rein-forcement composed of spirals, closed stirrups, or hoops and supplementary cross-ties provided to restrain the concrete and qualify the portion of the component, where used, as a confined region.
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STRENGTH, NOMINAL.
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The capacity of a structure or member to resist the effects of loads, as determined by computations using specified material strengths and dimensions and formulas derived from accepted principles of structural mechanics or by field tests or laboratory tests of scaled models, allowing for modeling effects and differences between laboratory and field conditions.
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STRENGTH, REQUIRED.
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Strength of a member, cross section or connection required to resist factored loads or related internal moments and forces in such combinations as stipulated by these provisions.
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STRENGTH DESIGN.
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A method of proportioning structural members such that the computed forces produced in the members by factored loads do not exceed the member design strength (also called load and resistance factor design.) The term "strength design" is used in the design of concrete and masonry structural elements.
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wind loading
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affected by:
wind velocity height of wind above the ground (wind speed taken at 10m above ground, will be more at higher points) surroundings (taken into account by variables in code) shape of building |
|
wind effects
|
positive pressure windward side, negative on leeward side (suction)
local areas may be greater or less |
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earthquake general effects
|
lateral and vertical movement
(lateral the most important and vertical is generally ignored) |
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dynamic structural analysis
|
use computer to model the building and earthquakes to study the response
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dynamic load
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treated as a static load multiplied by an impact factor
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resonant load
|
rythmic application of force to a structure with the same fundamental period as the structure itself
utilize dampenner to minimize the effect |
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fundamental period
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time taken for a structure to complete one full oscillation
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coefficient of expansion
|
amount of change in a material expressed in inches per inch per degree F
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retaining walls
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minimum lateral pressures are given by code
must resist these forces in addition to other vertical loads at the top of wall must be designed to resist sliding by 1.5x the lateral force and resist overturn by at least 1.5x the overturn moment |
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pressure at bottom of retaining wall
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P=p(h/2)
lateral force= max soil pressure (height of wall/ 2) |
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hydrostatic pressure
|
load developed by water
62 lbf/sq ft |
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b
|
base of recatngular section , in
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d
|
depth of rectangular section, in
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e
|
total deformation (strain), in
|
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E
|
modulus of elasticity, lbf/sq ft
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f
|
unit stress, lbf/sq ft
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F
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force, lbf
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I
|
moment of inertia, in to 4th
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In
|
moment of inertia of transferred area, in to 4th
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Ir
|
moment of inertia of area about neutral axis, in to 4th
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L
|
original length, in
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P
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total force, lbf
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R
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reaction force, lbf
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W
|
weight, lbf
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fish symbol
|
coefficient of linear expansion,
in/in-degree F |
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squiggly E
|
unit strain, decimal
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statics
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mechanics dealing with bodies at equilibrium
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principles of equilibrium for buildings
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sum of all vertical forces acting on a body must equal zero
sum of al horizontal forces actin on a body must equal zero sum of all moments acting on a body must equal zero |
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vector quantity
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direction and magnitude of a force
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stress
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resistance to external forces
tension compression shear |
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shear
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stress in which particles of a member slide past each other
force acts parallel to the area resisting the force |
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stress formula
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f=P/A
stress= total force/ total area |
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torsion
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type of shear in which the member is twisted
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bending
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combination of tension and compression
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thermal stress formula
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e= fishy x delta T
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internal thermal stress
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not dependent on cross section if only force applied
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strain
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deformation of a material caused by external forces
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hookes law
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strain is directly proportional to the stess up to a certain point
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elastic limit
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point at which material will change shape at a faster ratio than the applied force
above this limit the member will be permenently deformed, even when force is removed |
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yield point
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point at which the material continues to deform with very little increase in load
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ultimate strength
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stess just before the member ruptures
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modulus of elasticity
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measure of a materials resistance to deformation (stiffness)
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moment
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tendancy of a force to cause rotation about a point. dependent on force by the perpendicular distance acting on the point, shown in lbf
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centroid
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point on a plane (at cross section of member) that corresponds to the center of gravity
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moment of inertia
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measure of the bending stiffness of a structural members cross-sectional shape
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neutral axis
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asix passing through the centroid
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load combinations
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some structural materials are designed using strength design, some w/ allowable stress design
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c
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distance from extreme fiber in bending to neutral axis, in
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f
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allowable fiber stress in bending, lbf/in sq
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f sub b
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extreme fiber stress in bending, lbf/in sq
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Fa
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allowable axial unit stress, lbf/in sq
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M
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bending moment, in-lbf
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Q
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statical moment about neutral axis of the area above the plan under consideration, in cubed
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r
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radius of gyration, in
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S
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section modulus, in cubed
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v sub h
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horizontal shera stress, lbf/in sq
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V
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vertical shearing force, lbf
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basic theory of bending
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internal resisting moments at any pont in the beam must equal the bending moments produced by the external loads on the beam
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section modulus
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ratio of the beams moment of inertia to the distance from the neutral axis to the outermost part of the section (extreme fiber)
S=I/ c S=M/ f |
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horizontal shear causes problem in...
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wood beam where the horizontal fibers may split and shear
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selection of beam size dependent on...
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bending and horizontal shear, but not affected by deflection
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kip
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1000 pounds
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uniformly distr. loads
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resultant force is at the center of load
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statically determinate
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the reactions can be found using the equations of equilibrium
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total load
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total load= W= wL
total load= uniform distr. load x length of beam |
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principles of equilibrium
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sum:
-vertical forces = 0 -horiz. forces = 0 -moments acting on the body = 0 |
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shear diagram provides...
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maximum shear
point on the beam where the value of shear is zero |
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maximum moment occurs...
