Deflection Limits

Deflection limits are the maximum allowable vertical deformation of beams, slabs, and overhanging members under load. Per IS 456:2000 Cl. 23.2: total long-term deflection ≤ span/250 (final, including creep + shrinkage); deflection occurring after partition installation ≤ span/350 (to prevent cracking of sensitive partitions and finishes). For a 5 m simply-supported beam: max final deflection = 20 mm; max post-partition deflection = 14.3 mm. Per IS 800:2007 Cl. 5.6.1 for steel: similar limits — span/350 for general buildings, span/250 for industrial.

Three principal deflection components: (1) Short-term elastic deflection — occurs immediately under load, computed from force methods or finite element with cracked section moment of inertia (Ieff per IS 456 Annex C); (2) Long-term creep deflection — slow, time-dependent increase under sustained load due to concrete creep; typical multiplier 1.5-2.5 of short-term. (3) Shrinkage curvature deflection — additional deflection from concrete drying shrinkage; typically smaller component (5-15% of total). Total long-term = short-term × (1 + creep multiplier) + shrinkage curvature × span²/(8E×Ieff).

Per IS 456 Cl. 23.2.1, the most-used method to satisfy deflection without explicit calculation is the span/depth ratio table — basic ratios for simply-supported (20), continuous (26), cantilever (7), and span/L ratios for two-way slabs. These are modified by tension steel percentage and tension steel multiplier. A typical residential 5 m simply-supported beam needs effective depth ≥ 5000/20 = 250 mm to satisfy deflection by span/depth ratio, simplifying design without explicit deflection calculation. For unusual situations (long spans, heavy loads, sensitive partitions), explicit deflection calculation per Cl. 23.2.2 + Annex C is required. Most-violated rule: many residential designs use span/depth = 22-25 for simply-supported beams (vs 20 limit) to save concrete depth; deflection then exceeds limits and partition cracks appear within 3-5 years.