Experimental investigation on failure mechanism of stepped footings rocking on bedrock

The gravity pier with a multiple-step footing is a common substructure system in railway bridges in China, particularly on bedrock. Rocking of this system benefits its seismic performance during the earthquake, but the cantilever part of the stepped footing is vulnerable since it is loaded with a large eccentricity, which has not been well studied. The present paper investigates the failure mechanisms and load capacity of two-layer stepped footing, based on two quasi-static tests and twenty finite element analyses, where four parameters in the stepped footing are studied: the longitudinal reinforcement ratio, the shear span to depth ratio, the critical contact area ratio, and the aspect ratio. In all investigations, the bottom step of the footings fails in flexure. The two quasi-static tests reveal that with the increase of the longitudinal reinforcement ratio in the footing, the load capacity at peak increases, and the damages at the footing are slighter, whereas the damages at the pier body become severe. The finite element models of the two test specimens are first built, and the validations show the finite element results agree well with the test results. The remaining finite element analyses further find that increasing the critical contact area ratio, increasing the aspect ratio, or decreasing the shear span to depth ratio significantly reduces the extent of damage to the footing and increases the load capacities at different damage states. Moreover, the investigations also find damages at the pier body are influenced by the four parameters studied. As the longitudinal reinforcement ratio or critical contact area ratio increases, the damages at the pier body become severe. Whereas, increasing the shear span to depth ratio or the aspect ratio results in slighter damages at the pier body.