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where the shear diagram passes through zero, and is indicated by the high point on the moment diagram
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shear diagram vs moment diagram
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shear: horizontal line between two concentrated loads or reactions
moment: straight constant sloped line |
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deflection
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change in vertical position of a beam due to a load
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deflection limitations
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1/360 of the beam span for live load
1/240 for total, live and dead load |
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column buckle
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tendancy of long, slender column to bend laterally under load
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column considerations
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buckling
combined forces- compressive + lateral loads flexural stress- caused by load eccentricity |
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radius of gyration
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combine the properties of area and moment of inertia
for non symmetic columns there are two radii, the lesser is the more important |
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slenderness ratio
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most important factor in column design
= l/ r =length of column/ radius of gyration |
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Eulers equation
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give the max. stress a column can resist w/o buckling
not applicable to all materials w/o adjustment |
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column types
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short compressive members
intermediate columns slender columns |
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column fixed ends
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fixed against rotation and movement from side to side (translate)- strongest
able to rotate but not translate translate but not rotate free to rotate and translate weakest is column free at one end |
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truss
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composed of straight member to form triangles
typ depth to span: 1:10-1:20 spaced at 10-40ft oc depending on the capabilities of the purlins |
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trusses are subject to buckling...
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must be installed w/ bridging
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truss types
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flat truss
pitched truss bowstring truss scissors truss |
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net area
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area of member minus the area of bolt hole
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truss members to be designed...
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concentric, so the member is symmetric on both sides of the centroid axis in the plane of the truss
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horizontal force formula (truss)
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Fx= F cos a
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vertical force formula (truss)
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Fy= F cos b
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methods to determine force in truss
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method of joints
method of sections graphic method |
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first step in designing a foundation
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determine bearing capacity of underlying soil, through exploration and testing
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water in soil
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reduce load carrying capacity in general
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hydrostatic pressure
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puts additional loads on the structural elements
makes waterproofing more difficult because the pressure tends to force water into any imperfection in the structure |
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minimize problems w/ excess soil moisture
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slope the ground away and drain from the bldg
below grade: drain tile, gravel, open-web matting at foundation underslab: large gravel below slab |
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soil treatment to increase bearing capacity or decrease settlement
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drainage
fill- remove and replace compaction- densification- vibration, weight, pounding piles and filling displacement w/ sand surcharging- load site with fill material to settle and remove |
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additional soil concerns
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frost
expansive soil- caissons to soil below, grade beams spanning btwn piers repose- natural slope of soil, max. practical angle of grade w/o other means of holding back soil |
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types of foundations
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spread footings
pile or caissons |
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independant column footing
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supports single column
size of footing found by dividing total load by bearing capacity of soil |
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combined footing
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support 2 or more columns where spaced too closely for independent
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strap footing
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variation on combined, where columns are far apar
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raft foundation
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low soil bearing, two way slab below with support columns above
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pile foundation
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xfer load through unsuitable soil to secure bearing
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pier
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drilled piles or caissons
formed by drilling out a hole and filling w/ concrete |
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pile cap
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xfers load from building to piles located in groups or a line beneath cap
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pile group
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designed so that centroid of group is located w/ center of gravity of the column load
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grade beam
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xfer building loads to piles drilled in a line
often w/ expansive soils at surface. form creates a void btwn beam and soil to avoid vertical pressures from soil |
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unit loading
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verify that soil bearing in not exceeded and diff. settlement is eliminated
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footing shear
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if a column punches through the footing
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footing bending
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when lower surface cracks under flexural loading
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footing design factors
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unit loading
shear bending |
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footing tendancies
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act like inverted beams w/ upward soil pressure resisted by downward column load
causes bending in upward direction, with top fiber in compression and bottom in tension |
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area of spread footing
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divide total wall or column load (plus own weight and soil weight) by allowable soil bearing pressure
additional formulas take into account live, lateral, fluid loads, etc |
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required footing strength load
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U, equals 1.4D + 1.7L
(1.4 actual dead load + 1.7 actual live load) |
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effective depth of footing
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d, distance from top of footing to the centroid of reinforcing steel
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width and thicknees of footings designed to...
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resist the wall load and shear force using only strength of unreinforced concrete
not economical to provide tension reinforcing |
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types of retaining walls
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gravity
cantilever conterfort |
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cantilevered retaining wall
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arm, heel and toe act as cant. slabs. thickness and reinforcement increase with length as greater moments are developed
limit to 20-25' high |
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counterfort wall
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reinforce concrete web that acts as diagonal bracing (winston salem walls)
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earth pressure (P) on retaining wall increases...
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proportionall with the depth from the surface at zero to greatest at base of wall in a triangular distribution
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retaining wall surcharges
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additional dead and live loads acting on retaining wall (i.e. nearby driveway)
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retaining wall failure
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overturn or sliding
use a safety factor of 1.5 to avoid |
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eliminate or reduce buildup of water behind retaining wall by...
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providing weeps
layer of gravel next to wall install drain is necessary |
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majority of failure...
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in connections, not member
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wood connection design variables
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load bearing capacity of the connector
species of wood load type weather fire resistant treated angle of load to grain critical net section type of shear the joint is subjected to spacing fo connectors end and edge distances |
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species and density of wood determines...
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type of connector
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wood connector types
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timber connector- split ring and shear plate (type A,B,C,D)
screws, nails, spikes, mtl plates, etc (type I, II, III, IV) |
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design value for connectors can be adjusted...
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for duration of loading. wood can carry greater max. loads for short duration than for long
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wood design values are for...
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seasoned, dry wood, less than 19% moisture
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service conditions
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environment in which wood joint will be used
any conditions other than dry or continuously wet reduce holding power of connection |
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angle of load
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angle of connector to grain of wood. important variable in design
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hankinson formula
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provides compressive stress at specified angle
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critical net section
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section where wood is removed for connector, load must be checked against this section
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min. connector spacing
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given by National Design Specification for Wood Construction
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end distance
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distance measured parallel to the grain from the center of the connector to the square cut end of the member
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edge distance
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distance from the edge of the member to the center of the connector closest to the edge of the member measured perpendicular to the edge
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perpendicular grain loading
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make distinction btwn loaded and unloaded edges, min. values given in tables
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fastener orientation
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fastener loaded lateraly in side grain where the holding power is the greatest
not allowed in end grain by code |
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increase nails and screws by....
