------------- | ------------------ | ------------------------- | ------------------- | ------------------- | | Gravity dominant | 1.2D + 1.6L | 1.2G + 1.5Q | 1.35G + 1.5Q | 1.25D + 1.5L | | Wind + gravity | 1.2D + 1.0W + 0.5L | 1.2G + W_u + 0.4Q | 1.35G + 1.5W + 0.7Q | 1.25D + 1.4W + 0.5L | | Uplift (wind) | 0.9D + 1.0W | 0.9G + W_u | 1.0G + 1.5W | 0.9D + 1.4W | | Seismic | 1.2D + 1.0E + 0.5L | 1.0G + E_u + 0.3Q | 1.0G + 1.0E + 0.3Q | 1.0D + 1.0E + 0.5L | | Uplift (seismic) | 0.9D + 1.0E | 0.9G + E_u | 1.0G + 1.0E | 1.0D + 1.0E | | Dead load only | 1.4D | 1.35G | 1.35G (for EQU) | 1.4D |

Key differences: ASCE 7-16 uses a wind load factor of 1.0 because the mapped wind speeds already represent strength-level events. Eurocode EN 1990 uses a wind load factor of 1.5 because EN wind speeds are at a lower return period. Australian AS/NZS 1170 uses ultimate-level wind speeds (similar to ASCE 7) with factor 1.0. Canadian NBCC uses 1.4 on wind because NBCC wind pressures are specified at a 50-year return period (not strength-level). These differences mean load factors from different codes must never be mixed.

Step-by-Step Example

Problem: Determine the governing LRFD load combination for a roof beam in Chicago, IL with the following service loads: D = 25 psf, Lr = 20 psf, S = 25 psf, W = +/-30 psf (net), E = 0 (low seismic).

Step 1 -- Evaluate all LRFD combinations:

Step 2 -- Identify governing combinations: Maximum gravity: Combination 3b at 85.0 psf (1.2D + 1.6S + 0.5W). This governs beam strength. Maximum uplift: Combination 6 at -7.5 psf net upward. This governs connection and anchorage design.

Step 3 -- Convert to line load for a 10-ft tributary width: wu = 85.0 _ 10 / 1000 = 0.85 kip/ft (factored). wu_uplift = 7.5 _ 10 / 1000 = 0.075 kip/ft (net uplift).

Result: Design beam for 0.85 kip/ft factored gravity (Comb. 3b). Design roof-to-beam connections for 0.075 kip/ft net uplift (Comb. 6). The snow load combined with wind produces the highest gravity demand, not the dead + live combination -- which is why all combinations must be checked.

Common Design Mistakes

• Checking only Combination 2 (1.2D + 1.6L) for all members: Combination 2 governs for interior floor beams with no environmental loads. For roof members, exterior columns, and foundations, wind or snow combinations (3, 4, 6) frequently govern. Engineers who only check Combination 2 miss the controlling case.
• Forgetting to check uplift combinations (0.9D + 1.0W): Net uplift on roof beams, edge columns, and foundations is caused by wind suction exceeding the stabilizing dead load. This is the most commonly missed combination and has caused connection failures where bolts or welds were not designed for tension reversal.
• Mixing load factors between LRFD and ASD: Using a 1.6 factor on live load (LRFD) with ASD allowable stresses, or using service loads with LRFD resistance values, produces incorrect results. The entire design must be performed consistently in one method.
• Applying the 0.75 three-load factor to LRFD instead of ASD: The 0.75 concurrent load factor (D + 0.75L + 0.75S + 0.75W) is an ASD-specific provision in ASCE 7 Section 2.4. LRFD combinations use specific factors for each load type without a blanket reduction.
• Not identifying which combination governs for each member: Different members in the same structure can be governed by different combinations. A roof beam may be governed by snow + wind (Comb. 3), while an interior column is governed by gravity only (Comb. 2). The governing combination must be identified per member, not for the whole building.
• Using 1.0W with pre-ASCE 7-10 wind speeds: ASCE 7-10 and later use strength-level (ultimate) wind speeds with a load factor of 1.0. Older editions (ASCE 7-05 and earlier) used nominal wind speeds that require a 1.6 factor on W. Using 1.0W with old wind speed maps underestimates wind load by 37%.

