Wood/Timber

Question 1

What resists tensile loading of wood parallel to the grain at the microstructural level?

Answer 1

Strong primary bonds:

Microfibrils in the S2 wall

They are parallel to the axis of the cell (along the grain)

They are a collection of cellulose molecules bonded together with strong covalent bonds

Question 2

What resists tensile loading of wood perpendicular to the grain at the microstructural level?

Answer 2

Weak secondary bonds:

Lignin between cell walls

The hydrogen bonds between the microfibrils in the S2 wall

The fibers in the S1 and S3 wall (they are weaker since they are shorter)

Question 3

What are the three methods of failure of wood?

Answer 3

Shear
Compression
Tension

Question 4

What is the most common method of failure in wood and how does it occur at the micro and microscale?

Answer 4

Shear

Microscale: shear off lignin between cell walls
Macroscale: separation along growth rings

Question 5

Assuming no other design adjustments are relevant, why is it a bad idea to use the mean strength of wood cubes when designing for a structure?

Answer 5

Half of the specimens would be weaker than the average

Half of the members would fail at the designed maximum stress

Question 6

What is the other statistical method used for the strength of wood besides the mean and how is it used?

Answer 6

5% exclusion limit

Only 5% of the specimens will be of less strength than what is designed for

This is potentially also applicable to other materials, but it is most relevant for wood

Question 7

What is the mechanism that allows thick members of timber to survive fire?

Answer 7

A layer of char is formed immediately next to the fire (this layer is not flammable and is a good insulator)

Beyond the char there is the pyrolysis zone

Beyond the pyrolysis zone is regular wood

Question 8

Explain the main methods of applying fire regarding methods on wood. Explain the mechanisms for each and how effective they are.

Answer 8

Pressure impregnation:

Can be used for a new structure (since members can be fully dipped into a liquid)

Steps:
1. Place wood into a vacuum
2. Dip the wood into fire retardant
3. Release the vacuum (thus, pushing the fire retardant into the wood)

Result - 1.3 cm or more penetrated

Surface coating:

Only method that is effective on existing structures

Paint it on and capillary suction will suck the fire retardant into the wood

Only penetrates 2-3 mm deep

Question 9

What are the environmental and economic benefits of using wood for construction?

Answer 9

Renewable and cheap

Question 10

Why will wood never be able to compete with concrete as a construction material?

Answer 10

Volume requirements for how much wood would be needed

Concrete is manageable bc it’s just rocks

Would need to cut down too many trees to match that demand for wood

Question 11

What are the two main limitations of using structural timber in construction?

Answer 11

Size and shape: you are limited by the size and shape of the tree from which the wood is sourced

A lot of defects in the timber mean that you must limit the allowable stresses

Question 12

How can one get around the main limitations of using structural timber in construction?

Answer 12

Composites

Make them any shape/size
Reduce the amount of defects

Question 13

Explain why it is more practical to curve parallam or glulam members rather than timber members?

Answer 13

They have thinner individual members

Smaller distance from the neutral axis

Stress from bending moment reduced (since it increases linearly from the NA)

Question 14

What is common between molds and bacteria in wood construction?

Answer 14

Both are harmful to humans BUT do not significantly damage the timber (bacteria can over like 100 years but not 10 years)

Question 15

Explain why and how “dry-rot” fungi is a misnomer.

Answer 15

Wood has to be saturated for a fungus to exist

For dry-rot fungus, there doesn’t have to be water at the site - the fungus will take in water from 2-3 m away

Question 16

What protects a structure from the outside environment?

Answer 16

Building envelope

Walls, sheeting, and roofing
Anything that blocks the outside, external environment from internal sources

Question 17

What are the two main poor construction practices that can degrade the building envelope?

Answer 17

Holes for the dryer duct on the outside of a building

Balcony railings (you have a hole in the wall to anchor the railing to the building)

Both of these can actually be done without damaging the envelope with proper sealing but this is usually not done

Question 18

How does the deterioration of the building envelope affect the susceptibility of timber to fungal and bacterial attack?

Answer 18

More susceptible since more moisture will get into the structure

Question 19

Why is it that if we test a hardwood cube, the strength will be similar to or exceed concrete but if we test a structural member made of the same material, its strength is lower? What is the difference in nomenclature between these two cases?

Answer 19

The cube is wood: no macroscopic defects so greater strength

The structural member is timber: a lot of macroscopic defects so strength is reduced

Question 20

The same vertical compressive load is applied on two specimens. Specimen 1 has growth rings orientated vertically. Specimen 2 has growth rings orientated horizontally. Which specimen is stronger and why?

Answer 20

Specimen 2 is stronger

The vertical compression will result in tensile stresses in the other directions because of Poisson

In Specimen 1, the tension will split apart the growth rings (the weak point). In Specimen 2, the tension will split apart along the growth rings.

(It’s better to just know the picture)

Question 21

Do you want to place a knot near the top or bottom of a beam? Why?

Answer 21

Near the top

Knots are an issue in tension but not compression

Top of a beam is in compression, bottom is in tension

Question 22

Why can wall studs be a lower grade material than a structural beam?

Answer 22

Load sharing: if one stud is weak the other ones will make up for that lost strength

They are compression members only: defects like knots have less of an effect

Question 23

Why does only some wood float? Why does a saturated wood sink?

Answer 23

The wood material itself is 1.5 specific gravity (SG) for all species

Different species have different SG due to different amount of voids

As long as SG below 1, you float

If there are enough voids filled with air (SG - 0), will balance out with the wood material (SG + 1.5) to have an overall SG < 1 so will float

If voids fill up with water, their SG is now 1. Average of 1 and 1.5 will be > 1 so will sink

Question 24

In wood, which part (cell wall or cell cavity) absorbs water first and which part releases water first?

Answer 24

Cell wall absorbs first

Cell cavity releases first

Question 25

What is the fiber saturation point?

Answer 25

The moisture content at which the cell walls are 100% saturated with water BUT the cell cavities are 100% empty

Question 26

What happens to wood when moisture drops below FSP in terms of its mechanical properties, shape, and durability?

Answer 26

Mechanical properties: stronger

Shape: shrinks

Durability: increased (no bacterial or fungal attack)

Question 27

Explain why counting growth rings is a good but not foolproof approximation for determining tree age.

Answer 27

In general, sunny and wet growing conditions (spring) give less dense earlywood

Dry and less sunny growing conditions (summer) give more dense latewood

Alternation between earlywood and latewood gives appearance of rings

In many climates, per year there will be one earlywood layer and one latewood layer

However, this can be affected by abnormal environments such as droughts or extra moisture which will give you more or less than 1 growth ring per year

Question 28

What is green wood?

Answer 28

Freshly cut wood

Question 29

Why is it a bad idea to build with green wood?

Answer 29

Green wood is usually at or above FSP

As it dries it will shrink and warp

If you build something with green wood, once it starts shrinking there are stresses applied

Question 30

If a square is cut from a trunk, how is this piece of wood going to shrink and what will it look like (which direction has more shrinkage)?

Answer 30

Tangential: more shrinkage

Radial: less shrinkage