Soil-structure interaction for the seismic design of integral abutment bridges: from advanced numerical modelling to simplified procedures

Integral abutment bridges are characterised by a monolithic connection between the deck and the abutments. Because of this connection, their behaviour during a seismic event is controlled by the interaction of the entire structure with the surrounding soil, and markedly with the approach embankment. Although this is becoming a popular design solution due its low maintenance requirements, procedures for the seismic design are still characterised by substantial uncertainties, mostly because of a lack of comprehension of the dynamic response of the soil-structure system. This study provides a contribution to the interpretation of the seismic behaviour of integral abutment bridges, focusing on a single-span structural scheme type that has received significant attention in recent years. The dynamic interaction between the bridge and the soil is studied with global numerical models of the soil-bridge systems developed in OpenSees and subjected to a variety of ground motions. The results of these dynamic calculations, interpreted also with the aid of a modal analysis of the system, are used to develop simplified design procedures aimed at evaluating the deformation and the internal forces in the structure. In details, a novel seismic design approach based on a nonlinear static analysis is provided for the longitudinal component of the seismic motion, that typically dominates the design of this type of bridges. In addition, a second simplified procedure is developed to take into account the influence of the transverse component of the seismic motion as well as of the wing walls. Two-directional time-domain analyses carried out on the full soilstructure model show that these complementary procedures lead to a seismic design of these bridges without any considerable underestimation. Finally, the possibility to use an average response spectrum prescribed by technical provisions makes this approach immediately applicable to the ordinary design.