Wood & Timber Materials Flashcards
Where do softwoods come from?
cone-bearing (coniferous) trees
Describe the cell structure of softwoods.
relatively simple
Describe the grain structure of softwoods.
plain grain structure
Where are softwoods typically sourced?
North American forests
What are some advantages of softwoods? (4)
- fast-growing
- plentiful
- relatively inexpensive
- easily worked
What are softwoods used for? (5)
- structural wood products
- finish trim
- shingles
- siding
- flooring
Are all softwoods soft?
no - Douglas Fir is a softwood but is harder than some hardwoods
Where do hardwoods come from?
from broad-leafed (deciduous) trees
Describe the cell structure of hardwoods.
more complex
Describe the grain structure of hardwoods.
more interesting
Where are hardwoods typically sourced?
around the world
What are some advantages and disadvantages of hardwoods?
- advantages: denser, with a greater variety of colors and figure
- disadvantages: slower growing, generally more expensive than softwoods
Why is wood the most sustainable of construction materials? (4)
- wood is the only major structural material that is renewable
- in North America, annual tree growth exceeds the volume of harvested trees
- manufactured wood products efficiently use waste
- if the wood frame of a building is kept dry, it can last indefinitely
What sort of strength does wood possess?
wood has both tensile and compressive strength
What natural factors can affect wood strength? (3)
- direction of grain
- species
- presence of knots or other defects
What man-made factors can affect wood strength? (5)
- duration of load
- moisture content
- chemical treatments
- temperature
- size and shape
Why are timber engineering properties often unique?
they’re relative to the grain orientation
What do the mechanical properties of timber often depend upon, and why? (2)
- mechanical properties depend on load direction
- mechanical properties along the longitudinal axis are usually greater than those along the radial and tangential axis
What are the five steps of sawn wood production?
- sawing into desired shapes
- seasoning
- surfacing
- grading
- preservative treatments
What is the difference between plainsawn and quartersawn wood?
- plainsawn: growth rings roughly parallel to wider face of board
- quartersawn: growth rings close to perpendicular to wider face of board
What are some pros and cons of plainsawn lumber?
- pros: most economical sawing method; produces maximum yield of useful pieces
- cons: lumber experiences greater distortion during drying process
What are some pros and cons of quartersawn lumber?
- cons: requires more handling of the log during sawing; not as efficient, generates more waste
- pros: lumber is more dimensionally stable; produces more pleasing grain figure
Why is kiln drying, or seasoning, generally preferred to air drying?
kiln drying uses more energy but it is faster and produces more stable, uniform lumber
Why is lumber dried/seasoned?
seasoned lumber is lighter, stronger, and stiffer than green, or unseasoned, lumber
What is moisture content (MC)?
the amount of water present in the wood
What formula is used to determine moisture content (MC)?
MC = (wet weight - dry weight) / dry weight x 100
What is the standard seasoning for framing lumber?
moisture content (MC) 19
What does the KD19 stamp on lumber mean?
- KD: kiln dried
- 19: moisture content (MC)
How widely does the moisture content (MC) of growing wood vary?
from 30% to 200%
What moisture content (MC) is required to prevent decay-causing fungi?
below 20%
What is advantageous about the manufactured moisture content (MC) of lumber?
it’s close to the in-service conditions, resulting in dimensional and structural stability
How do changes in moisture content (MC) affect the lumber?
wood does not shrink or swell uniformly with changes in MC
What parts of the wood experience the worst distortion with changes in moisture content (MC)? (3)
- shrinkage along the length of the log is minimal
- radial shrinkage is larger
- tangential shrinkage is larger again by half or more and causes radial cracks called checks
Why is lumber surfaced?
lumber is surfaced to make it smooth and more dimensionally precise
How is framing lumber surfaced?
usually surfaced on four sides, or S4S
How is finish lumber surfaced?
may S4S, or surfaced on two sides, or S2S, the other sides to be sawn and surfaced by the woodworker
What is surfacing after seasoning, or S-DRY?
most common; removes some drying distortions
What is surfacing before seasoning, or S-GRN?
sometimes more economical; best for wood species that don’t distort excessively as they dry
What is construction planking?
unsurfaced; no material has been removed and more slip-resistant surface
What is structural grading?
framing lumber is graded and stamped for structural strength and stiffness
What does a higher structural grade indicate?
higher structural grades have fewer defects and, when left exposed, are generally more attractive
How is structural grading performed?
structural grading can be performed visually or it may be done by machines
What is appearance grading, and how is it performed?
ranks the visual qualities of lumber intended for flooring, trim, or cabinetry; always performed visually
How are lumber dimensions calculated in the U.S.?
lumber sizes are given as nominal dimensions in inches
How do actual sizes differ from nominal sizes?
actual sizes are less than nominal size
How is lumber priced in the U.S.?
lumber is priced by the board foot based on nominal dimensions
What is the board footage (BF) formula?
nominal dimension / 12 x length of board
