Flexural members - Reinforced Concrete with FRP Bars: Mechanics and Design - page 126

The theoretical and experimental values of the constant K were computed
and compared based on a database of 370 beams. Based on these results, it
was suggested to adopt a value of K = 5 for design purposes.

Design requirements. For the case of FRP reinforced concrete beams,
shear reinforcement is not required when the following condition is verified:

φ V

c

V ≤ (4.94)

u

2

For the case of slabs and footings, this threshold is raised to ϕ V c . Typically,
slabs and footings do not include shear reinforcement.

COMMENTARY

FRP RC members without shear reinforcement exhibit a size effect in shear
whose extent and relevant parameters are similar to those of steel RC mem-
bers [35]. Experimental evidence gained using smaller specimens may lead to
overestimating strength when used for validation purposes and in practice on
larger members without shear reinforcement. When used, longitudinal skin
reinforcement or minimum shear reinforcement contributes to mitigating the
size effect. In particular, the former option improves the flexural stiffness,
allowing the formation of more closely spaced cracks. The size effect is exac-
erbated as the amount of flexural reinforcement and the maximum aggregate
size are reduced, both resulting in smaller shear strength values. However, the
ACI 440.1R-06 algorithm remains conservative due to an implicit strength-
reduction factor that offsets size effect and is also effective for large beams
(with effective depth limited to that in the research presented) with minimum
FRP shear reinforcement [35]. Figures 4.15 and 4.16 show setup and postmor-
tem crack patterns for a large-sized beam that failed in shear.

Punching shear. ACI 440.1R-06 proposes the following equation to
calculate the concentric punching shear capacity of FRP reinforced two-
way concrete slabs that are supported by interior columns or subjected to
concentrated loads, either square or circular in shape:

V = 10 f bc ′ (4.95)

c c o

4
5

[or V = f bc ′ in SI units]

c c o