A stirrup hook on Canadian metric bar adds 100 mm for 10M and 140 mm for 15M, for both 90° and 135° hooks, bent around a 45 mm pin for 10M and a 65 mm pin for 15M. Those are the stirrup and tie hook dimensions in the Reinforcing Steel Institute of Canada (RSIC) Manual of Standard Practice, which Canadian detailers and fabricators work to. Seismic hooks run longer, and stirrups of 20M and larger use the standard 180° hook dimensions instead.
This post is the numbers reference. For shapes, ordering and what to put on a stirrup schedule, see our guide to ordering custom rebar stirrups.
Stirrup and tie hook dimensions
The RSIC table for stirrup and tie hooks applies to all grades of steel:
| Bar size | Bar diameter | Pin diameter | 90° hook | 135° hook | H (approx.) |
|---|---|---|---|---|---|
| 10M | 11.3 mm | 45 mm | 100 mm | 100 mm | 70 mm |
| 15M | 16.0 mm | 65 mm | 140 mm | 140 mm | 100 mm |
| 20M and larger | same as standard 180° hook |
How to read it:
- Pin diameter is the diameter of the bending pin, which sets the inside bend diameter. Both work out to about four bar diameters (4 × 11.3 = 45 mm; 4 × 16 = 64 mm, rounded to 65).
- Hook dimension (A or G in RSIC notation) is the detailing allowance added to the bar length for each hooked end. It is what you add when working out a cut length, not the length of the straight tail.
- H is the approximate distance the 135° hook projects, which tells you how much room it takes up inside the member.
Note that the 90° and 135° allowances are the same for 10M and 15M in this table, so the hook type does not change the cut length of a standard stirrup. It does change how the stirrup performs, which is why the drawings specify it.
Seismic hooks
Where an engineer designs a frame or wall for seismic ductility under CSA A23.3, closed stirrups (hoops) and crossties usually need seismic hooks: 135° hooks with a longer tail that anchors deep into the confined core, so the hook does not open up when the cover spalls in an earthquake. RSIC detailing dimensions for seismic hooks:
| Bar size | 135° standard hook | 135° seismic hook |
|---|---|---|
| 10M | 100 mm | 140 mm |
| 15M | 140 mm | 150 mm |
Two practical points. First, the drawings must say which hook type is required: seismic hooks cannot be added after a stirrup is bent. Second, seismic detailing also changes spacing near joints, as covered in our post on stirrup spacing in beams and columns.
Hooks for 20M and larger stirrups
Heavier members sometimes call for 20M or larger stirrups and ties. These do not use the stirrup table. RSIC uses the standard 180° hook dimensions for them, which come with larger bend diameters. For Grade 400W or 500W bar:
| Bar size | Finished bend diameter | 90° hook allowance | 180° hook allowance |
|---|---|---|---|
| 20M | 100 mm | 300 mm | 200 mm |
| 25M | 150 mm | 400 mm | 280 mm |
These are the same values used for hooks on main bars. Our post on standard rebar hooks has the full table from 10M to 55M, and the rebar bending chart covers minimum bend diameters. Larger stirrups are much harder to fit around main bars, which is one reason engineers keep stirrups at 10M or 15M wherever the design allows.
What size rebar for stirrups?
On houses, small commercial buildings, grade beams and most beams in Ontario, stirrups and ties are 10M. 15M stirrups appear in deeper or heavily loaded beams, transfer girders and large footings. The choice is the engineer's, and it is driven by three things:
- Shear demand. CSA A23.3 sets a minimum amount of shear reinforcement wherever stirrups are required: Av is at least 0.06 × √f'c × bw × s ÷ fy, where Av is the area of all the stirrup legs, bw is the web width and s is the spacing. Bigger shear means more Av, which means a bigger bar, more legs or closer spacing.
- Spacing limits. A23.3 caps stirrup spacing, so a designer who needs more steel often goes to closer spacing first and only steps up the bar size when spacing gets too tight to place concrete well.
- Main bar size, for column ties. A23.3 ties the minimum tie size to the size of the vertical bars it encloses, so very large column bars and bundled bars need heavier ties. The column schedule will show it.
Bar diameter in millimetres is the other half of the "stirrup size in mm" question: 10M is 11.3 mm and 15M is 16.0 mm nominal. US drawings use #3 (9.5 mm) and #4 (12.7 mm) stirrups, which are not the same bars; see our Canadian vs US rebar sizes guide before substituting.
Stirrup dimensions and the stirrup angle
Stirrups are sized out-to-out: outside of one leg to outside of the other. When the drawings do not give a size, RSIC practice details stirrups 80 mm smaller than the beam in width and depth, and ties 80 mm smaller than the column. That only works when the cover suits it, so if your cover is different, size from the actual cover.
On angles: in buildings, stirrups are normally set perpendicular to the main bars, and the "angle" people search for is usually the hook angle (90° or 135°). Closed stirrups and ties use 135° hooks so the hook anchors into the core rather than the cover. Open U stirrups often use 90° hooks or are closed at the top by a separate cap tie.
Working out a stirrup cut length
The cut length of a stirrup is the sum of its out-to-out sides plus the hook allowances. Detailers then apply their standard deductions for bends, but the simple sum is what most estimates use, and it slightly overstates the true length.
Example: closed 15M stirrup in a 400 mm wide by 600 mm deep beam, 135° hooks, sized by the 80 mm rule.
- Out-to-out size: 320 mm by 520 mm
- Perimeter: 2 × (320 + 520) = 1,680 mm
- Hooks: 2 × 140 mm = 280 mm
- Approximate cut length: 1,960 mm, or about 3.1 kg of 15M at 1.570 kg/m
The same stirrup in 10M with 100 mm hooks would be about 1,880 mm. Multiply by the count along the beam, which comes from the spacing on the drawings, and you have the stirrup tonnage. Our rebar estimating guide shows how to count them.
Getting stirrups made
Stirrups are the part of a rebar order with the most marks and the least room for error: a stirrup 20 mm too large pushes the main bars into the cover, and one 20 mm too small will not fit around them. Shop bending to the RSIC hook and pin dimensions keeps every piece consistent.
We fabricate stirrups, ties, crossties and dowels to your drawings or your own schedule, tagged and bundled by mark. Send us your drawings or stirrup schedule for a free quote, or see our rebar products for fabricated rebar, straight bar and GFRP.
Frequently asked questions
What is the hook length for a rebar stirrup?
Under RSIC detailing practice, a 10M stirrup hook adds 100 mm and a 15M stirrup hook adds 140 mm, for both 90° and 135° hooks. Seismic 135° hooks are longer: about 140 mm for 10M and 150 mm for 15M.
What size rebar is used for stirrups?
10M is the most common stirrup and tie size on residential and light commercial work, with 15M used for larger beams and heavier shear. The size always comes from the structural drawings.
What is the bend diameter for 10M stirrups?
RSIC stirrup and tie hooks are bent around a 45 mm pin for 10M and a 65 mm pin for 15M, roughly four bar diameters. Main bars use larger bend diameters.
What angle are stirrup hooks?
Stirrup and tie hooks are 90° or 135°. A 135° hook anchors back into the concrete core and is used for closed stirrups, ties and anywhere the engineer needs more confinement. Seismic hooks are 135° with a longer tail.
What is a stirrup's size in mm?
A 10M stirrup is bent from bar with a nominal diameter of 11.3 mm and a 15M stirrup from 16.0 mm bar. The stirrup's overall size is given as out-to-out width and height, usually 80 mm less than the concrete member when the drawings do not say otherwise.