Frequently Asked Questions

Which ASCE 7-16 LRFD combination governs for a floor beam with D = 20 psf and L = 50 psf? With only dead and live load and no environmental loads, compare LRFD Combination 1 (1.4D = 28 psf) against Combination 2 (1.2D + 1.6L = 1.2×20 + 1.6×50 = 24 + 80 = 104 psf). Combination 2 governs at 104 psf — 3.7 times the service dead load alone. The load factor on live load is 1.6 because live load has greater variability than dead load. For the equivalent ASD check, Combination 2 (D + L = 70 psf) governs, and the member is designed for Rn/Ω ≥ 70 psf rather than φRn ≥ 104 psf.

Why does LRFD Combination 6 (0.9D + 1.0W) use 0.9D instead of 1.2D? Combination 6 is the uplift or overturning case where wind load acts opposite to dead load. Using 0.9D — a reduced dead load — is intentionally conservative: it accounts for the possibility that actual dead load may be 10% less than nominal due to material tolerances or weight estimates. If 1.2D were used, the self-weight would appear to resist the wind more strongly than it actually does, which is unconservative for net uplift checks. The 0.9 factor is therefore a lower-bound on dead load applied only when dead load is stabilizing, not when it is additive with the design load.

What is the governing LRFD combination for a roof beam with D = 15 psf, S = 30 psf, and W = ±25 psf? Evaluate Combination 3 (1.2D + 1.6S + 0.5W): 1.2×15 + 1.6×30 + 0.5×25 = 18 + 48 + 12.5 = 78.5 psf. Compare Combination 4 (1.2D + 1.0W + 0.5S): 1.2×15 + 1.0×25 + 0.5×30 = 18 + 25 + 15 = 58 psf. Combination 3 governs for the gravity-plus-snow case at 78.5 psf. For net uplift, check Combination 6 (0.9D + 1.0W): 0.9×15 − 25 = 13.5 − 25 = −11.5 psf net uplift — this governs the connection and anchorage design with a net upward demand of 11.5 psf.

What is the combined LRFD factored load for a column with D = 100 kips, L = 80 kips, and E = 60 kips? Check LRFD Combination 5 (1.2D + 1.0E + 0.5L): 1.2×100 + 1.0×60 + 0.5×80 = 120 + 60 + 40 = 220 kips. Check Combination 2 (1.2D + 1.6L): 1.2×100 + 1.6×80 = 120 + 128 = 248 kips. Combination 2 governs for gravity-only at 248 kips; Combination 5 governs when seismic is present if the seismic component is larger. Also check Combination 7 (0.9D + 1.0E): 0.9×100 − 60 = 30 kips net tension — this minimum axial case governs column splice and anchor bolt design when seismic creates net uplift.

How does the 0.75 live load reduction factor work in ASD Combination 8? ASD Combination 8 is D + 0.75W + 0.75L + 0.75(Lr or S or R). The 0.75 factors reflect statistical improbability of simultaneous maximum wind, live, and roof environmental loads. If D = 20 psf, L = 50 psf, W = 30 psf, and S = 20 psf (with no Lr or R): combination = 20 + 0.75×30 + 0.75×50 + 0.75×20 = 20 + 22.5 + 37.5 + 15 = 95 psf. Compare to Combination 2 (D + L = 70 psf) and Combination 3 (D + S = 40 psf): Combination 8 governs at 95 psf for this loading scenario.

When does load combination 1.4D control over combinations with live load? LRFD Combination 1 (1.4D) only governs when dead load is very large relative to live load — typically for heavy equipment foundations, mass concrete structures, or members that support predominantly self-weight. The crossover occurs when 1.4D > 1.2D + 1.6L, which simplifies to 0.2D > 1.6L, or D > 8L. For a typical floor beam with D = 50 psf and L = 50 psf, Combination 1 gives 70 psf versus Combination 2 gives 140 psf — Combination 2 dominates by 2:1. Combination 1 rarely governs in building design except for extremely heavy dead loads with minimal live load.

Related pages

• Snow load calculator
• Wind load calculator
• Seismic load calculator
• Beam capacity calculator
• Load combinations — ASCE 7 LRFD & ASD reference
• Snow load calculation — ASCE 7 design procedure
• Live load reference — IBC and ASCE 7 occupancy table
• Wind load calculation — ASCE 7 MWFRS and C&C
• Seismic design categories — ASCE 7 SDC reference
• How to verify calculator results
• Disclaimer (educational use only)
• Load Combinations CSA S16
• Load combinations AS 4100
• Load combinations ASCE 7-16
• Wood timber design calculator

Disclaimer (educational use only)

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