Lateral restraint & beam segment calculator

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 i icons for guidance.

Section i Segment length l (m) i Design moment M* (kNm) i End A restraint i End B restraint i Lateral rotation restraint i Load height i Load position i α_m method i

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β_m for full-lateral-restraint check i Parallel members on one bracing line, n i

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Method (AS 4100:2020)

Effective length l_e = k_tk_lk_rl (Cl. 5.6.3, Tables 5.6.3(A)–(C)). k_t = 1 + [(d₁/l)(t_f/2t_w)³]/n_w for FP, PL and PU, and 1 + [2(d₁/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/√(M₂² + M₃² + M₄²) ≤ 2.5.
φM_b = φα_mα_sM_s ≤ φM_s with M_o = √[(π²EI_y/l_e²)(GJ + π²EI_w/l_e²)] (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 and data sources.