Timber Flashcards

Question

With further drying of timber what happens to Vessel Cells?

Answer 1

Vessell Walls (Cellulose) begin to dry out and they shrink - Thus it is drying out the vessel cell walls (drying below 28% moisture) which causes the moisture movement

Answer 2

Because the vessel cell walls are arranged in rings, the moisture movement will be greatest in the tangential (circumferential) direction

Answer 3

Cupping, Diamonding and Twist

Answer 4

``` Air seasoning (can reduce to 20% in UK) or kiln seasoning (hot chamber - can reduce as low as 10%). In the UK, hardwood planks are usually air-dried for 18 to 24 months before being kiln-dried. ```

Answer 5

Biological (fungal & insect attack), Chemical, Photochemical, Thermal, Fire and Mechanical

Answer 6

The beetles eat the organic material causing loss of mechanical strength. Strength is lost because the cellulose cell walls are cut through, and can be very serious if the wood becomes badly infested.

Answer 7

At least one-third of all timber harvested goes into construction, the rest goes into paper-production, like fuel or is wasted during the logging process

Answer 8

Because of the way that timber grows, with most of the vessel cells running along the length of the tree, the timber has different properties in different directions. In other words, the properties are different along the grain from those measured across the grain - this is called ANISOTROPY

Answer 9

This swelling and shrinkage is called Moisture Movement. The amount of movement will depend on the species of wood, dense hardwoods such as Teak will show little movement, whereas Yellow Canadian Birch shows high movement. Other species such as English Elm give medium movement effects

Answer 10

The lamellae produced by peeling are glued together timber sheets. Notice that there is always an odd number of lamellae

Answer 11

8 years. Whereas Softwood 25mm thick, if air-dried in the Spring, should dry to about 20% moisture content in 1.5 to 3 months. Hardwood 25mm thick, if stacked in the Autumn, will take 7 to 9 months to dry to 20%, and 50mm hardwood will take about 1 year

Answer 12

The optimum temperature for growth varies for different species of fungi but is usually within the range from 20 to 30°C. Little growth takes place below 5°C and fungi are usually killed by prolonged heating to 40°C

Answer 13

The acid resistance of impermeable timbers is greater than that of most common metals; iron begins to corrode below pH 5, whereas an attack on wood commences below pH 2, and even at lower pH values, proceeds at a slow rate. In alkaline conditions wood has good resistance up to pH 11

Answer 14

Exposure to sunlight causes the colouration of the heartwood of most timbers to lighten. A few like Rhodesian teak and Douglas fir will darken.

Answer 15

There is some uncertainty about what minimum temperature will degrade timber, but it would appear that temperatures as low as 60°C can induce degradation over many years of exposure. The rate of degradation will rise markedly with an increase in temperature and time of exposure; hardwoods appear to be more susceptible to thermal degradation than softwoods

Answer 16

Wood is an organic (hydrocarbon) material and in its dry state it will burn. When the wood is heated to 160 - 200°C, it will pyrolyse and give off flammable gases

Answer 17

After 50 years the load which a piece of timber can withstand is less than half the stress it can carry at the onset of loading. Similarly, there is a reduction in stiffness (Young’s Modulus of Elasticity, E) with the time that shows itself as an increase in deflection with time under constant stress Timber overstressed in compression in the longitudinal direction forms kink bands and compression creases. Such defects can reduce the tensile strength of the wood by 10 - 15%, but the loss of toughness under impact conditions can be as high as 50%. Mistreatment of wooden ladders and scaffold boards on site has led to injury and loss of life on numerous occasions.