What is Us Load Combinations used for in structural engineering?

Us Load Combinations provides values required for steel design calculations including capacity checks, connection design, and detailing. It is referenced in structural design codes and used by practicing engineers during the design of buildings, bridges, and industrial structures.

Can I use these reference values directly in my design?

Yes, provided the values match your governing code edition and project specifications. Always cross-reference against the primary source (code document, manufacturer data, or published standard). The information on this page is for educational use — final verification by a licensed engineer is required.

How often are these reference values updated?

Reference content is maintained to align with current code editions. When a new edition of AISC 360, AS 4100, EN 1993, or CSA S16 is published, values are reviewed and updated. Check the last-modified date at the bottom of the page and verify against the current edition for your jurisdiction.

US Load Combinations — ASCE 7-22 LRFD and ASD Guide

Complete reference for load combinations per ASCE 7-22 Chapter 2 and IBC Section 1605 for US structural steel design with AISC 360. Covers all seven LRFD Strength combinations, all seven ASD Allowable Stress combinations, when to apply each, companion load factors, exceptions for wind and seismic, and a step-by-step worked example for a steel building frame.

LRFD vs ASD — Which Method to Use?

AISC 360 provides two parallel design methods with equivalent reliability:

LRFD (Load and Resistance Factor Design):

Factored loads compared against factored resistance (phi*Rn)
Resistance factors phi are less than 1.0 (0.75-0.90)
More widely used in current practice
Required by most US building codes for new construction
All AISC 360 provisions are written in terms of LRFD

ASD (Allowable Strength Design):

Service loads compared against allowable strength (Rn/Omega)
Safety factors Omega are greater than 1.0 (1.67-2.00)
Still permitted by IBC for steel structures
Some engineers prefer ASD for serviceability-controlled designs
ASD results can be obtained by dividing LRFD results by 1.5 (approximate)

The ASCE 7-22 load combinations are designed so that both methods produce structures with the same target reliability index (beta approximately 3.0 for typical members).

LRFD Strength Combinations — ASCE 7-22 Section 2.3.1

The seven basic LRFD load combinations for steel design per IBC 1605.2:

Comb	Load Combination	Typical Use
1	1.4D	Dead-dominant structures (tanks, retaining walls)
2	1.2D + 1.6L + 0.5(Lr or S or R)	Most common — gravity-controlled buildings
3	1.2D + 1.6(Lr or S or R) + (1.0L or 0.5W)	Roof-dominant with snow or rain
4	1.2D + 1.0W + 1.0L + 0.5(Lr or S or R)	Wind-governed lateral systems
5	1.2D + 1.0E + 1.0L + 0.2S	Seismic-governed structures
6	0.9D + 1.0W	Uplift and overturning (wind reversal)
7	0.9D + 1.0E	Uplift and overturning (seismic reversal)

Where:

D = dead load (self-weight, superimposed dead, fixed equipment)
L = live load (occupancy, per IBC Table 1607.1 or ASCE 7 Chapter 4)
Lr = roof live load (per ASCE 7 Chapter 4, minimum 12 psf for flat roofs)
S = snow load (per ASCE 7 Chapter 7, ground snow _ exposure _ thermal * importance)
R = rain load (per ASCE 7 Chapter 8, usually negligible unless ponding)
W = wind load (per ASCE 7 Chapter 26-31, directional or envelope procedure)
E = seismic load (per ASCE 7 Chapter 12 or 15-23)

Key Differences from Prior Editions

ASCE 7-22 made important changes to load combinations:

L factor in Combination 3: The live load factor changed from 0.5L to (1.0L or 0.5W). This means when L is the dominant companion load in a roof-controlled combination, it uses the full 1.0 factor. This was a significant change from ASCE 7-16 where L had a 0.5 factor in this combination.

No 0.6D for wind: The familiar 0.6D factor in the overturning combination is now 0.9D. This reflects updated statistical analysis of dead load uncertainty. Engineers accustomed to 0.6D should verify their combinations are updated.

Snow importance factor absorbed: The snow importance factor Is is now incorporated directly into the ground-to-roof snow conversion, not applied as a separate factor in the load combinations.