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25% if a metal side plate is used
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lag screw
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threaded w/ a point, but head like a bolt
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bolt design variables
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thickness of main and side members
ratio of bolt length in main member to bolt diameter number of members joined |
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split ring
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2 1/2 or 4" diam. and are cut through in one lace in the circumference to form a tongue and slot
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shear plate
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2 5/8 or 4" diam. and are flat plates w/ a flange extending from the face of the plate. a 3/4 or 7/8" bolt extends through the hole to hold the members together
can hold either 2 pieces of wood or peice of wood and piece of stl spec. suited for construction that must be disassembled |
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common methods of stl connections
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bolting and welding
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bearing type connection
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resist shear load on the bolt through friction betw surfaces but may also produce direct bearing btw the stl being fastended and the sides of the bolts
holes slightly larger and over time may slip and bear on bolt |
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slip critical connections
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those where any amount of slip would be detrimental to the serviceability of the structure
entire load taken by friction the nuts are tightended to develop a high tensile stress in the bolt, causing the connected members to develop a high friction btw them that resists the shear |
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bolt thread classifications
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included or excluded in shear plane, affects the amount of material to resist shear
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bolt types
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ASTM A307- unfinished bolts (low load bearing)
A325 and A490- high strength bolts (must be used in slip critical connection) |
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bolt used conditions
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SC- slip critical
N- bearing type X- bearing type S- bolt in single shear D- bolt in double shear |
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standard round holes
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1/16 larger than diam of bolt
5/8" larger w/ high strength |
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oversize holes
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3/16 larger than 7/8 bolts
1/4 larger than 1" bolts 5/16 larger than 1 1/8" bolts only used in slip critical connections |
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short slotted holes
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1/16 wider than bolt diam. and length that does not exceed hole by more tahn 1/16
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long slotted holes
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1/16 wider than bolt and length not to exceed 2.5x bolt diam.
only used on one of the joints, other must be standard or welded |
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slotted hole connections
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used where some amount of adjustment is req.
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net area
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area reduced in beam from bolt holes
increase tendancy for shear failure parallet to the load and tension failure perpendicular to the load |
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total tearing force
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sum required to cause both shear and tension failure in a beam at a bolt connection, shown as Tt
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typ, the angle used to connect beam to column...
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is shop welded to column and bolted to the other member in the field
slotted holes sometimes used to allow minor adjustments |
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moment connection (steel)
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welding more suitable, but can be accomplished with bolted T-Sections
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bolt spacing for steel
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min. spacing is 2 2/3x the diam. of the bolt being used, w/ 3x being the pref. dim.
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edge distance for steel
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1.25" for all bolts having a diam. up to 1"
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reasons for using weld
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gross section can be used, not net
efficient construction, no angles, bolts, wrenches, etc. more practical for moment connections |
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to hold member in place to complete connection...
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bolting is often used w/ welding
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electric arc weld
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most common in buildings
electrode from power source connects to members being joined, other electrode in the welding rod. base metal and the end of electrod melt into joint due to intense heat |
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penetration
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depth from the surface of the base metal to the point where fusion stops
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types of electrodes
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E60- 18 ksi
E70- 21 ksi |
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common weld type
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fillet most common- isosceles triangle w/ the two equal legs being the sides of the weld
butt joint- throat dim. is the thickness of the material is both are same thickness |
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weld on both side
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symbol repeated above and below line
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field weld symbol
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flag place at the junction of teh horiz line and the arrowhead line. circle at the joint indicates weld all around
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weld load
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fillet- considered shear regardless of direction of load
butt weld- allowable stress is same as base metal |
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AISC weld design req.
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max size of fillet is 1/16" less that nominal thickness of material connected
minimum length must not be less that 4x the weld size intermittent weld must be at least 1 1/2" min. size of fillet welds are given by table |
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no connectors in...
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CIP concrete
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precast constr. connectors
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embed weld plates
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CIP conc. joints
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reinforcing bars
keyed joints |
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dowel
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reinforcing only used to connect two pours
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keyed joint
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used alone or w/ reinforcing to strengthen a joint betw pours
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precast movement joint
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precast typically bear on elastomeric bearing pads (UNL)
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concrete shear connector
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anchors a composite system. often called headed anchor studs
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IBC requires construction method based on...
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rational anlaysis in accordance with well established principles of mechanics and that analysis provide a bath from point of origin to load resisting elements
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anchorage
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resiste the uplift and sliding forces on a structure
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IBC live load provisions
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design floors to accomodate concentrated loads
where unif. dead and live loads, design may be limited to full dead load on all spans and full live load on adjacent spans live floor loads in commercial and industrial buildings must be posted interior walls, partitions over 6' must be designed to resist all loads on them, but not less than 5psf perpendicular |
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IBC office building distr live load
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20 psf
access floors may be design to take an additional 10 psf |
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IBC wind tunnel req.
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for buildings sensitive to dynamic effects, wind load ocillations, and over 400ft.
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IBC req. use of higher load...
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between earthquake and wind loads
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repetitive use
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a factor equal to 1.15 is used when several beam members are used together
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duration of load
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amount of stress a wood member can withstand is dependent on the time during which the load producing the stress acts
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fire retardent treatment affects
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design values for wood members must be obtained from manuf. treatement reduces bearing capacity of members
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size factor adjustment
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IBC req. forces parallel w/ grain to be multiplied by size factors given
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IBC allowable stress for bolts
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based on type of load
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IBC allowable stress for welds
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based on either yield strnth of base metal or nom. tensile strength of the weld mtl.
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IBC concrete req.
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complex and detailed covering formwork, reinforcing, mixing placing and curing
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IBC important concrete factors
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ultimate load above dead and live load
strength reduction factor |
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IBC concrete sample req/
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sample taken not less than once per day, nor less than on ce per 150 sq yd., nor less than once for each 5000 sq ft of surface area for wall and slab
no test can be less tha 500 psi below compressive strength value |
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IBC wood req
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bottom of joist 18" above ground, girder 12" unless treated or natural resistant
provide 1/2" airspace around girder in masonry or concrete unless treated foundation plates and sills must be treated or redwood crawl space must be ventilated, min. 1sq ft for every 150 sq ft all stuct. wood must be weather protected fire stops req. at ceiling and floor levels at 10ft. intervals vert and horiz fire stops req in concealed spaces in stairway and vert. openings |
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IBC stl construction req.
