Stirrups / Shear Reinforcement

Stirrups (also called shear reinforcement, hoops, or links) are closed loops of reinforcement encircling the longitudinal main bars in beams and columns. Their primary function is to resist shear forces — without stirrups, beams fail by diagonal tension cracking long before flexural failure. Per IS 456:2000 Cl. 26.5: minimum stirrup spacing in beams ≤ 0.75d (where d is effective depth) or 300 mm; maximum spacing in columns 300 mm. For seismic frames per IS 13920:2016 Cl. 7.4: tighter spacing at confining zones (typically 100 mm c/c at the ends of beams and columns near joints).

Stirrup geometry: (a) Single stirrup — simple closed loop; standard for typical residential beams. (b) Two-legged stirrup — same as single. (c) Four-legged stirrup — used in heavily-loaded sections. (d) Helical / spiral stirrup — for circular columns; provides continuous confinement. (e) 135° hook stirrups — IS 13920 mandate for seismic frames; the 135° bend prevents the stirrup from opening under cyclic loading. The hook leg must extend ≥ 6× bar diameter (or 65 mm minimum) into the concrete core after the bend.

Design: shear reinforcement provided when applied shear Vu exceeds concrete shear capacity τc × b × d (where τc is from IS 456 Table 19 depending on concrete grade and tension steel percentage). Stirrup spacing: Sv = (0.87 × fy × Asv × d) / (Vu - Vc), where Asv is the area of stirrup legs at one location. For seismic frames, additional rules apply: stirrup spacing in confining zone ≤ d/4 or 100 mm; legs ≥ 8 mm dia; 135° hooks. The most-overlooked stirrup issue on Indian construction sites: spacing relaxation in the middle of beams or columns. Workers often pull stirrups apart in the middle to ease placement of main bars; this reduces shear capacity and causes brittle failure under load. Site QC must verify stirrup spacing against the BBS pre-pour. Random tightening or loosening is unacceptable.