Cold-Formed Steel Beam (AS/NZS 4600:2018)

How it calculates

The Cold-Formed Steel Beam calculator designs CFS beams using the Direct Strength Method (DSM) per AS/NZS 4600:2018 Cl. 7. Rather than the traditional effective width method, DSM uses Finite Strip Method (FSM) analysis to calculate elastic buckling moments for all relevant modes, then applies DSM strength curves to determine design capacities.

Section moment capacity (AS/NZS 4600:2018, Cl. 3.3.2)

The section moment capacity phi_b × M_s is computed from the FSM-determined local and distortional buckling loads and the full cross-section yield moment M_y. For back-to-back or boxed beam configurations, section capacity is doubled.

utilization = M / (phi × M_s) ≤ 1.0*

Member buckling capacity (AS/NZS 4600:2018, Cl. 7.2.2)

Three member buckling checks are performed separately using the DSM interaction curves:

• Global (lateral-torsional) buckling (Cl. 7.2.2.1): capacity phi × M_b,glob from FSM global buckling load, accounting for unbraced length and moment gradient
• Local buckling (Cl. 7.2.2.2): capacity phi × M_b,local based on interaction between local and global modes
• Distortional buckling (Cl. 7.2.2.3): capacity phi × M_b,dist based on the distortional buckling load from the FSM signature curve

Positive and negative moment member capacities are checked separately. The governing check uses the lowest capacity.

utilization = M+gov / (phi × M_b,gov+) ≤ 1.0* utilization = M-gov / (phi × M_b,gov-) ≤ 1.0*

Shear capacity and web holes (AS/NZS 4600:2018, Cl. 7.2.3)

Shear capacity phi × V_v is checked at the critical section. If web holes are present (depth d_wh, spacing s_wh), a reduced shear capacity phi × q_s × V_v is also checked at the hole locations. No web holes are permitted within bearing lengths.

utilization = V / (phi × V_v) ≤ 1.0*

Bearing capacity (AS/NZS 4600:2018, Cl. 3.3.6)

Bearing (web crippling) capacity phi × R_b is checked for reactions at supports and concentrated loads, based on section type and bearing length. Bearing is only checked for standard Cee or Zed sections bent about the X-X axis.

Combined bending and shear (Cl. 7.2.3.5) and bending and bearing (Cl. 3.3.7)

Interaction checks are performed where both effects are significant:

utilization (MV) = combined interaction ratio ≤ 1.0 utilization (MR) = combined interaction ratio ≤ 1.0

Deflection analysis (AS/NZS 4600:2018, Cl. 7.1.4)

Deflections use an effective second moment of area I_eff that accounts for local and distortional buckling under service loads. Short-term, long-term, and imposed-load deflections are each checked against span/limit criteria.