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roof syst. w/o proper drainage slope must be invest. for stability under ponding
horiz framing desinged for deflection and ponding |
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IBC conc construction req.
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constr. loads cannot be supported or shoring removed until conc has sufficient strength to support its own weight
limits on amount and placement of conduits and pipe penetrations to not decrease the load resisting area size and bending of reinforcement between the concrete and steel min. concrete cover over reinforcing |
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steel fire reaction
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non combustible but loses strength under high heat
must be protected, unless 25ft above floor |
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Type IV construction
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heavy timber construction
col. at least 8" in any dim. beams and girders 6" wide x 10" deep floor decking min. 3" deep |
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wind load type
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resonant load (ocillating load) perpendicular to the direction of the wind
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fundamental period
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time it takes the structur to complete one full swing from side to side
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wind measurement
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fastest mile wind- average speed of a column of air 1 mi long over a given point, measured w/ anemometer
three second peak gust wind- max peak gust at 33 ft abive ground ASCE 7 speed- ultimate wind sped that would occur every 700 years |
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surface conditions affecting wind load
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open country- most sever wind as there is nothing to slow the wind
suburban areas metropolitan areas |
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gradien height
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height above which the friction from the ground and other obstruction no longer affects wind speed
open country- 900 ft suburb- 1200 ft metro- 1500 ft |
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adjacent buildings on wind load
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ASCE does not allow reduction due to adjacency, but will affect a wind tunnel test
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wind G factor
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takes into account both atmospheric and aerodynamic effects
value of G= .85 for rigid structures |
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wind Kz factor
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combines effects of height, exposure and wind gusting into one factor
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wind exposures
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D- most severe, flat unobstructed facing large body of water at least 500ft or 10x the building height (exclude hurricane areas)
C- open terrain w/ scatter obstructions, less than 30ft heigh and inlcudes all hurricane areas B- urban and suburban wooded area w/ obstruction upwind for at least 2630 ft or 10x the height, whichever greatest |
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wind Kz factor on bldg sides
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varies w/ theigh on the wineward side
leeward side Kh is used over the entire surface- Kh= Kz at the mean roof height |
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wind Cp factor
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account for differing effect of wind on various parts of the building
Cpi- evaluate internal pressures = +/-0.18 for enclosed bldg = +/-0.55 for partially enclosed bldg |
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ASCE 7 methods
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method 1- normal force method
method 2- projected area method |
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normal force method
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must be used for gabled rigid frames and may be used for any structure
wind forces act simultaneous normal to all ext. surfaces. on leeward side, heigh is taken at the mean roof heigh and constant for the full height of bldg |
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projected are method
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assume horiz pressure act on the full vert projected area of the structure and that vert pressures act simult. on the full horiz projected area
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wind stagnation factor
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q factor- effect of wind speed on pressure
based on the 3-sec. peak gust wind at heigh of 33 ft. |
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Kzt
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topographic factor accounting for hills and escarpements. level ground = 1.0
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Kd
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accounts for type of structure being analyzed, use 0.85 for buildings
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importance factor
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I= safety factor for essential facilities that must be safe and usable for emergency purposes
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component wind loads
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higher pressures are experienced by individual elements than on building as a whole and must be reviewed seperately
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lateral force distribution (wind)
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wind strikes side of building, pressure xfer through cladding to connections with floor and roof. horizontal surfaces (diaphragms) xfer to lateral force resisters (side walls, interior walls or frame)
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diaphragm
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horizontal surface, floor or roof that xfers load to vertical load bearing system
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shear wall
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walls designed to carry horizontal shear to foundations
act as beams cantilevered out of the ground, deeper section is more efficient and req less struct. material |
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bent
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column and beam lines designed to carry wind loads to foundations
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diaphragm chords
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extreme fibers of diaphragm experiencing compression (windward) and tension (leeward)
useful to see as a sideways acting beam |
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framing methods resisting both wind and earthquake forces
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moment resisting frame
knee bracing x-brace/ chevron portal frame framed tube trussed tube combined stl/shear wall |
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moment resisting frame-
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simplest, w/ connections btwn col and beams. welds or gusset plts with higher loads.
used for low-rise and high rise under 30-story |
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x-brace/ chevron
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useful for resisting both directions, typically tension members.
chevron results in lower horizontal drift overall |
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knee bracing
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short diagonal struts, sim. in depth to gusset plt
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portal frame
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trusses at each floor level w/ knee braces connecting the truss to the column
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framed tube
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creates a large cant. tube from the ground
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trussed tube
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combination of rigid fame and diag braces on the exterior wall
(john hancock) x brace spans 5-10 floors |
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stl/shear wall combo
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steel frame carries most of the vertical loads while the shear wall xmit lateral loads to the foundation
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variations to basic framing methods
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incorporate belt trusses at intermed. floors and tapered buildings
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concrete rigid frames systems
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can be built but moment resisting connections are difficult and are limited to 20-30 stories
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chord force
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force distributed along the depth of a diagram. determines the type of connection btwn the diaphragm and the shear wall
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overturning moment (wind)
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lateral force that the shear walls and building must resist. additionally shear wall must be attached to foundation and footings in such a way to prevent uplift and/or sliding on the footing
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drift
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lateral displacement of building caused by lateral load from true vertical line
max drift= 0.0025x the story height |
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in all connections, the resisting moment...
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increases with increased distance btwn centroids of the top and bottom portions of the connection
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due to the strength of earthquake forces...
