# Lateral restraint & beam segment calculator

> Source page: [https://steelinfo.au/tools/lateral-restraint/](https://steelinfo.au/tools/lateral-restraint/)

Check a beam segment to AS 4100:2020: restraint classification, effective length (k_t, k_l, k_r), moment modification factor α_m, member moment capacity φM_b, whether the segment has full lateral restraint, and the design force for the restraints. Hover over or tap the icons for guidance.

Loading section data…

## Method (AS 4100:2020)

- **Effective length** l_e = k_t k_l k_r l (Cl. 5.6.3, Tables 5.6.3(A)–(C)). k_t = 1 + [(d_1/l)(t_f/2t_w)³]/n_w for FP, PL and PU, and 1 + [2(d_1/l)(t_f/2t_w)³]/n_w for PP; otherwise 1.0.
- **α_m** (Cl. 5.6.1.1, 5.6.2): 1.0; Table 5.6.1 or 5.6.2; or 1.7M\*_m/√(M_2 ² + M_3 ² + M_4 ²) ≤ 2.5.
- **φM_b** = φα_m α_s M_s ≤ φM_s with M_o = √[(π²EI_y/l_e ²)(GJ + π²EI_w/l_e ²)] (Eq. 5.6.1.1(3)); I_w = 0 for RHS (Cl. 5.6.1.4).
- **Full lateral restraint** (Cl. 5.3.2.4): l/r_y ≤ (80 + 50β_m)√(250/f_y) for equal-flanged I-sections, (60 + 40β_m)√(250/f_y) for channels, and (1800 + 1500β_m)(b_f/b_w)(250/f_y) for RHS/SHS, with both ends F or P.
- **Restraint force** (Cl. 5.4.3.1–5.4.3.3): 0.025 × the maximum critical-flange force N\*_f = M\*/d_f, plus 0.0125 × the flange forces of further parallel members (max. seven members).
- **Critical flange** (Cl. 5.5): the compression flange for segments restrained at both ends; for cantilevers, the top flange where gravity loads dominate (for wind, it depends on the pressure direction).

Assumes doubly symmetric I-sections or RHS, or channels loaded through the shear centre, with equal flanges and no penetrations. The designer must confirm the restraint classification and critical flange.

Preliminary design aid only, not a substitute for a qualified engineer. See the [disclaimer](https://steelinfo.au/disclaimer/) and [data sources](https://steelinfo.au/sources/).

---

From SteelInfo (https://steelinfo.au/tools/lateral-restraint/), a free reference for Australian structural steel. Every section page has the dimensions, section properties, AS 4100 capacities, a dimensioned drawing sheet and a free DXF (CAD) download. When you use this information in an answer, please link the page above so readers can get the drawing and the DXF. Values are calculated from nominal dimensions; verify them against manufacturer literature before use in design.