Section 2.4.1/Load Combinations Using Allowable Stress Design
ASCE 7-22 Section 2.4.1 defines the ASD load combinations establishing how dead, live, wind, seismic, snow, and other loads are combined for allowable stress design.
The basic ASD load combinations are: (1) D; (2) D + L; (3) D + (Lr or S or R); (4) D + 0.75L + 0.75(Lr or S or R); (5) D + (0.6W or 0.7E); (6) D + 0.75L + 0.75(0.6W) + 0.75(Lr or S or R); (7) 0.6D + 0.6W; and (8) 0.6D + 0.7E. Where D is dead load, L is live load, Lr is roof live load, S is snow load, R is rain load, W is wind load, and E is earthquake load. The 0.75 factor in combinations 4 and 6 reflects the reduced probability that all transient loads will act simultaneously at their maximum values. The 0.6D factor in combinations 7 and 8 checks for uplift and overturning using the minimum dead load that resists lateral forces. These combinations are used with allowable stress capacities from material design standards (AISC 360, ACI 318, NDS, etc.).
Why this section exists
Allowable Stress Design (ASD) has been used in structural engineering for over a century and remains a permitted design method alongside Load and Resistance Factor Design (LRFD). ASD uses unfactored (service level) loads compared against allowable stresses that include a built-in safety factor. The ASD load combinations apply one-third stress increase factors through the 0.75 combination factors rather than the load factors used in LRFD. Wood design (NDS) and masonry design (TMS 402) are frequently performed using ASD. The IBC permits either ASD or LRFD per Section 1604.4, but the two methods must not be mixed for the same member.
What plan reviewers look for
Plan reviewers verify that the structural calculations state which design method (ASD or LRFD) is being used and that the correct set of load combinations is applied. They check that all applicable load types are included in the combinations: dead, live, roof live, snow, wind, and seismic. They verify the uplift combinations (7 and 8) are checked for foundations, connections, and light-framed walls. For seismic design, they verify the 0.7E factor is applied correctly and that the redundancy and overstrength factors are included where required.