PEER has just published Report No. 2015/10 titled “Structural Behavior of Column-Bent Cap Beam-Box Girder Systems in Reinforced Concrete Bridges Subjected to Gravity and Seismic Loads, Part II: Hybrid Simulation and Post-Test Analysis”
This report presents the second half of the discussion of a mixed experimental-computational study that aimed at investigating the structural and seismic behavior of integral bent caps in reinforced concrete (RC) box-girder bridges. The main objective of the study was to accurately estimate the contributions of the deck and soffit slabs framing into the bent cap in as-built and retrofitted RC bridge systems under the combined effect of vertical and lateral loading. In particular, the study estimated the effective flange width of the bent cap beam due to the box-girder slabs contributions for more accurate and effective consideration of the stiffness and capacity of a bent cap. Computational and experimental methods were utilized to investigate the problem at hand. The finite element (FE) 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 sub-assemblies using quasi-static and hybrid simulation (HS) testing methods. This report presents the results of HS tests conducted on the second specimen and post-test analysis, which involved calibration of the FE model of the specimen and extended parametric studies at the specimen and full prototype bridge levels. The pre-test analysis phase and the first part of the experimental program that involved cyclic loading testing of the first specimen are discussed in the companion report (Part I of the companion report).
To conduct the HS tests, this study developed a new practical approach that utilized readily available laboratory data acquisition system as a middleware for feasible HS communication. The proper communication among the HS components and 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 entire HS system. A retrofit scheme was adopted for the second specimen that used a carbon fiber reinforced polymer (CFRP) column jacket before any HS testing was begun to study the behavior of the bent cap at higher moment demands 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 bi-directional 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 FE 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 at the specimen level, 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. 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 prototype bridge is presented. To optimize the box-girder geometry for the most efficient slabs contribution to the bent cap structural behavior, another parametric study was conducted of the entire bridge level to investigate the effect of the box-girder geometry on the slab contributions and the bent cap effective width.