Rebar Hook Length Calculator

How to work it out

1. Set the extension by hook type: a 180° semicircular hook adds 6.25d, including a 3d straight tail.
2. A 90° hook adds the straight tail plus 0.5d for the bend.
3. A 135° hook (stirrups) adds the tail plus 0.5d, with the tail at max(10d, 75 mm) in seismic work and max(5d, 75 mm) otherwise.
4. Check the tail: seismic stirrup tails are at least 10d and at least 75 mm, whichever governs.
5. Total length = net member dimension + hook extensions, counted per hook.

Where it goes wrong

• Using 10d alone for seismic stirrup tails — on Φ6, 10d is only 60 mm, below the 75 mm minimum, so 75 mm governs; this is commonly missed.
• Omitting hooks on plain round bars — HPB300 relies on hooks for anchorage and must have 180° hooks; only ribbed bars can be anchored straight.
• Not checking hook orientation — bottom bars hook up, slab bottom bars hook up, top bars of cantilevers hook down; wrong orientation means wrong cover.

Material and cost

Hooks add 3-8% to bar length — small, but they must never be missed. Fabrication adds about 0.1-0.3 per bar per hook.

On site

• Plain round HPB300 bars must be 180° hooked at the ends — mandatory, and not interchangeable with ribbed bar practice.
• Both tails of a 135° stirrup hook must be parallel; the wrong included angle bears on the formwork and distorts cover.
• Mechanically coupled bars have no end hooks, so sort which bars are bent and which are cut square before fabrication.

Basis

• GB 50010-2010 Code for design of concrete structures, Section 8.3 (anchorage and hooks of tension bars)
• GB 50666-2011 Code for construction of concrete structures, Section 5.3
• 16G101-1 Standard drawings for flat-slab detailing, pages 57-58