Seismic performance of Accelerated Bridge Construction (ABC) precast bulb-tee girder-to-bent cap connection.

摘要

With the total number of structurally deficient or functionally obsolete bridges at more than 20% of the nation's 607,380 bridges in 2013, the nation need to work effectively to decrease the total number below to 15% over the next decade. Accelerated Bridge Construction (ABC) techniques using the precast components are beneficial to effectively repair and replace the deficient bridges by reducing the period of onsite construction and improving the construction quality. However, the knowledge gap regarding the seismic performance of connections between the precast components limits the extensive implementation of ABC techniques in the moderate-to-high seismic regions of the country. Current Caltrans Seismic Design Criteria stipulates to degrade the connection between the precast girder and bent cap to a pinned connection under sever seismic event, which results in an inefficient bridge design. To overcome this issue, two innovative details for the precast bulb-tee girder to cast-in-place bent cap connection have been experimentally tested at Iowa State University. These two moment resisting connection details, named as the Extend Strand with a Mechanical Splice (ESMS) connection and the Extended Strand with a Lap Splice (ESLS) connection, consist of deck reinforcement, unstressed strands extended from the precast girder, and dowel bars grouted through the web of the girder as shown in following schematic. The deck reinforcement placed over the connection region provides the negative moment resistance. The strands spliced by mechanical splice chucks and lap splices within the ESMS and ESLS connection respectively develop the tension continuity to resist the positive moment, and the dowel bars are also designed to withstand the applied positive moments. Both connection details exhibited the adequate capacity to develop the moment resisting connection between the girders and the bent cap under target seismic demands. The testing results of this study reveal that the concrete crushing at the bottom of girder-to-cap interface induced the strength softening of the connection under the negative moments beyond the target value. For the connection behavior in the positive moment direction, the shear friction behavior was developed at the interface between the precast girder and the cast-in-place diaphragm poured around the girder, which provided the positive moment resistance. Meanwhile, the tension force generated by the strands extended from the girder contribute to the positive moment resistance as well. Based on the understanding of the behavior of the ESMS and ESLS connections exhibited within the experimental tests, an accurate but straightforward design methodology was then established to develop a guide to help with field implementation for these innovative connection details. The success in evaluating the seismic performance of the ESMS and ESLS connections confirms that both connections are adequate to develop the moment resisting connection for the precast bulb-tee girders subjected to the vertical and lateral loads and can be used within the routine bridge designs.