Steel Deflection Limits — L/360, L/240, L/300, L/250 Reference

Deflection limits control the serviceability of steel structures. A beam or frame that satisfies strength requirements can still be unserviceable if it deflects enough to crack finishes, cause ponding, or produce visible sag. This page tabulates the deflection limits from AISC 360, AS 4100, EN 1993, and CSA S16 with guidance on which limit applies to each situation.

Why deflection limits matter

Deflection is a serviceability limit state, not a strength limit state. A beam at L/180 deflection is not about to collapse -- but it may crack plaster ceilings and drywall partitions, cause ponding on flat roofs, create visible sag that occupants perceive as structural distress, misalign elevator rails or crane runways, or cause doors and windows to bind in their frames.

Most deflection limits are expressed as a fraction of span length (L/n), where a larger denominator means a stricter limit (less deflection permitted).

AISC 360-22 / IBC deflection limits

AISC 360 does not prescribe specific deflection limits in its specification. Instead, AISC Design Guide 3 and IBC Table 1604.3 provide recommended limits:

IBC Table 1604.3 quick reference

AS 4100-2020 deflection limits

AS 4100 Appendix B provides suggested limits, but AS/NZS 1170.0 and the project specification govern:

Key AS 4100 note: Australian practice generally uses span/250 for total deflection and span/300 for live load only. These are slightly less conservative than US L/240 and L/360 for the corresponding cases.

EN 1993-1-1 / EN 1990 deflection limits

Eurocode deflection limits are recommended values in National Annexes. The EN 1990 Annex A1.4 provides:

Key Eurocode note: Eurocode distinguishes between "reversible" (elastic, variable-action) and "irreversible" (creep, permanent) deflection. Pre-camber can offset permanent load deflection but not variable action deflection.

CSA S16-19 deflection limits

CSA S16 limits are nearly identical to IBC limits, reflecting the shared North American practice.

How to calculate beam deflection

For a simply supported beam under uniform load w (force per unit length):

delta_max = 5 * w * L^4 / (384 * E * I)

Where w = distributed load, L = span length, E = modulus of elasticity (29,000 ksi or 200,000 MPa for steel), I = moment of inertia about bending axis.

For a simply supported beam with a point load P at midspan:

delta_max = P * L^3 / (48 * E * I)

Worked example

Given: W16x40 beam, L = 30 ft, uniform live load w_L = 1.2 kip/ft

Properties: I_x = 518 in^4, E = 29,000 ksi

delta_LL = 5 * 0.100 * (360)^4 / (384 * 29,000 * 518)
         = 5 * 0.100 * 1.680e10 / (5.77e9)
         = 1.45 in

L/360 = 360/360 = 1.00 in

Result: delta_LL = 1.45 in > L/360 = 1.00 in -- FAILS. Select a deeper section (e.g., W18x50 with I_x = 800 in^4 gives delta = 0.94 in, which passes).

Common mistakes

1. Checking only live load deflection. Many engineers check L/360 for live load and forget to check L/240 for total load. When dead load is significant (heavy cladding, concrete topping), total load deflection often governs.

2. Ignoring camber offset. If the beam is cambered to offset dead load deflection, only the post-camber deflection needs to meet the limit. But if no camber is specified, total deflection must be checked.

3. Using unfactored loads inconsistently. Deflection is a serviceability check using unfactored (service) loads, not factored loads. A common error is applying load factors (1.2D + 1.6L) when calculating deflection.

4. Forgetting composite action. For composite beams, the effective moment of inertia is much larger than the bare steel section. Using the bare steel I_x for a composite beam is overly conservative.

5. Cantilever double-counting. A cantilever of length a has an equivalent simple span deflection limit. The tip deflection limit is typically L/180 or L/240 for the cantilever length, not L/360.

Frequently asked questions

What is the L/360 deflection limit? L/360 means the maximum permissible deflection is the span length divided by 360. For a 30-foot (360-inch) beam, the limit is 360/360 = 1.0 inch. This is the standard live load deflection limit for floor beams per IBC Table 1604.3.

When does deflection govern over strength? Deflection commonly governs for long-span beams (L > 25 ft), lightly loaded beams where moment capacity is underutilized, beams supporting sensitive finishes, and crane runway girders. As a rule of thumb, if the span-to-depth ratio (L/d) exceeds 20-24 for W-shapes, deflection is likely to govern.

Should I use factored or unfactored loads for deflection? Always use unfactored (service-level) loads. Deflection is a serviceability limit state, checked at service load levels. LRFD load factors (1.2D + 1.6L) are for strength limit states only.

Run this calculation

• Beam Deflection Calculator
• Beam Span Guide

Related references

• Beam Formulas
• Beam Sizes
• Steel Beam Load Tables
• How to Verify Calculations
• Floor Live Load Reference
• beam capacity calculator
• allowable beam span table

Disclaimer

This page is for educational and reference use only. It does not constitute professional engineering advice. All design values must be verified against the governing building code and project specification for your jurisdiction. The site operator disclaims liability for any loss arising from the use of this information.