Bending Rebar Into a Circle: Hoops, Arches and Spirals

Rebar is bent into a circle in one of two ways: a fabrication shop rolls or wraps the bar to the required diameter, or a straight bar is sprung into curved formwork on site when the radius is large enough. Column hoops, circular ties and spirals are always shop work. Long arcs for round walls, tanks and curved slab edges are shop bent up to a size limit, then shipped straight above it. Fibreglass (GFRP) hoops and rings can only be made at the factory.

This post covers the shapes, how to work out the length of a curved bar, the spiral rules you will see on drawings, and what to send when you order. For hooks and corner bends, see our rebar bending chart.

Three kinds of curved rebar

Curved reinforcement shows up on drawings in three main forms, each with its own shape code in the Reinforcing Steel Institute of Canada (RSIC) Manual of Standard Practice:

RSIC defines a radius bend as any bend with an inside radius of 300 mm or more. Anything tighter is a hook or a standard bend and follows the minimum bend diameters instead. Our guide to rebar bending shape codes explains how these types and their letters appear on a bar list.

Shop bending vs curving in the field

Whether a curved bar is bent in the shop or on site depends on the bar size and the radius. RSIC Table 21B sets the largest radius a shop is expected to fabricate for each size:

Bars that need a larger radius than the table are shipped straight and bent into place in the field. At that radius the bar is flexible enough to follow the formwork and be tied in position without a permanent bend. Bars that need a tighter radius are rolled in the shop.

Shipping is the other limit. A curved bar can't be laid flat on a truck if it is wider than the load. RSIC Table 21A works this out against a 2,400 mm loading width: a full circle can only be shipped up to a 1,200 mm radius, and for larger radii the maximum length of a curved piece is limited. Big circular walls are therefore made of several curved bars lapped together, not one continuous ring.

How to work out the length of a curved bar

The length of a curved bar is measured along its centreline, not its outside face.

For a full circle: Length = π × (outside diameter minus one bar diameter)

For an arc: Length = 2 × π × centreline radius × (angle in degrees ÷ 360)

Then add whatever closes or anchors the bar: a lap, hooks, or straight legs.

Example: a 10M circular tie with 135° hooks (T3A). Outside diameter 420 mm, which suits a 500 mm round column with 40 mm cover to the tie.

• Centreline diameter: 420 minus 11.3 = 408.7 mm
• Circumference: π × 408.7 = about 1,284 mm
• Overlap: RSIC sets the T3A lap at 150 mm minimum, or enough to hook around two adjacent vertical bars, whichever is greater. Use 150 mm here.
• Hooks: RSIC gives about 100 mm for a standard 135° hook on 10M, so add 2 × 100 mm.
• Total: about 1,634 mm per tie, or roughly 1.3 kg at 0.785 kg/m.

Example: a 15M bar on a 3,000 mm centreline radius, a quarter circle. 2 × π × 3,000 × (90 ÷ 360) = about 4,712 mm, plus laps to the neighbouring bars. This is well inside the shop limit for 15M, but check the shipping table before assuming it can go as one piece.

On a bar list the detailer gives the radius or diameter and the arc dimensions, and the shop works out the exact cut length. A lapped hoop (T3) uses a Class B tension splice for its lap, which depends on the concrete and the bar, so take that length from the engineer's drawings or the lap splice chart.

Spirals: what the drawings will specify

Spirals are continuous helical bars that wrap the vertical bars of a round column. The drawing should give the bar size, grade, outside diameter, pitch and the number of turns. A few rules come up every time:

• Diameter. RSIC practice makes the spiral 80 mm smaller than the column's outside diameter unless the drawings say otherwise. The diameter is measured to the outside of the spiral.
• Size range. RSIC recommends 10M and 15M bar for spirals, with 240 mm outside diameter as the recommended minimum. Diameters up to 1,800 mm are common.
• Pitch and clear spacing. CSA A23.3 limits the pitch to one sixth of the core diameter and keeps the clear space between turns between 25 mm and 75 mm. Those values are from the 2004 edition of A23.3; your engineer will design to the edition the project uses.
• End turns. RSIC practice finishes spirals with an extra one and a half turns at the top and at the bottom unless otherwise specified.
• Splices. Where a spiral must be spliced, RSIC calls for a tension lap of at least 50 bar diameters plus a 90° hook around a vertical bar at the free end. Welded splices are acceptable only where the engineer authorizes them.
• Spacers. Spacers hold the pitch during placing and are supplied when the engineer specifies them.

The fabricating tolerance on a spiral's diameter under RSIC practice is plus or minus 13 mm below 750 mm diameter and plus or minus 25 mm at 750 mm and over.

Bending circles and arches on site

For non-structural work, such as a garden arch, trellis hoop or a form for landscaping, a 10M bar can be curved by hand around a jig: a sheet of plywood with blocks or pipe stubs screwed down along the curve. Bend a little at a time along the length so the curve stays even. Smaller bars and smooth rod are easier still.

For structural reinforcement, the rules are stricter:

• Use the radius on the drawings. A curve that is too tight changes the bar's position and its cover.
• Don't kink it. A smooth curve is not the same as a series of sharp bends. Kinks put the bar out of line and can overstrain it.
• Don't heat it. Heating changes the steel's properties. Our post on whether bending rebar weakens it covers heating, rebending and field bends.
• Spring, don't force. Large-radius bars shipped straight are meant to be sprung into the forms and tied. If a bar won't follow the curve without being bent permanently, it probably should have been shop rolled.

Hoops and spirals are not worth making on site. Getting a consistent diameter, pitch and lap by hand is slow, and inspectors check those dimensions.

Fibreglass rebar hoops and rings

GFRP bar can't be bent after it is made. The RSIC manual states that FRP bends are made by the manufacturer before the resin cures and that field bending is not permitted. That applies to hoops, rings, spirals and curved bars just as it does to corners and stirrups.

So for a GFRP project:

• Detail every curved piece with its diameter or radius and lap before ordering.
• Check the manufacturer's limits. Bend radius and shape options vary between producers, and the design must follow them.
• Order early. A missing hoop can't be fixed with a bender on site.

Our guide to GFRP corners, L-bars and hooks explains how bent fibreglass shapes are ordered.

Ordering curved bars

To get curved bars made right the first time, give the shop:

1. The diameter or radius, and say whether it is to the outside of the bar or the centreline. Spiral diameters are to the outside.
2. The arc length or angle, and any straight legs.
3. How the bar is closed or joined: lap length, hooks, or a mechanical connection.
4. For spirals: bar size, grade, pitch, height or number of turns, and whether spacers are specified.
5. Quantities by mark, so curved pieces are tagged and bundled separately from straight bar.

We detail, cut and bend rebar from 10M to 55M, including stirrups, ties and custom bent shapes, and we supply GFRP rebar. For radius work, send us the radius and lengths and we will confirm which pieces we bend in the shop and which ship straight to be curved in place. Send your drawings or bar list for a free quote, or see our services.