Ultimate design capacity of bridge abutments

Under the large forces transmitted by a bridge structure during an earthquake, a bridge abutment may undergo significant displacements deriving from the mobilisation of both the soil and the structural strength. In some cases a controlled yielding of the abutment may produce favourable effects, dissipating seismic energy and limiting the seismic actions into the superstructure. In fact, the bridge design could explicitly account for dissipative features of the soil-abutment system, allowing a certain amount of irreversible displacements compatible with a prescribed performance level. The present paper focuses on the plastic response of the soil-abutment system, examining its potential plastic mechanisms. With the aid of numerically-evaluated limit analysis solutions, different plastic mechanisms are identified, including those associated with combined yielding of the soil and the abutment structure. On the basis of the above results, the paper presents a model describing the capacity surface of bridge abutments subjected to multi-axial loading conditions, that can be identified through the calibration of a limited number of parameters. It is shown that the inertial effects associated to the seismic loading can be incorporated into the model through a contraction and a rotation of the ultimate limit surface. The proposed model can be either included in a plasticity-based macro-element to model the bridge abutments, or it can be used for a direct evaluation of abutment in a force-based approach.