Shear Span (a/d Ratio)

The shear span 'a' is the distance from a support to the nearest major concentrated load (or, generally, the moment/shear ratio M/V at a section); the shear-span-to-effective-depth ratio a/d governs how a member carries shear. For a/d greater than about 2.5-3, the member behaves as a normal (slender) beam, carrying shear by the usual flexure-shear mechanism with stirrups designed per IS 456 Cl. 40. For a/d less than about 2 (and span/depth < 2 overall), arching/strut-and-tie action dominates and the member is a deep beam, designed per IS 456 Cl. 29 with quite different reinforcement detailing.

The a/d ratio also explains the well-known dip in apparent shear strength of beams around a/d ≈ 2.5-3 and is fundamental to choosing the correct design model — applying ordinary beam shear theory to a deep member, or vice-versa, is a basic but serious modelling error.