Effects of near-fault earthquakes on existing bridge performances

The vertical component of seismic acceleration, often overlooked in ordinary structures, plays a role of primary importance in the case of bridges and viaducts. In particular, it induces both the appearance of uncommon stress conditions on vertical structures, and in some cases, it is a really important factor for bearing device capacity of girders. In fact, seismic excitations may give rise to great relative displacement between deck and piers or abutment in bridges. Among many structural damages of bridges during past earthquakes, the unseating failure is one of the most severe and recurring damages of girder bridges. When relative displacements exceed a pre-assigned seating length, the unseating of span will then take place. Therefore, for seismic design of new bridges or for a check of existing bridge, to take into account the vertical component due to seismic acceleration is an important issue. This paper presents a numerical analyses about damage effects of near-fault seismic events on existing bridge performances. The near-fault earthquakes are characterised by own some fundamental characteristics, such as forward-directivity phenomena, relatively high acceleration amplitudes and elastic response spectra, which are very different with respect to the reference ones defined in the codes. With this background, the purpose of this paper is to highlight the role of this kind of analysis of understanding the response behavior of girder bridges subjected to near-fault earthquakes. Furthermore, a case study and parameter studies are performed to evaluate its effectiveness in preventing bridge spans from unseating failure and protecting the piers and the abutment of bridges from damage.