ASD Allowable Stress Combinations — ASCE 7-22 Section 2.4.1

The seven ASD combinations per IBC 1605.3:

Comb	Load Combination	Typical Use
1	D	Dead-dominant structures
2	D + L	Gravity-controlled buildings
3	D + (Lr or S or R)	Roof-dominant with snow or rain
4	D + 0.75L + 0.75(Lr or S or R)	Reduced gravity combinations
5	D + (0.6W or 0.7E)	Wind or seismic with dead
6a	D + 0.75L + 0.75(0.6W) + 0.75(Lr or S or R)	Wind as companion with 0.75 reduction
6b	D + 0.75L + 0.75(0.7E) + 0.75S	Seismic as companion with 0.75 reduction
7	0.6D + 0.6W	Uplift and overturning (wind)
8	0.6D + 0.7E	Uplift and overturning (seismic)

Note: Combinations 6a and 6b are alternatives — use whichever applies based on whether wind or seismic governs.

ASD-to-LRFD Conversion

For preliminary design, ASD results can be converted to LRFD equivalents using approximate factors:

Load Type	ASD to LRFD Factor
Dead load only	x 1.4
Dead + Live (gravity)	x 1.5 (approximate)
Dead + Wind	x 1.3-1.5 (depends on D/W ratio)
Dead + Seismic	x 1.3-1.5

These are approximate. Always verify with the actual load combinations.

When to Use Which Combination

Gravity- Controlled Structures (Low Wind, Low Seismic)

Combination 2 (1.2D + 1.6L + 0.5Lr) governs most floor members. For roof members, Combination 3 (1.2D + 1.6Lr + 1.0L) may govern when the roof live load or snow load exceeds the floor live load effect.

Wind-Controlled Structures (Tall or Flexible)

Combinations 4 and 6 govern. Combination 4 (1.2D + 1.0W + 1.0L) checks the maximum effect with wind. Combination 6 (0.9D + 1.0W) checks uplift, overturning, and sliding — critical for anchor bolt and foundation design.

Seismic-Controlled Structures

Combinations 5 and 7 govern. Per ASCE 7 Section 12.4.2.3, the seismic load E includes both horizontal and vertical components: E = rhoQe + 0.2SDS*D

Where rho is the redundancy factor (1.0 or 1.3), Qe is the horizontal seismic force, and 0.2SDSD accounts for vertical seismic effects.

Worked Example — Two-Story Steel Office Building

Given:

Two-story steel frame, Risk Category II
Roof dead load D = 25 psf (including steel, deck, roofing, MEP)
Floor dead load D = 80 psf (including steel, concrete deck, finishes)
Roof live load Lr = 20 psf (reducible, flat roof)
Floor live load L = 50 psf (office, reducible per ASCE 7 Table 4.2)
Ground snow load pg = 30 psf (flat roof, Cs = 1.0, Ct = 1.0, Is = 1.0)
Basic wind speed V = 115 mph, Exposure B
Seismic Design Category B, SDS = 0.50g
Interior column tributary area: 30 ft x 30 ft = 900 sf per story

Gravity Loads on Interior Column

Dead load at base: D = (25 + 80) _ 900 _ 2 = 189,000 lb = 189 kips (two stories)

Live load (reducible per ASCE 7 Section 4.7): Roof: Lr = 20 psf (not reducible for flat roof with R1 = 1.0) Floor: L = 50 _ (0.25 + 15/sqrt(900)) = 50 _ 0.75 = 37.5 psf (reduced) L_total = (20 + 37.5) * 900 = 51,750 lb = 51.75 kips

Snow load: S = pg _ Cs _ Ct _ Is = 30 _ 1.0 _ 1.0 _ 1.0 = 30 psf S_total = 30 * 900 = 27,000 lb = 27 kips (roof only)

