Comparison of non-linear analytical and experimental curvature distributions in two-column bridge bents
Creators: name: English, Daryl S.
Publisher: name: University of British Columbia
Year: 2009

description: The Ministry of Transportation and Highways of British Columbia has conducted a seismic assessment and upgrade initiative for many of its major bridges. Many deficiencies had been noted in the reinforced concrete approach bents of these bridges and the consequences of recent failures caused by earthquake loading have emphasized the need for retrofitting to be carried out on bridges with deficiencies. Since the costs of rehabilitating the bridge bents are significant, a scale model testing program was devised. Models of bents comprising the details of the approach spans of the Oak Street Bridge were cast and then subjected to slow cyclic lateral load testing. The specimens were instrumented externally with linear potentiometers, and internally using strain gauges bonded to the reinforcement. The objectives of the test program were primarily to confirm the seismic deficiencies in the as-built bents and to prove the adequacy of proposed economical retrofit schemes for twocolumn bridge bents. Particular to this thesis, the test program was also intended to produce data for further research that would contribute to the art of retrofit design. The strain gauge data obtained from the test program presented the opportunity to derive section curvatures at discrete locations wifJiin the specimens for various stages of loading. The data obtained from the strain gauges of the models were analyzed. Curvature distributions for two of the retrofit schemes that performed particularly well were derived. The distributions were integrated to give deflections which were then compared with the measured displacements. The errors in the calculated displacements ranged from -3% to +21%.Using the theoretical member properties, combined with the known material properties, analytical curvature distributions were derived using the non-linear analysis program DRAIN-2DX. The shapes of the distributions and the peak curvature values were the focus of interest and using moment-curvature relationships, estimates of peak concrete strains were predicted and compared with peak strain capacities. The strain capacities were derived from theory that accounts for the level of confinement provided by transverse reinforcement in a section. It was estimated that the architectural fillet region of the beamcolumn joint region, when in compression, was able to provide confinement enough to sustain concrete strains of the order of 0.013. The same fillet region retrofitted with high strength fiberglass wraps was estimated to be capable of ultimate concrete strains of approximately 0.027. The experimentally derived curvatures were then compared with those obtained analytically. It was found that the curvature distributions and the peak values compared reasonably well, which increased the confidence in the ability of the analysis to predict the flexural behaviour of retrofitted two-column bridge bents. The inclusion of joint shear deformations reduced the curvature demand in the plastic hinge regions and improved the agreement between the experimental and analytical curvatures. It is felt that the deterioration of the bond between the concrete and the reinforcement, caused by the cyclic nature of the tests, facilitated the derivation of reasonable approximations to the curvature distributions by reducing the tension stiffening effect on the reinforcement. This deterioration of the bond, particularly in the plastic hinge regions, decreased the variations of experimental curvatures occurring between cracks.