Wood And Timber Flashcards
What are some advantages and disadvantages of using timber?
Economic and cost effective
Very useful and good be stronger than concrete in some cases
BUT
Lots of variability
Different types of wood
Cannot be enough wood to make up for the amount of concrete we use
What is the difference between wood and timber?
Wood has no macroscopic defects. It’s a small clear specimen. Stronger than concrete. Stronger density/strength than steel even
Timber is a structural with a wide variety of defects
How are growth rings formed?
Did you know that a tree goes from the outside out. Newest rings are closer to the bark.
Rings formed during less favourable conditions are thinner than the rings formed at favourable conditions and we can see this difference in ring thickness.
New ring per season (fall and spring) so two per year - theoretically
Why are growth rings an inaccurate way to determine a tree’s age?
Some weather conditions can lead to the tree not producing any growth increment or to produce multiple growth increments in the span of one year which makes the number of rings shown unrepresentative of the tree’s age.
What are the two types of wood
Softwood and hardwood
What is softwood. Give 2 examples
Spruce, pine, fir, cedar, EVERGREENS
Less strong and lower mechanical properties and less dense
Grow very long a straight so more efficient to build with because already like in the shape of a beam
Commonly used as lumber
What is hardwood. Give 2 examples
Oak, maple, walnut, ash, birch, elm
More complicated cell structure
Deciduous - sheds leaves
Denser, stronger, more resistant to decay
Has shorter members so less ideal for construction
Describe the microstructure of wood
Hollow, elongated, spindle shaped cells arranged parallel to each other along trunk
Anisotropic because these long narrow cells go in one direction so strength is different based on the direction
What is earlywood and latewood in growth rings?
Earlywood is the large cells formed during springtime. Lots of water and sun. Latewood is when the cells get smaller and more cell walls are visible than cell cavities and this is what forms the growth ring
Describe the microstructure of wood on a more cellular level
Fibre reinforced cells
Outer layer (S1) has x shape
Middle and inner layers have S2 formation which provides most of the strength to the wood because
S2 has fibres in the same direction as the cell
We have lignin in between the cells but it’s the weakest point because not fibre reinforced
What is grain?
The arrangement of a wood’s fibres resulting from the growth of a tree
Wood is an orthotropic material, what does that mean?
Part of wood being anisotropic. Their properties depend on the direction in which they are measured. Opposite of isotopic
What are the three axes of wood and describe them
longitudinal: parallel to the fibre (grain) along the trunk
radial: normal to the growth rings (perp to the grain in radial direction) like drawing a line from the centre of tree outwards perp to growth rings
tangential: perp to grain buy tangent to growth rings (think tangential acceleration on circle)
In wood. How are mechanical properties affected by direction of loading?
Mechanical properties along longitudinal axis are higher than in radial and tangential
In compression, buckling along grain (on outside first), flattening across grain
Low compressive strength!
What is a knot?
A visible imperfection (usually darker)
Caused when a branch falls off from the tree. The tree continues to grow around the hole where the branch has detached.
Why are defects not ideal for construction? Mention flaw distribution
Mechanical properties are affected. Defects have a tendency to decrease strength of wood and change its mode of failure. Flaws are not uniformly distributed so large variability in strength due to flaws.
Name two types of knots
Spike
Loose
Sound right
What is wane
The lack of wood on the face of a piece
Cutting rectangular cross section from a circular tree trunk
what is check
A check is a crack that occurs along the wood’s growth rings. It does not extend through the entire thickness of the board. You may notice that the corners of a board do not touch the ground when the board is flat on a surface.
what is shake
A shake is when the grain between the wood’s growth rings separates.
what is pitch pocket
Pockets of resin or openings in the bark
These are voids and become defects
Is a knot at the top or the bottom going to be weaker?
At the bottom because it experiences tension so the knot gets pulled apart and is weaker. compression at the top of the beam makes the effect of the knot less significant (design standard)
where are the bonds stronger in wood microstructure?
in cell walls > between cell walls
bond between knot and wood is weak point so is tested first
what are the three types of grading?
visual (variability), mechanical (deformation and stiffness), and nondestructive
explain the grade of wall studs
wall studs are only ever used in compression. wood is stronger in compression than in tension so wood only in compression (a wall stud) can be of a lower grade because it is already very strong at a lower (and cheaper) grade
explain specific gravity of wood
SG of cell wall material is around 1.5 (that of water is 1)
it depends on void space and porosity
why do some woods float and other sink
It depends on the specific gravity of the wood and its density. some woods have no void space so they are denser and sink. (lots of 1.5 causes sinking) some have more voids and float (mix of 1.5 and 0 can give average below 1)
what are the two states that moisture can be found in wood?
- free water within the cell cavities
- bound water absorbed into the cell walls
what is FSP?
Fibre saturation point
only water in cell wall and none in the cavity. all water in cell cavities has evaporated but cells walls are still saturated
