Wood

Question 1

why do we use softwoods in construction?

Answer 1

cheaper, it grows straight, grows fast.

Question 2

what are bark, cambium and sapwood

Answer 2

sapwood: active cells in the wood and food storage
cambium: new cell growth area
Bark: outside surface of the wood that protects it from environmental

Question 3

why is wood anisotropic?

Answer 3

wood is composed of hollow, elongated, spindle-shaped cells that are placed parallel to each other, making its properties (such as strength) different depending on the direction of these cells.

Question 4

what are the three main components of wood and their roles?

Answer 4

cellulite, hemi-cellulite and lignin.
Cellulite: thin long fibers that compose 50% of wood. It also attracts water molecules and is responsible for shrinkage and swelling
Hemi-Cellulite: enables fiber-to-fiber bonding
lignin: acts as a glue for the wood cells.

Question 5

what does anisotropic mean?

Answer 5

different material resistance depending on different directions of loading.

Question 6

what is the difference between compression along main axis and perpendicular to the main axis

Answer 6

along axis: buckling of the cells, adjacent cells provide support
perpendicular: crushes the cells, microstructure is weak (hollow cells)

Question 7

what is the difference between tension along main axis and perpendicular to the main axis

Answer 7

along axis: resisted by cellulose and hemicellulose, weak planes between growth rings are not loaded
perpendicualr: pulls apart the lignin bonds, weak planes are loaded.

Question 8

whats the difference between orthotropic and anisotropic

Answer 8

Orthotropic is more specifically in three perpendicular direction. Wood is orthotropic, and has a radial, longitudinal and tangential directional strength

Question 9

what are growth rings

Answer 9

change in density of the cells due the change of environmental conditions (seasons)

Question 10

what does applying a load along the grain mean?

Answer 10

the load is applied on the longitudinal axis

Question 11

what is the difference between latewood and earlywood?

Answer 11

Latewood is more compacted and dense, whereas earlywood is less compacted and takes up more space.

Question 12

how are the cell walls arranged and how does it impact wood strength?

Answer 12

S1 and S3 have microfibrils running perpendicular to the S2 microfibrils (longitudinal), S3 being 15% off the horizontal axes. the S2 layer is by far the thickest

Question 13

what happens when cell walls are compressed along the grain?

Answer 13

the fail in buckling

Question 14

does wood have any defects?

Answer 14

No, but timber does ;) Wood is used to study the relationship between its properties and the microstructure, therefore it needs to be free of defects

Question 15

how are knots formed?

Answer 15

a branch grows laterally, but eventually dies. The lateral growth is still there though, and the trunk keeps growing and surrounds it. In the end there is a laterally grown defect in the middle of the trunk, which is also refered as a knot.

Question 16

how do defects affect wood properties

Answer 16

they decrease the strength and change the mode of failure.

Question 17

name as many types of defects and describe them

Answer 17

fiber and ring orientation: the axes of the board have and angle with the grain orientation
knots: two types - spike and loose knots. Portion of a limb that was surrounded by wood cells. They distort the fibers which lead to tensile stresses perpendicular to the grain. Their effect depends on size, type, frequency and location of the knot.
Checks: lengthwise separation of the wood.
Wane: lack of wood on the face of a piece.
Shake: separation along the grain (growth ring detaches)
pitch pocket: hole between growth rings, filled with resin or bark.

Question 18

what is the difference between a spike and a loose knot

Answer 18

A spike knot is when the stem of a branch makes an accute angle in the stem of the trunk.
A loose knot cannot vind properly to the wood cells surrounding it because of the bark. Bonus knowledge: the position of the knot is very important when planning a structure, as it makes the mode of failure vary drastically.

Question 19

What are the different types of grading?

Answer 19

visual grading: just look for variability, most commonly used. Can check for-
densitiy
decay
Heartwood or sapwood
knots
slope of grain
any other defects

mechanical grading: stiffness
non-destructive grading: Modulus of elasticity

Question 20

how are mechanical properties related to relative density

Answer 20

mechanical properties are a function of porosity (the more the wood is porous, the smaller the mechanical properties). Relative density is also a function of void space (porosity). It is to be noted that the relative density of cell walls is always 1.5, but the overall density of species varies dramatically.

Question 21

In what ways can moisture exist in wood? What are the differences between these two states?

Answer 21

As bound water absorbed by cell walls, and as free water between the cavites. The free water evaporates first, because the bound water is inside the cell, so it takes more energy for heat to get through and dry it.

Question 22

What is the fiber saturation point?

Answer 22

when all the free water is evaporated but the cell walls are still completely filled with water (bound water).

Question 23

what happens when we keep drying a piece of wood that is in the FSP.

Answer 23

It shrinks and gets stronger due to compaction of the molecules and the creation of more Hbonds. passed the FSP, the properties don’t change significantly. Below, the strength increases exponentianlly, along with the shrinking.

Question 24

why is it important to keep wood dry in construction? how can we prevent it from being filled with water?

Answer 24

because if we use greenwood (freshly sawn with water in it), it will shrink and cause big changes to the structure in terms of loading. This can be prevented by curing the wood (drying it), or having a building envelope.

Question 25

how does wood generally shrink? what are the consequences of shrinkage?

