Wood And Timber

Question 1

What are some advantages and disadvantages of using timber?

Answer 1

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

Question 2

What is the difference between wood and timber?

Answer 2

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

Question 3

How are growth rings formed?

Answer 3

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

Question 4

Why are growth rings an inaccurate way to determine a tree’s age?

Answer 4

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.

Question 5

What are the two types of wood

Answer 5

Softwood and hardwood

Question 6

What is softwood. Give 2 examples

Answer 6

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

Question 7

What is hardwood. Give 2 examples

Answer 7

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

Question 8

Describe the microstructure of wood

Answer 8

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

Question 9

What is earlywood and latewood in growth rings?

Answer 9

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

Question 10

Describe the microstructure of wood on a more cellular level

Answer 10

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

Question 11

What is grain?

Answer 11

The arrangement of a wood’s fibres resulting from the growth of a tree

Question 12

Wood is an orthotropic material, what does that mean?

Answer 12

Part of wood being anisotropic. Their properties depend on the direction in which they are measured. Opposite of isotopic

Question 13

What are the three axes of wood and describe them

Answer 13

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)

Question 14

In wood. How are mechanical properties affected by direction of loading?

Answer 14

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!

Question 15

What is a knot?

Answer 15

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.

Question 16

Why are defects not ideal for construction? Mention flaw distribution

Answer 16

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.

Question 17

Name two types of knots

Answer 17

Spike
Loose
Sound right

Question 18

What is wane

Answer 18

The lack of wood on the face of a piece
Cutting rectangular cross section from a circular tree trunk

Question 19

what is check

Answer 19

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.

Question 20

what is shake

Answer 20

A shake is when the grain between the wood’s growth rings separates.

Question 21

what is pitch pocket

Answer 21

Pockets of resin or openings in the bark
These are voids and become defects

Question 22

Is a knot at the top or the bottom going to be weaker?

Answer 22

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)

Question 23

where are the bonds stronger in wood microstructure?

Answer 23

in cell walls > between cell walls
bond between knot and wood is weak point so is tested first

Question 24

what are the three types of grading?

Answer 24

visual (variability), mechanical (deformation and stiffness), and nondestructive

Question 25

explain the grade of wall studs

Answer 25

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

Question 26

explain specific gravity of wood

Answer 26

SG of cell wall material is around 1.5 (that of water is 1) it depends on void space and porosity

Question 27

why do some woods float and other sink

Answer 27

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)

Question 28

what are the two states that moisture can be found in wood?

Answer 28

1. free water within the cell cavities 2. bound water absorbed into the cell walls

Question 29

what is FSP?

Answer 29

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

Question 30

why is FSP a critical point?

Answer 30

any more water lost from the wood causes shrinkage (*shrinkage causes strength*). before, all water is inside the cell wall so no shrinkage

Answer 31

High strength when dry Constant lower strength past FSP When it's dry more shrinkage and denser and more material to carry load (stronger with less water) Constant because cell cavities are full of water and cell wall is saturated. Within cell wall you get maximum expansion at 25%. At max cell wall saturation, that's all the expansion and strength that you'll get. More water in the cell cavities doesn't affect material properties so constant FSP when it switches from curved to straight line

Question 32

what is greenwood?

Answer 32

freshly cut wood that is at FSP. has high moisture content because was just cut.

Question 33

why is it bad to build with greenwood?

Answer 33

it is wood at FSP (or past it) and if you build with it, the water in the wood will start evaporating when it is already inside the member and this evaporation causes shrinkage which causes stresses in the structure

Question 34

what happens to wood when moisture content increases?

Answer 34

thermal and electrical conductivity increase. rate of creep increases. wood is not conductive - but water makes it conductive

Question 35

how can you protect wood from outside environment?

Answer 35

Building envelope. design for worst case - sheathing and walls, and moisture barriers too.

Question 36

how do we test our wood?

Answer 36

worst case scenario: SATURATED

Question 37

shrinkage is different in different directions. see answer

Answer 37

wood is dimensionally stable when moisture content is greater than the FSP. wood shrinks when losing moisture and swells when gaining.

Question 38

is what direction does wood shrink the most?

Answer 38

in tangential direction (with the growth rings) because of bigger proportion of cell wall. greater density = greater shrinkage

Question 39

CHECK OUT NOTES ON SLIDE 13 LECTURE 57

Question 40

Why is curing timber important?

Answer 40

improves its properties and protects it form interaction with external elements. prevents shrinkage because part of moisture is already removed. wood is usually dried before usage. prevents shape reconfiguration

Question 41

difference between viscous and elastic?

Answer 41

Viscous: resists strain linearly with time. grows with time as long as the stress is applied. Elastic: return to their original state once the stress is removed

Question 42

describe the tensile strength of wood

Answer 42

strong in primary bonds, longitudinally, parallel to grain. weak in secondary bonds, perpendicular to grain (see image)

Question 43

describe compressive strength of timber

Answer 43

stronger longitudinally than radial but still weaker than tension. this is due to buckling. longitudinal is slender :/. also due to poisson ratio, compression causes tension which leads to growth ring separation (weak point). outer rings buckle before inner ones back in compression perpendicular to grain because cell walls are hollow and they will collapse

Question 44

Describe COLLAPSE

Answer 44

Cell walls will collapse one by one so no lateral support Hollow cells collapse and microstructure is weak Cells are relatively uniform No macrostructure defect. See L14S9 for picture

Question 45

how does timber fail?

