Web Bearing Capacity — BS 5950-1 Clause 4.5.2

The bearing capacity of an unstiffened web:

P_bw = (b_1 + n_k) × t × p_yw

Where:

• b_1 = stiff bearing length (length of applied load or support seating)
• n_k = dispersion length through flange and root radius: n_k = 2.5 × (T + r) for rolled I/H sections; n_k = 2 × (T + r) for welded sections
• t = web thickness
• p_yw = design strength of web

For a 406x178x54 UB (t = 7.7 mm, T = 10.9 mm, r = 10.2 mm) with 100 mm bearing: n_k = 2.5 × (10.9 + 10.2) = 52.75 mm. b_1 + n_k = 100 + 52.75 = 152.75 mm. P_bw = 152.75 × 7.7 × 275 / 1000 = 323 kN.

This is the bearing yield capacity — the load at which the web starts to yield in bearing.

Web Buckling Resistance — BS 5950-1 Clause 4.5.3

Web buckling checks the web as a compression strut over depth d. The buckling resistance:

P_x = (b_1 + n_k) × t × p_c where p_c is the compression strength from BS 5950-1 Table 24 evaluated at slenderness λ = 2.5 × d / t.

The factor 2.5 represents the effective length coefficient for a web acting as a strut between flanges. For a 406x178x54 UB:

d = D - 2T - 2r = 403 - 21.8 - 20.4 = 360.8 mm. d/t = 360.8 / 7.7 = 46.9. λ = 2.5 × 46.9 = 117.2.

From BS 5950-1 Table 24(a), S275, λ = 117.2: p_c ≈ 98 MPa.

P_x = 152.75 × 7.7 × 98 / 1000 = 115.3 kN.

The buckling resistance (115.3 kN) is often much lower than bearing yield (323 kN), especially for slender webs.

Stiff Bearing Length — BS 5950-1 Clause 4.5.1

The stiff bearing length b_1 is the length over which the applied load or reaction is effectively distributed to the web:

1. Support reaction on bearing plate: b_1 = actual bearing length (minimum 50 mm recommended for UK construction).

2. Concentrated load from beam framing in: b_1 = seated connection width, typically 100-150 mm for a UK fin plate or end plate connection.

3. Point load from above: b_1 = length of load spread through any packing, spreader plate, or beam flange. For a beam seated on top flange: b_1 = flange width of supported beam.

Worked Example — UK Beam Web at Support

Problem: A 457x191x67 UB (S275) is supported on a 150 mm bearing plate at each end. Factored support reaction V_Ed = 280 kN. Check web bearing and buckling. If the web is inadequate, design bearing stiffeners.

Section Properties:

Step 1 — Web Bearing Capacity:

n_k = 2.5 × (12.7 + 10.2) = 57.25 mm. b_1 + n_k = 150 + 57.25 = 207.25 mm. P_bw = 207.25 × 8.5 × 275 / 1000 = 484 kN.

V_Ed = 280 < 484 kN. OK.

Step 2 — Web Buckling Resistance:

d = 454 - 2 × 12.7 - 2 × 10.2 = 408.2 mm. d/t = 408.2 / 8.5 = 48.0. λ = 2.5 × 48.0 = 120.0.

From BS 5950-1 Table 24(a), p_c ≈ 95 MPa. P_x = 207.25 × 8.5 × 95 / 1000 = 167 kN.

V_Ed = 280 > 167 kN. FAIL — Web buckling governs. Provide bearing stiffeners.

Step 3 — Bearing Stiffener Design (BS 5950-1 Clause 4.5.4):

Provide 2 × 90 × 12 mm flat stiffeners either side of web, S275. Stiffener outstand b_s = 90 mm ≤ 13 × t_s × ε = 13 × 12 × 1.0 = 156 mm. OK.

Effective cruciform section for buckling: A_eff = (2 × 90 × 12) + (15 × 8.5 × 8.5) = 2160 + 1084 = 3244 mm² (web portion 15t_w each side per BS 5950-1 Clause 4.5.4.2).

