Design for FRP-Based Structural Strengthening: How Safe Is Safe Enough?

Abstract

FRP-based structural strengthening has emerged as a popular approach for rehabilitating the distressed and underperforming concrete structures. Design of FRP-based structural strengthening systems involves an interesting interplay of uncertainties between those inherent in the existing structure being strengthened and those arising from the lack of complete knowledge and time testimony of using FRP composites for structural strengthening. The safety formats prescribed by most strengthening design standards remain largely cloned from that used for the design of new concrete structures, with a set of safety factors which are significantly higher (up to 2–3 times) than the conventional norm in design of concrete structures. These higher safety factors are proposed to account for the additional uncertainties associated with FRP composites. However, FRP composites in general, and their use as externally bonded reinforcement for strengthening in particular, involve substantially peculiar characteristics compared to reinforced concrete (RC). Therefore, the design processes for strength (for new RC structures) and additional strength (for strengthening existing RC structures) have conflicting design requirements and objectives. Under these conditions, a blind extension of the conventional safety format to design for FRP-based structural strengthening will not only fall short in providing the required safety margins, but could also instigate negative implications and undesirable side effects under some design scenarios. This paper highlights important sources of conservatism in strengthening design arising from the design treatments of uncertainties and mechanics of FRP, and their implications on the course and quality of strengthening design solutions and that on their resultant safety contents are discussed.