1623 Design Basis for FRP Reinforced Concrete

A design basis for FRP reinforced concrete has been recommended by a number of standards and professional organizations. The LRFD basis is recommended by the ACI 440.1R-03; however, at this time resistance factors are not probabilistically based. Load factors are those recommended for all concrete structures by ACI 318-95 (1995).

For the design of flexural members reinforced with FRP rebars the ACI 440.1R-03 recommends the following resistance factors:

Flexural capacity (tensile reinforcement only):

f — 0.5 for an underreinforced beam section (pf < pft) f — 0.7 for a substantially overreinforced beam section (pf > 1.4pfb) f — 0.5pf/pfb for a lightly overreinforced beam section (p^ < p < 1.4pfb)

Shear capacity (stirrups): f — 0.85 per ACI 318-95.

where pf is the FRP reinforcement ratio and pfb is the balanced FRP reinforcement ratio.

Characteristic strength (also called the guaranteed strength) and strain to failure of FRP rebars are defined as the mean minus three standard deviations of a minimum of 25 test samples. The design strength, ffu, and design failure strain, efu, are obtained from the characteristic strength and failure strain by multiplying them by an environmental reduction factor, CE, which depends on the fiber type in the bar and the type of intended service of the structure. For example, for weather exposed concrete with glass FRP rebars, CE is 0.7 (ACI 440.1R-03 2003).

Since FRP rebars typically have a lower modulus than steel rebars, the serviceability limit state (deflections and crack widths) can often control the design of FRP reinforced concrete sections. The ACI 440.1R-03 provides procedures for calculating deflections and crack widths in FRP reinforced members.

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