Wood Flashcards
Why is wood used in construction?
- Easy to use
- Durable
- High Strength
- Low weight
- Widely Available
- Low Cost
In what is wood widely used?
- Building Frames
- Bridges
- Utility Poles
- Floors
- Roofs
- Trusses
- Piles
Classification of Trees?
- Endogenous
- Exogenous
- Decidious
- Coniferous
Describe Endogenous trees.
Trees that grow inwards in a longitudinal fibrous mass
- Very strongm flexible and lightweight
- Not generally used for engineering applications
Describe Exogenous trees.
Fibers grow from the center outward by adding concentric layers (annual rings) which gives more predictable engineering properties.
Describe Decidious trees and give an example.
- Broad Leaf
- Hardwood
- Expensive slow growing
- Example: Ash, Oak, Maple.
Describe Coniferous trees and give an example.
- Cone Bearing
- Softwood
- Example: Fir, Pine and Spruce
List the main Structural Features of tree stem.
- Pith, which is the center stem
- Heartwood, which is darker and provides structural support
- Sapwood, which is lighter and transports the sap (Vascular tissue)
- Cambium, which is a very thin layer and is the location of the wood growth
- Inner Bark
- Outer Bark
What are each annual rings of exogenous tree composed of?
- Earlywood, which is a light ring that has a rapid spring growth of hollow thinwalled cells.
- Latewood, which is a dark ring that has a dense summer growth of thickwalled cells which are much harder and stronger.
What is the Chemical Composition of wood?
- Cellulose, which is 40-50% by weight.
- Hemicellulose, which is 15-20% of softwood and 20-30% of hardwood.
- Lignin, which serves as glue. It is 23-33% of softwood and 16-25% of hardwood.
- Others, which include Extractives (5-30% by weight), Ash-forming (mineralsm 0.1-3% by weight).
What does wood being anisotropic mean?
Properties change with direction
- Longitudinal, which is parallel to the long axis (grain) and is the strongest and least in shrinkage.
- Radial, which is perpendicular to the growth rings.
- Tangential, which is tangent to the growth rings and the weakest and with the most shrinkage.
Which properties does direction influence?
- Strength
- Modulus
- Thermal Expansion
- Conductivity
- Shrinkage
Descibe what bound water is.
Water held by surface attraction.
Describe what free water is.
Water or water vapour inside the cell cavities.
What is the formula for Moisture content?
(Weight of water/Weight of wood) x 100
Moisture Content of wood is directly related to?
Humidity and temperature of the surrounding air.
What depends on moisture content?
Shrinkage, strength and weight.
Explain the Equilibrium Moisture Content.
Occurs when the wood has reached a water content equilibrium with its environment and is no longer gaining or losing moisture.
Explain the Fiber Saturation Point.
Moisture content when cells are completely saturated with bound water but no free water inside cell cavities.
What is the range in % of the FSP?
21-32%
What happens when it goes above the FSP?
Changes affect only wet weight.
What happens when it goes below the FSP?
Small changes strongly affect all physical and mechanical properties.
Where is the largest shrinkage?
Tangential Direction.
Where is the smallest shrinkage?
Longitudinal Direction.
What happens to the shrinkage above FSP?
Zero Shrinkage regardless of direction.
What are the step to produce wood?
- Harvesting
- Sawing
- Seasoning (drying)
- Surfacing (Planing) (Optional)
- Grading
- Preservative Treating (Optional)
Describe Dimensional Lumber and its use.
- 2” to 5” thick.
- Used for light framing such as studs, joists, beams, rafters, trusses, decking.
Describe Heavy Timber and its use.
- 4x6, 6x6, 8x8 and larger
- Usually rough sawn (actual sizes)
- Used for heavy framing, railroad ties and landscaping.
What are the different type of sawing?
- Live (Plain) Sawing, which is the most rapid and economic
- Quarter Sawing, which has the maximum amount of prime (vertical) cuts.
- Combination of the two, which is the most typical one.
What are the three types of board cut?
- Flat-sawn (grain is less than 45 degree from flat side), which is the worst quality, has the most problems and defects.
- Rift-sawn (45-80degree)
- Quarter-sawn (vertical- or edge-sawn) (80-90), which is the best quality, least shrinkage problems.
What the methods of seasoning?
- Air drying, which is the cheapest and slowest.
- Kiln Drying, which is the fastest and most expensive
- Usually a combination.
What does uneven shrinkage in different directions during seasoning causes?
- Warping
- Checks
- shakes
How can you control the uneven shrinkage?
With the different types of cuts (vertical is the best)
What is meant by surfacing?
- Surfacing takes 1/4” (or more) from each side.
What is meant by S4S?
Surfaced 4 sides = Dressed
What are some agencies that grade lumber?
- BC Lumber Manufactureres Association
- West Coast Lumber Inspection Bureau
- Western Wood Products Association
- Eastern Spruce Grading Commitee
What is lumber graded on?
- Visual (appearance) Grading
- Stress (structural) Grading (Strength and durability)
- Imperfections (number of defects: knots, checks)
What causes lumber defects?
- Natural Wood growth
- Seasoning too fast
- Wood Diseases
- Animal Parasites
- Faulty Processing
What are the lumber defects?
- Knots: branch base that degrades mechanical properties
- Bow: curve along the face of a board that usually runs from end to end.
- Blue stain: A bluish grey discoloration on the woods surface. This feature is most common in woods like Holly, Pine and Sycamore.
- Checks or Splits: Breaks at the end of a board that run along the grain.
Explain Engineered wood product
Made by bonding together wood strands, veneers, lumber or other form of wood fibers to produce large units.
Why are Glulam preferred?
- Ease of Manufacturing Large Members from standard commercial lumber
- Can vary the cross section along the length
- Special architectural designs
- Can use lower wood grade in less stressed areas
- Minimizes shrinkage defects
