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Auto-size (AS 4100) ​

CivilKit Studio can automatically size your steel members - it searches the catalogue for the lightest section that still passes the AS 4100 design check, commonly known as automatic member design.

Works with your region's code

Auto-size optimises against the active design code, not only AS 4100. Set the region in Settings and it sizes to AISC 360 (US) or Eurocode 3 (EU) instead

How to use it ​

  1. Assign catalogue sections to your members (each member's section defines its design group, e.g. all Universal Beams).
  2. Apply your loads.
  3. Click Auto-size in the toolbar.

The log reports each member that changed (e.g. M3: 410UB53.7 → 250UB25.7), the final utilisation, and the total steel-mass saving.

How it works ​

It is the analyse-design loop from steel design practice:

  1. Analyse - solve for the member forces with the current sections.
  2. Design - for each member, scan its design group from lightest to heaviest and choose the first section whose governing AS 4100 utilisation (Cl 8.3/8.4 combined actions on both axes, plus shear) is ≤ 1.0.
  3. Re-analyse - changing sections shifts the stiffness and the forces, so the model is solved again and the loop repeats until the chosen sizes stop changing (converged).

Member capacities depend only on a member's own section, length and grade - not on the applied forces - so each candidate section is evaluated once per design group per iteration and the result re-used for every member of that length. This keeps auto-size fast (≈ one solve per candidate section per iteration).

What if nothing fits? ​

If no section in a member's group can carry the load at utilisation ≤ 1.0, the member is left at the heaviest available section and flagged OVER in the log and Design panel - auto-size never hides an inadequate member. Add a heavier group, change the grade, or add restraints, then re-run.

Verify before professional use

Auto-size uses the same simplifying assumptions as the design check (effective length = member length, αm = 1.0 unless set, no intermediate restraints). Always confirm restraints, grade and the governing load combination before relying on the chosen sizes. See Design checks.