Steel Beam (old AS 4100:1998)

How it calculates

Structural model

The calculator uses a finite element method (FEM) engine to model the steel beam. Supports are defined at any position along the member as pins or rollers. The FEM solver produces moment, shear, and deflection diagrams for every AS/NZS 1170.0 load case combination, and the governing case for each check is reported.

Load combinations (AS/NZS 1170.0)

Loads are entered by type: permanent action (G), imposed action (Q), wind (W), and earthquake (E). The character of Q (floor, roof, storage) determines the combination factors psi_s and psi_l. The calculator generates all required strength and serviceability load case combinations and evaluates every check against each.

Section compactness (Clause 5.2)

For bending about the major and minor axes, flange and web element slenderness ratios (lambda_e) are compared to the yield limit (lambda_ey) and plastic limit (lambda_ep) from AS 4100:1998. The section is classified as compact, non-compact, or slender in each axis:

• Compact (lambda_s ≤ lambda_sp): effective section modulus Z_e = S (plastic)
• Non-compact: Z_e interpolated between S and Z_11
• Slender: Z_e = Z_11 reduced by the slenderness ratio

Design moment capacity (Clauses 5.1 to 5.3)

Section moment capacity (phiM_s): the product of phi, Z_e, and yield stress f_y.

Member moment capacity (phiM_bx): reduced from phiM_s to account for lateral torsional buckling. The calculation uses:

• Restraint classification for each segment (F, P, L, or U type at each end)
• Moment modification factor alpha_m, computed from the quarter-point moment values M2, M3, M4 across the segment
• Slenderness reduction factor alpha_s, derived from the reference buckling moment M_oa

Proportioning method: For sections with a slender compression flange (AS 4100:1998 Cl. 5.2.4), the calculator computes the effective compression flange area A_fc, effective tension flange area A_ft, the minimum flange effective area A_fm, and the moment capacity of the flanges alone M_f. This is an important check for asymmetric sections including PFC profiles.

Utilization: moment utilization = M*x / phiM_sx ≤ 1.0

Shear capacity (Clause 5.11)

Nominal shear yield capacity V_w is derived from the web area A_w and yield stress. The shear buckling capacity V_b accounts for panel aspect ratio via the buckling coefficient alpha_v. The governing shear capacity V_v is the lesser of V_w and V_b.

A combined shear-moment interaction check is performed where both M* and V* are significant.

Bearing capacity (Clause 5.13)

Bearing capacity at each support is checked in two modes: end bearing and interior bearing. The calculation includes the web buckling contribution via the member section constant alpha_b, the form factor k_f, and the member slenderness reduction factor alpha_c. The bending-bearing interaction is then checked at each support.

Deflection checks (Clause 2.3)

Three deflection limits are checked independently per span:

1. Short-term deflection (delta_s) - under short-term service load case
2. Long-term deflection (delta_l) - accounting for creep and shrinkage
3. Imposed load deflection (delta_Q) - Q component only

Each is compared to the user-defined L/n criterion or optional absolute limit.

Section properties

Section properties (I_11, I_22, Z_11, S_11, J, I_w, A_g, A_w, f_y, f_u) are drawn from the AU/NZ hot-rolled steel section database. The principal axis angle alpha and centroid offsets x_L, y_L handle asymmetric sections such as angles and PFCs.

Load linking

Support reactions are made available for linking to downstream column and footing calculations. Changing any load input propagates updated reactions automatically through the project.