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Structural Behavior of Bent Cap Beams in As-built and Retrofitted Reinforced Concrete Box-Girder Bridges

  • Author(s): Moustafa, Mohamed Aly Abdel-Razik
  • Advisor(s): Mosalam, Khalid M
  • et al.
Abstract

Research on resilient infrastructure systems is expanding. As we experience more infrastructure deterioration in the US, numerous efforts are ongoing for building the nation's new infrastructure and maintaining the existing one. Bridges are key components of infrastructure that are vulnerable to earthquakes and are undergoing retrofit or complete replacement. Thus, optimized seismic design of new bridges and informed retrofit decisions are indispensable. A specific design issue that is concerned with the structural response of bent cap beams in as-built and retrofitted box-girder bridges under gravity and seismic loads is tackled in this dissertation. The lack of proper account of box-girder slabs contribution to the integral bent cap can lead to an uneconomical seismic design of new bridges or unfavorable mode of failure in retrofitting existing ones. A combined experimental and computational research was undertaken in this study to investigate the structural behavior and seismic response of bent cap beams in as-built and retrofitted reinforced concrete box-girder bridges under the combined effect of vertical and lateral loading. In particular, the contribution of the box-girder slabs to the stiffness and strength of the integral bent caps was evaluated for optimized design and enhanced capacity estimation. The computational part of the study consisted of two phases: pre-test and post-test analyses. The experimental program involved testing two 1/4-scale column-bent cap beam-box girder subassembly using quasi-static and Hybrid Simulation (HS) testing methods. The test specimens were adopted from a typical California bridge that is modified from the Caltrans Academy Bridge, and were designed in light of the most recent AASHTO and Caltrans provisions.

The pre-test analysis phase of the computational research utilized one-, two-, and three-dimensional finite element models to carry out different linear and nonlinear static and time history analyses for both of the full prototype bridge and the test specimen. The pre-test analysis successfully verified the expected subassembly behavior and provided beneficial input for the experimental program. The first stage of the experimental program involved quasi-static cyclic loading tests of the first specimen in as-built and repaired conditions. Bidirectional cyclic loading tests in both transverse and longitudinal directions were conducted under constant gravity load. A rapid repair scheme was adopted for the tested specimen using a Carbon Fiber Reinforced Polymer (CFRP) column jacket. A similar quasi-static cyclic test to the as-built specimen was carried out for the repaired specimen for comparison purposes and to verify the essentially elastic status of the bent cap beam. The second stage of the experimental study embraced the HS testing technique for providing the lateral earthquake loading to the test specimens. A new practical approach that utilized readily available laboratory data acquisition systems as a middleware for feasible HS communication was achieved as part of this study. The proper communication among the HS components and the verification of the HS system were first performed using tests conducted on standalone hydraulic actuators. A full specimen HS trial test was conducted using the previously tested repaired specimen to validate the whole HS system. The last phase in the experimental program involved retrofitting the column of the second specimen using CFRP jacketing before any testing to increase the demands on the bent cap beam for further investigation into its inelastic range of structural response. The retrofitted second specimen was then tested using multi-degree of freedom HS under constant gravity load using several scales of unidirectional and bidirectional near-fault ground motions.

The post-test analysis was the final stage of this study. The results from the as-built first specimen cyclic tests were used to calibrate the most detailed three-dimensional finite element model, which was previously developed as part of the pre-test analysis stage. The calibrated model was used to explore the effect of reducing the bent cap reinforcement on the overall system behavior and to investigate the box-girder contribution at higher levels of bent cap seismic demand. Based on the computational and experimental results obtained in this study, the effective slab width for integral bent caps was revisited. The study concluded that the slab reinforcement within an effective width, especially in tension, should be included for accurate bent cap capacity estimation. The study was finalized with an illustrative design example to investigate the design implications of the revised effective slab width and bent cap capacity estimation on the optimization of the bent cap design for a full-scale bridge.

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