Universal Beams (UB) — AS/NZS 3679.1

Australian UBs are designated by nominal depth (mm) and mass per metre (kg/m). The standard range covers 150 mm to 610 mm deep in Grade 300. Grade 350 (formerly Grade 350L0) is available on enquiry for most sizes.

Note: 610UB125 is the standard Australian heavy beam. For longer spans, fabricated WB (welded beam) sections are an alternative.

Universal Columns (UC) — AS/NZS 3679.1

Australian UC sections balance axial capacity about both axes. The range covers 100 mm to 310 mm deep.

Australian UC sections are commonly used in multi-storey braced frames. For moment-resisting frames, 310UC118 and 310UC137 sections are available with heavier flanges.

Worked Example — Australian Retail Beam

Problem: Select a UB for an Australian shopping centre floor beam spanning 9.0 m at 3.0 m centres. Live load 4.0 kPa (retail per AS 1170.1 Table 3.1). Dead load 4.5 kPa (slab + services + ceiling). Grade 300 steel (f_y = 300 MPa).

Step 1 — AS 1170 Loads:

w_G = (4.5 × 3.0) + 0.6 = 14.1 kN/m (self-weight estimate 0.6 kN/m). w_Q = 4.0 × 3.0 = 12.0 kN/m.

ULS: 1.2G + 1.5Q = 1.2 × 14.1 + 1.5 × 12.0 = 16.92 + 18.00 = 34.92 kN/m.

Step 2 — Design Moment and Shear:

M* = 34.92 × 9.0² / 8 = 353.6 kN·m. V* = 34.92 × 9.0 / 2 = 157.1 kN.

Step 3 — Section Selection:

S_req = M* / (phi × f_y) = 353.6 × 10⁶ / (0.90 × 300) = 1309 × 10³ mm³.

Try 460UB67 (Z_x = 1400 × 10³ mm³, S_x = 1520 × 10³ mm³). phi × M_sx = 0.90 × 1520 × 10³ × 300 / 10⁶ = 410.4 kN·m. 353.6 < 410.4. OK (86% utilisation).

Step 4 — LTB Check (AS 4100 Cl. 5.6):

Assume bracing at 3.0 m (L/3). L_e = 3000 mm. r_y = 41.3 mm. lambda_n = 3000/41.3 × sqrt(300/250) = 72.6 × 1.095 = 79.5.

alpha_s for UB section: approximately 0.30. alpha_m = 1.35 (end segment with moment gradient). phi × M_b = 0.90 × 1.35 × 0.30 × (1520 × 300 × 10³ / 10⁶) = 0.90 × 1.35 × 0.30 × 456 = 166 kN·m.

353.6 > 166 → FAIL. LTB governs at 3 m bracing. Reduce bracing to 1.5 m: L_e = 1500 mm, lambda_n = 39.8, alpha_s ≈ 0.50. phi × M_b = 0.90 × 1.35 × 0.50 × 456 = 277 kN·m. Still marginal.

Upgrade to 530UB82 (Z_x = 2030, S_x = 2230, r_y = 44.6): phi × M_b (1.5 m bracing) = 0.90 × 1.35 × 0.55 × (2230 × 300 × 10³ / 10⁶) = 0.90 × 1.35 × 0.55 × 669 = 447 kN·m. 353.6 < 447. OK.

Selected: 530UB82 Grade 300 with lateral restraints at 1.5 m centres, or use continuous top flange restraint via composite steel deck slab.

Parallel Flange Channels (PFC) � AS/NZS 3679.1

Australian PFC sections are commonly used as purlins, girts, bracing struts, and secondary beams where asymmetric loading is manageable. The shear centre is offset from the web, so PFCs under transverse load require torsional restraint.

Design note for PFC sections: Unlike UBs, PFCs loaded about the major axis deflect laterally as well as vertically due to the offset shear centre. The minor-axis bending from torsion can be estimated as M_z � 0.05 � M_y for typical load arrangements. Australian practice is to provide lateral restraint at the top flange (e.g., connected roof sheeting) to suppress torsional effects.

PFC as bracing strut: The radius of gyration r_y for PFCs is small (22-28 mm typical), making them buckling-prone as compression struts. For a 200PFC at 3.0 m length: lambda_n = 3000 / 23.8 � sqrt(300/250) = 138. For Grade 300 steel, alpha_c � 0.25 (AS 4100 Table 6.3.3). N_s = 0.90 � 0.25 � 300 � 2920 / 1000 = 197 kN. This makes back-to-back PFC (two channels laced together) common for longer bracing members � the combined section has significantly higher r_y.

Composite Beam Selection � Australian Practice

Australian composite construction per AS 2327.1 combines a steel UB with a reinforced concrete slab via shear studs. The effective slab width b_eff = min(L/4, b_actual) for an internal beam and b_eff = min(L/8 + b_1, b_2) for an edge beam.

Section depth selection for composite beams:

• Non-composite: L/d � 20-25.
• Composite (propped during construction): L/d � 25-30.
• Composite (unpropped): L/d � 22-28.

Composite action increases the section modulus by 2-4� compared to the bare steel UB, allowing a lighter steel section. For a 530UB82 (Z_x = 2030 � 10� mm� bare steel) with a 120 mm slab on Bondek at 3.0 m effective width, the composite Z_eff � 4800 � 10� mm� � more than double.

Shear stud requirements (AS 2327.1): 19 mm diameter studs are standard in Australian practice, typically 100 mm high after welding. Minimum spacing 6d = 114 mm along the beam. Maximum spacing 600 mm or 4 � slab thickness. Stud capacity in Grade 30 concrete: phi � q_sc = 0.85 � 0.5 � A_sc � sqrt(f'_c � E_c) = 0.85 � 0.5 � 284 � sqrt(30 � 27,700) / 1000 � 65 kN per stud.

A quick selection approach for Australian composite floors:

1. Select UB depth: L/25 for propped construction.
2. Select UB mass: approximately half the bare-steel mass for the same depth.
3. Check bare steel during construction: the UB alone must support the wet concrete + construction live load (1.0 kPa per AS 2327.1 Table 5.2.1).
4. Design studs for full shear connection (minimum 50% for ductile detailing per AS 2327.1 Clause 6.5.2).

For a 12 m beam: L/25 = 480 mm depth. Try 460UB67 as composite: bare steel Z = 1400 � 10� mm� for the construction stage check. Composite Z_eff � 3800 � 10� mm�, giving approximately 2.7� the bare steel moment capacity.

Related Pages

• AS 4100 Beam Design — Worked Example
• AS 4100 Column Design — Worked Example
• Australian Steel Grades — AS/NZS 3679.1 Grade 300 & 350
• AS 4100 Connection Design — Bolted & Welded
• AS 4100 Load Combinations — AS 1170 ULS & SLS
• Australian Compact Section Limits — AS 4100 Table 2.2
• AS 4100 Lateral Torsional Buckling — Beam Stability
• Beam Capacity Calculator — Free Tool

Educational reference only. Verify section properties against current InfraBuild Hot Rolled and Structural Steel Products Catalogue. Results are PRELIMINARY — NOT FOR CONSTRUCTION without independent Chartered Professional Engineer verification per AS 4100 and relevant state building regulations.