LRFD Load Combinations at Column Base

Comb	Formula	Pu (kips)	Notes
1	1.4D	264.6	Dead only
2	1.2D + 1.6L + 0.5S	226.8 + 82.8 + 13.5 = 323.1	Gravity governs
3	1.2D + 1.6S + 1.0L	226.8 + 43.2 + 51.75 = 321.8	Snow dominant
4	1.2D + 1.0W + 1.0L + 0.5S	226.8 + W + 51.75 + 13.5	Wind companion (calculate W separately)
5	1.2D + 1.0E + 1.0L + 0.2S	226.8 + E + 51.75 + 5.4	Seismic companion
6	0.9D + 1.0W	170.1 + W	Overturning check
7	0.9D + 1.0E	170.1 + E	Overturning check

For this building in a low seismic and moderate wind area, Combination 2 governs the column design at 323.1 kips.

ASD Load Combinations at Column Base

Comb	Formula	Pa (kips)
1	D	189.0
2	D + L	240.75
3	D + S	216.0
4	D + 0.75L + 0.75S	189.0 + 38.8 + 20.25 = 248.1
5	D + 0.6W	189.0 + 0.6W

ASD Combination 4 governs at 248.1 kips (service level).

Verify Equivalence

LRFD/ASD ratio = 323.1/248.1 = 1.30 (close to the approximate factor of 1.5 for dead+live, but lower because the live load reduction and snow companion factor reduce the difference).

Companion Load Factors — Special Cases

Pattern Loading

For continuous beams and frames, ASCE 7-22 Section 4.2 requires pattern loading — alternate spans loaded and unloaded to maximize negative moments at supports and positive moments at midspan. The pattern load factor is 0.75L for the companion spans (per ACI 318 for concrete; steel design typically uses 1.0L for all spans per AISC Design Guide 3).

Hydrostatic Load (H)

When hydrostatic pressure is significant: Add H to Combinations 1-5 with factor 1.6H if it adds to the load effect, or 0.9H if it counteracts.

Differential Settlement (T)

When differential settlement T is significant: Add T to Combinations 1-5 with factor 1.2T for loads that cause settlement effects.

Calculator

Generate factored load combinations instantly with our free calculator. Enter dead, live, snow, wind, and seismic loads to get all LRFD and ASD combinations with governing cases highlighted.

Try the Load Combination Calculator

FAQ

Q: What is the difference between LRFD and ASD load combinations? A: LRFD uses factored loads (greater than service) compared against factored resistance (phi*Rn, less than nominal). ASD uses service loads compared against allowable strength (Rn/Omega, less than nominal). Both methods produce equivalent reliability. LRFD factors up the loads (1.2D + 1.6L); ASD keeps loads at service level and divides the capacity by a safety factor.

Q: When did ASCE 7 change the dead load factor in the overturning combination from 0.6D to 0.9D? A: ASCE 7-22 changed the dead load factor from 0.6D to 0.9D in the wind overturning combination (Combination 6/7). This reflects updated statistical analysis showing that the coefficient of variation for dead load is lower than previously assumed. The 0.9D factor is less conservative than 0.6D for uplift checks.

Q: Do I need to check all seven load combinations? A: In practice, engineers typically identify 2-3 governing combinations based on the structural system and loading. For gravity-controlled low-rise buildings, Combination 2 governs members and Combination 6 governs anchors. For wind-controlled structures, Combinations 4 and 6 govern. For seismic, Combinations 5 and 7 govern. Always check all combinations for columns and foundations where multiple load effects interact.

Q: How is the seismic load E defined in ASCE 7-22 load combinations? A: Per ASCE 7 Section 12.4.2.3, E = rhoQe +/- 0.2SDSD. The horizontal component is rhoQe (redundancy factor times horizontal seismic force). The vertical component is 0.2SDSD (acts up or down). In the load combinations, E already includes both components. For D + E: use +0.2SDSD if it increases the effect; for 0.9D + E: use -0.2SDSD if it decreases the effect.

Q: Can I use ASD for new steel construction? A: Yes, ASD is permitted by the IBC for structural steel. AISC 360 provides both LRFD and ASD provisions. However, most US design firms use LRFD for new construction because it is more widely taught, better supported by software, and aligns with the load combinations in ASCE 7-22. ASD is sometimes preferred for renovation projects where existing members were designed by ASD.