Clause 6.3 is the most referenced provision of IS 13920, governing stirrup spacing in seismic beams. Closed-loop stirrups (hoops with 135° hooks) are mandatory. In the plastic hinge zone (2d from face of column on both sides), stirrup spacing is the minimum of d/4, 8 times the smallest longitudinal bar diameter, 24 times the hoop bar diameter, and 300 mm. Outside this zone, spacing shall not exceed d/2. The first hoop must be placed at 50 mm from the column face.
Key Requirements
•Only vertical hoops (closed stirrups with 135° hooks and 6d or 75 mm extension, whichever is more) shall be used as transverse reinforcement
•In the plastic hinge zone (2d from face of column on each side), maximum hoop spacing shall be the minimum of: (a) d/4, (b) 8 × diameter of smallest longitudinal bar, (c) 24 × diameter of hoop bar, (d) 300 mm
•The first hoop shall be placed at not more than 50 mm from the face of the column/joint
•Outside the plastic hinge zone, maximum spacing of hoops shall not exceed d/2
•Minimum diameter of hoop bar shall be 8 mm
•The shear force to be resisted by transverse reinforcement shall be computed from the equilibrium of the beam assuming plastic hinge formation at both ends with 1.4 times fy for steel
•At locations of lap splices, the spacing of hoops shall not exceed d/4 or 100 mm, whichever is less
Reference Tables
Stirrup Spacing in Plastic Hinge Zone — Worked Examples
Beam Size (mm)
Effective Depth d (mm)
Longitudinal Bar φ (mm)
Hoop Bar φ (mm)
d/4 (mm)
8φ_long (mm)
24φ_hoop (mm)
Governing Spacing (mm)
230 × 450
410
16
8
102
128
192
100
300 × 500
460
20
8
115
160
192
115
300 × 600
560
20
10
140
160
240
140
300 × 600
560
25
10
140
200
240
140
350 × 750
700
25
12
175
200
288
175
The 300 mm upper limit rarely governs. For most practical beam sizes, d/4 controls the spacing in the plastic hinge zone.
Maximum stirrup spacing in the plastic hinge zone (2d from column face)
d = Effective depth of beam in mmφ_longitudinal = Diameter of smallest longitudinal bar in mmφ_hoop = Diameter of hoop (stirrup) bar in mm
Vu = 1.4 × (Mu_As + Mu_Bs) / L_clear + Vgravity
Design shear force from plastic hinge mechanism with capacity-based design
Vu = Design shear force in kNMu_As = Plastic moment capacity at end A using 1.4fy for steelMu_Bs = Plastic moment capacity at end B using 1.4fy for steelL_clear = Clear span of beam in mmVgravity = Shear due to factored gravity loads on the span
Practical Notes
✓For a typical 300 × 600 mm beam with 20 mm bars and 8 mm stirrups, the governing spacing in the hinge zone is d/4 ≈ 140 mm — round down to 125 mm or 100 mm for ease of construction.
✓The 50 mm first-hoop rule is often missed on site. Mark this dimension clearly on drawings and BBS. Many engineers specify 50 mm c/c from column face for the first stirrup.
✓Use 135° hooks (not 90°) on all stirrups — 90° hooks can open up under reversed cyclic loading, which defeats the purpose of confinement.
Common Mistakes
⚠Using 90° bent hooks instead of 135° seismic hooks on stirrups — 90° hooks provide inadequate anchorage under cyclic loading and can pop open.
⚠Not reducing stirrup spacing at lap splice locations to d/4 or 100 mm — splices need extra confinement to prevent bond failure.
⚠Applying the d/2 mid-span spacing throughout the beam — the 2d zone from each column face must have the tighter spacing, which can extend well into the span for short beams.