Civil Engineering Reference
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where
b
o
is the perimeter of the critical section for slabs and footings
(computed at
d
/2 away from the column face), and
c
is the cracked trans-
formed-section neutral axis depth.
Equation (4.95) corresponds to the basic ACI 318-11 concentric punching
shear equation for steel reinforced slabs modified by the factor 5/2
k
that
accounts for the axial stiffness of the FRP reinforcement as shown:
5
2
=
Vk
4
fbd
′
(4.96)
c
c
o
COMMENTARY
Ospina [36] demonstrated that the one-way shear design model proposed by
Tureyen and Frosch [34], which accounts for reinforcement stiffness, can be
modified to account for the shear transfer in two-way concrete slabs. Dulude
et al. [37] investigated the punching shear behavior of full-scale, interior,
GFRP RC two-way slabs. All slabs showed punching shear failure and similar
crack patterns, regardless of the reinforcement ratio. The slabs with low
reinforcement ratios showed some ductility and large deformation before
the punching shear failure. Both slab thickness and reinforcement ratio sig-
nificantly affected punching shear capacity. Figure 4.17 shows an example of
a GFRP RC two-way slab tested to simulate the effects of punching shear.
Figure 4.17a depicts the test setup showing the restraint at the perimeter of
the slab that would correspond to the load in a field condition. Figure 4.17b
displays the extensive crack pattern visible on the slab top surface, and the
third one portrays the punching cone after failure.
4.10.2 Shear reinforcement contribution,
V
f
The ACI 318-11 method used to compute the shear contribution of steel
stirrups is adopted by ACI 440.1R-06 to compute the contribution to the
shear capacity due to the FRP stirrups,
V
f
:
Afd
s
ffv
ffv
f
V
=
(4.97)
f
where
A
ffv
is the area of FRP stirrups within a spacing of
s.
The tensile
strength of FRP for shear design,
f
ffv
,
is calculated as
f
ffv
= 0.004
E
f
≤
f
ffb
(4.98)
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