Vertical ground motions effects on balanced cantilever box-girder bridges during construction

Abstract

The effects of vertical ground motions (VGMs) on bridges have been studied mainly for completed structures. In contrast, the response of bridges during staged construction under the vertical earthquake component has not been investigated systematically. In this work, the seismic response of a two-span balanced-cantilever box girder bridge under VGMs in service and during three construction stages (50%, 75%, and 100% of the cantilever progress of each span) is studied. This bridge type is of interest because its mass and stiffness evolve during construction, reaching their highest values upon completion. The response of a 150-m-long bridge was analyzed using a computational model developed in the software Midas Civil. Multimodal response spectrum analyses were performed to determine internal forces in the girder and pier. The analyses were first conducted using horizontal (H) response spectra acceleration as defined in the Colombian bridge design Code and then including the vertical (V) component. The latter was defined based on V/H spectral relationships for far-field and near-field earthquakes available in the literature. The results show that the VGMs significantly increase the girder bending moment and the axial force in the pier during construction and service, with the largest variations occurring when the cantilever progress is 50%. This suggests that the VGM should be considered in the design of stage-construction bridges.