Section B3.1/Design Basis (LRFD and ASD)
AISC 360-22 Section B3 establishes LRFD and ASD as the two permitted steel design methods with their respective factors.
Steel structures must be designed using either LRFD or ASD, applied consistently throughout the structure. LRFD uses factored loads (from ASCE 7 load combinations in Section 2.3) and a resistance factor phi (typically 0.9 for yielding, 0.75 for fracture) applied to the nominal strength. ASD uses unfactored loads (from ASCE 7 Section 2.4 combinations) and a safety factor omega (typically 1.67 for yielding, 2.00 for fracture) dividing the nominal strength. Both methods produce similar designs for typical structures. The design method must be stated on the structural drawings.
Why this section exists
LRFD and ASD represent two philosophies for ensuring structural safety. LRFD applies different load factors to each load type (reflecting the uncertainty in each) and a single resistance factor to the capacity. ASD applies a single safety factor to the capacity and uses unfactored loads. LRFD is more theoretically rigorous and can produce slightly more economical designs for structures with high ratios of live load to dead load. ASD is simpler and more familiar to some engineers. Both methods achieve equivalent safety levels when properly applied.
What plan reviewers look for
Plan reviewers check the structural general notes for the stated design method (LRFD or ASD). They verify the load combinations match the stated method (ASCE 7 Section 2.3 for LRFD, Section 2.4 for ASD). They check that the calculations consistently apply the correct factors (phi for LRFD, omega for ASD) and that the two methods are not mixed within the same structural system.