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no building can be economically designed to completely resist all loads in a major earthquake w/o damage
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earthquake design approach
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structure should not collapse
components of building should not cause other damage or personal injury even though they are damaged should be able to withstand minor EQ w/o damage |
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methods of seismic design
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simplified analysis
equivilant lateral force dynamic analysis |
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equivilent lateral force method
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treats seismic loads as equivalent lateral loads acting on various level of the building
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dynamic method
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uses computer to mathematically model the building
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simplified analysis
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only be used in seismic zone A and zone B for 3-story light framed buildings and other 2-story buildings in group 1
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equiv. lateral force categories
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used for buildings in seismic categories A,B and C
used in D, E, and F if: regular structure w/ T<3.5T irregular structure w/ T<3.5T and having only plan irregularities 2,3,4,5 or vert irregularities 4 or 5 |
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hypocenter
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stress is at a max. several miles below the surface of the earth
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epicenter
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point on the earth surface directly above the hypocenter
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type of EQ waves
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P- pressure waves
S- shear waves surface waves |
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pressure waves
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relatively small movement in the direction of wave travel
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shear waves
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produce a sideways or up-and-down motion that shakes the ground in three dirctions
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surface waves
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travel at or near the surface and can cause both vert and horiz earth movement
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ground movement measurement
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acceleration
velocity displacement |
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acceleration of the ground
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induces forces on a structure
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vertical EQ forces
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weight of a structure is usually enough to resist vertical forces. side-to-side movement causes the most damage
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EQ scale
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require measure of acceleration or duration of EQ
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strong motion accelerograph
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provides objective quantified data useful for building design
used for research on EQ reactions and specifies amounts to be installed |
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IBC EQ maps
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give the mapped spectral response acceleration at a period of 0.2 and 1.0 sec.
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period of lateral motion dependant on...
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mass and stiffness of the building
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lateral force induced
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flexible, long period buildings have less
stiff, short period buildings have more |
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as a building moves, the forces applied to it are...
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xmitted through the structure to the foundation absorbed by the building components or released i nother ways such as collapse of structural elements
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goal of seismic design is to...
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build a structure that can safely xfer the loads to the foundation and back to the ground and absorv some of the energy present rather than suffering damage
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ductility
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ability of structure to absorb some of the energy
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ductility of steel
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has the ability to deform under a load above the elastic limit w/o collapsing
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IBC load resisting structural systems
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table 1617.2 gives structural types, R factor and max heigh of each type of system
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bearing wall system
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system w/o a complete vertical load carrying space frame in which the lateral loads are resisted by shear walls or braced frames
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shear wall symmetry
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important aspect of shear walls and vertical elements in general that determines whether torsional effects are produced
does not need to be symmetrical, but is preferred |
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interior shear walls
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not as efficient from structural point of view, interior shear walls do leave exterior open for windows
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shear wall openings
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may have them, but calculations become difficult as larger percent of open area is used
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building frame system
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essentially complete space frame that provides support for gravity loads in which lateral loads are resisted by shear walls or braced frames
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braced frame
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truss system of the concentric or eccentric type in which the lateral forces are resisted through axial stresses in the members
can be placed on interior or exterior of structure |
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bracing in one direction
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member must work as either compression or tension
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x-bracing
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tension diagonal members, with one member in tension while the other has no load; opposite for other direction
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moment resisting frame system
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carry lateral loads primarily by flexure in the members and joints
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special moment resisting frame
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specially detailed moment resist frame to provide ductile behavior and comply with IBC
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intermediate moment resisting frame
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fewer restrictions than special frames, but cannot be used in seismic design category D, E or F
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dual system
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essentially complete frame provides support for gravity loads and resistance to lateral loads is provided by a specially detailed moment resisting frame and shear walls or braced frames
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diaphragm
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acts as a horizontal beam resisting forces with shear and bending force
flexible and rigid |
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types of horizontal members
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diaphragm, trussed frames, horiz. moment resisting frames
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flexible diagphragm
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has max. lateral deformation more than two times the average story drift of that story
wood deck, mtl deck (depend on detailing) |
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rigid diaphragm
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shear foces xmitted from the diaph. to the vert elements will be in proportion to the relative stiffness of the vert elements (assuming no torsion)
concrete floor, composite steel deck |
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building configuration
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refers to overall building size and shape and the size and arrangement of primary structural frame as well as the size and location of the nonsturctural components of the building that may affect its structural performance
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horizontal and vertical irregularities
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specifically defined by IBC so it is clear which structures must be designed with the dynamic method and which by static analysis method
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irregular building
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generally req desingn by the dynamic method and additional detailed desing req are imposed depending on the type of irregularity
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torsional irregularity
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notes that diaphragms are not flexible
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non parallel systems
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includes buildings in which a column forms part of two or more intersecting lateral force resisting systems, unless the axial load due to seismic forces actin in either directio is less than 20% of the colun allowable axial load
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lateral forces on a portion of a building...
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are assumed to be unifromly distributed and can be resolved into a single line of action acting on a building
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for symmetric buildings w/ vert resisting element of equal regiditys, lines of later forces pass through...
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the same point
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if shear wall or other vert. elements are not symmetric or of unequal rigidity, the resultant of their resisting forces...
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does not coincide with the applied force. since the force acting in the opposite direction acts with eccentricity, torsion force is developed
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accidental torsion
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IBC req symmetrical buildings have some amount of torsion designed for
requires the mass at each level to be assumed displace from center of mass by a distance of 5% |
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plan irregularities
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can create torsion and concentration of streess, both of which shuld be avoided whenever possible
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reentrant corner
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stress concentrated at the inside corner of shape
since the center of mass and center of rigidity do not coincide there is an eccentricitty est. that results in twisting of the entire struct |
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reentrant corner resolution
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portions of the building can be seperated w/ a seismic joint, tied together across the connection or inside corner splayed
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ideal seismic elevation
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symmetrical, continuous and matches other elevations; setbacks to be avoided, corners
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discontinuous shear wall
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elevation problem, shear wall stops before reaching the foundation or offsets
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soft story
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elevation problem, when ground floor is weaker than floors above. can happen anywhere, but is most serious at ground.
can happen if: columns do not extend to ground heavy ext cladding above first story and ground open |
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with elevation irregularities...
