LECTURE 8 Flashcards
why wood
availability
low cost
ease of use
durability
application
buildings
bridges
utility
floors
roofs
trusses
piles
Worlds tallest timber residential tower
rocket and tigerli 100 meters tall
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
Endogenous wood
growing or originating from within an organism
not for engineering applications
includes palm trees
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
exogenous trees are broadly classified as
decidious and conifers
conifers (___ wood) include
soft wood, fir, pine, larch
decidious trees…
shed their leaves at the end of each growing season
used for furniture
uneconemical for construction
hard to cut
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
Common lumber sizes
5/6 6/4 8/4 12/4
structure of wood
anisotropic sctructure
the wood produced in one growing season constitutes…
a single growth ring
each annual ring is composed of
earlywood from rapid spring growth during the spring and late wood from summer growth
latewood consists of
dense dark and thick walled cells producing a stronger structure than earlywood
physical features of the tree stem
bark
cambium
wood
pith
the exterior covering of the tree has an __ and ____ layer
outer and inner
the outer bark leather is
dead and corky and has great variability in thickness
the inner bark is
the growth layer of the bark but is not part of the wood section of the tree
The cambium is
a thin layer of cells situated between the wood and the bark and is the location of all wood growth
Sapwood is
A pipeline to transport sap and a storehouse for starches, it grows faster
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
species with durable heartwood include
redwood, cedar
The extractives in some heartwood can protect wood from
termites
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
