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, the same idea SpaceGass calls automatic member design.

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.