Ra. Dunham et al., The effect of growth rate on the strength properties of sawn beams of silver birch (Betula pendula Roth), SC J FOR R, 14(1), 1999, pp. 18-26
A study was carried out into the effects of growth rate of silver birch (Be
tula pendula Roth) on strength properties of sawn wood beams. Despite the l
arge variation in growth rate, with average ring width ranging from <2 mm t
o >4 mm, density of beams from the slowest-grown trees was only about 10% g
reater than that of the fastest. While density was significantly correlated
with hardness (r(2) = 0.45), the small and non-significant differences in
density found in beams from the slowest-to the fastest-growing trees are un
likely to have any economically deleterious effects on hardness, for exampl
e for the purpose of flooring. The high modulus of rupture (MOR) of the bir
ch beams, with average values ranging from 47 to 63, N mm(-2), highlights t
he suitability of birch for load-bearing structures even though beams from
fast-grown trees were significantly weaker than those from trees grown less
quickly. The lower 5% quantile values, which represent the reliable timber
strength and are more important than average strength, show that beams fro
m the fastest-growing trees have a safe strength only 55% of that of beams
from the slowest-growing trees and this was linked to a higher knot area ra
tio and sloping grain. Clear wood strength of the fast-grown trees, however
, was only approximately 10% lower than that for slower-growing trees, poss
ibly reflecting differences in the proportion and cell anotomy of juvenile
wood. Beams from fast-growing birch trees are capable of being used for str
uctural purposes. Strength, however, could be improved if silvicultural pra
ctices, such as pruning or appropriate early spacing, were adopted to reduc
ed knot size.