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

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