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Seismic Response of Precast Bridge Columns with Energy Dissipating Joints
Civil and Environmental Engineering
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Accelerated bridge construction (ABC) is attractive in congested urban areas and environmentally sensitive regions because it minimizes traffic delays and construction site safety risk. Precast bridge components are an essential for ABC to succeed. However, knowledge of the behavior and performance of precast bridge columns and their connections during earthquakes is lacking, and consequently their widespread use in high seismic hazard regions is yet to be realized. ABC in seismic areas requires particular attention to connections because of the need to dissipate energy. The purpose of this study was to develop precast column details that are able to dissipate energy under seismic loads. Several innovative precast concrete columns were designed, and studied experimentally on a shake table and analyzed. Two types of precast bridge columns were studied, including segmental columns and monolithic columns.The first part of the project included studying four segmental concrete cantilever column models with plastic hinges incorporating different advanced materials to reduce damage under earthquake loads. All the models were of one-third scale with longitudinal steel dowels connecting the base segment to the footing. Unbonded post-tensioning was used to connect the segments and to minimize residual displacements. Energy dissipation took place mostly through the yielding of the longitudinal bars in the base segment. The columns were tested on one of the shake tables at the University of Nevada, Reno and were subjected to the Sylmar hospital ground motion (Northridge, California earthquake of 1994) with increasing amplitudes until failure. One of the four column models constituted the benchmark case (SC-2). In this column conventional reinforced concrete detail was used in the base segment. The performance of other specimens having innovative materials in plastic hinges was compared with SC-2 to evaluate their merit relative to SC-2. The second specimen was a segmental concrete column incorporating an elastomeric bearing pad in the plastic hinge (SBR-1). The other two columns incorporated ECC (engineered cementitious composite) and unidirectional CFRP (carbon fiber reinforced polymer) fabrics in the lower two segments (SE-2 and SF-2), respectively. The purpose of using the elastomeric pad was to minimize damage while dissipating energy through yielding of the longitudinal bars and deformation of the rubber. Ductile behavior of the ECC resulted in less damage at the interface of the base and second segments in SE-2, and the column was able to sustain its lateral capacity under large drifts. The FRP wrapping provided confinement for the concrete and increased the displacement ductility capacity. The concrete damage in SF-2 was minimal and yielding of the longitudinal bars in the plastic hinge was more extensive. Compared to standard precast concrete segmental columns (those with no monolithic connection between the base segment and the footing), all specimens showed superior performance with minimal residual displacement and larger energy dissipation. The effectiveness of repair with CFRP wraps was also studied by repairing and retesting SC-2. The results showed that the strength and ductility capacity of the repaired model were improved compared to the original column, although the initial stiffness was lower. The relatively simple and effective repair procedure demonstrated that it is possible to quickly repair and restore the bridge.The second part of the project was testing and analysis of a 0.3-scale two-column bent incorporating two precast columns, precast footing, and a precast cap beam. Two openings were formed in the footing during the construction to allow for placement of precast columns. The embedment length was designed in such a way as to transfer the full plastic moment of the column to the footing. One column was built with conventional reinforced concrete, but incorporated ECC in the plastic hinge zone instead of concrete (RC-ECC column). The other column consisted of a GFRP (glass fiber reinforced polymer) tube with +/- 55-degree fibers filled with concrete (FRP column). The column-pier cap connection was a telescopic steel pipe-pin to facilitate construction. The bent was tested to failure, which was due to fracture of longitudinal bars in the RC-ECC column, and rupture of GFRP fibers in the FRP column. Test results showed that the embedment length was sufficient to develop the plastic moment completely in both columns. It was further found that the seismic performance of both columns was satisfactory and that the pipe-pin connections performed well in that they remained damage free, as intended.Comprehensive analytical models were developed using program OpenSees for all the test models and acceptable correlation was achieved between the measured and calculated data. This program is used for nonlinear dynamic analysis of structures using a variety of element models. The test results showed that the proposed models are suitable for accelerated bridge construction in high seismic zones (where large drifts are expected during earthquakes) because of their superior performance, such as fast construction, large energy dissipation, minimal damage in the plastic hinge zone and minimal residual displacement. Extensive parametric studies were performed to develop design methods for precast columns and to understand the influence of important factors on the capacity and performance of specimens. Seismic design methods for segmental columns and precast bent based on the test observations, measured data, and parametric studies were developed.