Members subjected to combined axial load and bending moment - Reinforced Concrete with FRP Bars: Mechanics and Design - page 178

A fv is the area of FRP ties with a spacing of s (or spiral with a pitch of s ).
For FRP RC columns, the spacing is limited to the least dimension of the
column, 12 longitudinal bar diameters or 24 tie bar diameters. Other
parameters are only briefly discussed herein as they are similar to flexural
members. The tensile strength of the FRP tie for shear design, f fv , is calcu-
lated as

f fv = 0.004 E ffv ≤ f fb (5.61)

where E fv is the modulus of elasticity of the FRP ties. To prevent loss of
aggregate interlock in the concrete as the result of a wide crack, the maxi-
mum usable tensile strain of the FRP ties is limited to 0.004. f fb , strength of
the bent portion of FRP tie, depends on r b / d b , the ratio of the internal radius
of the bend to the diameter of the tie, which has to be at least equal to 3,
the minimum recommended by the ACI 440.1R-06 guidelines. f fb is given
by the following expression:

r
d

b

f = 0.05 + 0.3 f ≤ f

(5.62)

fb ffu ffu

b

where f ffu is the tensile strength of the straight portion of the GFRP tie.

5.9.3 Shear strength-reduction factor
Jawaheri Zadeh and Nanni [32] showed that the current shear strength-
reduction factor of 0.75 may be maintained so long as the maximum effec-
tive level of shear resistance of a member with FRP shear reinforcement
does not exceed four times the strength provided by concrete ( V n ≤ 4 V c
or, equally, V f ≤ 3 V c ). Accordingly, the shear strength-reduction factor for
columns is 0.75.

5.9.4 Examples—Shear strength for square columns
Example 5.2

Calculate the design shear strength of the following square column
with no P u effect:

Concrete: f´ c = 4.0 ksi

E c (ksi) = 57 √ f ′ c (psi) = 3600 ksi

Longitudinal GFRP reinf.: f ffu = f ffd = 60 ksi

E f = 6000 ksi

Continued