Document Type : Seismology and Engineering Seismology
Department of Earth Sciences, University of Trieste
ICTP and University of Trieste
Frequently long-span bridges provide deep valley crossings, which require special consideration due to the possibility of local amplification of the ground motion as a consequence of topographical irregularities and local soil conditions. This does in fact cause locally enhanced seismic input with the possibility for the bridge piers to respond asynchronously. This introduces special design requirements so that possible out-of-phase ground displacements and the associated large relative displacements of adjacent piers can be accommodated without excessive damage. Assessment of the local variability of the ground motion due to local lateral heterogeneities and to attenuation properties is thus crucial toward the realistic definition of the asynchronous motion at the base of the bridge piers. We illustrate the work done in the framework of a large international cooperation to assess the importance of non-synchronous seismic excitation of long structures. To accomplish this task a complete synthetic accelerogram dataset was computed by using as input a set of parameters that describes, to the best of our knowledge, the geological structure and seismotectonic setting of the investigated area. The results show that lateral heterogeneities can produce strong spatial variations in the ground motion even at small incremental distances. In absolute terms, the differential motion amplitude is comparable with the input motion amplitude when displacement, velocity and acceleration domains are considered. Thus, on the base of the existing empirical regression relations between Intensity and peak values of ground motion a general result of our modeling is that the effect of the differential motion can cause an increment greater than one unit in the seismic intensity experienced by the bridge, with respect to the average intensity affecting the area where the bridge is built.