Mechanical Properties of Timber:

The intelligent use of wood for any structural purpose requires a general knowledge of the mechanical properties of different woods:
1- Tensile strength: Timber in construction is practically never subjected to pure tensile stresses for the simple reason that the end connections cannot be so devised that they do not involve either shear along the grain or compression across the grain. Failure in tension across the grain involves principally the resistance offered by the thinner-walled wood elements to being torn apart longitudinal.
2- Compressive strength: The compressive strength of wood in a direction normal to the grain is simply a matte the resistance offered by the wood elements to being crushed or flattened. The cells with thinnest walls collapse first, end the action proceeds gradually. Compressive strength of wood in a direction //to the grain depends upon the internal structure and the moisture content of the wood and the manner of failure is fixed by these same factors.
3- Flexural strength: The flexural strength of timber is determined by the following formula:
Sb = 3/2 (PL/bh2) The tensile strength of all timber is greatly an excess of its compressive strength (about 3 times as much the average), and the latter will usually be the determining factor in limiting the cross- breaking strength.
4- Stiffness: Stiffness of timber largely upon the same factors as strength. Dense woods are always stiffer than open, porous woods, and heavy woods are stiffer than lightwoods.
Physical Properties of Wood:
1- Bulk Density: depends on the volume of pores and moisture content of the wood. Bulk density value is used to determine the quality factor, which is the ratio of compressive strength to the bulk density. it is 0.6 for pine and 0.57 for oak.
2- Shrinkage: is the reduction in linear and volumetric dimensions in drying of wood. Evaporation of capillary water is not accompanied by shrinkage