GFRP stirrups and dowels do the same jobs as their steel counterparts, but they are factory-bent, much weaker at the bend, less stiff, and designed under FRP rules rather than steel ones. In a corrosive exposure that is often worth it: stirrups sit closest to the concrete surface and are usually the first bars to rust, so a GFRP cage with steel stirrups only solves half the problem. Outside corrosive or electrically sensitive work, steel stirrups and dowels are simpler, cheaper to change and well covered by everyday design rules.
Why stirrups and dowels are where GFRP matters
In a beam, column or wall, the stirrups or ties wrap around the main bars, so they have the least concrete cover of anything in the section. Dowels cross joints, and joints are where water and de-icing salt get in first. That makes both of them the front line for corrosion.
If an engineer chooses GFRP main bars to keep a bridge barrier, parking deck or marine structure corrosion-free, the stirrups, ties and dowels usually have to be GFRP too. Otherwise the first steel bar to rust cracks the cover over the GFRP that was meant to last. For a primer on the material itself, see our GFRP pros and cons.
GFRP vs steel stirrups and ties
| Factor | Steel stirrups and ties | GFRP stirrups and ties |
|---|---|---|
| Made | Shop or field bent from straight bar | Factory formed before resin cures |
| Changes on site | Easy: re-bend or bend new pieces | Not possible; reorder from the plant |
| Strength at bend | Full bar strength develops around a standard hook | Much lower than the straight bar |
| Stiffness | About 200 GPa | Lower than straight GFRP bar |
| Hooks | 90° and 135° standard and seismic hooks | Larger bend radii, longer tails, per manufacturer and design standard |
| Closed shapes | Single bar with overlapping hooks | Closed shapes, or two U-shapes lapped to form a closed stirrup, depending on the maker |
| Design standard | CSA A23.3 | CSA S806 (buildings), CSA S6 (bridges) |
| Corrosion | Rusts once chlorides reach it | Does not rust |
Bend strength is the big difference
When steel is bent, it keeps its strength through the bend. GFRP does not. The fibres on the inside and outside of the bend are stressed unevenly, and the bent portion fails at a fraction of the straight bar's capacity. MTO's draft material specification for GFRP reinforcement (OPSS.PROV 1640, November 2023 draft) sets the minimum bend strength of Grade III bent bars at less than half of the straight-bar strength for each bar size, and a lower minimum modulus for bent bars than for straight bars. CSA S807 includes a test method specifically for the strength of bent bars and stirrups, so manufacturers qualify their bent shapes separately from straight bar.
The same draft MTO specification sets a limit on bulging on the inside of a bend and rejects bent bars that exceed it, and it applies the RSIC Manual of Standard Practice fabricating tolerances to GFRP bars, straight or bent. Inspect bent shapes on delivery the same way you would check steel shapes against the bar list.
Design limits
Because the bend is the weak point and GFRP is less stiff, FRP design standards limit the strain a GFRP stirrup is counted on for in shear. The practical results:
- More or larger stirrups than a steel design for the same beam.
- Closer spacing in some members.
- Longer hook tails and larger bend radii.
- Detailing that follows the GFRP manufacturer's shapes, because not every steel shape can be produced in GFRP.
None of that can be worked out by converting a steel stirrup schedule bar for bar. The engineer has to design the transverse reinforcement to CSA S806 or CSA S6. Our post on fiberglass rebar and CSA S807 and S806 explains those standards.
For the general difference between stirrups (beams, shear) and ties (columns, confinement), see rebar stirrups vs ties.
GFRP vs steel dowels
"Dowel" covers two different things, and GFRP comes in both. Our post on dowel bars vs rebar explains the difference in detail.
Smooth dowel bars at movement joints. In pavements and slabs, dowel bars pass load across a joint while letting it open and close. GFRP dowels cannot rust, so they cannot seize in the joint or stain and spall the concrete. MTO's draft GFRP material specification covers GFRP dowel bars along with straight bars and spirals. The catch is stiffness: a GFRP dowel bends more under a wheel load than a steel dowel of the same diameter, so designs typically use larger diameters or closer spacing. Steel dowels rely on coatings or stainless steel for the same purpose.
Deformed dowels and starter bars. GFRP bars bent into L-shapes can act as starter bars from a footing or slab into a wall, and straight GFRP bars can be set in drilled holes with adhesive. Things to check:
- Bends are factory-made, so starter bar shapes and projections must be fixed before ordering.
- Embedment and laps follow FRP development rules, not steel ones.
- Adhesive compatibility. For drilled-in GFRP dowels, the adhesive maker must support use with FRP bars and provide design data for it.
- No welding, so GFRP dowels cannot be welded to embedded plates or steel bars.
Mixed systems: GFRP and steel in the same element
Some designs deliberately combine materials, for example GFRP in the top mat of a deck exposed to salt and steel lower down. That is a valid engineering choice, but it is the engineer's choice. On site:
- Do not swap stirrups or dowels between materials to solve a shortage. A steel stirrup in a GFRP cage, or a GFRP stirrup in a steel design, changes the design.
- Keep accessories consistent. GFRP assemblies use plastic chairs and coated or nylon tie wire so nothing in the cage corrodes.
- Tie GFRP cages down well. Light bar can float during concrete placement.
Ordering GFRP stirrups and dowels
Getting GFRP transverse steel right is mostly about planning:
- Detail every shape from the drawings with the designated bar diameter, grade and the manufacturer's bend dimensions.
- Count every piece, including extras at openings, corners and joints. Nothing can be bent later.
- Order early. Bent GFRP shapes are produced at the plant and have their own lead times.
- Check deliveries against the bar list, including bend dimensions and bulging at bends.
Our post on GFRP corners, L-bars and hooks covers ordering bent GFRP shapes in more detail.
When to stay with steel
Steel stirrups and dowels remain the better choice when the concrete is not exposed to chlorides, when the design relies on ductile behaviour and standard seismic hooks, when the schedule needs quick changes on site, or when the building rules are prescriptive steel rules (most Part 9 houses).
Pricing GFRP or steel shapes
We fabricate steel stirrups, ties, dowels and custom bent shapes from 10M to 55M, cut and tagged to your bar list, and we supply GFRP rebar. Ask us about bent GFRP stirrups and dowels and the lead time for your job. Send us your drawings or see our rebar products.
Frequently asked questions
Can fiberglass rebar be bent into stirrups?
Yes, but only at the factory. GFRP stirrups, ties, spirals and hooks are formed before the resin cures, so every shape has to be detailed and ordered in advance. GFRP bar cannot be bent on site.
Are GFRP stirrups as strong as steel stirrups?
Not at the bend. The bent portion of a GFRP bar carries much less than the straight portion, and the design standards also limit how much strain a GFRP stirrup can be counted on for in shear. Designs usually need more or larger GFRP stirrups than steel.
Can I use steel stirrups with GFRP main bars?
Only if the engineer designs it that way. Stirrups are the bars closest to the surface, so steel stirrups in a GFRP cage are usually the first thing to corrode, which defeats the purpose of using GFRP in a corrosive exposure.
Can GFRP bars be used as dowels?
Yes. GFRP dowel bars are used at joints in pavements and slabs, and deformed GFRP bars can be used as starter bars or drilled-in dowels. Because GFRP is less stiff than steel, the size, spacing and embedment must come from an FRP design, not a steel detail.