Answer 45

in shear! macro - shear micro - shearing between cells and lignin because these weak points are loaded different young's moduli bases on direction observed

Question 46

how do we overdesign in wood?

Answer 46

too much variability so we overdesign **5% exclusion limit** adjustment factor strength ratio modification factors

Question 47

describe 5% exclusion limit

Answer 47

Overdesign s.t. 5% exclusion means only 5% will be less than the specified value Means that 95% of specimens won't fail so this is acceptable DON'T USE THE MEAN

Question 48

What is adjustment factor? Wood and timber

Answer 48

Multiply the applied stress by a given factor. Design to resist a lot more Clear wood has no defects and timber has a lot of defects So when going from wood to timber multiply by a given factor

Question 49

What is strength ratio?

Answer 49

What percentage of bending stress you can take Lower quality: more defects and less dependable Down the list = down in quality Down in quality = down in bending stress when you design

Question 50

how does strength ratio apply to load sharing?

Answer 50

Sheds/shares loads because connected Low probability of two consecutive weak studs (stud that compresses or buckles more)

Question 51

Most structures designed to be permanently loaded (designed for forever)

Question 52

Describe load duration

Answer 52

Creep: longer load, lower stress to failure The shorter the term the higher the allowed resistance Lower allowed resistance for long-term duration

Question 53

What are the main durability concerns for wood?

Answer 53

fungi bacteria animals weather chemical attack fire

Question 54

what is fungi

Answer 54

non-flowering plants without chlorophyll. cannot produce their own food and can only grow on dead or living organic material (for food source)

Question 55

how do fungi reproduce?

Answer 55

through spores. these spread and develop into the wood and secrete an enzyme that de-polymerizes which leads to softening and weakening of the wood

Question 56

name the types of fungi and what wood they affect all wood must be damp for rotting to begin

Answer 56

brow rot - softwood white rot - hardwood soft rot - less severe (just get a thicker piece of wood). needs wet wood so they're inactive during dry periods dry rot - water conducting strands carry water to wood mould and stain fungi - not that bad for wood but awful for humans so need to prevent. also ugly

Question 57

how can you prevent fungi and bacterial attack

Answer 57

stay below fsp. Plumbers/pipes with moisture barriers/building envelope Keeps water inside bathrooms and the wetter rooms Drier ducts pierce holes through building envelopes so we need to seal these holes well

Question 58

describe bacteria for wood durability

Answer 58

Damp wood Bacteria can be significant for long term strength loss Could be hazardous to humans

Question 59

describe how termites deteriorate wood

Answer 59

Termites excavate the cellulose in wood and eat it huge threat to structural images

Question 60

describe how carpenter ants deteriorate wood

Answer 60

Don't eat wood but do build houses inside the wood and make nests in it Grow in colonies so they are many and that causes more damage

Question 61

there are beetles and marine borers too 🤷‍♀️

Question 62

What is wood chemical attack?

Answer 62

concrete in contact with wood damages wood anything corroding on wood does damage (staining and softening) acids weaken wood too

Question 63

describe the wood burning fire process

Answer 63

Starts off as pyrolysis zone and is when wood degrades/ hydrolysis is deterioration Char is layer of black material which is decomposed wood Char is insulator Once wood burns and turns into char is stops burning

Question 64

what are some wood preservatives and what do they do? Wow

Answer 64

waterborne: salts that are toxic to fungi and insects oil borne: also toxic. stain surface of wood weather resistant coatings: protective barrier (stains, varnishes, paints, ...)

Question 65

What are the two methods of applying fire retardants. which is the better method?

Answer 65

Surface coating and **pressure impregnation**

Question 66

Describe surface coating

Answer 66

Applied to surface and penetrate in due to capillary suction (not very large penetration distance however). Less effective but relevant for already built structures

Question 67

Describe the process of pressure impregnation

Answer 67

1) Put wood in vacuum chamber (pores = vacuum) 2) Immerse in fire retardant 3) Release vacuum

Question 68

what are some advantages of using wood?

Answer 68

high strength/weight ratio good insulation pretty easily repaired, remolded, altered resists many corrosive agents Redundancy: is members fail it still won't cause the whole structure to collapse Sustainable, economic and high in salvage value can be combined with many other materials

Question 69

disadvantages of using wood?

Answer 69

We use so much concrete and there's just not enough wood to replace the concrete too variable

Question 70

what is plywood?

Answer 70

Plywood is a composite material manufactured from thin layers, or "plies", of wood veneer that are glued together with adjacent layers, having both glued with each other at right angle or at 90 degrees angle.

Question 71

what are advantages of plywood? Mention moisture changes

Answer 71

split resistant: If to split along the grain in one layer the other layer having a grain in the opposite grain will resist that Has same properties in two directions because grain is oriented in two directions Longitudinal direction does not experience shrinking or expanding during moisture changes. Restrains plate

Question 72

what is glulam and its advantages?

Answer 72

constituted by layers of dimensional lumber bonded together with durable, moisture-resistant structural adhesives so that all of the grain runs parallel to the longitudinal axis good for curved members effect of knots is minimized

Question 73

what is parallam?

Answer 73

made from parallel wood strands bonded together with adhesive then sawed to desired shape. curves easily :)

Question 74

explain the I-beam concept

Answer 74

missing material would not carry much stress if present. Therefore cheaper to have less material because as efficient but cheaper moment of i-beam also same as moment of rectangle