Load Combinations

Load combinations are the specified mathematical combinations of dead load (DL), live load (LL), wind load (WL), seismic load (EL), and other applicable loads used to find the worst-case design force at every section in a structure. The combinations exist because the actual load combinations a structure experiences are uncertain — different loads may peak simultaneously or non-simultaneously. IS 800:2007 Cl. 5.2 (steel) and IS 456:2000 Cl. 36.4 (RCC) specify the partial safety factors and combinations for limit-state design.

For RCC per IS 456 Cl. 36.4.1, the principal combinations are: 1.5 (DL + IL) — dead + imposed (live) load, ultimate limit state strength. 1.5 (DL + WL or EL) — dead + wind or earthquake. 1.2 (DL + IL + WL or EL) — dead + live + wind or earthquake. 0.9 DL + 1.5 (WL or EL) — dead minus environmental (uplift / overturning check). For serviceability: 1.0 DL + 1.0 IL — service combination for deflection and crack-width check. The factor 0.9 in the overturning check is critical — when wind or earthquake causes uplift, the dead load works against the structure (resists overturning), so design uses the LOWER bound of dead load (0.9× nominal) to be conservative.

For steel per IS 800:2007 Cl. 5.3.3: similar combinations with γf = 1.5 for DL+LL, 1.5 for DL+WL or EL, 1.2 for DL+LL+WL or EL, and 1.5 for DL−WL/EL (uplift). Material partial safety factors γm = 1.10 (yield), 1.25 (ultimate) for steel; 1.5 for concrete (IS 456). Modern structural software (ETABS, STAAD, SAFE) applies all combinations automatically and reports envelope (critical) responses at every section. Engineers must verify that wind / seismic forces are applied in both directions (X+, X−, Y+, Y−) and that bi-directional combinations are included for irregular structures.