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Engineers, architects and carpenters must be well versed with the mechanical properties of
timber. In order that the engineer may properly design columns and beams for various parts
of wooden structures, he must be thoroughly conversant with the strength and stiffness of the
available classes of timber. He must also know how various defects and conditions influence
these properties. The architect must not only appreciates the beauty of various species, the
relative ease with which each may be worked, the tendency to shrink, warp, and check; but he
must likewise be prepared to proportion joints and rafters to carry the imposed loads without
excessive deflection. The wheelwright must understand how the toughness and strength of his
axles, spokes, and shafts are influenced by species, rate of growth, density, and defects. The
carpenter and the craftsman must also have knowledge of the mechanical properties of wood
in order that they may woork it to best advantage. The mechanical properties of timber that
need elaboration are as follows.
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Wood has three principal axes—longitudinal, radial and tangential— along which properties
are fairly constant. Since wood is a nonistropic material, it has three values of modulus of
elasticity varying by as much as 150 to 1, three shear moduli varying by 20 to 1, and six
Poisson’s ratios varying by 40 to 1. There is no sharply defined elastic limit in wood but there
is a proportional limit. However, the stress-strain diagram in any direction is fairly straight
over a considerable range before it gradually curves off. It is a ductile material.
The relative stress-strain curves for direct tension, direct compression and bending stress
intensities parallel to the grain in Fig. 4.22 show that in both, direct compression and bending,
the proportional limit is in the vicinity of 65 to 75 per cent of the ultimate strength. For all
practical purposes, there is no proportional limit in direct tension.
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Modulus of elasticity of the grain is practically the same in direct tension, direct compression
and bending, if shear deformation in bending is eliminated. Becaus modulus of rigidity of
wood is low (approximately 1/15 to 1/20 the modulus of elasticity), the apparent bending
modulus of elasticity varies somewhat with the type of loading and span, but in no case it is
less than 90 to 95 per cent of the true bending modulus of elasticity. The mechanical properties
of timber that need elaboration are as follows.
g!& X When subjected to compressive force acting parallel to the axis of
growth, wood is found to be one of the strongest structural material. Columns and posts are,