Wood and Timber

Question 1

What is glulam made of?

Answer 1

Small pieces of timber glued together to form a big composite piece

Question 2

What is the difference between carpenter ants and termites?

Answer 2

1. Termites eat wood but not carpenter ants
2. Carpenter ants make their nesting sites inside wood by burrowing deep

Question 3

Why does wood exhibit anisotropic properties when it comes to resisting loads?
In what principal direction and loading condition (Compression or Tension) does wood perform the best?

Answer 3

1. Wood is anisotropic because of the orientation of its microstructure. The cells are formed in the longitudinal direction meaning that its strength is greater in that direction
2. Wood is weaker against the grain because there is only lignin and cell buckling to prevent failure. Wood is stronger along the grain.
3. Tension along the grain > Compression along the grain > Compression against the grain = Tension against the grain

Question 4

Why is it wrong to count the growth rings of a tree to determine its age?

Answer 4

Abnormal environmental conditions can lead to abnormal formation of growth rings.
For example, you can have a rainy season and a drought which creates 2 growth rings or neither causing no rings to form

Question 5

What is the difference between wood and timber?

Answer 5

1. Wood refers to the small pieces of material without macroscopic defects.
2. Timber refers to wood that has been specifically processed and manufactured for construction. It contains macroscopic defects.
3. All timber is wood but not all wood is timber

Question 6

What is the difference between earlywood and latewood?

Answer 6

1. Earlywood grows during spring which has good growing conditions. Wood grows fast which leads to a less dense material and less strength. It is lighter coloured
2. Latewood refers to wood grown during summer with slower growth periods which leads to a denser and darker coloured wood. It has a higher strength

Question 7

What is the difference between softwood and hardwood?

Answer 7

1. Softwood refers to trees that grow straight and are generally less dense which make them weigh less and less strength
2. Hardwood refers to trees that grow weirdly and are generally more dense which makes them weigh more. They have a higher strength compared to softwood

Question 8

Why do we use softwood in construction instead of hardwood?

Answer 8

1. Softwood is easier to cut into long and straight sections which makes it more economical to use

Question 9

Why does it look like there’s a line between earlywood and latewood?

Answer 9

1. Earlywood grows faster which makes it less dense and lighter coloured
2. Latewood grows slower which makes it more dense and darker coloured
3. The juxtaposition of earlywood and latewood makes it seems as if there’s a line seperate the two because of the difference in density and colour.

Question 10

What is the difference between the S2 layer and the S1 layer in the microstructure?

Answer 10

S2 layer cells grow longitudinally providing strength in that direction whereas S1 cells grow horizontally

Question 11

What is the difference between anisotropic and orthotropic?

Answer 11

1. Anisotropic = not same strength in all directions
2. Orthotropic = different strength for each directions (x, y, z)
3. When a material is orthotropic, it necessarily means it is anisotropic
4. When a material is anisotropic, it does not necessarily mean it is orthotropic

Question 12

What does lateral support do in the microstructure of wood?

Answer 12

Provides support around each cell which increases strength because it reduces the chances of buckling

Question 13

What are the failure modes involved when loading wood longitudinally vs laterally?

Answer 13

1. Longitudinally = buckling
2. Laterally = collapse of cells or flattening

Question 14

What are the 3 main limitations when using timber in construction?

Answer 14

1. Low availability
2. Wood has more chances of containing flaws like knots
3. Size of the tree limits the size of the beam (you cannot make a beam thicker than the tree itself)

Question 15

Why do thick members of wood survive in a fire?

Answer 15

1. When wood is burnt, a char layer is formed at the surface
2. The char layer surrounds the wood
3. The char layer is inflammable and insulates the wood from the heat of the fire
4. Directly beneath the char layer is the pyrolysis zone which is also inflammable
   5 . Wood beneath pyrolysis zone will remain cool and protected from the fire
5. A thicker member of wood has an increased cross-sectional area, which means they have more wood below the char layer that can be protected

Question 16

Why is it bad to use mean strength with wood?

