Earlier versions of the Steel Construction Manual published by the American Institute of Steel Construction (AISC) contained design procedures based on the allowable stress design (ASD) method. In 1986, the first edition of a "Load & Resistance Factor Design" (LRFD) Steel Construction Manual became available as an alternative to the traditional ASD Manual. More recent AISC Manuals have discarded the Allowable Stress Design concept in favor of an Allowable Strength Design method (still ASD); in both cases (allowable stress and allowable strength), determination of loads and load combinations is the same (see Appendix Table A-2.7 Part B). What changes are the limit states and factors of safety: whereas the allowable stress method defines the limit state as the moment when any part of the cross section yields, the newer allowable strength method defines the limit state as the point where all available strength is exhausted — i.e., the moment when the entire cross section has yielded.
Of course, the LRFD method is still published as an alternative to ASD, but one that will not be used in this text. In fact, the two methods (ASD and LRFD) yield identical results for structural elements for which the magnitude of live loads is exactly three times that of dead loads — safety factors for ASD and LRDF have been calibrated to achieve that result. However, because the LRFD safety factor for live load is greater than that for dead load, structural elements designed with the ASD method and with a live-to-dead load ratio greater than three will be slightly conservative compared with the same elements designed with LRDF; conversely, elements designed with the ASD method and with a live-to-dead load ratio less than three will end up smaller (and therefore slightly less strong) than elements designed with LRFD. For most structures, these differences are small and will not compromise the safety of buildings designed using either method.
© 2020 Jonathan Ochshorn; all rights reserved. This section first posted November 15, 2020; last updated November 15, 2020.