Effect of soil-structure interaction on the dynamic identification of a prestressed concrete bridge

In recent years, safety assessment of existing bridges is becoming an urgent and critical issue in light of the increased number of collapses observed worldwide, most of which related to the high vulnerability of the structure itself due to material (concrete or steel) degradation over the years. In seismically-active areas, vulnerability of existing bridges might combine to the high seismic hazard of the site. The fatal consequence is that after a strong earthquake, total or partial interruption of the road infrastructure could occur with the bridge behaving as the weakest component of the chain. In order to shed light on the seismic response of existing r.c. bridges, the paper illustrates the numerical procedure developed by the authors to investigate the dynamic response of a pre-stressed reinforced concrete bridge placed in the highly seismic area of Benevento (IT). The paper investigates the effect of soil-structure interaction on the dynamic behavior of the bridge. A detailed 3D model representing the structure and the soil deposit was developed and solved through the finite element method with the software MIDAS FEA NX. The use of a 3D continuum approach also for the subsoil was motivated by the huge stratigraphic and topographical variability characterizing the soil deposit. After the calibration of the soil model, the bridge was added. At this stage of the work, the structure was modelled through solid elements and a perfect contact was imposed between the piers and the soil. A modal analysis was conducted on the complete model made of both soil and bridge to get the natural oscillation frequencies and mode shapes of the coupled system. These results were later compared to those provided by the analyses in which the superstructure was modelled with a fixed base or a compliant base with springs.