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Seismic Performance of Prestressed Pile-To-Bent Cap Connections
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A portion of the bridge structures in South Carolina use a pile-to-bent cap connection detail in their bridge bents which relies completely on mechanical bond to develop a load path from the superstructure of the bridge to the foundation. Typical construction practices involve driving precast prestressed concrete piles to a specified depth and leaving a portion of the pile above ground to be used for an intermediate superstructure support. The portions of the piles above ground are leveled within tolerance, and a bent cap cage is constructed around them with a specified pile embedment depth into the bent cap. Once this is completed, the bent cap is cast-in-place. No dowel bars or brooming of prestressing strands are used in the pile-to-bent cap connection. As part of a SCDOT sponsored research project to study the seismic performance of precast prestressed concrete piles plainly embedded into cast-in-place bent caps, a full scale 3-pile bridge bent was constructed based on typical SCDOT designs and subjected to earthquake loading at the Large Scale Structures Laboratory at the University of Nevada, Reno (UNR). The objective of the simulation was to study the seismic behavior of the pile-to-bent cap connection within an axial load range of 100 kips of compression to 50 kips of tension. This axial load range was specified by the SCDOT as previous tests investigating the pile-to-bent cap connections were performed at unrealistically high compressive axial loads. Additionally, the seismic performances of two conventional bent cap reinforcing details were investigated. One bent cap design was based on typical details used by the SCDOT, and the other was a heavier reinforcing detail proposed by UNR. Several analytical models were created in SAP2000 in an attempt to accurately capture the response of the 3-pile bent observed during the experiment. Three main inelastic model types were explored: the fiber hinge, the coupled P-M hinge, and the multi-linear plastic link element. Of these three types, the nonlinear plastic link element produced the closest correlation between observed and calculated results. Based on observations made during the 3-pile bent experiment, it was determined that bridge bent designs using prestressed concrete piles plainly embedded into cast-in-place bent caps can be used in seismic areas. Reasonable axial tension forces do not adversely affect the performance of the connections. Both of the bent cap reinforcing details were shown to be conservative, and the choice between either detail should be left to the bridge designer.
Report No. CCEER-12-04