A concrete beam reinforced with basalt FRP (BFRP) bars for both flexure and shear, designed per fib Bulletin 40's provisions, needs its shear design approached differently from steel stirrup design — FRP stirrups can't rely on the same bend-radius and anchorage assumptions steel does, since FRP loses a meaningful portion of its straight-bar strength at a bent corner.
Every check above runs automatically inside the Structyze FRP-Reinforced Concrete Beam module — this particular module is part of the full desktop catalogue rather than the free browser trial, since the trial only carries a curated flagship set. You can still try the free trial modules right now to see the same calculation style in action on RCC Beam, Column, Slab and a handful of others.
The Example We'll Use
- Beam: 300 mm × 550 mm, span 5.5 m, BFRP longitudinal bars for flexure, BFRP stirrups for shear
- BFRP stirrup: 10 mm diameter, bent at 90°, straight-bar guaranteed strength f_fu*: 1,100 MPa
- Factored shear demand at the critical section: V_u = 112 kN
- f'c: 30 MPa
Working Through It
FRP's strength at a bent portion (like a stirrup corner) is significantly lower than its straight-bar strength, since the bending process introduces fibre misalignment and stress concentration at the bend that FRP's brittle fibres handle far less gracefully than ductile steel — fib Bulletin 40 requires using a bent-bar strength reduction specific to the bend radius, giving an effective bent-portion strength of only about 40% of the straight-bar value for this stirrup's bend radius: 440 MPa.
Concrete shear contribution V_c, calculated using FRP-specific provisions that account for the longitudinal FRP reinforcement's lower stiffness (which affects the concrete shear mechanism through reduced dowel action and shallower compression zone depth compared to steel), gives V_c = 48 kN.
FRP stirrup shear contribution, using the reduced bent-portion strength (not the straight-bar strength) at the design stress level: V_f = 58 kN.
Total shear capacity: φ(V_c + V_f) = 0.75 × (48 + 58) = 80 kN.
Utilization: V_u / capacity = 112 / 80 = 1.40 — this beam fails its shear check as initially stirrup-detailed, driven significantly by the bent-portion strength reduction that a straight-bar-strength-only calculation would have missed entirely.
What Governs, and Why
Shear fails at utilization 1.40, and the root cause traces directly back to using the stirrup's bent-corner strength rather than its straight-bar strength — a calculation using the full 1,100 MPa straight-bar strength instead of the reduced 440 MPa bent value would have shown a comfortable pass, which is the kind of error that's easy to make by carrying over steel stirrup design intuition (where bend strength reduction isn't nearly as significant a concern) into FRP stirrup design.
Where Structyze Takes Over
Every check above runs automatically inside the Structyze FRP-Reinforced Concrete Beam module — this particular module is part of the full desktop catalogue rather than the free browser trial, since the trial only carries a curated flagship set. You can still try the free trial modules right now to see the same calculation style in action on RCC Beam, Column, Slab and a handful of others.
For FRP-Reinforced Concrete Beam and the rest of the full catalogue — 80+ components across RCC, Steel, Composite, Timber, Masonry and more, across 20+ design codes — buy the full Structyze desktop app or see everything it does on the Software page.
Quick FAQ
Why does bending reduce FRP strength so much more than it reduces steel strength?
Steel's ductility lets it redistribute stress around a bend without significant strength loss, while FRP's fibres (especially in a pultruded bar bent during or after manufacture) experience fibre misalignment and resin stress concentration at the bend that the material's inherently brittle failure mode handles far less forgivingly, resulting in a much larger proportional strength reduction at the bend.
Is there a way to avoid relying on FRP stirrup bent-corner strength entirely?
Yes — closed FRP stirrups manufactured with the bend already cured into the fibre layout during production (rather than field-bent) can achieve higher bent-portion strength than field bending, and some designs use straight FRP bars combined with mechanical anchorage details specifically to avoid relying on a bent corner's reduced strength at all.
Final Thoughts
An FRP stirrup's usable strength at its bent corner can be a fraction of its straight-bar rating, which is the kind of FRP-specific behavior that has no real equivalent in steel stirrup design and is easy to miss if a shear calculation defaults to the material's headline tensile strength instead of the bend-reduced value that's actually relevant at the stirrup corner doing the work.