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when earthquake load occur forces and deformation are concentrated on weak floor instead of being uniformly distributed
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soft story fixes
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extra horiz and diag bracing
framing above to be same as below, making lighter more flexible intermediate levels |
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determine base shear
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or total later force using equivalent lateral force procedure
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total lateral force formula
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V= CsW
total lateral force= seismic response coefficient x weight of building given in W factor |
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W factor
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total dead load of the building
warehouse/storage: 25% of dead load must be added partition loads shall not be less than 10 psf 20% of flat roof snaow load must be added where the snow load exceds 30 psi total weight of permanent equip must be added |
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once total base shear is known
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it is used to determine the force on the various building elements
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displacement taking place during EQ
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results in an inverted force triangle varyingg from zero at the base to max. at the top
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EQ: the sum of loads at each floor equals the...
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total base shear:
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EQ: the greatest force is at the top of the building, ...
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the shear increases from zero at the top to its max. at the base
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IBC req. for tall buildings w/o uniform disttribution
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the height variable in the distribution equation be raised by an exponential power
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IBC req. attachement of elements beyond structural frame
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includes connections, partitions to floors, ceilings, millwork, equipment, piping, ducts, suspended lighting
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rigid elements
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have a fixed base period less than or eq. to 0.06 sec.
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nonrigid elements
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flexibly supported items have a fixed base period greater than 0.06 sec.
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EQ: overturn moment
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due to the inertial force acting through the center of mass of the building there is a tendency for the moment created by the force acting above the based
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EQ overturn: dead weight of building is...
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normally sufficient to resist the overturn force, but must be checked. only 90% of dead weight may be used.
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story drift
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displacement of one level relative to the level above or below
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EQ: response displacement
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static analysis shal be used to determine
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drift
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is the limiting factor for earthquakes in order to ensure that exterior facades do not break or crack excessively
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long span systems
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over 60 ft
one way and two way |
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long span issues
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temperature
expansion and contraction shipping deflection lack of redundancy |
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redundancy
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failure of one portion would cause a larger area of damage if not overall failure
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one way long span systems
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linear members that span in one direction and resist loads primarily by beam action or bending
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two way long span systems
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distribute loads to supports in both directions and involve complex 3D methods of resisting loads
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types of one way systems
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most are steel or concrete
wood are either truss or glulam beams |
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steel girders
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span up to 72ft. and can be strengthened by adding stl plates to top of bottom of beam
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plate girder
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beam built up of plt and angles welded together. efficient due to the flanges being seperated resulting in a high moment of inertia
often 8ft deep or more |
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plate girders as roof beams
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can be tapered to the middle where the moment is the greatest
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rigid frame
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structura systme in which the vertical and horizontal members and joints resist load primarily by flexure and moments are xferd from beams to columns
long span rigid system has a sloped roof w/ a rigid moment resisting connection btwn columns and roof |
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pinned connection
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allows frame to be determinate and does not develop secondary stresses from temperature differences
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tapering structural system
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moment is greatest at the junction where more material is needed to resist forces in the structure
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long span trusses
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good for long span due to stress in compression and tension rather than bending
high strength to weight ratio relatively lightweight prefabed efficient use of material |
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long span steel joists
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LH and DLH series
LH- suitalbe for floor support DLH- suitable for roof support standardized sizes by Steel Joist Institute |
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joist bearing depth
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5 in bearing for LH and DLH up to 17" deep
7.5" for DLH above 17 |
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joist camber
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rise in the beam at center to account for deflection
1/4 per 20' length to 8.5" for 144' span |
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joist bearing systems
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joist can bear on steel, concrete, masonry or joist girders
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joist girder
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depts of 20-120 in.
spacn up to 100' |
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vierendeel truss
|
composed of s series of rigid rect frmaes
no diagonals and must resist bending as well as tension and compression must use larger members than a truss may req. knee bracing |
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prestressed concrete longspan
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reduced cracking and deflection and allows longer spans w/ smaller sections
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prestressed types
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single tee, double tee, AASHTO girder
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AASHTO
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american association of state highway and transportation officials
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single tee
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4-8ft wide
8-12" web depths from 1-4' span of up to 120' |
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double tee
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8-10' wide
8-32" web span up to 60-80' function as both structure and decking |
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arches
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one of oldest longspan systems
mainly compressive force w/ some bending |
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long span design and selection criteria
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function
cost and economy shipping acoustics assembly and erection fire protection |
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cost considerations of long span
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structural system
material labor equipment construction time integration w/ building systems |
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structural costs
|
cheaper to build more columns than deeper beams
more efficient to design for compression and tension than for flexure, x-bracing, deeper bending membera are more efficient that shallow ones |
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construction time costs
|
anything that speeds construction results in substantial cost savings
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shipping limits
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60ft. for truck shipping
80 ft. for rail shipping max height for truck is 14ft. |
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assembly costs
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at times stress on members is greater for erection than under use
w/o redundancy, erection seq. is important size of members need to be considered in relation to crane limitations |
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long span technical considerations
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connections
envelope attachment ponding temperature movement and stresses tolerances stability shop drawing review construction observation |
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long span connections
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most failure at these locations
less redundancy, so more critical verify no changes by fabricator per shops must observe installation |
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long span envelope attachment
|
more prone to larger movement stresses
temperature, deflection, moment rotation greater in long span |
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ponding considerations
|
if roof deflects and does not allow drainage as designed, deflection can increase and ponding become worse
require appropriate slope or camber desinged to allow for long term deflection |
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temperture movement
|
increases in proportion to the length of the member
more critical if exposed to weather |
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long span stability
|
often need temporary shoring to assure stability when complete system not in place
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two way structural system
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distr. loads in two directions and all members are required. load is distributed to more supports and is therefore more efficient than one way
square is most effiecient shape when proportion approaches 2:1, the short dimension carries almost all of the load allows for some amount of redunancy |
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long span two way systems used primarily for...
