Influence of Changes in Cross Section on the Effectiveness of Externally Bonded FRP Strengthening

There are many situations where strengthening might be required for a nonprismatic reinforced concrete section (i.e., a beam or slab where the depth of the section varies along its length). For example, many bridges in the United Kingdom have inadequate capacity to carry accidental vehicle loads on verges. These shallow depth verges are often cantilevered from the much deeper main bridge deck. The cantilever might be strengthened by applying fiber-reinforced polymer (FRP) composites to the top surface of the cantilever, extending transversely onto the bridge deck. However, a problem may exist with such a situation due to the potential for a dramatic reduction in the degree of strengthening which is achievable. This is due to the effects of cracking, and longitudinal shear stresses. Tests presented in this paper demonstrate that in regions where little or no cracking occurs, local or global debonding of the external FRP may result. Therefore, the strength of some nonprismatic beams, as predicted by current design guidelines, is often shown to be overly conservative and, in one case significantly unconservative. However, more importantly, the predicted failure modes and FRP strains often do not correspond to those observed. Advice on the best approach for analyzing these beams is given.