Flexural members - Reinforced Concrete with FRP Bars: Mechanics and Design - page 108

Table 4.5 Standard bends of ASLAN 100 GFRP bar
4.8 SPECIAL CONSIDERATIONS
4.8.1 Multiple layers of reinforcement
Because FRP reinforcement is linear-elastic to failure, when multiple layers
of FRP reinforcement are used, rupture of the bars at the outermost layer
controls overall reinforcement failure; in such a case, the strain compat-
ibility approach should be used to determine the flexural resistance of the
member. If different FRP bars (in terms of size or material) are used in a
multiple-layer configuration in the same concrete component, failure may
not occur in the outermost layer of reinforcement; in fact, failure will occur
in the FRP bar that first reaches its ultimate tensile strain irrespectively of
its position within the concrete cross section. This latter case, however, has
little significance as concrete sections with different FRP bars have rare, if
any, practical application.
The methodology followed with traditional steel reinforcement of locating
the centroid of the layers of steel bars and determining a result of magni-
tude equal to the total area times the yield strength cannot be used with
FRP bars because each layer of FRP reinforcement is subjected to different
levels of strain and, therefore, different values of the tensile stress due to the
linear-elastic behavior of the FRP reinforcement. Nonetheless, the strain
compatibility approach should also be used with traditional steel reinforce-
ment if the strain in one of the layers does not reach the yielding strain.
A rectangular cross section with multiple layers of FRP reinforcement
as indicated in Figure 4.11 is selected. Assume that the FRP reinforcement is
made of the same material and bar size so that failure of the reinforcement