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CCEER-20-06: Development And Seismic Evaluation Of Cast-in-place Emulating Abc Pocket Connections And Sma Reinforced Two-way Column Hinges
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Utilization of precast members in bridge construction is also known as accelerated bridge construction (ABC) and may be preferable to conventional cast-in-place (CIP) method because it can significantly expedite project delivery. The design of precast member connections is critical as they must maintain structural integrity and have ductile behavior in regions of moderate and high seismic activity. Among different connections for ABC, pocket/socket connections are preferred because they allow precast members to be inserted into adjacent members uninhibited eliminating the need to thread bars through the column cage, which can be time consuming and cause congestion. The primary goal of this study was to evaluate the performance and viability of a CIP column-cap joint that was designed using existing ABC pocket/socket connection design guidelines. Depending on the performance of the connection, this alternative connection may be used to simplify and expedite CIP construction as well. Another objective of the study was to determine the seismic performance of a new generation of shape memory alloy (SMA) bars. Bridge designers may be inclined to use SMA bars within critical regions of a column to reduce residual drifts due to the superelastic characteristic of SMA. Reducing the residual drifts of structures may minimize damage and allow structures to remain in service after seismic events. Due to the significantly higher costs of SMA bars relative to traditional steel bars, SMAs have only been implemented in critical regions of bridge columns in past studies. Recent research has shown that post-earthquake serviceability is significantly enhanced when SMA is combined with ductile types of concrete in plastic hinges. As a secondary objective of this study, the performance and viability of two-way hinges reinforced with Copper-Aluminum-Manganese (CAM) shape memory alloy (SMA) bars coupled to traditional rebar using headed rebar couplers was evaluated. To achieve the primary goal of the study, a CIP emulating ABC pocket connection design was developed and implemented in a 0.33-scale test model. The test model was a two-column bent, wherein one of the column-cap connections utilized the novel CIP pocket connection and the other utilized a traditional ABC pocket connection. Both connections were designed identically according to existing guidelines for ABC pocket connections. Auxiliary transverse cap beam reinforcement was placed around the connections to reduce joints stresses and address prying forces. The secondary objective of the study was achieved by implementing two-way hinges reinforced with CAM bars at the base of both columns. CAM bars were attached to #4 (Ø13 mm) steel bars at both ends using headed bar couplers and were placed at the two-way hinges at the base of each column. As CAM bars can only be manufactured to limited lengths, are smooth, and are expensive relative to traditional steel bars, they were connected to steel bars using headed bar couplers to ensure sufficient development and anchorage. Before the design of the bent was finalized, a nonlinear analytical model of the bent was created in OpenSees. The response history of the analytical model under various ground motions was analyzed to develop the shake table testing protocol and to ensure the bent could be tested to failure. The test specimen was instrumented with strain gauges, displacement transducers, accelerometers, and cameras to measure local and global responses. The experimental study involved shake table testing of the test bent. The specimen was subjected to multiple runs of simulated, scaled versions of the 1940 Imperial Valley-02 earthquake event recorded at the El Centro Array #9 Station. The displacement history and peak curvatures of the columns obtained from the experimental study showed the bent reached a drift ratio of 11.2 percent, and the connection was effective in forming the plastic hinge in the column while the joint remained damage-free. The strains and curvatures of the connections and columns were comparable up until the final two runs, where the CIP connection had smaller longitudinal bar strains due to its lower stiffness. Capacity- protected behavior was also observed in the cap beam for both connections as the cap beam reinforcement around the connection remained elastic. The CAM reinforced two- way hinges performed well under seismic loading as they maintained the integrity of the column bases with substantial yielding and energy dissipation. Post-test analytical studies of the bent were conducted by developing a more accurate representation of the bent in OpenSees by using actual material properties and shake table motions. Comparison of the calculated and measured results showed that a relatively routine OpenSees model can be used to obtain an approximate response for moderate and strong earthquakes of up to approximately twice the design earthquake. Finally, the design of CIP pocket connections emulating ABC was evaluated in light of the measured seismic performance of the bent.
Report No. CCEER 20-06