Load Cases and Load Combinations

If you'd like to follow along, open WebStructural in a separate tab and take a few minutes to familiarize yourself with the app. We're going to focus on the Design Specification card.

Serviceability

In structural engineering, the two main design criteria to consider are serviceability and capacity. Serviceability refers to the performance of a structure under load as perceived by its occupants. Due to the elastic nature of steel and wood, elements that are meeting loading demands can still feel uncomfortable if they deflect too much under live loads. Furthermore, floors and roofs must limit deflection in order to be safely serviced. Typically, serviceability is maintained by ensuring structural elements remain within a deflection limit. In WebStructural's Beam Designer module, these limits can be specified from the Design Specification card.

The deflection limits in the Design Specification dialog

Capacity

Capacity, as the name implies, refers to the structural element's ability to meet the loading demands it is expected to experience over its lifetime. While serviceability is based on human perception and general structural behavior, capacity calculations keep structural elements from failing by yielding, buckling or breaking. All design specifications include some type of safety factor to modify capacity and demand, so beams and columns keep a margin over the largest loads they are expected to see. These safety factors are built into the loads applied to the structure via Load Combinations (LRFD and ASD). Safety factors are also included in the design specifications used to determine capacity (AISC and NDS). In this article, we'll focus on Load Combinations. Specifically, Load and Resistance Factor Design (LRFD) and Allowable Stress Design (ASD). In WebStructural, load combinations can be specified from the Design Specification card.

The Design Specification dialog with the ASCE 7-22 LRFD load combinations

Load Cases

LRFD and ASD share the concept of Load Cases. Load Cases are a way to break loads up based on their type, allowing loads to be combined in different ways to simulate different interactions. These combinations, called Load Combinations, account for extreme events, the maximum load a member is likely to experience over its lifetime. It is important to remember that this is not the only place safety factors are applied. Safety factors are also applied to capacity checks to further ensure that even in extreme conditions, structural elements keep a margin against failure.

D	Dead Loads
L	Live Loads
Lr	Roof Live Loads
S	Snow Loads
W	Wind Loads (strength level, as ASCE 7-10 and later give them)
E	Earthquake Loads
H	Lateral pressure due to water or dirt typically pressing on the sides of a structure
R	Rain Loads

LRFD vs ASD

So what's the difference between LRFD and ASD? In its simplest form, LRFD focuses on increasing load, accounting for uncertainty in dynamic loads like wind and snow. ASD, on the other hand, focuses on reducing load (and capacity) based on the probability of multiple load cases acting together at exactly the same time. WebStructural's Beam Designer module supports a comprehensive set of combinations for both LRFD and ASD.

LRFD Combinations (ASCE 7-16 2.3.1)
#	Combination
1	1.4D
2	1.2D + 1.6L + 0.5(Lr or S or R)
3	1.2D + 1.6(Lr or S or R) + (1.0L or 0.5W)
4	1.2D + 1.0W + 1.0L + 0.5(Lr or S or R)
5	1.2D + 1.0E + 1.0L + 0.2S
6	0.9D + 1.0W
7	0.9D + 1.0E

ASD Combinations (ASCE 7-16 2.4.1)
#	Combination
1	D
2	D + L
3	D + (Lr or S or R)
4	D + 0.75L + 0.75(Lr or S or R)
5	D + 0.6W or D + 0.7E
6	D + 0.75L + 0.75(0.6W) + 0.75(Lr or S or R)
7	D + 0.75L + 0.75(0.7E) + 0.75S
8	0.6D + 0.6W
9	0.6D + 0.7E

Wind loads have been strength-level loads since ASCE 7-10, so they take 1.0W in LRFD and 0.6W in ASD. Older editions used service-level wind with 1.6W and 1.0W. Wind and earthquake loads can act either way, so the Beam Designer also checks uplift, with the load reversed: 0.9D − 1.0W in LRFD, for example.

A final important piece to all of this is understanding how each method adjusts capacity. The key difference becomes clear when we look at the controlling equations for each method:

LRFD

Mu ≤ φ × Mn

Where Mu is the moment from the LRFD combinations and φ is a resistance factor specific to the material and stress state (bending, shear or normal).

ASD

Ma ≤ Mn / Ω

Where Ma is the moment from the ASD combinations and Ω is a factor of safety.

Which combination usually governs

Every combination has to be checked, but a few usually decide the size of a member:

Floor beams and joists: 1.2D + 1.6L in LRFD, and D + L in ASD.
Roofs with snow: 1.2D + 1.6S + 0.5W in LRFD, and D + S in ASD. Where the roof live load is larger than the snow load, Lr takes the place of S.
Light roofs in high wind: 0.9D − 1.0W in LRFD, and 0.6D − 0.6W in ASD. The member lifts, so its bottom flange needs bracing, and the connections, posts and anchors must hold it down.
Mostly dead load: 1.4D governs in LRFD when the live load is less than an eighth of the dead load, such as a beam carrying a masonry wall.
Braces, shear walls and frames: the wind and earthquake combinations, 1.2D + 1.0W + L and 1.2D + 1.0E + L, along with their 0.9D uplift partners.

