Design Approach for Calculating Deflection of FRP-Reinforced Concrete

Abstract

Deflection of reinforced concrete is typically computed with an effective moment of inertia Ie that accounts for nonlinear behavior after the concrete cracks. Existing expressions for Ie tend to overpredict the member stiffness of concrete reinforced with fiber-reinforced polymer (FRP) bars, and an alternative expression is used as the basis for developing a practical design approach to compute deflection. The proposed expression has a rational basis that incorporates basic concepts of tension stiffening to provide a reasonable estimate of deflection for both steel and FRP-reinforced concrete without the need for empirically derived correction factors. Calculation of deflection with the proposed expression for Ie is recommended using the code value for the elastic modulus Ec of concrete because computed values of deflection are relatively insensitive to variations in Ec, and shrinkage restraint is taken into account by using a reduced cracking moment less than the code-based value of the cracking moment Mcr. Ie is conservatively based on the moment at the critical section (where the member stiffness is lowest), unless more accuracy is required with an integration-based expression that gives an equivalent moment of inertia Ie′ to account for the variation in stiffness along the member length. Recommendations are validated by comparison with a database of deflection test results for FRP-reinforced concrete.