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73 Cards in this Set

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  • Back

What are the ideal properties of wood and their corresponding molecular structure?

Polymer Chains - Cellulose

Alignment to form fibres - Micro-fibrils of cellulose

Maximize strength/weight ratio by forming tubes in matrix of glue - Tube "cells" made of micro-fibrils embedded in resin matrix (lignin)

What are the major advantages of wood?

- Reusable, recyclable, biodegradable

- CO2 Storage

- Aesthetics

- Cost effective

- Renewable resource

What are the major disadvantages of wood?

- Food source for fungi, insects, and bacteria

- Many imperfections that tend to control structural behavior

What orientation type does wood fall under? What does this orientation type imply?

Orthotropic - properties vary between parallel, tangential and radial axis.

** Mechanical properties for each axis may vary by a factor of 20

What is the difference between wood, timber, and lumber?

Wood - defect free material, small clear pieces

Timber - natural material which includes defects

Lumber - timber sawn into structural members

What molecular structure is wood composed of?

49% Carbon

Composed of the following organic polymers

- Cellulose 40-44%

- Hemicellulose 15-35%

- Lignin 18-35%

- Extratives 1-5%

What is cellulose and their main contributions to the mechanical properties of wood?

- Linear polymer chain of glucose units (covalently bonding leads to high strength and stiffness)

- Laterally bonded into linear bundles

- Provides stength and framework of cells

Hydroxyl units (OH-) provide bonding for what part of wood?

- It attracts cellulose molecules making microfibrils

- Attracts water which is largely responsible for swelling and shrinking

What are lignins and their main contributions to the mechanical properties of wood?

- Complex high molecular weight polymer of phenolpropane units

- Permeate cellulose microfibrils in cell walls

- Lignins and hemicellulose surrond cellulose units, bonding them together (similar to a surrounding matrix)

- Imparts rigidity and compressibe strength to cell walls

- Adds to woods toxicity (resistant to decay and insect attack)

Bonus ** Lignins are washed out during the paper making process; black liquor is used as an admixture for water reduction in concrete.


What are Extratives and their main contributions to mechanical properties of wood?

- Variety of chemical substances

- Provides properties such as colour, odour, taste, strength, flammability, hyrgroscopicity

Not part of basic wood structure

What are Microfibrils and their main contributions to mechanical properties of wood?

- Threadlike bundles of cellulose molecules that are arranged parallel

- Consist of cellulose, hemi-cellulose, and lignin

Describe the orientation of the different layers of microfibrils.

From Outside to Inside:

Middle Lamella - Lignin rich area which joins neighbouring cells

Primary wall - thin, oriented randomly

Outer Layer (S1) - left and right hand helix 50-70% inclination

Middle Layer** (S2) - Very important layer for elastic modulus and stiffness of wood; Steeper angled S2 increased stiffness and wood from older trees tend to be steeper

Inner Layer (S3) - left and right hand helix 50-70% inclination

Page 1-14 of notes- Schematic Illustration of parts of a tree

What is the purpose of outer bark?


What is the purpose of inner bark?

Transport sap from leaves to the growth parts

What is Cambium?

Layer of tissue 1 cell thick; between bark and wood - cell division forms new wood

What is sapwood?

Roughly 1/3 of a trees's wood; responsible for moisture conduction and food storage

What is heartwood?

Inner, non-living core. More resistant to decay than sapwood and is drier and harder

What is the difference between Spring wood (early) and Summer wood (late)?

Spring wood- rapid growth of large cells with thin walls

Summer wood- thicker cell walls; stronger mechanically

What is a pith?

Center of wood

What is the difference between softwood and hardwood?

Softwood- Needle or scale-like leaves (SPF, D-F, Hemlock, Cedar, Cypress, Redwood, Larch)

Hardwood- Broadleaf trees (lose leaves in winter), Alder, Poplar, Birch, Oak... etc.

Used more for furniture, rather than structural purposes

What is a Tracheid?

- Longitudinally Oriented cells

- 2 to 5mm long with aspect ratio of 100

- Make of 90% of volume of wood in softwoods

** Responsible for mechanical support of tree and vertical conduction of water

What is a Parenchyma?

- Transversely oriented cells

- 30um wide by 200um long

- located in rays extending radially within a tree

** Stores food and transports horizontally

What is the density of wood controlled by?

Void spaces located within the wood

Why is density important when taking about mechanical properties of wood?

Void space (porosity) control density, which also results in large changes in mechanical properties

Moisture Content exists in wood in what two states?

Free water - which liquid is trapped in cell cavities (>28% M.C)

Bound water - physically absorb water by the cell walls (<28% M.C)

**Bigger holes dry more quickly

What does green wood entail?

Green wood has high moisture content due because it contains sap and the cell cavities are full or partially full

What is fibre saturation point?

When all free water is evaporated, but the cell walls are still fully saturated

As drying occurs below Fiber Saturation Point, what wood properties change?

Wood becomes stronger mechanically, but starts to shrink

below the FSP the volumetric shrinkage is approximately proportional to the volume of water lost

Drying wood too quickly leads to what?

Cracking and warping

What are the codes and relative M.C of Dry and Green Lumber

S-DRY - < 19%

S-GRN - > 19%

How do shrinkage minimized in wood

- Slower rate of drying

- Coatings

- Protective wraps

- Protect from rain

- Use wood near EMC such as Kiln Dried Lumber

What are the tensile properties of wood?

70 to 150 MPa parallel to grain

- Requires rupture of primary bonds

- Small failure strain (1%)

2 to 9 MPa perpendicular to grain

- Secondary bonds between tubs are weaker

- large failure strain due to distortion of tubes

What are the compressive properties of wood?