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roofs due to primary shape
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|
long span two way types
|
space frame
domes- geodesic, thin shelled hyberbolic paraboloids barrel vault lamella arch folded plates suspended cable structures |
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space frame
|
3 dimensional structure xfers load through network of nodal connections
efficient due to large number of members in primarily tension and compression redundancy of members |
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offset grid
|
type of space frame
bottom grid offset from top by one half bay |
|
space frame optimum depth to module ratio
|
0.707:1
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space frame connections
|
sollow or soild sections w/ screws
bolted/welded bent plates prefab units over spanning members |
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space frame supports
|
can be supported an any node
best is symmetrical can cantilever 15-30% of span |
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dome types
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geodesic
frame dome thin shell |
|
dome forces
|
meridian lines act as individual arches xfering loads to the ground through compression, meridians are supported by horizontal hoops
very shallow low rise domes- all compression, not tensile forces high rise dome- under loads meridian compress at top and expand in lower (tension in hoops) if hoops large enough, no thrust passed on to foundation |
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dome stress changes from compression to tension
|
under dead load at 52 degrees
under snow load at 45 degrees |
|
framed dome
|
areas between meridians and hoops braced w/ diagonal members
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geodesic dome
|
designed by buckminster fuller
spherical space frames enclose greatest surface area w/ least surface area |
|
thin shelled structures
|
strength is a result of ability to support loads through compression tension and shear in the plane of the shell
single or double curve |
|
single curved thin shell dome
|
barrel vault
w/ end frames act as a curved beam, w/ compression at top and tensions at bottom w/o end frames, vault will deform inward. vault will req. additional thickness |
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lamella roof
|
single curved barrel vault w/ rounded hips ,created by intersecting grids of parallel skewed arches covering a rectangular area
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|
double curved thin shell dome types
|
synclastic shell- curves on the same side of the surface (dome)
anticlastic shell- main curves on opposite sides of the surface (hyperbolic paraboloid) |
|
double curved thin shell dome
|
stable for either symmetric or asymmetric loads. behave similar to frame domes
|
|
hyperbolic paraboloid
|
anticlastic shell formed by moving a vertical parabola along an upward curving parabola perpendicular to it
|
|
hyperbolic paraboloid characteristics
|
thin shells can be made w/ minimal thickness and very efficient
not often used in US due to labor intensive construction |
|
membrane structures
|
can only resist forces in tension
must be anchored to compression taking members loads change due to wind |
|
counteract membrane forces
|
prestress the membrane w/ anticlastic shapes
|
|
air supported structures
|
pneumatic roof
air supported rather than by cables |
|
folded plate structure
|
thin slab bent to increase load capacity
in longitudinal direction folded plate acts as a beam requires a stiffening slab at each end to compensate for additional stress |
|
suspension structures
|
cable structures support span, efficient as the full cable section is used in resistance
|
|
the sag of a suspension structure cable is...
|
inversely related to the amount of tensile force
|
|
ideal proportion of cable sag
|
1/2 the span so that it create an angle of 45 deg.
|
|
structural lumber
|
primary concern is the amount of stress that a particular grad of lumber of a species will carry
|
|
beams and stringers
|
members 5in wide or greater w/ depth of 2" greater than width
|
|
post and timbers
|
5in by 5" or larger w/ a depth not more than 2" greater than width
|
|
visual graded design values
|
based on the species of wood, size category, grade and direction of loading
also whether used alone or in group |
|
repetitive member loading
|
at least 3 members spaced not more than 24 in apart with some method of distr. load
|
|
moisture content
|
weight of water in wood as a graction of the weight of oven-dry wood
|
|
fiber saturation point
|
when cell walls are completely saturated but no water exist in the cell cavities
abgout 30% moisture content |
|
ideal moisture content
|
the same as exposed prevailing humidity
|
|
wood shrinks
|
prependicular to the grain, very little parallel
|
|
wood beam flexure
|
S= M/Fb
Moment/ extreme fiber stress in bending |
|
wood design shear
|
must be checked against the design unit shear stressw Fv
shear will always occur before vertical shear so only necessary to check vert. if beam is notched at supports |
|
wood deflection
|
not as stiff as other materials and must be checked
|
|
wood deflection reduction
|
code allows 1/2 reduction, but common practice to use full load for long term deflection
|
|
notched beam
|
cannot exceed 1/6 the depth of the member and cannnot be located in middle third of the span
when at supports, notch can be 1/4 of the beam |
|
wood size factor
|
as depth of a beam increase ther is a light decrease in bending strength
|
|
lateral support
|
to allow for tendency to buckle laterally, code allows a reduction if aspects are not met
(sheating at compression edge, bridging at tension) |
|
wood bearing
|
1 1/2" bearing on wood or metal
3" bearing on masonry |
|
wood slenderness ratio
|
laterally unsupported length divided by the least dimension of the column
|
|
wood joists
|
standardized sizes and design values typically referenced from tables
|
|
glulam grades
|
do not affect structural characteristics, but are aesthetic
|
|
two methods of structural stl design
|
allowable stress design
load and resistance factor design |
|
disadvantage to stl structure
|
reduction in stregth due to exposure to fire
will rust |
|
most common steel type
|
ASTM A572, yeild point of 50 kips
|
|
design of steel beams
|
finding the lightest weight section that will resist bending and shear forces and have a minimum amount of deflection for the use
|
|
lateral support
|
support of top member that tends to buckle
steel deck, conc. slab, other beams provide support |
|
compact or non-compact sections
|
based on yeild strength of the steel and width to thickness ratio
if non compact lower bending stress must be used |
|
stl shear design
|
typically the bending stress is enough to account for shear, but needs to be checked for short heavily loaded beams w/ heavy loads near supports
|
|
stl deflections design
|
beam may sag enough to be objectionable for use
|
|
stl slenderness ratio
|
slenderness= length of column/ radius of gyration
|
|
plate girder
|
stl plate as a web and stl plates welded to it for flanges
thin web req. vert stiffeners to prevent buckling |
|
concrete design
|
more complicated than steel or wood
assumptions must be made and then tested use strength design method |
|
min. concrete mix
|
0.35 to 0.40 by weight
4-4.5 gallons of water per 94 lb sack |
|
aggregate percentage
|
70-75% of mix by volume
|
|
typ design strength
|
2000, 3000 or 4000
|
|
conc. admixtures
|
air entraining
accelerators plasticizers |
|
concrete combination loads and strength reduction reduce...
