Wood/Timber Flashcards
(30 cards)
What resists tensile loading of wood parallel to the grain at the microstructural level?
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
What resists tensile loading of wood perpendicular to the grain at the microstructural level?
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)
What are the three methods of failure of wood?
Shear
Compression
Tension
What is the most common method of failure in wood and how does it occur at the micro and microscale?
Shear
Microscale: shear off lignin between cell walls
Macroscale: separation along growth rings
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?
Half of the specimens would be weaker than the average
Half of the members would fail at the designed maximum stress
What is the other statistical method used for the strength of wood besides the mean and how is it used?
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
What is the mechanism that allows thick members of timber to survive fire?
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
Explain the main methods of applying fire regarding methods on wood. Explain the mechanisms for each and how effective they are.
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
What are the environmental and economic benefits of using wood for construction?
Renewable and cheap
Why will wood never be able to compete with concrete as a construction material?
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
What are the two main limitations of using structural timber in construction?
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
How can one get around the main limitations of using structural timber in construction?
Composites
Make them any shape/size
Reduce the amount of defects
Explain why it is more practical to curve parallam or glulam members rather than timber members?
They have thinner individual members
Smaller distance from the neutral axis
Stress from bending moment reduced (since it increases linearly from the NA)
What is common between molds and bacteria in wood construction?
Both are harmful to humans BUT do not significantly damage the timber (bacteria can over like 100 years but not 10 years)
Explain why and how “dry-rot” fungi is a misnomer.
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
What protects a structure from the outside environment?
Building envelope
Walls, sheeting, and roofing
Anything that blocks the outside, external environment from internal sources
What are the two main poor construction practices that can degrade the building envelope?
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
How does the deterioration of the building envelope affect the susceptibility of timber to fungal and bacterial attack?
More susceptible since more moisture will get into the structure
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?
The cube is wood: no macroscopic defects so greater strength
The structural member is timber: a lot of macroscopic defects so strength is reduced
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?
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)
Do you want to place a knot near the top or bottom of a beam? Why?
Near the top
Knots are an issue in tension but not compression
Top of a beam is in compression, bottom is in tension
Why can wall studs be a lower grade material than a structural beam?
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
Why does only some wood float? Why does a saturated wood sink?
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
In wood, which part (cell wall or cell cavity) absorbs water first and which part releases water first?
Cell wall absorbs first
Cell cavity releases first
