Figure 1.5 shows a simply supported wood beam with a concentrated load applied at the midpoint. This process results in bend
ing. The lumber is stressed internally to resist the external loads. Bending in a member causes tension forces in the extreme fibers along the face farthest from the load and causes compression in the fiber along the side closest to the applied load. The maximum stress induced in the fibers, which occurs at the edges, is referred to as the ‘‘extreme fiber stress in bending.’’ This stress is highly dependent on the parallel-to-grain strength of the wood in both tension and compression. The allowable bending stresses are based on a clear specimen having no defects. Allowable bending stresses are then factored to account for defects.