A stirrup is a closed or open loop of reinforcing bar wrapped around the main bars of a concrete beam. It resists shear and torsion, holds the cage in position while concrete is placed, and stops compression bars from buckling. The same loop in a column is called a tie or hoop.
What stirrups do inside a beam
Under load a beam bends, and near its supports it also wants to shear. Shear shows up as diagonal cracks at roughly 45° that start low near the support and climb toward the load. Concrete alone resists some of it, then the crack opens and the beam can fail suddenly with no warning. A stirrup crosses the crack and ties the two sides together.
Stirrups have four jobs. They carry shear across diagonal cracks. They resist torsion, but only when they are closed. They hold the top and bottom bars at the right depth while workers walk, vibrate and pour. And they stop compression bars from bowing out. Of these, shear is the one that governs spacing and size.
Types of stirrups
Most articles list shapes (square, rectangular, triangular, diamond) and then leg counts from one to eight. On site, three questions sort everything: is the loop closed, how many legs cross the section, and is the bar straight or spiral?
| Type | Where it is used | Notes |
|---|---|---|
| 2-legged closed | Beams up to about 300 mm wide, 4 main bars | The default. Carries shear and torsion. |
| 3-legged (stirrup + cross-tie) | Beams with 3 bars per face | The middle bar is held by a cross-tie with a 135° hook at one end and a 90° at the other. |
| 4-legged | Beams 400 mm wide or more, heavy shear | Two overlapping closed stirrups. Doubles the steel crossing each crack. |
| 6-legged and above | Transfer beams, bridge girders, pile caps | Chosen when shear is too high for 4 legs at a workable spacing. |
| Open U-stirrup | Precast beams, slab ribs, with a cap bar | No torsion resistance. Not for edge beams. |
| Rectangular or square tie | Columns | Called a tie or hoop. See stirrups vs ties below. |
| Circular tie or spiral | Round columns, piles | Confines the core, improves ductility. |
Triangular and diamond stirrups appear in drawings for odd sections such as trapezoidal beams, and for corner bars in non-rectangular columns. They are detail solutions, not standard types, and each bend has to be anchored like any other hook.
Which one to choose
- Beam width up to about 300 mm, bars on the corners only: 2-legged.
- Three or more bars across the top or bottom layer: add a cross-tie so no bar sits more than about 150 mm clear from a supported one (ACI 318 sets this limit for columns, and designers copy it for beams).
- Width 400 mm and above, or shear steel at maximum density at 100 mm: 4-legged before squeezing the spacing below 100 mm. Tight stirrups block the vibrator and make honeycombing likely.
- Edge beams, balconies, anything with torsion: closed loops only, with 135° hooks.
- Round column or pile: spiral if the design is for ductility, circular ties if you want easier fabrication.
Stirrup spacing in beams
Three layers of rules set spacing. The code gives a ceiling. The seismic detail gives a floor near the supports. And the concrete’s need to flow gives a practical minimum.
- Upper limit. Beyond it a diagonal crack could pass between two stirrups without touching either.
- Confinement zone. In seismic design, the length of 2d (IS 13920) or 2h (ACI 318) from the support face has closer hoops. First hoop 50 mm from the face.
- Practical minimum. About 75–100 mm. Below this the vibrator cannot enter and aggregate bridges the bars.
Spacing also controls the minimum shear steel. In IS 456 the check is Asv / (b × sv) ≥ 0.4 / (0.87 fy). For a 300 mm wide beam with 2L-8 bars and Fe 500, that gives a maximum spacing of about 365 mm, so the 300 mm cap governs.
Code limits side by side
| IS 456 / IS 13920 (India) | ACI 318 (US) | Eurocode 2 / 8 | |
|---|---|---|---|
| Max spacing, beam | 0.75d or 300 mm, smaller of the two | d/2 or 600 mm; d/4 or 300 mm when shear steel carries a high share | 0.75d for vertical stirrups |
| Minimum shear steel | Asv/(b·sv) ≥ 0.4/(0.87 fy) | Larger of 0.062√f′c and 0.35, times bw/fyt | ρw ≥ 0.08√fck / fyk |
| Seismic zone length | 2d from the face | 2h from the face | hw (medium ductility class) |
| Seismic spacing | ≤ d/4, ≤ 8 × smallest bar, ≥ 100 mm | ≤ d/4, ≤ 6 × smallest bar (typical for 60 ksi), ≤ 150 mm | ≤ hw/4, 24 dbw, 225 mm, 8 dbL |
| Hook tail | 135°, ≥ 6d and ≥ 65 mm | 135°, ≥ 6d and ≥ 75 mm | Set in national annex and detailing rules |
| Column ties, max spacing | Least of: smallest dimension, 16 × smallest bar, 300 mm | Least of: 16 × bar, 48 × tie, smallest dimension | Least of: 20 × smallest bar, smallest dimension, 400 mm |
Use the table to cross-check a drawing, never as a substitute for the design code that governs your project. Editions change, and national annexes change the numbers again.
