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In the design of railway bridges, it is necessary to be able to predict their dynamic behavior under a moving train load so as to avoid a resonance state from repetitive moving axle forces with uniform intervals. According to design trends, newly developed girder bridges weigh less and have longer spans. Since the dynamic interaction between bridge superstructures and passing trains is one of the critical issues concerning such railway bridges that are designed with greater flexibility, it is very important to evaluate the modal parameters of newly designed PSC girders before carrying out dynamic analyses. In this paper, a full scale incrementally prestressed 25-meter long concrete girder was fabricated as a test specimen and modal testing was performed at every prestressing stage in order to evaluate the modal parameters, including the natural frequency and the modal damping ratio. Young's modulus was also obtained from the global stiffness of the test specimen. During the modal testing, a digitally controlled vibration exciter and an impact hammer were applied in order to obtain precise frequency response functions, and the modal parameters were evaluated at various construction stages. With the availability of reliable properties from the modal experiments, dynamic performance estimation of a PSC girder railway bridge during the passage of a moving train can be carried out.

The forced-free responses of nonuniform beams under moving point loads are analyzed in this paper. Simple approximate analytical formulae for the forced responses of undamped nonuniform beams, derived using the fundamental mode by the Rayleigh–Ritz (R–R) method, are presented. The responses of both simply supported and clamped–clamped beams are analyzed. The responses are also determined by the finite element method (FEM) in which nonuniform elements are used for fast convergence. It is found that the present method yields results that are very close to those obtained by the FEM. As this method does not require time integration, it is faster and computationally more efficient. Though the single-mode analysis of forced vibration of uniform beams under moving loads has been done by several researchers, its application to nonuniform beams has not been reported.

Pile foundation with elevated cap is always applied for railway bridges in permafrost region to minimize the thermal disturbance to vulnerable permafrost. Permafrost can influence the failure characteristics of the pile foundation under earthquakes and complicate the seismic performance evaluation of railway bridges with pile foundations in seismically active zones. Quasi-static tests of reduced scale models were carried out to investigate failure characteristics and hysteretic behaviors of the pile foundation with elevated cap in permafrost. Test results showed that the existence of permafrost can increase the stiffness of the foundation soil and then inhibit crack propagation of the unfrozen topsoil around the bridge pile foundation. Quite differently from the pile foundation in unfrozen soils, severer damages occurred at the pile foundation in soils with permafrost. Thus, the topsoil crack will be mitigated while the pile failure will be aggravated if strong earthquake occurs in permafrost regions for railway bridge pile foundation with elevated cap. Otherwise, the permafrost significantly influenced the loading bearing capacity and deformation features of the pile-soil interaction (PSI) system under seismic loads. The PSI system with permafrost has better energy dissipation than that with unfrozen soils due to the severe damage of the pile foundation. A simplified formula was developed to estimate the equivalent stiffness of the PSI system. It is indicated that the stiffness of the PSI system with permafrost degraded more severely than that with unfrozen soils. Therefore, permafrost effect cannot be neglected in seismic design of railway bridge pile foundations with elevated cap.