Choosing a Steel Section

Quick pick (TL;DR)

Beam → wide-flange (W/UB) sized so φMn ≥ Mu and deflection ≤ L/360.
Column → W/UC or HSS (good in both axes).
Bracing → angle or HSS.
Purlin → channel or cold-formed C/Z.
Then check compactness and lateral-torsional bracing.
A992 (Fy 345) is the default grade.

Step 1 — Shape by action

Member	Section	Why
Beam	Wide-flange W / UB	High Zx per weight.
Column	W / UC or HSS	Balanced both axes.
Bracing	Angle / HSS	Light, easy connections.
Purlin	Channel / cold-formed C-Z	Economical light flexure.

Step 2 — Size for bending strength

Required section modulus

Given Beam M_u = 120 kN·m , span 6 m A992 steel: Fy = 345 MPa , φ_b = 0.90 Step 1 — Required Zx Zx ≥ M_u / (φ_b · Fy) Zx ≥ 120×10⁶ / (0.90 × 345) Zx ≥ 386,000 mm³ = 386 cm³ Step 2 — Pick a section W360×57 has Zx ≈ 897 cm³ ≥ 386 → strength OK (also covers self-weight margin) Step 3 — Deflection δ_live ≤ L/360 = 6000/360 = 16.7 mm compute δ from Ix; if > limit → deeper section

Deflection / lateral-torsional buckling often govern — check both.

Step 3 — Stability checks

Compactness — compact reaches full Mp.
Lateral-torsional buckling — brace the compression flange.
Columns: KL/r and P-M interaction.
Web shear, bearing stiffeners.

Beam section calculator (required Zx)

Required Zx & deflection limit

Zx ≥ Mu/(φb·Fy); plus the L/360 deflection limit.

Mu (kN·m)

Fy (MPa)

Span L (m)

Zx provided (cm³)

Assumes compact, fully braced. Check LTB, shear & deflection separately.

Common mistakes

Strength-only sizing.
No lateral bracing.
Slender elements without reduction.

Requisition & datasheet

Member, loads, span; section & designation.
Grade, Zx, utilization; connection & protection.

Frequently asked questions

What should I know about Choosing a Steel Section?

Beam → wide-flange (W/UB) sized so φMn ≥ Mu and deflection ≤ L/360. Column → W/UC or HSS (good in both axes). Bracing → angle or HSS. Purlin → channel or cold-formed C/Z. Then check compactness and lateral-torsional bracing. A992 (Fy 345) is the default grade.

Which standards or codes apply?

AISC 360 (LRFD/ASD), ASCE 7 (loads); ASTM A992/A36/A572 (grades); Eurocode 3; Thai DPT/EIT. Section properties from the AISC Steel Construction Manual.

Standards — AISC 360 / ASCE 7; ASTM A992/A36; Eurocode 3.