Answer 16

1. When wood is cut, it is graded based on its mechanical properties such as strength
2. Mean strength does not consider the lower half of the strength curve = 50% of wood structures will fail when using mean strength
3. Choose wood so that only 5% of specimens fail below a specified strength value
4. 5% is a reasonable limit based on the risk of having a failure below 5% strength requirement for a structure

Question 17

How does plywood resists shrinkage and splitting?

Answer 17

1. Plywood is arranged in planes of 90 degrees perpendicular to each other
2. The 90 degree arrangement allows plywood to resist shrinkage and splitting because the opposite direction grain minimizes shrinkage and resists splitting

Question 18

What is the overall density of wood and how do you calculate it?

Answer 18

1. Overall density = cell wall density + void space
2. All wood have a cell wall density of 1.5
3. What differs is the amount of void space

Question 19

Why does wood sink?

Answer 19

1. For a material to sink, it needs to have a density higher than water (>1)
2. Putting wood in water will fill the void space of wood with water
3. Overall density = 1.5 (cell wall) + 1 (void space filled with water)
4. When wood is fully saturated, it has an overall density of 2.5 which is bigger than 1, making it sink in water

Question 20

What are the states of moisture (states of water) in wood?

Answer 20

1. Free water within the cell cavities (void space filled with water)
2. Bound water (absorbed by cells)

Question 21

What is the fiber saturation point and how is it acquired?

Answer 21

1. Cell walls have fully absorbed water but there is no free water in cell cavities
2. Water evaporates from void space first, then from cells
3. Water is always absorbed by cells first, then fills the void space

Question 22

What is the effect of moisture content above or below FSP?

Answer 22

1. When moisture is below FSP (about 25%), water is removed from cell walls causing the wood to shrink and increase its strength
2. When moisture is above FSP (about 25%), no shrinkage occurs and anything above FSP has the same mechanical properties

Question 23

What is green wood and why is it bad for construction?

Answer 23

1. Wood that is freshly sawn (cut) has a lot of water in it (high moisture content)
2. We use green wood for construction
3. Green wood will dry out after construction and will shrink

Question 24

What is the solution for green wood in construction?

Answer 24

1. We dry out the wood before using it for construction to prevent shrinkage
2. To prevent shrinkage and warping, we pile the dry wood in a certain way
3. After shrinking the green wood, we build a building envelope (walls, sheathing) to protect the dry wood from moisture changes with the outside world

Question 25

How does wood shrink and what are the consequences of shrinking?

Answer 25

1. Shrinks most in tangential direction of the growth rings
2. Shrinks half as much in the radial direction
3. Longitudinal direction shrinks the least
4. Shrinkage causes warping, checking, splitting

Question 26

Why is wood stronger in tension than in compression?

Answer 26

1. In compression, buckling occurs which reduces strength of wood
2. Lignin is less effective in compression than in tension

Question 27

What happens when wood is compressed in the parallel direction?

Answer 27

1. Poisson effect = material expands in the perpendicular direction to the loading
2. Indirect tension occurs which separates growth rings

Question 28

What are some durability issues for wood?

Answer 28

1. Fungi
2. Bacteria
3. Insects
4. Fire
5. Chemical attack

Question 29

How do you control fungi?

Answer 29

1. Fungi occurs above FSP when moisture content is higher
2. Make sure that wood is below FSP to reduce fungi
3. Sometimes, water enters wood due to poor construction practices

Question 30

How does pressure impregnation protects from fire?

Answer 30

1. Put wood in a vacuum chamber where pores are no longer filled with air
2. Immerse the wood in a fire retardant
3. Release the vacuum which pushes the fire retardant deep into the wood

Question 31

What is the effect of bending glulam vs bending a regular beam?

Answer 31

1. Under bending, you have less compressive and tensile stresses because your glulam is less thick than a regular beam
2. You should bend all your small glulam pieces first and then combine them
3. Do not combine first and then bend because it will be like a regular beam

Question 32

Why are wood studs defined as a different grade from wood?

Answer 32

Lowest quality of wood mostly knots, knots in compression have no effect but in tension they can fail

Stud grade wood always have to be in compression

Question 33

How does fungi grow?

Answer 33

They consume wood and breaks down the wood
They need moisture and temperature to grow