Wood Flashcards
Why is wood used?
Easy to use
Durable
High strength
Low weight
Widely available
Low cost
What is an Endogenous tree?
Tree that grow inward in a longitudinal fibrous mass
Very strong, flexible and lightweight
Not used for engineering application
What is an Exogenous tree?
Fibers grow from the center outward by adding concentric (annual rings) which gives more predictable engineering properties
What are the main structural features of a tree?
Pith - center stem
Heartwood (darker) - provides structural support
Sapwood (lighter) - transports the sap
Cambium (very thin layer) - location of wood growth
Inner bark
Outer bark
What are the 2 types of annual ring?
Earlywood (light ring): rapid growth
Latewood (dark ring): dense summer growth (much harder & stronger)
What are the chemical composition of wood?
Cellulose (40-50% by weight)
Hemicellulose
Lignin (GLUE)
Others
Wood is anisotropic. Explain
the properties change with direction
Longitudinal (Strongest and least shrinkage)
Radial (rayon)
Tangential (weakest and most shrinkage)
Direction influence strength, modulus, thermal expansion, etc.
What is free water and bound water?
Free water is water within cavities
Bound water is on wall cells
What is green moisture content?
Highest moisture content (30 to 200%)
on what does the moisture content depend on?
Humidity and temperature of the surrounding air
When does the equilibrium moisture content (EMC) occurs?
no longer gaining or loosing moisture with its env.
Note: as wood absorbs or bleed moisture, it will expand or contract
What is Fiber saturation point?
cells are completely saturated with bound water BUT no free water
What are the wood product in construction?
Dimensional lumber
Heavy timber
Round stock
What is the best type of board cut?
Quarter-sawn
How do they do seasoning (drying) of wood?
Air drying
Kiln drying
Combination
How much thickness (min) do you need to do surfacing?
1/4”
Note: Nominal dimension is b4 surfacing
What are lumber defects?
Knots
What is the specific gravity of the cell walls of wood?
1.5 (closest to 1.5–> denser & stronger)
Wood has low ___ diffusivity
Wood has low thermal diffusivity
What happens when applying heat to wood?
First expands the wood from thermal expansion
Then shrinks from moisture loss
Note: 5-10 times greater across grain than parallel to it
What is the modulus of elasticity // grain?
7-14MPa
Note: depends on species variation, specific gravity, moisture content, direction of grains
_____ strength of wood is greater than _____ strength of wood.
Tensile strength of wood is greater than compressive strength of wood.
Tensile strength ____ to grains is 20 x greater than _____ to grains.
Tensile strength // to grains is 20 x greater than perpendicular to grains.
Wood supports higher load better on short duration or longer duration
Short duration
Under sustained load, wood deforms
