How to design a steel beam

A steel beam carries load across a gap by bending. Designing one means choosing a section that is strong enough (it doesn't yield, buckle or shear) and stiff enough (it doesn't sag so much that floors bounce or finishes crack). This tutorial walks through each check with a worked example, then shows how Struxt3D does the same thing for every beam in a model.

#What a beam has to resist

Under a uniform load w on a simply supported span L, the beam develops a bending moment that peaks at midspan and a shear force that peaks at the supports:

Struxt3D doesn't need these hand formulas — the 3D analysis gives the real moment and shear at every station along each member, for every load combination. But knowing them lets you sanity-check the results (see Reading results).

#The four checks

#1. Bending resistance

A compact (Class 1 or 2) steel section can develop its full plastic moment — the whole section yields, top in compression, bottom in tension:

Slender plates (Class 3 and 4, or "noncompact/slender" in AISC) buckle locally before reaching the plastic moment, so the elastic modulus or an effective section is used instead. Hot-rolled IPE and W shapes in bending are almost always compact.

#2. Shear resistance

Shear is carried mostly by the web. The plastic shear resistance is the web area times the shear yield stress, f_y/√3:

#3. Lateral-torsional buckling (LTB)

The compression flange of a beam behaves like a column: if nothing holds it sideways, it can buckle out of plane and twist the whole section. The resistance drops with the unbraced length L_b between lateral restraints.

The elastic critical moment M_cr rises sharply as L_b falls, so the cheapest fix for an LTB problem is usually a restraint, not a heavier beam. A concrete slab or metal deck fixed to the top flange restrains it along its whole length, which is why most floor beams don't suffer from LTB in their sagging zones.

#4. Deflection

Serviceability is checked under unfactored loads. A common limit is span/250 for the total deflection and span/360 for the live load alone on floors with brittle finishes. Deflection depends on I, not on strength, so a higher steel grade doesn't help at all — and on long spans deflection, not strength, often decides the section.

#Worked example: a 6 m floor beam (Eurocode)

An office floor beam spans 6.0 m between pinned connections, at 3.0 m centres, carrying a 150 mm concrete slab. Steel S355 (f_y = 355 MPa), γ_M0 = γ_M1 = 1.0.

#Loads

Ultimate limit state, EN 1990 eq. 6.10: w_Ed = 1.35 × 16.15 + 1.5 × 9.00 = 21.80 + 13.50 = 35.30 kN/m.

#First try: IPE 300

IPE 300: W_pl,y = 628 cm³, I_y = 8356 cm⁴, A_v,z = 25.7 cm², Class 1 in bending.

• Bending: M_c,Rd = 628 × 10³ mm³ × 355 MPa = 222.9 kN·m → 158.9 / 222.9 = 0.71 ✓
• Shear: V_pl,Rd = 2570 × 355 / √3 = 526.8 kN → 105.9 / 526.8 = 0.20 ✓ (below 0.5, so no bending–shear interaction)
• LTB: the slab holds the top flange, which is in compression everywhere on a simply supported span → χ_LT = 1.0 ✓
• Deflection under g_k + q_k = 25.15 kN/m:

The IPE 300 is strong enough but slightly too flexible: utilization 24.2 / 24.0 = 1.01.

#Second try: IPE 330

IPE 330: W_pl,y = 804 cm³, I_y = 11 770 cm⁴.

• Bending: 804 × 10³ × 355 = 285.4 kN·m → 0.56 ✓
• Deflection: 24.2 × 8356 / 11 770 = 17.2 mm → 17.2 / 24.0 = 0.72 ✓

#The same beam in AISC 360

The US route differs in factors, not in physics. LRFD combination 1.2D + 1.6L, then φ_bM_n = 0.9 F_y Z_x for a compact, braced beam, and φ_vV_n = 0.6 F_y A_w for the web. For an unbraced beam, AISC compares L_b with L_p (full plastic moment) and L_r (elastic LTB), interpolating between them and scaling by C_b for the moment shape. See Designing to AISC / ACI.

#Design it in Struxt3D

Struxt3D runs these checks on every steel member, for every strength combination, at stations along the member — with the full code check from its steel engine (classification, flexural and torsional buckling, LTB, shear and interaction) to EN 1993-1-1, AISC 360, KDS, CSA S16 or AS 4100.

1. Start a model. Open the Continuous beam template, or any frame. Set spans in the Size step.
2. Pick the code and sections. In Materials, choose the Design code, the steel grade, and a starting section for each group (beams, columns…). See Materials and sections.
3. Set the loads. In Supports & loads, choose the floor use (it sets the live load from the code) and the finishes. Self-weight and combinations are added for you.
4. Run the design. Press F6, or Design › Run design in the Full interface. F5 only analyses; the results panel then offers Run design.
5. Read the utilization. In the Show panel pick Utilization: every member is coloured by its ratio, segment by segment along its length. The summary names the Highest utilization — click it to select that member.
6. Inspect one beam. Click a beam. The inspector shows the governing check, its clause and combination, and the deflection ratio. Calculation report opens its step-by-step calculation.
7. Let it size the beams. With Size members automatically on (Design settings), Struxt3D picks the lightest section in each group that passes strength and deflection. The summary lists what changed under Sized for you.

#How Struxt3D handles restraint and deflection

• Unbraced length: L_b is the largest gap between points where other members or supports frame into the beam. A beam carrying a slab or deck is taken as restrained at close spacing (L/10, at most 1 m).
• Deflection: checked under D + L, relative to the nearest held points, against span/250 by default. Change the limit in Design settings › Deflection span / (100 to 1000).
• Steel beams under a concrete slab can also be designed as composite — see How to design a composite beam.

#Common mistakes

• Forgetting deflection — checking strength only and ending up with a bouncy floor.
• Assuming the slab braces everything — it doesn't brace the bottom flange in hogging zones.
• Mixing factored and unfactored loads — deflection uses service loads; strength uses factored ones. See Loads and load combinations.
• Ignoring the connections — a beam designed as pinned needs end connections that actually behave as pins. See How to design steel connections.