Seismic response characteristics and whiplash effect mechanism of continuous rigid-frame bridges subjected to near-fault ground motions

Abstract Continuous rigid-frame bridge (CRFB) is widely constructed in western China with high seismicity areas. To investigate the seismic response characteristics and whiplash effect mechanism of CRFBs under near-fault ground motions, this study selects a long-span CRFB with high piers as the prototype bridge and develops the nonlinear finite element model based on OpenSees. In this work, three groups of near-fault ground motions having forward directivity pulse, fling-step pulse and non-pulse are selected as seismic inputs. These records are intercepted using significant duration index and scaled to 0.2 g, 0.4 g, and 0.64 g, representing basic ground motions, frequent ground motions and rare ground motions, respectively. The study analyzes the seismic response characteristics of CRFBs and discusses the effects of bearing constraints, ground motion components and vertical excitations on the seismic responses. The numerical results show that the longitudinal vibration, transverse whiplash effect and vertical uplift behavior of main girder are main deformation characteristics of CRFBs. Compared with non-pulse earthquakes, the structural displacements, lateral drift angles, bearing deformations, internal forces and pounding effects all significantly increase under pulse-like earthquakes. There are spatial torsional effects in mid-span girder and main piers and pounding effects between girder ends and transition pier top. The perfectly-free and fixed bearings in transverse direction are not recommended for the seismic design of CRFBs. An optimal stiffness ratio in friction pendulum systems may exist that can minimize bending degree of the main girder. Furthermore, the side-span girder under pure longitudinal excitations can uplift that is closely related to the pier-girder consolidation form and rotation of the main piers. The main piers may be tensioned under strong vertical excitations, which is an uncommon problem worthy of attentions.