Suitable diameter of stirrups
Diameter comes from three limits: shear demand, the main bar size, and what can be bent and handled. Ties in columns have an explicit rule. In IS 456 and Eurocode 2 the tie must be at least one quarter of the largest longitudinal bar and never under 6 mm. ACI 318 sets 10 mm for longitudinal bars up to 32 mm and 13 mm above that.
| Situation | Common stirrup size | Reason |
|---|---|---|
| Lintels, light beams, spans under 3 m | 6–8 mm | Low shear. 6 mm is fine when the design says so. |
| Residential and small commercial beams | 8 mm | Stiff enough to hold the cage, easy to bend by hand. |
| Long-span beams, transfer beams, 600 mm+ deep | 10–12 mm | Higher shear, more legs at wider spacing. |
| Girders, pile caps, deep beams | 12–16 mm | Heavy shear, often with 4 or 6 legs. |
| Column ties, long bars up to 32 mm | 8–10 mm | ¼ of the largest bar, ACI 10 mm. |
| Column ties, long bars 36 mm+ | 10–12 mm | Keeps ties stiff enough to restrain big bars. |
When shear demand rises, you have three levers: bigger diameter, more legs, tighter spacing. Raising the diameter is usually the cheapest on site because the number of pieces stays the same.
What spacing and legs do to shear capacity
Steel contribution to shear under IS 456 is Vus = 0.87 fy Asv d / sv. Take a beam with effective depth 450 mm and Fe 500 steel. The chart shows what three common arrangements carry, and how fast capacity drops as spacing opens up. The concrete’s own share, Vc, is added on top and comes from the IS 456 tables.
Hooks and anchorage
A stirrup is only as strong as its anchorage. If a hook opens, the loop stops confining and the shear steel above the crack loses its grip. That is why codes ask for 135° hooks on closed stirrups.
- Bend diameter: inside bend diameter at least 4d for bars up to 16 mm, which is the usual rule on mandrels at bar-bending yards.
- Tail length: 6d minimum, 65 mm (IS 13920) or 75 mm (ACI 318), whichever is larger. Many drawings call for 10d.
- Hook location: in the compression zone at the top of a beam. Never inside a beam–column joint where it clashes with column bars.
- Laps in stirrups: do not lap a stirrup in the zone where it works hardest. Use a closed shape bent from one bar.
Cutting length and weight
The popular shortcut of subtracting the cover from the beam perimeter hides every bend allowance. This method follows the bar: measure the loop out-to-out, add the hook tails, subtract what the bends save.
Different bar-bending schedules use different bend allowances. Britain follows BS 8666 shape codes (a closed stirrup is typically shape code 51), and US detailers use ACI/CRSI standard hooks. Check which table your schedule uses before comparing lengths. A 10 mm variation per piece over 2,000 stirrups is 20 m of steel.
Stirrups vs ties
| Stirrup | Tie / hoop | |
|---|---|---|
| Member | Beams, girders, slabs ribs | Columns, piers, piles |
| Main job | Shear and torsion | Confinement and bar restraint |
| Governing check | Shear force diagram | Bar buckling, ductility |
| Dense zones | Near supports (2d) | Ends of column and the joint |
| Shapes | Closed, U, multi-leg | Closed, circular, spiral, with cross-ties |
Some older sources say stirrups go in “horizontal members like columns”. That is a slip: columns are vertical, and their loops are ties. The shapes are identical, but the design logic is not, and that matters when you read a drawing legend.
Walls and retaining structures rarely use classic stirrups. Stem shear is checked on a unit strip and usually carried by concrete alone, with ties only at counterforts and buttresses. See retaining wall types for where that changes. Steel framed floors skip stirrups too, because shear travels through the web of the section; the mezzanine floor types guide shows that comparison.
