A practical guide to ASCE 7 load combinations — understanding dead load, live load, wind, seismic, and snow load combinations, and how LRFD and ASD methods determine structural member sizes.
What Is ASCE 7?
ASCE 7, "Minimum Design Loads and Associated Criteria for Buildings and Other Structures," is the primary loading standard for building structural design in the United States. Published by the American Society of Civil Engineers and updated every 6 years (current edition: ASCE 7-22), it defines the minimum loads that structures must resist — gravity, wind, seismic, snow, rain, flood, and blast. The International Building Code (IBC) references ASCE 7 as its basis for structural loading.
Types of Structural Loads
Dead Load (D) — the permanent weight of the structure itself: concrete slabs, steel framing, roofing, mechanical equipment, and any permanently attached finishes. Dead loads are well-defined and remain constant throughout the structure's life.
Live Load (L) — the variable loads from occupancy: people, furniture, movable equipment. ASCE 7 Table 4.3-1 provides minimum uniform live loads by occupancy: 50 psf for office areas, 100 psf for public assembly, 125 psf for light storage, and 250 psf for heavy storage. Live loads can be reduced for large floor areas and high tributary areas using ASCE 7 Section 4.7 live load reduction.
Roof Live Load (L_r) — maintenance personnel and equipment on roofs, not to be confused with snow load.
Snow Load (S) — determined from the ground snow load map (ASCE 7 Figure 7.2-1), then adjusted for roof slope, exposure, thermal factor, and importance factor.
Wind Load (W) — determined from wind speed maps (based on return period) and adjusted for exposure category, building height, and aerodynamic shape.
Seismic Load (E) — determined from spectral acceleration maps and site class, producing a seismic base shear applied to the building's weight.
LRFD Load Combinations (ASCE 7 Section 2.3)
Load and Resistance Factor Design (LRFD) applies load factors greater than 1.0 to each load type, then compares the factored demand to the factored (reduced) capacity of the structural member. The controlling combination is the one that produces the maximum demand. Key LRFD combinations (from ASCE 7-22):
- 1.4D
- 1.2D + 1.6L + 0.5(L_r or S or R)
- 1.2D + 1.6(L_r or S or R) + (L or 0.5W)
- 1.2D + 1.0W + L + 0.5(L_r or S or R)
- 0.9D + 1.0W
- 1.2D + 1.0E + L + 0.2S
- 0.9D + 1.0E
The 0.9D + 1.0W and 0.9D + 1.0E combinations check overturning and uplift — when minimum gravity load combined with lateral forces might cause a column or foundation to lift off the ground.
ASD Load Combinations (ASCE 7 Section 2.4)
Allowable Stress Design (ASD) uses unfactored (service-level) loads but compares demand to allowable stress that is a fraction of the material strength. ASD combinations are:
- D
- D + L
- D + L_r (or S or R)
- D + 0.75L + 0.75(L_r or S or R)
- D + 0.6W (or 0.7E)
- D + 0.75L + 0.75(0.6W) + 0.75(L_r or S or R)
Which Method to Use?
LRFD is required for steel design (AISC 360) and reinforced concrete design (ACI 318). ASD may be used for timber (NDS) and masonry (TMS 402), though LRFD options exist for both. Most modern structural engineering practice uses LRFD for its better calibrated reliability and consistency across materials.