Beam Deflection Limits — Eurocode Reference

L/250 means the maximum allowable deflection is the span length divided by 250. For a 4m beam, L/250 = 4000mm ÷ 250 = 16mm maximum deflection under the full design load. This is a serviceability limit — the beam is not at risk of failure, but greater deflection could cause cracking of finishes, visible sagging, or user discomfort. Different limit values apply depending on the element type and finish materials.

For a domestic floor beam supporting plasterboard ceiling below (a brittle finish), use L/360 to prevent cracking of the ceiling finish. If the ceiling is flexible or there is no ceiling below, L/250 is generally acceptable. For timber floor joists, the TRADA/Eurocode 5 approach limits total deflection to L/250 and also checks that the instantaneous deflection under variable loads does not exceed L/300.

For a simply supported beam with a uniformly distributed load (UDL), deflection = (5 × w × L⁴) ÷ (384 × E × I), where w = load per metre (N/mm), L = span (mm), E = elastic modulus (N/mm²), and I = second moment of area (cm⁴ or mm⁴). For steel, E = 210,000 N/mm². For a point load at mid-span, deflection = (P × L³) ÷ (48 × E × I). These formulae assume ideal simply supported conditions.

Instantaneous deflection occurs immediately when load is applied. Final deflection includes creep — the long-term increase in deflection under sustained load. Creep is significant for timber (kdef factor of 0.6 for service class 1, 0.8 for class 2 per EC5) and concrete but negligible for steel. For timber design, final deflection = instantaneous deflection × (1 + kdef) for permanent loads, plus instantaneous deflection for variable loads.