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238 COMMENTS ABOUT PERMANENT DISPLACEMENTS The parametric analysis showed that the GRS abutments sustained small settlements (less than 5 cm) while sustaining very significant permanent lateral displacements following Kobe and Northridge earthquakes. Up to 20 cm lateral displacements at the top of some of the GRS abutments were calculated following the application of the Northridge earthquake. It is important to note, however, that in all parametric analysis when one of the two abutments deformed forward (in the longitudinal direction of the bridge), the other abutment, on the opposite side of the bridge, deformed backward--i.e., the two abutments along with the bridge superstructure deformed in a near "simple shear" manner. Likely, this type of deformation does not exert significant additional stresses in the bridge during an earthquake- -the bridge girders and two abutments move in unison. With the bridge superstructure being safe and stable, the permanent deformations of the GRS abutments are deemed acceptable following destructive earthquakes such as Kobe and Northridge. This observation is only relevant to loading conditions similar to the one used in the present parametric study in which the earthquake motion was assumed to be in the longitudinal direction of the bridge. In the parametric analysis a 7.5-cm wide expansion joint is assumed at both ends of the bridge. These expansion joints were set up to serve two purposes: (1) allow for thermal expansion of the single span bridge, and (2) allow the bridge to oscillate horizontally via the bearing (elastomeric) pads. The elastomeric pads can deform laterally up to 7.5 cm in any horizontal direction (in extreme load cases) without loss of functionality. The 7.5 cm expansion gaps allow for this to take place. During the parametric analysis, special attention was paid to the expansion gap width on both sides of the bridge. All analyses revealed that the gaps were always "open" during seismic analysis (i.e., gap width>0), indicating that the bridge was never in contact with the abutment back wall.
