Uf Structures 1 Rocke Hill

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–pinned connections at all joints

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In general, a truss should have:

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–external forces acting only on the joints

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In general, a truss should have:

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–zero moment at each joint

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In general, a truss should have:

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–members subjected only to axial forces

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In general, a truss should have:

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trusses are usually constructed in

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wood or steel. Concrete is possible but generally not practical.

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A rule of thumb in determining truss depth is a depth to span ratio of

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1/10 –1/15

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a 50’span would have a depth of approximately

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3.3’–5’depending on the load

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Trusses can be looked at in several categories:

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planar truss:
two-way truss system:
space truss:
space frame:

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two dimensional

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planar truss:

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planar trusses connected perpendicular to each other, forming a truss grid.

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two-way truss system

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three dimensional truss

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space truss:

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space trusses connected to form a three dimensional grid

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space frame:

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two-dimensional in their section and connected with pin joints.

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planar trusses

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framed beam structures having vertical web members rigidly connected to parallel top and bottom chords

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Verendeel Trusses

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Are not true trusses because their members are subject to nonaxial bending forces

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verendeel trusses

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connected with rigid joints

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VierendeelTruss

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resist bendingmomentlike the flanges in a steel beam.

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top and bottom chord of the truss

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top chord is in compression while the bottom chord is in tension.

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like the flanges of a steel beam in a simple span condition, the truss

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The web members in the truss resist

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shear,the same as the web of the steel beam.

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Loads on a truss should be applied at

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the panel points.

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By placing loads at the panel points, forces act

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axially. in all the members of the truss so that members are all in tension or compression.

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Loads applied to the member rather than at the panel point introduces

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bending in the member making it behave like a beam.

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Now the members must resist both bending and axial forces--not very efficient.

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Loads applied to the member

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Uniform loads applied directly to the truss members also introduce

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bending moment into the members.

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secondary members that span between trusses,

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Purlins,

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can be used to transfer uniform loads to the panel points.

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Purlins,

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ideally, forces are distributed axially in tension or compression

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Truss Force Distribution

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forces increase towards the centerwhere bending moment increases in beams.

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top and bottom chord:

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forces increase towards the supportswhere shear increases in beams.

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web members:

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b=

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number of members

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n =

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number of joints

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is stable, generally
(equation)

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b = 2n-3

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is stable but indeterminate
(equation)

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b > 2n-3

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is unstable
(equation)

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b < 2n-3

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In order for the truss assembly to be stable,

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the truss or any part of the truss must be in equilibrium.

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A joint, shown in a free-body, must also be in

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equilibrium

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ΣFV= 0 ΣFH= 0 ΣM = 0

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laws of static equilibrium:

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all joints are pinned with zero moment, in the case of

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trusses,

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Diagonal members have

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both vertical and horizontal components.

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AEach member of the truss has

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axial, tensile or compressive,internal forces.

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the relationship of the
rise to run on the
diagonal member
determines the
relationship between the

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vertical and horizontal
components.

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find the resulting diagonal force by using the

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Pythagorean Theorem.

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All forces moving to the joint are

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compressive

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All forces moving away from the joint are

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tensile

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A cable with a constant cross section carrying its own
deadweight will form a

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catenary curve

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A cable carrying a uniform load projected horizontally along its length will form a

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parabolic curve.

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Cables can be looked at in several categories:

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–single cable
–double curved cables
–cable-stayed

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Only_____ ______are developed in cable structures

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tension forces

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they offer no resistance to compression or bending.

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cable structures

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the shallower the cable slope,

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the larger the force in the cable

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____ load generates an upward load on the cable (suction).

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wind load

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(wind loads on cable systems)

The shape of the roof changes in response to _______,resulting in ________ Pressure caused by the effect of the wind on the new shape.

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suction,
downward

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The alternating effects of the wind on the changing shape of the roof cause

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fluttering

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a dynamic and cyclical process.

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fluttering

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-increase dead load
(1 of 4)

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Stabilizing wind loadson simple cable structures to prevent fluttering.:

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-use a stiffened tension element to resist the deformation
(1 of 4)

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Stabilizing wind loadson simple cable structures to prevent fluttering.:

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-use anchor cables to tie the system to the ground
(1 of 4)

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Stabilizing wind loadson simple cable structures to prevent fluttering.:

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-use double-curved cable systems
(1 of 4)

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Stabilizing wind loadson simple cable structures to prevent fluttering.:

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are connected by secondary members.(cables)

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double concave cables, pretensioned cables

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are separated by compression struts. (cables)

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double convex cables,
pretensioned cables

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Similar to the analysis we looked at in trusses, we can also analyze cables using a series of

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free-body diagrams.

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Each free-body must also be in equilibrium satisfying the laws of

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static equilibrium:

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In the case of cables, all supports are pinned with

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zero moment.

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Unlike truss analysis, cables generally have

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pinned
connections, meaning there are a total of 4 unknown
reactions, not 3.

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A free-body diagram is necessary in determining

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the external reactions.

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Determine the vertical reactions either by ________ or by ____________.
(cable problem 1)

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symmetry

the ΣM at the reaction.

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The relationship between the vertical and horizontal force components is the same as the

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relationship between the rise and run for the slope of the line.

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general stress formula,

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f = P/A,

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A = P/Ft
what is Ft

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Ft is the allowable tensile stressfor steel cable.

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For a given shape, cable structures only develop _______________________ and no ___________________

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axial tensile forces
no interior bending moment.

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result in corners
(cable summary)

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pointloads

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horizontally projected, result in parabolic curves
(cable summary)

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uniform loads,

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results in catenary curves
(cable summary)

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self-weight