Designing a bearing stiffener to Eurocode 3
A concentrated load or support reaction on an I-beam has to pass through the thin web. If the web cannot carry it, it crushes locally or buckles sideways, and a pair of bearing stiffeners is welded in so the load is carried as a short strut. This calculator checks the web on its own first, then designs the stiffeners to EN 1993-1-5 and EN 1993-1-1.
When do you need a bearing stiffener?
EN 1993-1-5 §6 gives the resistance of an unstiffened web as F_Rd = f_yw·L_eff·t_w/γ_M1. The effective loaded length L_eff = χ_F·ℓ_y, where ℓ_y is the stiff bearing length spread through the flange — s_s + 2t_f(1 + √(m1 + m2)) for a load away from the ends of the beam — and χ_F allows for buckling of the web.
If F_Ed exceeds F_Rd, or the interaction with bending η2 + 0.8η1 ≤ 1.4 (§7.2) is not met, stiffeners are needed. Loads at or close to the end of a beam (load type c) have a much lower resistance, so end reactions on deep, thin webs are the usual case.
What the stiffener checks are
- Outstand: b_s/t_s ≤ 14ε keeps a flat at least class 3. The torsional buckling rule (t_s/b_s)² ≥ 5.3f_y/E of §9.2.1(8) is stricter above about S275 — it limits b_s/t_s to about 12.0 in S275 and 10.6 in S355.
- Strut buckling: the stiffeners plus 15εt_w of web each side act as a column buckling out of the plane of the web, with L_cr = 0.75h_w and buckling curve c (§9.4).
- End bearing: the stiffener ends are fitted to the loaded flange, and only the area beyond the corner snipes bears: F_Ed ≤ A_s,net·f_y/γ_M0.
- Welds: the fillet welds pass the whole load into the web over the stiffener length (EN 1993-1-8 §4.5.3.3).
- Bending: the beam must still resist the moment at the load point.
Worked example: 356 × 171 × 51 UB with a 300 kN point load
This is the "basic web stiffener design example" from a BS 5975 temporary works guide, which uses BS 449 permissible stresses, recast to the Eurocodes. A crossing beam applies the load at mid-span of a 2 m span (stiff bearing 30.5 mm), in S275. Taking the 300 kN as permanent, F_Ed = 1.35 × 300 = 405 kN.
| Check | Result | Outcome |
|---|---|---|
| Unstiffened web (type a) | F_Rd = 374 kN < 405 kN | Stiffeners needed |
| Pair of 65 × 6 mm flats: outstand | b_s/t_s = 10.8 ≤ 12.9 (class 3) and ≤ 12.0 (torsion) | OK |
| Strut: A = 2343 mm², λ̄ < 0.2 | N_b,Rd = 644 kN | 63% |
| End bearing: 2 × (65 − 15) × 6 mm | F_Rd = 165 kN | 245% — fails |
| Revised to 80 × 15 mm flats | Bearing 536 kN; outstand 80 ≤ 82 mm available | All checks pass (98%) |
The BS 449 version accepts the 6 mm stiffeners (481 kN as a strut) because it does not check bearing at the ends. Open the "BS 5975 example" and "revised to 80 × 15 mm" presets above to step through both.
Common mistakes
- Ignoring the corner snipe in the bearing area — on a 65 mm stiffener a 15 mm snipe removes almost a quarter of the contact.
- Using L_cr = 0.75h_w where the loaded flange is not held laterally; use h_w or more.
- Checking a single-sided stiffener as if it were a pair: the load is eccentric to the strut and its capacity drops.
- Stiffeners wider than (b − t_w)/2, which would project beyond the flange.
- Thin, wide flats in S355 that pass the class 3 limit but fail the torsional buckling rule.