How do I check deflection limits according to design codes?
Design & Analysis
Deflection limits are typically expressed as span/N where N depends on the type of structure and code. Common values: L/360 for floor beams under live load (AISC, NBR 8800), L/240 for roof beams, L/200 for members supporting plaster, H/400 to H/500 for column drift under service wind. Eurocode 3 uses similar values defined in the national annex. The code checks are performed under service (unfactored) loads, not ultimate (factored) loads. In CalcSteel, deflection is computed automatically from the FEM analysis and shown in the verification report alongside the limit — if deflection exceeds the limit, the member fails serviceability even if its strength is adequate.
Key takeaways
- Deflection is a serviceability limit state (SLS): checked under unfactored service loads, separate from the strength (ULS) check.
- The span-proportional idea (limit = L/n) dates to Thomas Tredgold around 1820 at L/480; 19th-century American practice relaxed it toward L/360 to control plaster cracking.
- Numerical limits live in different places per code: IBC Table 1604.3 (US), the National Annex for Eurocode 3, Table 6 of IS 800, and Annex C of NBR 8800 (informative since the 2024 revision).
- Software computes the elastic deflection from the same finite-element model used for strength, then compares it to the code ratio for each load case.
Full guide
Deflection Limits in Steel Design Codes
Step-by-step with figures and sources — 9 min read
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