Strengthening of critically designed girders with dapped ends

Reinforced concrete (RC) beams with dapped ends are frequently found in bridge girders and precast concrete construction. A reduction in depth near the supports tends to produce a stress concentration and hence requires special analysis and detailing of any reinforcement used. Improper dimensioning and distribution of reinforcing steel can lead to undesirable cracking and failure mechanisms that need to be predicted in order that a proper strengthening system be applied. The experimental programme described in this paper incorporated 52 specimens with dapped ends; three main defects were intentionally introduced at the recess zone and 12 different strengthening techniques were applied. The defects included an inadequate development length of bottom longitudinal reinforcement at the dapped ends, and elimination of either horizontal or vertical shear reinforcement at the ends. External bonding of the steel angle at the reentrant corner, unbonded bolt anchoring, external steel plate jacketing, exterior carbon fibre wrapping and/or stripping were applied in order to determine the best strengthening technique. A strut-and-tie model was used in the analysis of such discontinuity regions and a strength enhancement index procedure was introduced. A comparative study was undertaken in order to evaluate the various strengthening techniques applied.