Emulative precast bent cap connections for seismic regions: component tests - preliminary grouted duct specimen (unit 5)

Andrew G. Wilson

This report, Emulative Precast Bent Cap Connections for Seismic Regions: Component Tests—Preliminary Grouted Duct Specimen (Unit 5) (PGD), is a supplemental report to a series of four reports summarizing the California State University, Sacramento (CSUS) component tests supporting National Cooperative Highway Research Program (NCHRP) Project 12 74, “Development of Precast Bent Cap Systems for Seismic Regions”. CSUS investigated two emulative connections—cap pocket and grouted duct—for nonintegral precast bent caps. The test objectives of the PGD specimen were to examine the behavior of an additional grouted duct specimen for comparison to the Grouted Duct (GD) and Cast-in-Place (CIP) specimens, designed using the 2006 Recommended LRFD Guidelines for the Seismic Design of Highway Bridges. Design provisions were less conservative than current 2009 AASHTO Guide Specifications for LRFD Seismic Bridge Design—including a smaller area of vertical stirrups within the joint and smaller area of bent cap longitudinal reinforcement. In contrast to the GD specimen, the PGD used column longitudinal reinforcement with a 3.1% vs. 1.6% ratio (#7 vs. #5 bars) and with the bars offset within the ducts. In addition, the PGD used a full-scale 2.5-in bedding layer with steel shims and a reinforcing hoop, rather than the scaled 1.5-in bedding layer without shims or reinforcement. Additionally, the PGD was used to verify the performance of the test setup, actuator control, data acquisition system and instrumentation. Specimen response was dominated by plastic hinging of the column adjacent to the bent cap. At ultimate, column bars buckled on both the north and south sides of the column. The specimen exhibited excellent ductility to a large average drift of 5.4%, which corresponded to a nominal displacement ductility of 8, without appreciable strength degradation. Diagonal joint cracking was more extensive than other specimens, but only minor joint spalling occurred and bar slip was minimal. The bedding layer remained intact and performed integrally with the specimen. Displacement decomposition confirmed the dominance of plastic hinging. Although the flexure/shear displacement component ratio averaged only 5.5 and limited joint softening was evident, column bars were well anchored within the joint. The use of additional joint reinforcement required for development of a force transfer mechanism was justified, and strain records confirmed the effectiveness of hoop confinement in the joint. Conclusions include: 1) emulative performance was achieved for the PGD; 2) use of a large 3% column reinforcement ratio did not unacceptably affect system performance; 3) column bar offset did not affect bar anchorage; 4) the full-scale bedding layer remained intact through testing and produced integral performance; 5) steel shim stacks within the bedding layer did not produce unusual behavior in the joint or specimen; 6) use of hoop reinforcement within the bedding layer helped achieve integral performance and overall system ductility; and 7) additional analysis is required to develop a new model that fully characterizes joint behavior, including joint forces, crack patterns, and strain distributions. Finally, the test setup, actuator control, data acquisition system and instrumentation were verified and improved through testing to benefit testing of NCHRP specimens.

Emulative precast bent cap connections for seismic regions: component tests - preliminary grouted duct specimen (unit 5)

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