UK Deflection Limits — EN 1993-1-1 NA Serviceability

Serviceability deflection limits ensure structural performance under normal use conditions without excessive deformation that could damage finishes or cause occupant discomfort.

Deflection limits are typically expressed as a fraction of the span length (L/###). UK NA to EN 1993-1-1 and EN 1990 NA provides recommended limits. National building codes may specify mandatory minimum limits.

Code Reference: UK NA to EN 1993-1-1 and EN 1990 NA

UK NA to EN 1993-1-1 and EN 1990 NA deflection limits are generally recommendations, not mandatory limits. Local building codes may override or supplement these.

Recommended Deflection Limits

Deflection Calculation — Common Formulas

Simply supported beam — UDL: [ \delta = \frac{5 w L^4}{384 E I} ]

Simply supported beam — point load at midspan: [ \delta = \frac{P L^3}{48 E I} ]

Cantilever — UDL: [ \delta = \frac{w L^4}{8 E I} ]

Cantilever — point load at tip: [ \delta = \frac{P L^3}{3 E I} ]

Worked Example — Floor Beam Deflection

Given:

• 533×210 UB 92 in S355
• Span: 8000 mm
• Imposed load: 5 kN/m (uniform)
• Iy = 55,200 cm⁴, E = 210,000 N/mm²

Vertical deflection under variable load: δ = 5 × 5.0 × 8000⁴ / (384 × 210,000 × 55,200 × 10⁴) = 23.0 mm

L/300 = 8000/300 = 26.7 mm δ = 23.0 mm < 26.7 mm — Satisfactory

If finishes require L/360: L/360 = 8000/360 = 22.2 mm δ = 23.0 mm > 22.2 mm — Not satisfactory. Need stiffer section.

Try 610×229 UB 101 (I = 66,200 cm⁴): δ = 5 × 5.0 × 8000⁴ / (384 × 210,000 × 66,200 × 10⁴) = 19.2 mm < 22.2 mm — OK

Alternatively, reduce spacing or add camber.

Total vs Variable Deflection

UK design distinguishes between:

δmax = δ0 + δ1 + δ2 where:

• δ0 = pre-camber (if provided, positive upwards)
• δ1 = deflection due to permanent loads (including creep)
• δ2 = deflection due to variable loads

Horizontal Drift Limits

For UK multi-storey buildings, the UK NA to EN 1990 and EN 1993-1-1 recommends:

Camber Guidelines per UK Practice

Camber typically = deflection under dead load + 50% of live load deflection. Maximum camber ≤ L/300 for practical rolling tolerance. Camber is not applied to secondary beams, cantilevers, or members subject to load reversal.

Design Resources

• UK Beam Design — Beam flexural design
• UK Steel Properties — Material data
• UK Steel Beam Sizes — Section properties
• UK Vibration — Floor vibration criteria
• UK Composite Beam — Composite construction
• All UK References

Frequently Asked Questions

What are the UK deflection limits per National Annex?

UK NA to EN 1993-1-1: vertical deflection under variable loads ≤ L/300 for floor beams, ≤ L/200 for roof beams and purlins. Cantilevers: L/150 for floor, L/100 for roof. Where brittle finishes are present (plastered ceilings, tiled floors), L/360 is recommended. These limits are recommendations in the UK NA, not mandatory requirements — the designer may adopt more or less stringent limits based on the specific building use and client requirements.

How does the UK NA address camber?

UK NA recommends camber for beams exceeding L/300 deflection. Camber typically = deflection under dead load + 50% of live load deflection. Maximum camber ≤ L/300 for practical rolling tolerance. Camber is achieved by rolling the section with a prescribed curvature. The economic camber is between 15-50 mm; deeper camber requires special rolling and is more costly. Camber tolerance is typically ±20% or ±5 mm (whichever greater).

What deflection limits apply to long-span beams (12-24 m)?

For long-span beams, the criteria are more stringent: total deflection ≤ L/250 (visual), live load deflection ≤ L/360 (finishes), and dynamic acceleration ≤ 0.5% g (for walking excitation per SCI P354). For spans > 15 m, the beam self-weight deflection can be significant and should be included in the δmax calculation. Pre-cambering is standard for long-span beams, typically offsetting 100% of permanent loads and 50% of variable loads.

How are deflection limits applied to cantilevers?

Cantilever deflection limits are typically L/150 for floor cantilevers and L/100 for roof cantilevers, where L is the cantilever projection. These are half the equivalent limits for simply supported spans of 2L because the end slope of a cantilever is twice that of a simply supported beam of the same back-span. For public access balconies, a more stringent L/200 under imposed load is common UK practice.

What horizontal drift limits apply to UK multi-storey steel frames?

The UK NA to EN 1990 recommends inter-storey drift ≤ h/300 under serviceability wind loads (1-year return period). Overall building drift ≤ H/500 for typical curtain wall cladding. For buildings with masonry infill walls, the limit is h/500 (inter-storey) to prevent cracking. Peak acceleration under 5-year wind should not exceed 0.005g (0.5% gravity) for office occupant comfort. These drift limits often govern the design of bracing and core walls in taller UK steel-framed buildings.

Reference only. Verify all values against the current edition of UK NA to EN 1993-1-1 and EN 1990 NA. This information does not constitute professional engineering advice.