1636 Design of FRP Axial Strengthening Systems

Concrete compression members can be strengthened to increase their axial load carrying capacity, their shear capacity, their steel rebar lap splice capacity, and their lateral load carrying deformation capacity (ductility). FRP strengthening of columns is most effective when applied to circular columns and always consists of complete wrapping to obtain confinement of the concrete. FRP strengthening systems for confinement of columns are classifieds as contact-critical applications. It is also important to note that FRP axial strengthening systems are regarded as ''passive'' systems. That is, they are not effective (or active) until the concrete reaches its transverse cracking strain in compression and begins to dilate, thus placing hoop stress on the FRP wrap. This is in contrast to the FRP flexural and shear strengthening systems, which must be active at all load levels.

For a nonslender, nonprestressed, normal-weight concrete column reinforced with spiral reinforcement and steel ties the nominal axial capacity is given as

Pn = 0.80(0.85Cff'c(Ag - Ast) + f,Ast) (16.89) respectively, with the confined compressive strength, f'c, given as

in terms of the confining pressure provide by the FRP wrap (or jacket), fl, given as f = fe (16.91)

where Ag is the gross area of the concrete, Ast is the area of the longitudinal steel, ka is an efficiency factor that depends on the shape of the column, and pf is the reinforcement ratio of the FRP strengthening system. For circular columns, ka is 1.0 and the reinforcement ratio is given as

Af 4ntf

where h is the diameter of the circular column. It is important to note that the fiber layers must all be oriented in the hoop direction around the column. If layers are also oriented in the longitudinal direction, these layers should not be considered to contribute to axial strengthening. For noncircular columns FRP strengthening for increasing axial capacity is much less effective due to stress concentrations at the corners (even when rounded) and the nonuniform confining pressure developed by the wrap.

Limits are placed on axial strengthening to ensure that the concrete does not approach its transverse cracking strain, or the steel its yield strain, in the service range (ACI 440.2R-02 2002).

The lateral displacement capacity (or ductility) of a concrete column can also be increased by confining it with FRP strengthening wraps. The maximum concrete compressive strain for an FRP wrapped circular column is given as

cc Ec where Ec is the elastic modulus of the concrete and fc is as defined previously (Equation 16.90).

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