CHAPTER XIII 
STRENGTH AND TESTING OF TIMBER 
Yaryiiig Results obtained in Testing Timber and Causes thereof — Tests 
on American and Australian Timber — Necessity for Tests on 
Large Pieces of Timber— Comparison of Strengths of Timber, 
Green and Dry — Great Increase of Strength in Timber caused by 
Drying — Tables of Strengths of Timber with Varying Degrees of 
Moisture— Effects of Kiln Drying — Long Time Tests on Beams- 
Various Methods of Testing — Tests by Lanza, H. D. Smith, 
C. Graham Smith, Major Bock, and others — Strengths of Timber 
in Side Compression and Shearing — Weight of Timber Variable — 
Weight Compared with Number of Rings per Inch —factors of 
Safety. 
The results obtained when testing timber vary very 
much more than those obtained with iron or steeL Of all 
structural material timber is the most variable in strength, 
two pieces from the same tree, or even the same log, often 
producing very different results; the main reason being 
that timber is a built-up structure and subject to internal 
strains, and these strains vary with each piece of timber. 
It is because of this great variation in tests that in the 
calculation of stresses for timber structures it has been 
usual to allow a much larger margin of safety than in the 
case of steel structures, and probably this is why we have 
had a singular immunity from failures in timber structures, 
the margin of safety allowed being much greater than was 
really necessary. 
Nearly all calculations for strength of timber, until quite 
recent years, were based upon tests made on small specimens 
of wood 1 or 2 inches square, and all who have had to 