Worked example: a roof beam

A steel roof beam spans 20 ft, with beams 8 ft apart. Each roof load in pounds per square foot (psf), times the 8 ft of roof the beam carries, gives its load along the beam in pounds per foot (lb/ft). There is no floor live load, rain or earthquake load.

Load	psf	lb/ft
D, roof deck, insulation and beam	15	120
Lr, roof live load	20	160
S, snow	25	200
W, wind pushing down	10	80
W, wind lifting up	25	200

In the LRFD combinations, snow is the largest of Lr, S and R, so it is the one used wherever the combination says "Lr or S or R":

#	Combination	Worked out	lb/ft
1	1.4D	1.4(120)	168
2	1.2D + 1.6L + 0.5S	1.2(120) + 0.5(200)	244
3	1.2D + 1.6S + 0.5W	1.2(120) + 1.6(200) + 0.5(80)	504
4	1.2D + 1.0W + L + 0.5S	1.2(120) + 1.0(80) + 0.5(200)	324
5	1.2D + 1.0E + L + 0.2S	1.2(120) + 0.2(200)	184
6	0.9D − 1.0W	0.9(120) − 1.0(200)	−92
7	0.9D + 1.0E	0.9(120)	108

Combination 3 governs, at 504 lb/ft. The moment at midspan is wL²/8 = 504 × 20² / 8 = 25,200 lb-ft, or 25.2 kip-ft. Combination 6 lifts the beam with a net 92 lb/ft, so its bottom flange is in compression and needs bracing, and its connections must hold it down.

In ASD, D + S governs at 120 + 200 = 320 lb/ft, a moment of 16.0 kip-ft, and 0.6D − 0.6W gives a net uplift of 72 − 120 = 48 lb/ft.

The two methods land on nearly the same beam. For steel bending, φ is 0.9 and Ω is 1.67, so the beam needs a nominal moment Mn of 25.2 / 0.9 = 28.0 kip-ft in LRFD and 16.0 × 1.67 = 26.7 kip-ft in ASD. LRFD asks for about 5% more here because most of the load is snow, which it treats as less certain than dead load.

Load combinations calculator

Enter your loads to see every ASCE 7-22 or 7-16 combination and which one governs. Use one unit for all of them: psf, lb/ft or kips. Wind and earthquake loads act both ways, so enter them as positive numbers. The values below are the roof beam above.

Wind is strength level, as ASCE 7-10 and later give it. Lateral soil and water pressure (H) is left out. To size the member as well, the Beam Designer applies these same combinations.

What changed in ASCE 7-22

ASCE 7-22, which the 2024 International Building Code adopts, maps ground snow loads at strength level, the way ASCE 7-10 did for wind. Its snow loads are larger, and the factor on snow in the snow-led combination drops from 1.6 to 1.0 in LRFD, and from 1.0 to 0.7 in ASD (D + 0.7S). The snow factors in the other combinations change to match, so use ASCE 7-22's own tables on a project built to it. Putting a 7-22 snow load into the 7-16 combinations above overstates snow by about 60%. The calculator on this page, and the beam, column and frame designers, let you pick ASCE 7-22 or 7-16; new designs start on 7-22, and designs saved before October 15, 2026 stay on 7-16.

Conclusion

So is one method better than the other? The short answer is no. For structures that experience more variable and dynamic loads, LRFD will typically result in a more conservative design. Conversely, if loads are less dynamic and predictable, ASD can result in a more conservative design. Both are still acceptable methods under the International Building Code (IBC). At WebStructural we've found that LRFD seems to be the more preferred method. That said, it's ultimately up to you, the engineer or designer, to decide which is best for your use case.

Load combination questions

What is the difference between a load case and a load combination?

A load case is one kind of load on its own, such as dead, live, snow or wind. A load combination adds the load cases together with factors, such as 1.2D + 1.6L, for the loads likely to act at the same time.

Why is wind 0.6W in ASD and 1.0W in LRFD?

Since ASCE 7-10, wind maps give strength-level wind loads, so LRFD uses them as they are, with a factor of 1.0. ASD multiplies them by 0.6 to bring them back to the service level that allowable stresses were set for.

What does 0.9D mean in a load combination?

In uplift and overturning, dead load helps hold the structure down. Using 0.9D in LRFD, or 0.6D in ASD, assumes there is less dead load than estimated, which is on the safe side.

Can I mix LRFD and ASD?

Not for one member. Use LRFD loads with LRFD capacities (with the resistance factor), or ASD loads with ASD capacities (with the factor of safety or allowable stresses). Steel can be designed either way; wood is usually designed in ASD.

Do I have to check every load combination?

Yes, but combinations with loads you don't have drop out, and a few usually govern: 1.2D + 1.6L for floors, 1.2D + 1.6S for snowy roofs and 0.9D − 1.0W for light roofs in wind. The calculator on this page checks them all.

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