Steel Beam (AS 4100:2020)

How it calculates

The Steel Beam (AS 4100:2020) calculator designs steel beams using limit state design (LRFD) per AS 4100:2020. Factored load demands are checked against design capacities for bending, shear, bearing, and deflection. A live FEA engine solves for moment, shear, and deflection under all AS/NZS 1170.0 load combinations simultaneously.

Structural analysis

The FEA solver computes moment, shear, reaction, and deflection envelopes for every load combination. Distributed loads, line loads, point loads, and moment loads are all supported. Wind loads are entered directly, using net pressure coefficients and a wind tributary width to convert pressure to a line load.

Moment section capacity

Section moment capacity phi × M_s is calculated for bending about both principal axes ('11' and '22'). For standard I-sections and channels, the section is classified as compact, non-compact, or slender based on flange and web slenderness ratios per Table 5.2. Effective section modulus Z_e is used for non-compact and slender sections.

utilization = M / (phi × M_s) ≤ 1.0*

Member moment capacity - LTB (AS 4100:2020, Cl. 5.6)

For positive and negative bending, lateral-torsional buckling (LTB) capacity is computed separately. The reference buckling moment M_oa and slenderness reduction factor alpha_s are calculated from the segment length between restraints, the restraint types, section properties, and moment modification factor alpha_m. A cantilever check per Table 5.6.2 is also available:

phi × M_b = phi × alpha_m × alpha_s × M_sx ≤ phi × M_s

Where moments act about both principal axes, each segment gets a section biaxial interaction and a member biaxial interaction (AS 4100:2020, Cl. 8.3.4 and 8.4.5). The governing interaction is reported as an effective moment capacity phi × M_gov.

utilization = M / (phi × M_gov) ≤ 1.0*

Shear capacity (AS 4100:2020, Cl. 5.11) and shear-moment interaction (Cl. 5.12)

Shear capacity phi × V_v accounts for web yield and web shear buckling. Where shear and moment are both significant at the same section, the interaction check per Cl. 5.12 is applied.

Bearing capacity and bearing-bending interaction

Web bearing capacity phi × R_gov is checked for web bearing yield and buckling at each support with a bearing length entered. For RHS and SHS sections, a combined bending and bearing interaction check is also performed. It pairs the worst-case bearing demand with the worst-case negative moment. Bearing is only checked for bending about the X-axis, and not for angle sections. Other cases must be checked separately by the engineer.

Deflection analysis

Short-term (delta_s), long-term (delta_l), and imposed-load (delta_Q) deflections are each evaluated and checked against the span/limit criteria. Precamber can be entered for single-span beams and is included in the deflection checks. Precamber above a set percentage of the dead load deflection is flagged (80% by default, editable).

Assumptions

Beam is uniform cross-section. Net areas equal gross area with maximum allowable holes. Member moment capacity about the minor principal axis equals section moment capacity (M_b22 = M_s22). Angle sections are assumed restrained from lateral deflection and rotation. Moments are calculated about principal axes per Cl. 5.7.