In recognition of the anisotropic nature of wood, it is traditional to consider the strength of wood along its three principal axes: longitudinal, radial and tangential. Parallel to grain (longitudinal) strength is significantly greater than strength across the grain; however, since there may be only minor difference between radial and tangential directions, it is appropriate to simply consider average strength perpendicular to the grain.
Strength measures the ability of a material to resist applied force or load. The strength of material is commonly expressed in terms of a stress value. Stress is defined as load per unit area, and is calculated by dividing the magnitude of the applied load or force by the cross-sectional area over which it is distributed. A general formula:
stress = load/area
In English-speaking countries, loads have traditionally been measured in pounds (lb) and areas in square inches (sq. in). In continental Europe, kilograms and centimetres were used. In SI units, Mega Newtons per square metre (MN/m2) are used.
The basic modes of load application are compression, tension and shear. The components in Figure 2.19 are subjected to a single type of stress. In practice, a combination of stresses may occur, as in the bending of a beam. A beam is an elongated member supported at various points along its length with one or more loads acting perpendicular to its axis. For example, the beam of Figure 2.20 is supported at each end with a concentrated load at its mid span. As a result of the consequent bending deformation, the upper surface is shortened and stressed in longitudinal compression; the lower surface is stretched and thereby stressed in tension. These axial stresses in tension or compression are referred to simply as bending stresses; they are maximal at the upper and lower surfaces of the beam and diminish to zero at the mid plane of the beam
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