LECTURE 8 Flashcards by Brendan Burke

why wood

availability
low cost
ease of use
durability

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application

buildings
bridges
utility
floors
roofs
trusses
piles

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Worlds tallest timber residential tower

rocket and tigerli 100 meters tall

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north american wood certification systems

sustainable forestry initiative
forest stewardship council
american tree farm system
canadian standards association
programme for the endorsement of forest certification

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Endogenous wood

growing or originating from within an organism
not for engineering applications
includes palm trees

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exogenous wood

growing or originating from outside an organism, grows from the center out by adding concentric layers of wood around the central core used for engineering applications

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exogenous trees are broadly classified as

decidious and conifers

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conifers (___ wood) include

soft wood, fir, pine, larch

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decidious trees…

shed their leaves at the end of each growing season

used for furniture
uneconemical for construction
hard to cut

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conifers…

or evergreens have need like leaves and normally do not shed them at the end of the growing season, grow continuously through the crown producing a uniform stem and homogenous characteristics
widely used in construction renewable economical

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Common lumber sizes

5/6 6/4 8/4 12/4

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structure of wood

anisotropic sctructure

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the wood produced in one growing season constitutes…

a single growth ring

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each annual ring is composed of

earlywood from rapid spring growth during the spring and late wood from summer growth

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latewood consists of

dense dark and thick walled cells producing a stronger structure than earlywood

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physical features of the tree stem

bark
cambium
wood
pith

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the exterior covering of the tree has an __ and ____ layer

outer and inner

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the outer bark leather is

dead and corky and has great variability in thickness

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the inner bark is

the growth layer of the bark but is not part of the wood section of the tree

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The cambium is

a thin layer of cells situated between the wood and the bark and is the location of all wood growth

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Sapwood is

A pipeline to transport sap and a storehouse for starches, it grows faster

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heartwood is

often darker colored wood at the center of the cross section and is surrounded by sapwood, it also provides structural strength for the tree and IS NOT LIVING. The heartwood of a few species is decay resistant

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species with durable heartwood include

redwood, cedar

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The extractives in some heartwood can protect wood from

termites

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The pith is

the central core of the tree the color ranges from black to whitish depending on species

The pith structure can be

solid porous chambered or hollow

wood is an anisotropic material meaning….

it has unique properties in each direction

the three axis orientations in wood are

longitudinal radial and tangential

the anisotropic nature of wood affects physical and mechanical properties such as

shrinkage, stiffness, and strength

the wood cells have a ____ cross section

rectangular

The centers of the tubes are ____ whereas the ends are ___

hollow, tapered

The length to width ratio can be as large as

100

The long dimension of most cells is

parallel to the trees trunk

A few cells in localized bundles

, grow radially from the center to the outside of the trunk

Cell orientation in one direction gives wood its

anisotropic properties

The hollow tube structure is

very efficient in resisting compressive and tensile stresses parallel to its length but redials deforms when loaded on its side

Fluctuations in moisture contents…

expand and contract the tube walls but have little effect on the length of the tubes

Chemical composition of wood

Cellulose (50%) Lignin (16-33%) Hemicellulose (20-30%) Extractives (5-30%) Ash producing minerals (0.1-30%)

The main building block of cellulose is

sugar or glucose

As the tree grows,

linear cellulose molecules arrange themselves into highly ordered strands called fibrils these ordered strands form the large structural elements that compose the cell walls of wood fibers

Lignin is

a mostly intercellular material is is intractable, insoulable material loosely bonded to cellulose The glue that holds the tubular cells together the longitudinal shear strength of the wood is limited by the shear strength of the lignin bounds

Moisture content of a wood sample is

weight of water in the specified expressed as a percentage of the oven dry weight

an oven dried wood is a sample that

has been dried at 100-105c until constant wieght

moisture exists in wood as either

bound or free water

bound water is

held within the cell walls by adsorption forces

free water is

condensed water or water vapour in the cell cavities

The FSP is

the fibre saturation point, the level of saturation at which the cell walls are completely saturated but no free water exists in the cell cavities (21-31%)

The FSP is of practical significance because

the addition or removal or water below the FSP has a large effect on practically all physical and mechanical properties, above fsp changes in moisture contents are insignificant

When the moisture content is above the fiber saturation point the wood is considered…

dimensionally stable

moisture fluctuations below the FSP always result in

dimensional changes

Shrinkage is caused by

loss of moisture content from the cell walls

Swelling is caused by

gain of moisture content in the cell walls

dimensional changes in the radial direction are generally

one half the change in the tangential direction

swelling and shrinkage in the longitudinal direction is

minimal, typically 0.1% to 0.2% for a change in the moisture content from the FSP to oven dry

The anisotropy of dimensional changes in wood causes

warping checking splitting

Moisture content in wood varies depending on

air temp, humidity, it doesn’t react instantaneously and tends towards the average. Think DE with fourier inputs

The EMC is

the equilibrium moisture content, the content for the average atmospheric conditions

The EMC is a function of

air temp and humidity

The emc ranges from

less than 1% at temps greater than 55C and 5% humidity to 20% at temps under 27C and 90% humidity

For engineering calculations the FSP is commonly assumed to be

30%

Tangential Shrinkage and radial shrinkage can be assumed to be

8% and 4%

For structural lumber where the growth ring or orientation is not known in advanced and can be reasonable expected to vary from piece to piece

6% is chosen for shrinkage from FSP(30%) to oven dry (0%)

Shrinkage can be estimated as

6% shrinkage with a 30% change in moisture content or 1% shrinkage per 5% change in moisture content