Cold-Formed Steel Member (Design Only)

How it calculates

The Cold-Formed Steel Member calculator takes pre-computed analysis forces and applies AS/NZS 4600:2018 capacity design checks. The design method is the Direct Strength Method (DSM) from Clause 7, which uses elastic buckling analysis - specifically Finite Strip Method (FSM) signature curves built into the section library - to determine local, distortional, and global buckling loads.

Section selection and configuration

You select a section from the comprehensive Australian CFS library (Cee, Zed, top-hat, hat, and custom shapes). The section configuration - Single, Back-to-Back, or Boxed - scales all capacity results by 1 or 2 as appropriate. The library stores the FSM-derived elastic buckling moments and compression loads needed for DSM without requiring you to run separate buckling software.

Moment capacity

The moment section capacity phiM_s is the upper-bound capacity assuming perfect continuous restraint. It is compared against the maximum applied moment demand.

Positive and negative moment member capacities account for lateral-torsional buckling:

utilization = |M| / phiM_b ≤ 1.0*

where phiM_b is governed by the lateral-torsional buckling load, the effective length between restraints, and the DSM strength curve. Separate checks are performed for positive and negative moment because top-flange and bottom-flange lateral bracing intervals are specified independently.

Shear capacity

Shear capacity follows AS/NZS 4600:2018 Clause 3.3.4:

utilization = |V| / phiV_v ≤ 1.0*

When circular or rectangular web holes are present, a reduced shear capacity at holes is also reported, with the reduction factor q_s depending on hole size and spacing relative to web height.

Bearing capacity

Bearing capacity at end supports and interior supports is checked for standard Cee and Zed sections. Bearing demand is the support reaction at the critical support location:

utilization = R / phiR_b ≤ 1.0*

Combined action checks

Two interaction checks guard against simultaneous high moment and high shear or moment and bearing:

MV interaction = MV_int / MV_int,max ≤ 1.0

MR interaction = MR_intsum / 1.0 ≤ 1.0

These are evaluated at the location where both moment and shear or moment and bearing reaction are highest, per AS/NZS 4600:2018.

Compression capacity

When the member carries axial compression, the compression section capacity (phiN_s) is checked first as an upper bound. The compression member capacity (phiN_c) accounts for local, distortional, and global buckling using the DSM strength curve.

For combined bending and compression:

MC interaction = MC_int ≤ 1.0