Seismic Retrofitting Of Concrete Bridge Columns By External Prestressing

Due to recent earthquakes and the ensuing damage, there has been some concern regarding the adequacy of older structures. In particular, there is apprehension as to the adequacy of bridges that were erected prior to the early 1970's. Before this time there was little or no design process which considered seismic behaviour of bridges. Recent codes have been adjusted to compensate for these earlier problems. However, it is not feasible to remove and rebuild all of the previously built bridges. Hence, the only remaining viable approach to improving seismic adequacy of existing bridges is seismic upgrading or retrofitting these structures to improve their strength and/or deformability. The research program presented in this thesis addresses the problem of reinforced concrete bridge column retrofitting. More specifically, the applicability of a new retrofitting technique, developed at the University of Ottawa for columns of different aspect ratios and reinforcement arrangements, will be verified in terms of concrete confinement and shear resistance. The retrofitting technique consists of wrapping and prestressing high strength steel 7-wire strands around the test columns in order to increase their ductility. The specimen's tested were based on pre-1971 design, and hence had larger transverse reinforcement spacing than the present practice. This results in a lack of concrete confinement which translates into columns with poor shear resistance which tend to have a brittle failure mode. By retrofitting with prestressing strands, placed at closer spacing than the original hoops, the concrete confinement is increased, diagonal tension resistance is improved and a more ductile response is attained. Experiments from the first phase of this research program, performed by Yalcin (1997), gave very favourable results for a given aspect ratio and reinforcement arrangement. The present experimental research, forming the second phase, examined columns with different aspect ratios and reinforcement arrangements and also gave favourable results.