I_eff ≈ 2 × [90 × 12³/12 + 90 × 12 × (6 + 4.25)²] = 2 × [12,960 + 90 × 12 × 105.1] = 2 × [12,960 + 113,508] = 252,936 mm⁴.

r_eff = sqrt(252,936 / 3244) = 8.83 mm. L_e = 0.7 × d = 0.7 × 408 = 286 mm. λ = 286 / 8.83 = 32.4.

p_c (λ = 32.4, S275) ≈ 268 MPa. P_stiff = 3244 × 268 / 1000 = 869 kN > 280 kN. OK.

Provide 2-90×12 mm bearing stiffeners at supports, both sides, fitted to underside of top flange and tight to bottom flange. Full penetration butt weld to bottom flange.

Concentrated Loads Away from Supports � BS 5950-1 Clause 4.5.2.1

When a concentrated load is applied away from a beam end, the web must resist both bearing and buckling without the stiffening benefit of the support. The bearing check uses the same formula as end bearing but with additional dispersion through the top flange and any spreader plate:

P_bw = (b_1 + n_k) � t � p_yw

However, for internal loads the dispersion length n_k is modified per SCI P362: n_k = 2 � (T + r) for loads applied to the top flange (reduced dispersion compared to support bearing where n_k = 2.5 � (T + r)).

For a 457x191x67 UB with a 75 kN point load from a secondary beam at mid-span (b_1 = 100 mm): n_k = 2 � (12.7 + 10.2) = 45.8 mm. b_1 + n_k = 145.8 mm. P_bw = 145.8 � 8.5 � 275 / 1000 = 341 kN. Load = 75 kN. OK.

Internal web buckling (BS 5950-1 Clause 4.5.3.1): For loads away from the end, the effective length of the web strut is: ? = 1.0 � d / t (rather than 2.5 � d / t for end supports).

For the 457x191x67 UB: ? = 1.0 � 48.0 = 48.0. p_c (? = 48, S275) � 233 MPa (from BS 5950-1 Table 24(a)). P_x = 145.8 � 8.5 � 233 / 1000 = 289 kN > 75 kN. OK.

The lower effective length factor (1.0 vs 2.5) means internal point loads have significantly higher buckling resistance than end reactions for the same bearing length. This is because the web is restrained against lateral movement by the flange continuity on both sides of the load.

Web Stiffener Proportioning Rules � BS 5950-1 Clause 4.5.4

When web bearing stiffeners are needed, the following proportioning rules apply per BS 5950-1:

Outstand limits:

• b_s / t_s <= 13 � epsilon for S275 (epsilon = 1.0), <= 11.8 � epsilon for S355 (epsilon = 0.87).
• Maximum b_s = 13 � t_s for S275, typically providing 90-120 mm leg for beams up to 610 mm deep.

Minimum stiffener width: b_s >= D/30 + 50 for full-depth stiffeners (load-bearing stiffeners at supports), and b_s >= b_f/2 for intermediate stiffeners at point loads.

Stiffener thickness: t_s >= max(b_s / 13, 8 mm) for S275. Minimum 10 mm for stiffeners bearing against the loaded flange.

Weld to web: Minimum 6 mm continuous fillet weld on both sides, sized to transfer the full stiffener force: Leg size >= F_stiff / (2 � 0.7 � L_weld � p_w) where p_w = 220 MPa for S275 electrode.

For a 280 kN stiffener force with 400 mm weld length each side: Leg = 280,000 / (2 � 0.7 � 400 � 220) = 2.3 mm. Use 6 mm FW minimum.

Fit requirement: Bearing stiffeners must be fitted to the underside of the loaded flange (top flange for support stiffeners) to ensure direct bearing. A 1-2 mm gap at the opposite flange is acceptable and prevents unintended load transfer. This "fit-to-bear" requirement means stiffeners must be accurately cut and ground � off-the-shelf flat bar may need machining for proper fit.

Related Pages

• UK Beam Design — BS 5950 Flexure & Shear
• UK Column Design — Compression Members
• UK Stub Column Capacity — Squash Load
• UK Connection Design — Bolted & Welded
• UK Compact Section Limits — Classification
• UK Weld Capacity — Fillet Weld Design
• UK Steel Properties — py & fu Values
• Beam Calculator — SFD, BMD & Reactions

Educational reference only. Verify web bearing and buckling checks against current BS 5950-1:2000 and SCI P362. Localised load arrangements may require additional checks per SCI P358. Results are PRELIMINARY — NOT FOR CONSTRUCTION without independent Chartered Engineer verification.