Timber Flashcards
What are the two main types of trees?
1) Deciduous- Broadleaf/fruit or nut-bearing- Hardwood
2) Coniferous- Needle and cone-bearing- Softwood
What primary softwoods are used in construction?
Spruce (whitewood) and pine (redwood).
What percentage of cellulose is in wood?
40-45%
What percentage of hemicellulose is in wood?
20-35%
What percentage of lignin is in wood?
20-30%
What are deciduous and coniferous tree separated into?
1) Sapwood
2) Heartwood
What does the sapwood do?
Encases the trunk directly under the bark and transports the sap up the trunk.
What does the heartwood do?
Provides strength and wood from this stem region is normally more resistant to decay or insect attack.
What are the colour difference between heartwood and sapwood?
Heartwood is darker in colour providing aesthetic value.
What is timbers specific strength compared to other materials?
Timber has a higher specific strength than concrete which is comparable to that of steel and aluminium alloys.
Is timber environmentally friendly per unit strength?
Yes, but concrete is better.
Name 6 advantages of timber.
1) High aesthetic value
2) Reasonable stiffness and strength
3) Resistant to fire
4) Resistant to corrosion and chemicals
5) Thermally stable
6) Locally available
Name 6 disadvantages of timber.
1) Creep
2) Flammable
3) Suseptical to insect attack
4) Deforestation
5) Poor biodiversity
6) Trees grow slowly hard to keep up with demand
What is does it mean when the wood is thermally stable?
Not change in dimension or lose strength or performance in extreme conditions hot or cold.
Why is wood used in marine environments?
It is resistant to most chemical attacks such as chlorine corrosion attacks.
Why is timber fire resistant?
Wood forms a charred layer which protects the remaining timber and slows the rate of combustion.
What effects the durability of timber?
1) Timber is properly sawn avoiding cracks
2) Timber is properly pre-dried, to avoid large drying shrinkage
3) Timber is properly assembled to allow for dimensional changes upon humidity changes.
How does timber prevent major shrinkage?
Wood is usually dried down before converted into building material, either by long exposure to drying air which unlikely to reduce moisture content below 18% and might take several years or done quickly in drying kilns which lower moisture content to 6% or less.
What are the two stages in the drying process?
1) The rapid loss of liquid water from the cell lumens which are hollow voids at the centre of wood cells.
2) The slower drying out of the surrounding cell walls.
What is anisotropic?
Movement due to changes in moisture content.
Give an example of engineered timber?
Glulam- alternate orientation of fibres therefore isotropic.
The advantage of engineered timber?
More flexibility and freedom.
The disadvantage of engineered timber?
Higher CO2 emissions than solid timber.
What type of timber has the lowest CO2 emission?
1) Sawn Softwood
2) Sawn Hardwood
How can you be sure the wood is sustainably sourced?
Certification from FSC, PEFC or “Forest Forever Campaign”.
