Loads & combinations

Loading is where most of the engineering judgement in a linear model lives. The solver combines whatever you give it; deciding what belongs in each case, and which cases can act together, is yours.

In this section

• 01ASCE 7 combinationsLRFD and ASD combinations to ASCE 7-22 — how the generator maps your load groups onto D, L, Lr, S, W and E, and pairing them with the AISC check.
• 02Automatic combinationsGenerating EN 1990 and ASCE 7 combinations, understanding the equations behind them, and reading the generated set critically.
• 03EnvelopesExtracting the governing value across dozens of combinations without reading every one — and the traps in doing so.
• 04Load casesActions, self-weight, imposed loads and load arrangements — how to build cases that stay separable.
• 05Load panelsAreas that hand a floor, roof or wall pressure to the members around them as line loads, one way or two way, without meshing a slab.
• 06Temperature loadsUniform temperature change and through-depth gradients — what each does to a free and a restrained structure, and the coefficients to use.
• 07Wind and snowWind to EN 1991-1-4 and snow to EN 1991-1-3, generated on a building envelope — site inputs, recognised roof shapes, the cases produced, and the limits.

The structure of a loaded model

Load case          a single, self-consistent action (e.g. "Imposed — offices")
    ↓
Load group         cases that share a factor and a relationship (leading/accompanying)
    ↓
Combination        factored sum of cases, one per code equation and leading action
    ↓
Envelope           the extreme of a result quantity across a set of combinations

Keep load cases pure. A case called "Dead + services" cannot later be factored separately from the dead load it contains, and you find that out at the worst moment: when the check fails and you need to know which part drove it.

Linearity, and what it buys you

Linear static analysis superposes. The solver solves each load case once and builds every combination as a factored sum of those results, which is why fifty combinations cost little more than one.

Common mistakes

• Self-weight counted twice: once by the automatic self-weight case and once in a manually applied dead load.
• Imposed load applied everywhere at once when the governing arrangement is a chequerboard pattern. Adjacent-span loading usually governs support moments.
• Wind treated as a single case when the code needs several directions and internal pressure coefficients.
• Combinations generated once and never regenerated after new cases were added. The combination set does not update itself silently; regenerate it and confirm the count.