Answer 25

Wood shrinks in different ways for every direction (radial, tangeantial and longitudinal). It tends to shrink more in the tangeantial direction, causing the wood to warp. A good way to prevent warping is by stacking wood properly.

Question 26

how does temperature affect wood properties?

Answer 26

temperature usually lowers the wood mechanical properties, and with seasons creep increases, as wood is a viscoelastic material (resist strain linearly with time when stress is applied)

Question 27

explain how wood behaves in bending

Answer 27

it is in compression/shrinkage on the side of the applied load, and in tension/expansion. Since the stress in tension is greater than compression, the fracture starts due to compression and failure occurs in tension. The bigger the timber is, the greater the stresses will be (further away from neutral axis)

Question 28

What is the 5% exclusion limit and what does it represent?

Answer 28

It is a factor used by engineers to determine what mechanical properties will be used to design their products. The mech props are chosen such that only 5% of specimens tested would be expected to fall below specified value. Its about allowable risk.

Question 29

adjustment factors, what are they?

Answer 29

A coefficient that is used to multiply the required strength needed by materials to ensure they don’t fail (always prepare for worst case scenario)

Question 30

Name some design adjustments and describe them

Answer 30

load sharing: putting three or more parallel members to share the load, low prob to have two weak members next to each other.
treatment: an example would be timber treated with chemicals
Member size: have to take into account a decrease in strength by x% for beams greater than 300 mm
service condition: is the wood wet, dry or in between?
temperature: moisture content increases greatly in cooler temps.

Question 31

Name all the factors that could lead to deterioration mechanisms

Answer 31

FUngi, Bacteria, Insects, Marine Borers, weathering,Chemical attacks and fire

Question 32

name the types of fungi and what they be doin

Answer 32

-Brown rot fungi: causes the most dame to woods, particularly softwoods. Causes a rapid loss of strength.
-white rot fungi: Attack primarly hardwood, loss of strength is slower
-soft rot fungi: only attacks the surface, so can be dealt with with use of thicker members
-dry rot fungi: has its name because wood doesnt have to be damp, the fungus produces water-conducting strands that can carry water to the wood.
mould and stain fungi: don’t affect wood too much but can be detrimental to humans.

Question 33

How does one proceed to control fungi?

Answer 33

for fungi to start acting, wood must be above FSP, so keep it dry by having good construction practices (wood usually gets damp because climate and poor insulation)

Question 34

how does bacteria affect durability of wood?

Answer 34

It can cause significant loss of strength over long periods of time (decades). It usually causes more problems for health hazards.

Question 35

name the insects that attack wood and cite their differences

Answer 35

• termites: live in large colonies, use wood as food - there can be dampwood and drywood termites, they can be found in warmer environments (in the south)
  -Carpenter ants: also live in colonies, but dont use wood as food but to build nests near food sources.
  -wood-boring beetles: they dont live in big colonies, therefore they are rarely problematic

Question 36

what is weathering?

Answer 36

it involves drying and wetting, exposure of light, freeze-thaw, chemical changes. It is a surface effect. The most worrisome effect is warping due to non-uniform moisture changes.

Question 37

what chemicals affect wood in an irreversible manner?

Answer 37

-strong acids: attack cellulose and hemicellulose
- Alkalis and alkili salts interfere with bonds between lignin and cellulose.
-iron salts: staining and softening of wood.

Question 38

How does fire burn?

Answer 38

At first a char layer develops on the surface, and under it is the pyrolysis zone, where it starts to decompose and emit both flammable and non flammable gases. This slowly progresses through the wood. Since wood is a good insulator, temperature falls off rapidly beyond the char layer.

Question 39

what are protective techniques that can be employed for wood?

Answer 39

waterborne preservatives: clean the wood surface to put paint or stain on it.
oilborne preservative: toxic to fungi and insects but stains the wood
weather-resistant coating: wide range of properties, usually water repellent, has to be replaced periodically.
fire retardants: two types - surface coating and pressure impregnation. Surface coating is mostly for existing structures, not as effective as impregnation

Question 40

what are the advantages of using wood in construction?

Answer 40

• high strength/weight ratio
• high durability
• good insulator
• absorbs vibrations well
  -easily shaped
• economic + sustainable (if used correctly)

Question 41

what are the advantages of composites

Answer 41

• allows for different sizes and shapes
• improved mechanical properties
• economic advantage (usage of a bigger volume of trees)

Question 42

what are some characteristics of plywood that make it an interesting material?

Answer 42

Every layer has an opposite grain direction from the previous one. This reduces shrinkage and lowers the consequences of a split, as it will only happen in one layer. Knots are also often found on only one ply (low chances of having two knotted plies next to each other)

Question 43

what is glued laminated timber and what is it used for?

Answer 43

fabricated by gluing together timber. This is used to make curved or straight members and to make larger sections. It is useful to reduce the bending stresses as it is now multiple neutral axes, therefore the distance is shorter and the tension/compression stresses are lower.

Question 44

parallam. What is it ?

Answer 44

Small strips of veneer glued together and pressed into a thick panel, which will then be sawn. Usually found in thin form, easily curved, easy to quality control, knots become irrelevant. It is more expensive than glued timber.