ABSTRACT
The ASD Method designs beams using only the elastic region. As a result, the allowable moment is limited to the yield moment, My = Fy(I/c) = FyS where S is the elastic modulus, also known as the section modulus. The LRFD Method considers stresses beyond the yield stress because failure doesn’t occur until a great deal of yielding occurs. The allowable moment therefore becomes the plastic moment, Mp = FyZ, where Z is the plastic modulus. To illustrate, consider the cross-section of a rectangular beam when yield stress is reached at the extreme fibers, the moment is derived by the compression and tension couple using the internal couple method as shown in Figure 20.1 The yield moment, My = (Fybd/4)(2d/3) = Fybd2/6. Yield moment https://s3-euw1-ap-pe-df-pch-content-public-p.s3.eu-west-1.amazonaws.com/9780203583159/43b499e8-36af-4c2c-ba25-59ab9f62f21b/content/fig20_1_B.tif" xmlns:xlink="https://www.w3.org/1999/xlink"/>
