Question 1

What does L/250 mean for deflection?

Accepted Answer

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.

Question 2

What deflection limit should I use for a domestic floor beam?

Accepted Answer

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.

Question 3

How do I calculate beam deflection?

Accepted Answer

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.

Question 4

What is the difference between instantaneous and final deflection?

Accepted Answer

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.

Element	Limit	At 4m Span (mm)	Notes
Floor beams (general)	L/250	16	Total deflection under all loads
Floor beams (brittle finishes below)	L/360	11.1	Plasterboard ceiling, tiles, render
Roof beams (no brittle finish)	L/200	20	Accessible only for maintenance
Roof beams (brittle finish below)	L/250	16	Plastered ceiling below roof
Purlins and cladding rails	L/200	20	Metal cladding systems
Cantilever beams	L/180	22.2	Cantilever length, not back span
Lintels (masonry above)	L/360	11.1	Prevent cracking of masonry and render

Section	Depth (mm)	Weight (kg/m)	Ixx (cm⁴)	Typical Domestic Use
152 x 89 x 16 UB	152.4	16.0	834	Short lintels, light loads
178 x 102 x 19 UB	177.8	19.0	1356	Lintels, small openings
203 x 133 x 25 UB	203.2	25.1	2340	Floor beams (short spans)
203 x 133 x 30 UB	206.8	30.0	2896	Floor beams, load-bearing lintels
254 x 146 x 31 UB	251.4	31.1	4413	Floor beams, medium spans
254 x 146 x 37 UB	256.0	37.0	5537	Floor beams, heavier loads
305 x 165 x 40 UB	303.4	40.3	8503	Longer spans, wall removal
305 x 165 x 46 UB	306.6	46.1	9899	Heavy floor loads, wide spans

Beam Deflection Limits — Eurocode Reference

Deflection Limits by Element Type

Common Steel Universal Beams (UB) — Section Properties

Quick Deflection Check (Steel UDL)

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