Flexural members - Reinforced Concrete with FRP Bars: Mechanics and Design - page 123

If the structure is subjected to fatigue regimes (i.e., repeated cycles of
loading and unloading), ACI 440.1R-06 recommends limiting the FRP
stress to the same threshold values adopted to prevent creep rupture, k creep-R
f fu . Equation (4.85) can be used with M s equal to the moment due to sus-
tained loads plus the maximum moment induced in a fatigue loading cycle.
4.10 SHEAR CAPACITY
When using FRP as shear reinforcement, one needs to recognize that FRP has
a relatively low modulus of elasticity, FRP has a high tensile strength and no
yield point, tensile strength of the bent portion of an FRP bar is significantly
lower than the straight portion, and FRP has low dowel resistance [22].
COMMENTARY
The use of FRP as shear reinforcement has to be further explored to provide
the foundation of a fully rational model to predict shear strength [31]. This
is also the case for a deeper understanding of the contributions of concrete,
aggregate interlock, and dowel effect of the longitudinal FRP reinforcement.
At present, most of the shear design provisions incorporated in existing
codes and guides are based on the design formulas of members reinforced
with conventional steel, considering some modifications to account for the
differences between FRP and steel reinforcement [32].
Compared with a steel reinforced section with equal areas of longitudinal rein-
forcement, a cross-section using FRP flexural reinforcement after cracking has
a smaller depth to the neutral axis because of the lower axial stiffness (that
is, product of reinforcement area and modulus of elasticity). The compression
region of the cross-section is reduced, and the crack widths are wider. As a
result, the shear resistance provided by both aggregate interlock and com-
pressed concrete is smaller [33].
The contribution of longitudinal FRP reinforcement in terms of dowel action
has not been determined. Because of the lower strength and stiffness of FRP
bars in the transverse direction, however, it is assumed that their dowel action
contribution is less than that of an equivalent steel area [33].
Recent research [31] showed that the presence of GFRP stirrups (similar to
steel stirrups) enhances the concrete contribution after the formation of the
first shear crack. The shear resistance is influenced by the spacing between the
stirrups. A small spacing contributes to enhance the confinement of the con-
crete, to control the shear cracks, and to improve the aggregate interlocking.