SEISMIC RESPONSE OF FRICTION SUPPORTED BODIES ESPECIALLY GIRDER BRIDGES

Abstract

Analysis has first been developed for evaluating the sliding and rocking responses of rigid objects resting on plane surfaces under combined horizontal and vertical compo nents of earthquake motion and then extended to the problem of multi-span elastic girder bridges with conventional and unconventional bearings. The horizontal ground motion has been considered in one direction in all the problems. Girder bridges have been analysed for the seismic motion acting in the longitudinal direction of the bridge. In the analysis of rigid objects the resistance to sliding motion has been expressed by Coulomb friction with a constant value in either direction. The energy loss in case of rocking motion has been characterised by the coefficient of restitution. The bridge problem could be reduced to the analysis of a single span by approximating the effect of the adjoining spans. The degrees of freedom of the system have been further reduced drastically by building it up from the elastic modes c£ the girder and the substructures constituting the span, while retaining the non-linear characteristics of the bearings. Isolation of the superstructure from the substructure has been attempted by introducing roller or sliding bearings at both ends of a span. Effect of hydraulic dampers at the expansion bearings has also been investigated. Girder bridges have shown indications of jumping of girders on the bearings during earthquakes. Vertical acce leration response much more than that due to gravity have been observed in the laboratory on models subjected to horizontal ground motion alone. With the mathematical model and analysis developed in the thesis it has been possible to explain this dynamic phenomenon. Use of roller bearings in parallel with small capacity dampers at both ends end a soft spring at one end of a bridge girder has been found to reduce the horizontcl inertia force on the girder by more than 95% of that in the conventional rocker-roller arrangement. The vertical acceleration response of the girder also gets reduced by about 50% thus reducing the stresses end also greatly improving their stability against jumping. The dampers are found to be very effective in limiting the rolling motion at bearings to the same order or even less than in the conventional bearing case. However, the substructure's horizontal displacement response does not reduce significantly in case of bridges with heavy caisson foundations, elthough a reduction of about 70% over the conventional bearing arrangement occurs in case of bridges with light substructures. Hence this isolation system reduces the seismic forces in all the elements of the girder bridges thereby providing an economical way of making them earthquake resistant. Whereas it is essential to provide external dampers in case of both end roller bearings, they are of little use for the case of conventional bearings as well as both end sliding be arings.