ASCE 7 sets the factored load combinations that actually size structural members. The service loads you collect in a takedown, dead, live and roof, are only the ingredients; a load combination multiplies each by a factor and adds them, and the largest result is your design reaction. StructLoads evaluates the gravity combinations for every support and tells you which one governs, but it helps to know what the factors mean.
Why combinations exist
No structure sees every structural load at its maximum at the same instant. Combinations are the code’s way of pairing loads at realistic simultaneous levels while keeping a safety margin. This is the practical face of limit state design: you check that the factored demand stays below the factored capacity for each credible loading case, rather than designing for one nominal load.
The gravity strength (LRFD) combinations
For a gravity load takedown, these strength combinations cover most early checks. Here D is dead, L is live, Lr is roof live, and S is snow.
| Combination | Formula | When it tends to govern |
|---|---|---|
| Dead only | 1.4 D | Light members with little live load |
| Live leading | 1.2 D + 1.6 L + 0.5 (Lr or S) | Most occupied floors |
| Roof or snow leading | 1.2 D + 1.6 (Lr or S) + 1.0 L | Roofs and snow regions |
Live load carries the largest factor, 1.6, so on a typical occupied floor the live leading combination usually controls. On a roof, or a member with heavy dead load and little live, one of the other two can win. The factors themselves are not round guesses: they were calibrated statistically in the National Bureau of Standards study Development of a Probability Based Load Criterion (NBS SP 577), which set 1.2 on dead and 1.6 on live so that different load mixes reach a consistent reliability.
A worked example
Take a column that has gathered 519 kN of dead load, 270 kN of live load and 30 kN of roof load through a load takedown. The dead only case gives 1.4 times 519, which is 727 kN. The live leading case gives 1.2 times 519 plus 1.6 times 270 plus 0.5 times 30, which is 623 plus 432 plus 15, or 1070 kN. The live leading combination governs, so the factored design reaction is about 1070 kN. That is the number that sizes the footing.
Why you keep the largest result
You run every relevant combination and design for the largest, because you cannot know in advance which one controls a given member. A heavily dead loaded transfer girder may be governed by 1.4 times dead, while a normal office column is governed by the live leading case, and a long span roof beam by the snow leading case, all in the same building. The combination that governs one member does not govern the next, so the safe habit is to evaluate each case and let the numbers pick the winner rather than guessing. This is also why software helps once the building has many members: the bookkeeping of several combinations across dozens of supports is exactly where hand work gets slow and error prone.
LRFD and ASD are two routes to the same goal
ASCE 7 also gives allowable stress design combinations, which keep loads near their service values, for example dead plus live, and compare against an allowable stress rather than a factored capacity. Materials standards decide which route you use: steel and concrete strength design use the LRFD combinations above, while some timber and masonry work still uses ASD. The dead and live split that feeds both is covered in dead load vs live load.
Live load reduction fits inside the combination
Before you apply the factors, the code may let you reduce the design live load on a member that supports a large area, because the full live load is unlikely everywhere at once. That reduction happens to L first, then the 1.6 factor applies to the reduced value. The rules and the formula are set out in ASCE 7 live load reduction.
Key takeaways: ASCE 7 load combinations
Collect service loads, then let the combinations turn them into a design reaction: 1.4 D, and 1.2 D plus 1.6 L plus 0.5 roof are the gravity workhorses, with the largest result governing. StructLoads applies these (and the Eurocode set) automatically and names the governing combination per support, so you can check the factored number against this by hand. The combinations are code minimums, and a qualified engineer confirms the full set for the real design.
Related guides
More in this series: a load combination worked example.
Quick answers
What are the ASCE 7 load combinations?
ASCE 7 gives two sets. Strength design combines factored loads, the main gravity ones being 1.4 times dead, and 1.2 times dead plus 1.6 times live plus 0.5 times roof or snow. Allowable stress design combines service loads at factors near one, such as dead plus live. You check each relevant combination and design for the largest.
What is the difference between LRFD and ASD?
LRFD applies factors above one to the loads and compares against a reduced member strength. ASD keeps loads near their service values and compares against an allowable stress. Both target safety; LRFD does it through load and resistance factors, ASD through a single safety factor on capacity.
Which ASCE 7 load combination governs a gravity takedown?
For most floors, 1.2 times dead plus 1.6 times live plus 0.5 times roof governs, because live load carries the largest factor. On a light, heavily dead loaded member, 1.4 times dead alone can control. Evaluate both and take the larger; StructLoads reports which governs each support.
Is it enough to check just one load combination?
No. You check every combination that could realistically govern, because the controlling case differs between members. For a pure gravity, early stage takedown the two or three strength gravity combinations are enough; a full design adds wind, seismic and load reversal. Skipping a governing case is unsafe, so evaluate each rather than assume it does not control.