TABLE OF CONTENTS The shear stress in a beam is often referred to as horizontal shear. From strength of materials it will be recalled that the shear stress at any point in the cross section of a beam can be computed by the formula
Recall also that the horizontal and vertical shear stresses at a given point are equal. The shear strength of wood parallel to the grain is much less than the shear strength across the grain, and in a wood beam the grain is parallel with the longitudinal axis. In the typical horizontal beam, then, the horizontal shear is critical.
It may be helpful to compare the shear stress distribution given by VQ/ Ib for a typical steel beam and a typical wood beam. See Example 6.12. Theoretically the formula applies to the calculation of shear stresses in both types of members. However, in design practice the shear stress in a steel W shape is approximated by a nominal (average web) shear calculation.
The average shear stress calculation gives reasonable results in typical steel beams, but it does not apply to rectangular wood beams. The maximum shear in a rectangular beam is 1.5 times the average shear stress. This difference is significant and cannot be disregarded.
EXAMPLE 6.12 Horizontal Shear Stress Distribution
For a steel W shape, a nominal check on shear is made by dividing the total shear by the...