|
probability of failure to about 1: 100,000
|
|
because of the many combinations possible with the load and reduction factors....
|
there is no single structural solution to a concrete design problem
|
|
because a concrete beam is made up of two materials...
|
the neutral axis is not at the midpoint of the beams depth and changes as the load on the beam is increased
|
|
effective depth of concrete beam
|
distance from the top of the beam to the centroid of the reinforcing stl
|
|
strength method of concrete design
|
to resist the bending moment caused by a load, the values of T and C will equal but will act in opposite directions
current thinking requires that steel strength T be designed to fail prior to concrete strength |
|
T (reinforcing stl resistance) reduction
|
reduce steel by 3/4 so that stl fails first
safety factor increases total loads so that concrete stregth is well above necessary amount |
|
if there are no functional or architectrual needs for a shallow beam...
|
reducing the steel percentage and increasing the beam depth generally results i a more economical design and may reduce deflection
|
|
economical concrete beam depth
|
2 to 3x the width
|
|
min clr btwn reinforcing and edge of beam
|
1 1/2"
|
|
concrete beam shear failure
|
often shown as diagonal cracking near support
also referred to as diagonal tension stress |
|
concrete beam shear reinforcement
|
web reinforcement:
bend up tension stel near the supports at a 45 degree angle vertical stirrups, forming a U around the tension steel |
|
double reinforced beams
|
reinforcement located in the top, compression, edge of a beam
reasons: low strength concrete reduces long term deflections caused by concrete creep may just be used to support stirrups prior to concrete pour in case of unexpected negative force on a positive member |
|
concrete beam lateral ties
|
encircle the compression and tension steel on all four sides across the top of the compression reinforcement to form a secure tie
|
|
concrete/rebar connection
|
due to mechanical (deformation in bars) means and chemical bonding
|
|
one of primary safety req. is that there is...
|
sufficient length of stl bar from any point of stress to the end of the bar to develop the necessary bond
|
|
development length
|
primarily dependent on the strenghth of concrete, the strength of steel, the size of the bar, the amount of concrete surround the bar and amount of transverse reinforcement surround the bar
|
|
case I equation use
|
clear cover is not lesss than the bar diameter and the clear spacing is not less than two times the bar diameter
clear cover is not less than the bar diameter and the transverse reinforcing meets the minimum for ties or stirrups |
|
min. development length
|
12 in.
|
|
concrete beam deflections
|
due to strength design method used and higher strength concretes and steels, both result in smaller structural members that are less stiff than in the past
|
|
two phases of concrete deflection
|
immediate deflection- cause by normal dead and live loads
long-term deflection- caused by shrinkage and creep. may be two or more rimes the initial deflection |
|
concrete continuity
|
extension of a structural member over one or more supports
typically continuous in vertical and horizontal directions |
|
continuous beam
|
statically indeterminant, but more efficient than a simply supported beam as the adjacent span tend to counteract each other to an extent
|
|
T-beam
|
effective shape of beam and slab poured together
if neutral axis is equal to or less thant the slab thicknees the sections is designed as a solid beam if teh neutral axis is in the web, special analysis is needed |
|
isolated T-beam
|
flange thickness shall not be less than one half the width of the web and the total flange wiedth shall not be more than four rimes the web width
|
|
symmetrical T-beams
|
smalle of three conditions determines the effective width:
not to exceed on-fourth the span of the beam overhanging slab width shall not exceed 8x the thickness of the slab nto to exceed one-half the clear distance to the next beam |
|
edge T-beams
|
effective overhanging slabg portion shal not exced 1/12 the span of the beam
overhanging slab shall not exceed 6x the thickness of the slab not to exceed one-half the clear distance to the next beam |
|
one way slab
|
reinforcement run in one direction perpendicular to the beams supporting
need extra reinforcement to resist shrinkage and temperature changes |
|
two way slab
|
reinforcement run in both directions
more efficient because the applied loasds are distributed in all directions supports must be near square for efficientcy when raio of length to width approaches 2:1 begins to act as a one way slab regardless |
|
temperature steel
|
min. amount of reinforcement set by ACI code
|
|
concrete compressive members
|
tied columns and spiral columns
|
|
concrete column design
|
primary concern w/ buckling caused by axial load
typical length to width ratio of 8:12, so slenderness ratio not usually an issue |
|
conc column reinforcement
|
tends to fail by buckling and pushing out the concrete coverr at face of column
lateral ties used to prevent this and hold longitudinal steel in place prior to pour |
|
tied column
|
vertical steel running parallel to the length of the column near its faces w/ lateral reinforcement consisting of individual rebgars tied to the vertical reinforcement
usually used for square or rectangle shapes |
|
strength reduction factor for tied column
|
0.70
|
|
spiral column
|
cont. spiral of steel in lieu of individual lateral ties
|
|
spiral pitch
|
dimension between center lines of turns. must be between 1 and 3" clr btwn rebars
|
|
strength reduction factor for spiral column
|
0.75, because spiral columns are slightly stronger than tied of same size and reinforcement
also more ductile and will fail in stages |
|
load bearing wall type
|
vertical load bearing
shear walls retaining walls |
|
allowable stress of masonry wall
|
not only by masonry strength but by slenderness ratio
|
|
load at wall opening
|
lintel only carries load directly above opening at 45 degree angle
if floor load falls w/i this triangle, than lintel carries as well |
|
CIP concrete wall
|
must be anchored to intersectin elements (floors roofs etc)
min ratio for vert rinf. area to gross conc area min ratio for horiz reinf. walls more than 10" exc. foundation walls, must have two layers of reinf reinf. cannot be more than 3x wall thickness no less than two bars around all openings min. thickness of bearing walls not less than 1/25 of height resul of the load must fall w/i the mid one third of the wall thickness |
|
building envelope
|
exterior cladding and its attachment must resist:
dead load horizontal wind load seismic load |
|
suction on a wall cladding element...
|
can be greater than the inward force caused by wind
|