25 to 60 MPa parallel to grain

- develop small kinks within microfibril at failure and localized buckling of cell walls

- kinks can be seen on surface

3 to 10 MPa perpendicular to grain

- cells collapse or flatten at low stress

What key properties of wood are important in regards to flexural strength?

Failure will occur first in compression, then neutral axis will lower and eventually tensile failure will occur

40 to 100 MPa

What are the three types of failure in shear wood exhibits?

Shear perpendicular to grain, parallel to grain (5 to 15 MPa), and rolling shear (25% of previous)

What does temperature do to properties of wood?

- Prolonged exposure at high temperature degrades wood irreversibly

- Made worse by higher M.C

** Rule of Thumb 1 Degree changes the mechanical property by 0.6 to 1 %

What effect does the rate of loading have on wood?

increase in apparent strength as the rate of load increases (aprrox. 8% for every 10 times increase)

What effect does duration of loading have on wood?

Increased load duration results in reduced strength

What effect does creep have on wood?

- initial deformation is elastic

- if load is maintained for long periods of time, creep will occur

- occurs at low stress levels

**higher temp and moisture condition increases creep by approx 10 times

**Wet/dry cycles further increases creep

What is fatigue?

Progressive damage that occurs in a material that is subjected to cyclical loads

What factors effect fatigue?

- Number of cycles

- Frequency of cycles

- Stress level relative to failure

- Stress differential (between max and min values)

- Stress reversal (tension and compression)

- Temperature, moisture, size

What is a knot in regards to defects?

portion of a limb surrounded by subsequent wood growth

**shape and type of knot (encased(dead) knot and intergrown knot) influence mechanical properties

What is a check in regards to defects?

Lengthwise separation of wood which usually extends across the growth rings

Caused by differential in temperature of the surface and inner core

What is a wane?

lack of wood on the face of a piece for any reason

What is a shake?

Separation of along the grain between the annual growth rings (caused by wind damage)

What is a pitch pocket?

Opening between growth rings containing resin or bark

What are the four types of warping observed in wood?

Bow, Crook, Cup, Twist

What are the visual grading criteria?

- Density

- Decay

- Heartwood or sapwood

- Slope of grain

- Defects

How does machine stress grading work?

Machine measures the stiffness of a piece of timber by using the empirical relationship between stiffness and strength

Why is it important to grade wood?

Permits the designer to choose appropriate mechanical properties for structural purposes

Mechanical properties of wood are factored or reduced to account for:

- Variability of wood (using 5% limit)

- Conversion from small wood to lumber or timber size

- Conversion from perfect lumber or timer to specific grade category

What stress modification factor does Kh account for?

Load Sharing Factor - Accounts for redistribution of load from a highly stressed member to surrounding members

What stress modification factor does Kd account for?

Dependent of duration/rate of loading. Reduces or increases allowable stress depending on short or long term loading

What stress modification factor does Kt account for?

Treatment factor; chemicals for preserving or fire retardant may reduce strength properties

What stress modification factor does Ks account for?

Service Condition Factor; strength dependent on M.C, used to modify conditions other than dry

What stress modification factor does Kz account for?

Used to further account for effect of size on strength (Reduces)

List 5 types of manufactured wood products.

- Fingerjoined Lumber

- Glulam

- Structural Composity Lumber (SCL) ; Includes Laminated Veneer Lumber (LVL), Laminated Strand Lumber (LSL), Parallel Strand Lumber (PSL), CLT, Nail Laminated Timber

- Light frame truss

- Wood I-Joists

- Plywood and OSB

What is CLT?

Cross Laminated Timber - Kiln dried lumber stacked 90* and glued in alternating layers

- 3 to 7 layers

- good dimensional stability

- Use for more strictly controlled fire rated occupancies

What is CLT used for?

- Create large panels for walls, floors, and composite systems

- rapid construction erection

What is NLT and its pros and cons?

Nail Laminated Timber

- Similar to CLT, but boards only run in one direction (nailed together)

- good for one way spans; very weak in second axis

- less cost than CLT

What is Glulam and its pros and cons?

Wood pieces glued together with adhesive to make long curved structures

- Strong for long spans

What is LVL and its pros and cons?

Laminated Veneer Lumber

- Layered composite of wood veneers and adhesive

- Good for headers, beams, and flanges

- Each grain runes in the longitudinal direction, making it strong as a beam or plank

What is LSL?

Oriented strands (like OSB) made from aspen or poplar

- For headers, beams, and columns

What is PSL?

Structural composite lumber product made by gluing long strands of wood together under pressure (high strength, less prone to shrinkage, not for exterior exposure)

What are the advantages and disadvantages of Wood I-Joists?

- Dimensionally stable

- Known engineering properties

- well suited for long span

- Has serviceability issues

What is sheathing?

Flat plates of sheets for roofs, walls, and floors

- Plywood


- Wafer Board

- Sawn Lumber

What is Plywood?

Built up sheets of softwood veneer glued together with adhesive

- Direction of each grain is rotated by 90*

What is the difference between OSB and waferboard?

Oriented Strand Board

- laminated thin chips of Poplar

Waferboard is randomly oriented

OSB is oriented in long direction

What are the major factors that decrease the durability of wood?

Decay, fire, termites, and marine borers

What are the three zones of burnt wood?

Char layer, Pyrolysis zone, Normal wood

What do termites do to wood? How do we mitigate?

Create shelters in soil and travel in tubes into the wood structure. Best protection is to block access for the soil to wood through appropriate construct ion techniques.

To thrive fungi need:

Source of food, warmth, moisture, and oxygen