Seismic shear strength of circular bridge piers.

Abstract

The project was carried out with the intention to study the shear strength of circular reinforced concrete bridge piers under seismic loading. Two series of tests were conducted. Initially, twenty-five column units were tested by subjecting them to static incremental reversed cyclic loading, to investigate the influence of some main parameters. The columns were loaded into the inelastic range to control­ led displacement ductility levels. The second stage of experimental work involved dynamic testing of bridge piers, which were half scale models of the static test units on a shake-table. Altogether eight single pier models and two twin-pier models were tested. The single pier models were subjected to sinusoidal excitation while the twin-pier models were tested using scaled earthquake accelerograms.

The performance of the test units was gauged mainly in terms of shear strength and displacement ductility capacity. Four failure modes were identified according to the displacement ductility level at which significant degradation occurred. The static test results indicated that existing code provisions for shear strength were conservative and suggested that the level of shear strength and the displacement ductility might be related.

The behaviour of single pier models in the dynamic tests was compatible with that of the static test units. The behaviour of the twin-pier models was less predictable, especially when axial tension was acting. The dynamic magnification effect on material strength due to higher strain rate was not significant in the tests.

A design method was proposed as an outcome of the static tests. The proposal allows the shear strength and the displacement ductility capacity to be determined, and has been incorporated into an integral flexure/shear design approach in which the provision of transverse reinforcement is considered for confinement as well as shear resistance.

Some theoretical study was also conducted using 'Diagonal Compression Field Theory'. The theory was adapted using a stress­ strain relationship developed for confined concrete. The agreement between the predicted and the experimental behaviour in terms of load­ displacement response was reasonable.