Site mistakes that weaken stirrups
| Mistake | What it causes | Check |
|---|---|---|
| 90° hooks on closed stirrups | Loop opens, cover spalls | Look at hooks before tying |
| Spacing opened up at supports | Shear cracks where force is highest | Mark 100 mm zones on the formwork |
| First stirrup placed 100–150 mm from the face | Gap at the critical section | First stirrup at 50 mm |
| Stirrups left out through the beam–column joint | Weak joint, poor confinement | Count ties in the joint on the drawing |
| Stirrups not tied to main bars | Cage racks while pouring | Tie at every corner, alternate ties on others |
| Wrong cut length | Cover problems, loose fit | Cut one test piece and fit it in the cage |
| Cover too thin on the stirrup | Rust stains, early corrosion | Spacers under and beside the cage |
| Bars substituted: 10 mm cut to 8 mm | Shear capacity drops 36% | Compare diameters against the bar schedule |
The last row follows from the area: 8 mm bar has 50.3 mm², 10 mm has 78.5 mm², and the difference is a 36% drop in shear steel.
Newer stirrup forms
- Prefabricated stirrup cages. Factory-bent and welded or tied cages, fitted whole. Fewer site cutting errors, faster fixing.
- Machine-bent stirrups. Bending to a ±2 mm tolerance cuts waste and improves fit in tight beams.
- GFRP stirrups. Used in marine, bridge and corrosive settings. Capacity depends on the bent portion, which is weaker than the straight leg, so designers apply a reduction factor.
- Stainless and coated steel. For sea-spray zones and parking decks.
- Recycled materials. Infrastructure is shifting toward waste-derived inputs, the same direction as plastic roads, though stirrup steel itself is already heavily recycled.
Frequently asked questions
What is the purpose of stirrups in a beam?
They resist shear and torsion, hold the main bars at the correct depth, and keep compression bars from buckling. Shear is the main design reason.
What size stirrups are used in beams?
8 mm is the common size for residential and light commercial beams. Large or heavily loaded beams use 10–16 mm. The structural drawing sets the final size.
What is the standard spacing of stirrups?
Typically 100 mm in the first 2d from each support and 150 mm in the middle. The maximum under IS 456 is 0.75d or 300 mm, whichever is smaller.
What is the difference between 2-legged and 4-legged stirrups?
A 2-legged stirrup puts two bar legs across the section. A 4-legged stirrup uses two overlapping loops, so four legs cross every diagonal crack. That doubles the shear steel at the same spacing.
Why are stirrups closer near the supports?
Shear force is highest at the supports and drops toward mid-span. Tight spacing there puts more steel where diagonal cracks form.
How do you calculate the cutting length of a stirrup?
Measure the loop out-to-out, add hook tails, then subtract bend deductions. For a closed stirrup with two 135° hooks, the result is 2(a + b) + 8d.
Why is a 135° hook required on a stirrup?
It stays anchored in the concrete core when the cover spalls, so the loop keeps confining the bars. A 90° hook can open.
Are stirrups needed in every beam?
Almost always. Codes require minimum shear reinforcement in most beams, and even where calculation shows none is needed, stirrups still position the cage during concreting.
What happens if stirrup spacing is too wide?
Diagonal cracks can pass between stirrups without crossing any, so the beam loses shear capacity and may fail suddenly.
What is the difference between stirrups and lateral ties?
Stirrups are in beams and mainly resist shear. Ties are in columns and mainly confine the core and stop bar buckling.
Sources and references
- IS 456:2000, Plain and Reinforced Concrete, Clauses 26.5.1.5–26.5.1.6 (stirrup spacing, minimum shear steel) and 40.4 (shear design).
- IS 13920:2016, Ductile Design and Detailing of Reinforced Concrete Structures Subjected to Seismic Forces.
- ACI 318-19, Building Code Requirements for Structural Concrete, Chapters 9, 18 and 25.
- EN 1992-1-1 (Eurocode 2), Clauses 9.2.2 and 9.5.3; EN 1998-1 (Eurocode 8), Clause 5.4.3.1.
- BS 8666, Scheduling, dimensioning, bending and cutting of steel reinforcement for concrete.