Load Combination Generator: ASCE 7 and Eurocode

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Load Combination Generator

Enter your unfactored load effects once and get every code combination worked out, ranked, with the governing case flagged. Works with ASCE 7 strength design, ASCE 7 allowable stress design and the Eurocode ultimate and service sets. Units do not matter as long as you are consistent, so enter kN, kNm or kPa and read the answer back in the same units.

Enter wind and seismic as positive numbers. The tool also runs the reduced dead load cases, which is where uplift and overturning show up.

Horizontal bar chart of ASCE 7 strength design load combinations for an example set of loads, with the governing wind plus live case highlighted, and a panel explaining why the reduced dead load combinations exist
The same set of loads run through every combination. Notice that the case most people expect to govern, dead plus live, is not the one that wins here.

How to use this generator

Enter each unfactored load effect once. It does not matter whether you are working with an axial force, a bending moment, a shear or a pressure, as long as every input is the same quantity in the same units. The tool applies each combination and returns the answer in those units.

You get all three codes as separate tabs. Strength design and allowable stress design are the two ASCE 7 sets, and they are not interchangeable: strength combinations go with factored resistances, service combinations go with allowable stresses. Mixing them produces a design that is either wildly conservative or unsafe.

The Eurocode tab exposes the psi zero factors, because those are set nationally and vary by occupancy. The defaults are typical values but you should check your national annex.

The result table is sorted, so the governing case is at the top with everything else ranked below it as a percentage. That ordering is more useful than the raw numbers, because it tells you at a glance how much of your design is being driven by one load.

Why there are so many combinations

Every load a structure carries is uncertain, but they are not uncertain in the same way or to the same degree. Self weight can be estimated within a few percent once the design is fixed. Live load depends on who moves in and what they bring. Wind and seismic are statistical events that may never reach their design value in the life of the building.

Combinations exist because it would be absurdly conservative to assume that the worst live load, the worst snow, the worst wind and the worst earthquake all turn up in the same instant. So the codes nominate one variable action as the leading one, apply its full factor, and reduce the others to a value that plausibly coincides with it. Then they repeat that with each variable action taking a turn at leading.

Dead load is treated differently because it is the one load you can count on. Its factor is smaller, which reflects how well it is known, and it appears on both sides of the argument.

The reduced dead load cases are the ones that catch people

Every combination set includes cases with the dead load factor below one. In ASCE 7 strength design that is 0.9, in allowable stress design 0.6, in the Eurocode 1.0 rather than 1.35.

These exist because dead load is sometimes the thing keeping the structure in place. A light roof under wind uplift, a retaining wall being overturned, a cantilever being lifted at its far end, a slab being pushed up by water pressure: in all of those the weight is helping, and underestimating it is the unsafe direction.

The consequence is that a member can pass every gravity combination comfortably and still fail on uplift. This is a routine cause of failures in light steel buildings, canopies and roof anchorage. If any of your load effects can reverse sign, enter them and look at the bottom of the ranked list rather than the top.

Reading the result properly

Look at the percentages more than the values. If the governing case is only two or three percent above the next one, your design is balanced and a small change to any load could shift which case wins, so it is worth rechecking after any revision. If one case is twenty percent above everything else, that load is driving the whole design, and effort spent refining it will pay off more than effort spent anywhere else.

Also watch which case governs where. In a multi storey frame, gravity usually governs beam design while wind or seismic governs the columns and the lateral system. Running the generator separately for a beam moment and a column axial force can give different answers, and that is expected rather than an error.

One warning about seismic. In many codes the seismic term already includes its own internal factors and a vertical component tied to the site coefficients, so the value you enter should be the one your seismic analysis produced for use in combinations, not a raw base shear. The same care applies to wind, where the pressure coefficients already build in directionality.

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