Load Combinations — ASCE 7, AS/NZS 1170, EN 1990, NBCC Reference
Load combinations define how individual loads are combined for structural design. Each code specifies factors reflecting the probability of simultaneous maximum occurrence. This page covers LRFD and ASD combinations from the major international standards.
ASCE 7-22 LRFD load combinations (IBC)
| Combo | Expression | Controls For |
|---|---|---|
| LC1 | 1.4D | Heavy dead load |
| LC2 | 1.2D + 1.6L + 0.5(Lr or S or R) | Gravity floors |
| LC3 | 1.2D + 1.6(Lr or S or R) + (L or 0.5W) | Gravity roofs |
| LC4 | 1.2D + 1.0W + L + 0.5(Lr or S or R) | Wind + gravity |
| LC5 | 1.2D + 1.0E + L + 0.2S | Seismic + gravity |
| LC6 | 0.9D + 1.0W | Wind uplift |
| LC7 | 0.9D + 1.0E | Seismic overturning |
D = dead, L = live (0.5L when L<=100 psf except garages/assembly), Lr = roof live, S = snow, R = rain, W = wind (strength-level in ASCE 7-16+), E = seismic = rhoQE + 0.2SDS*D.
ASCE 7-22 ASD load combinations
| Combo | Expression |
|---|---|
| LC1 | D |
| LC2 | D + L |
| LC3 | D + (Lr or S or R) |
| LC4 | D + 0.75L + 0.75(Lr or S or R) |
| LC5 | D + (0.6W or 0.7E) |
| LC6 | D + 0.75L + 0.75(0.6W) + 0.75(Lr or S or R) |
| LC7 | 0.6D + 0.6W |
| LC8 | 0.6D + 0.7E |
AS/NZS 1170.0 ULS combinations
| Combo | Expression |
|---|---|
| 1 | 1.35G |
| 2 | 1.2G + 1.5Q |
| 3 | 1.2G + Wu + psi_c*Q |
| 4 | G + Eu + psi_c*Q |
| 5 | 0.9G + Wu |
| 6 | 0.9G + Eu |
psi_c = combination factor: 0.4 for floors, 0.6 for storage, 0.0 for roofs.
EN 1990 fundamental combination (ULS)
sum(gamma_GGk) + gamma_Q1Qk1 + sum(gamma_Qipsi_0iQki), where gamma_G = 1.35 (unfavorable), gamma_Q = 1.50.
Common psi_0 factors
| Action | psi_0 | psi_1 | psi_2 |
|---|---|---|---|
| Offices (Cat. B) | 0.7 | 0.5 | 0.3 |
| Storage (Cat. E) | 1.0 | 0.9 | 0.8 |
| Snow (< 1000m) | 0.5 | 0.2 | 0.0 |
| Wind | 0.6 | 0.2 | 0.0 |
NBCC 2020 ULS combinations (Canada)
| Combo | Expression |
|---|---|
| 1 | 1.4D |
| 2 | 1.25D + 1.5L |
| 3 | 1.25D + 1.5S |
| 4 | 1.25D + 1.4W |
| 5 | 1.0D + 1.0E |
Key differences from ASCE 7: dead load factor 1.25 (not 1.2), live load factor 1.5 (not 1.6), wind factor 1.4 (not 1.0).
Governing combination guide
| Scenario | Typical Governing Combo |
|---|---|
| Floor beams | 1.2D + 1.6L |
| Roof beams (snow) | 1.2D + 1.6S |
| Interior columns | 1.2D + 1.6L |
| Exterior columns (wind) | 1.2D + 1.0W + L |
| Foundations (uplift) | 0.9D + 1.0W |
| Lateral system | 1.2D + 1.0E + L |
Common mistakes
- Not checking minimum gravity combinations. LC6 (0.9D+W) and LC7 (0.9D+E) govern for uplift and overturning.
- Applying live load reduction incorrectly. The 0.5L factor only applies when L<=100 psf and not in garages/assembly.
- Mixing LRFD and ASD. Never mix factored loads with ASD capacities or vice versa.
- Forgetting companion loads. In LC3, the companion load (L or 0.5W) must be included.
- Omitting vertical seismic component. E includes 0.2SDSD, which increases the effective dead load factor.
Frequently asked questions
What is the most common governing combination? For most interior floor beams and columns: 1.2D + 1.6L. For lateral system members: seismic or wind combinations.
Factored vs. unfactored loads? LRFD uses factored loads and phi factors. ASD uses service loads and Omega factors. Both target the same reliability.
Why so many combinations? Different scenarios of simultaneous loading. The probability that all loads are at maximum simultaneously is very low. Factors reflect the statistical likelihood.
Run this calculation
Related references
- Load Combinations ASCE 7 (detailed)
- Live Load Reference
- Snow Load Calculation
- Wind Load Calculation
- Seismic Design Categories
- How to Verify Calculations
Disclaimer
This page is for educational and reference use only. It does not constitute professional engineering advice. All design values must be verified against the governing building code and project specification. The site operator disclaims liability for any loss arising from the use of this information.