Up to this point, we have considered bending and shear stresses in beams
separately. We now explore the design of beams that satisfy the prescribed
design criteria for both bending and shear. In general, bending stress governs
the design of long beams, whereas shear stress is critical in short beams. We
can draw this conclusion by observing that the shear force V is determined
only by the magnitude of the loading, whereas the bending moment M depends
on the magnitude of the loading and the length L of the beam. In other
words, for a given loading, Vmax is independent of L, but Mmax increases as
L is increased.
Shear stress is of concern in timber beams because of the low shear
strength of wood along the grain; the typical ratio of shear strength to bending
strength is 1:10. Very thin webs in metal beams can also fail in shear or
by buckling caused by the shear stress.
The most direct method for satisfying both design criteria is to perform
two separate computations: one based on the bending stress criterion and
the other on the shear stress criterion. Examination of the results will then
reveal which of the designs satisfies both criteria.