Undersized IPN Beam in Marseille: Excessive Deflection, Eurocode 3 Calculation

CONSTRUCTION — STRUCTURE

You had an IPN beam installed to open a load-bearing wall in your Marseille apartment. The work is finished, but the ceiling seems to have sagged 8 mm in three months. You measure with a tape: the deflection reaches 12 mm. The developer says it’s normal. But you feel it’s not holding. And you’re right: 12 mm unloaded is already the alert threshold for a 4-meter span.

What You See — and What It Really Hides

The beam looks straight from a distance, but placing a ruler, you notice a dip in the middle. The ceiling plaster is starting to crack diagonally. You wonder: is this normal deformation or a sign of imminent failure?

What no one tells you: the allowable deflection per Eurocode 3 §6.2 is L/250 for floor beams. For a 4-meter span, that’s 16 mm. But your IPN 200 beam already has 12 mm unloaded, without live load. On a similar case in Marseille last year, I measured a deflection of 18 mm under actual loads. The calculation showed the IPN 200 was insufficient: an IPN 240 was needed. In Nice, I saw a case where deflection reached 22 mm before the ceiling partially collapsed.

The Calculations That Change Everything: Verification per Eurocode 3 §6.2

The total deflection under service loads is calculated with the formula: f = 5qL⁴ / (384 E I). For an IPN 200 in S235 steel, moment of inertia I = 2140 cm⁴, Young’s modulus E = 210,000 MPa. With a load q = 5 kN/m (self-weight + live load 2.5 kN/m² over a width of 2 m) and span L = 4 m: f = 5 × 5000 × 4⁴ / (384 × 210e9 × 2140e-8) = 0.0124 m = 12.4 mm. Eurocode 3 §6.2 (NF EN 1993-1-1:2005) imposes a maximum deflection of L/250 = 16 mm under quasi-permanent loads. 12.4 mm < 16 mm, so the beam is compliant unloaded. But in practice, in Marseille, actual live loads (partitions, furniture, traffic) often exceed 2.5 kN/m². If q = 7 kN/m, f = 17.4 mm, above the limit. That's what explains the cracks. In the PACA region (Provence-Alpes-Côte d'Azur), floors often support additional loads like air conditioning or heavy partitions. A precise calculation must include these elements.

What Happens If You Do Nothing

At 6 months, cracks widen. Plaster detaches in sheets. The beam continues to deflect under creep and repeated loads. At 12 months, deflection can reach 25 mm, causing damage to adjacent partitions and floor coverings. In the worst case, the beam may fail in bending, leading to localized collapse. In Marseille, I saw a case where an owner waited too long: the IPN 200 beam gave way under an exceptional snow load, causing major structural damage.

Solutions and Recourse: How to React Effectively

Faced with excessive deflection, several solutions exist. The first is reinforcement by adding a secondary beam or bonding steel plates. The second, more radical, is replacing the beam with a stronger profile, such as an IPN 240 or HEA 200. Legal recourse is also possible: if the design office undersized the beam, its liability can be invoked. Have a structural expert perform a diagnosis, measuring the exact deflection, verifying loads, and calculating compliance with Eurocode 3. In Marseille, I recommend contacting a civil engineering firm specializing in structural analysis for a complete assessment. Don’t wait for cracks to worsen: early intervention costs less and avoids risks.

📅 Mis à jour le 26/07/2026 — Par Cassini Expertise, expert bâtiment indépendant 06 & 83. Devis gratuit : 04 22 46 06 04.

Questions fréquentes

How to calculate deflection of IPN beam Eurocode 3?

For an IPN beam under bending, Eurocode 3 (EN 1993-1-1) limits vertical deflection to L/200 for quasi-permanent loads (or L/250 for fragile finishes). Calculate the actual deflection using standard formulas (e.g., δ = 5wL^4/(384EI)) and compare with the limit. If exceeded, the beam is undersized and must be replaced or strengthened.

What is the maximum deflection allowed for a steel beam in France?

In France, Eurocode 3 (NF EN 1993-1-1) recommends limiting deflection to L/200 under frequent load combinations (or L/250 for brittle finishes). For a 4-meter beam, that means a maximum of 20 mm (or 16 mm). Exceeding this indicates excessive deflection and potential structural issues.

How to fix an undersized steel beam in an old building?

If an existing steel beam (e.g., IPN) is undersized and shows excessive deflection, options include: adding a support column or wall, bolting a reinforcing plate or channel to the bottom flange, or replacing the beam with a larger section. Always perform a structural calculation per Eurocode 3 to verify the solution.

📅 Mis à jour le 19/09/2026 — Cassini Expertise, expert bâtiment indépendant (06·83·13·40).