Effect of Pretension on the Dynamic Response of Footbridges

In this paper, we intend to study the effects of geometric nonlinearities on concrete footbridges. Dynamic characteristics of structures depend on their stiffness and mass. With those, we determine their natural frequencies and modes of free vibrations. Nevertheless, the initial stiffness of a structure, computed in its unloaded state, is affected by the applied forces, the so-called geometric stiffness. Compressive forces usually reduce the stiffness and the frequencies and may lead to buckling, for zero frequencies. In the other hand, traction loads tend to increase stiffness and frequencies, a phenomenon resorted upon by the socalled tensostructures. A class of structures of economic-strategic importance is footbridges, excited by vibrations induced by motions of persons. These vibrations may affect the structures but, in general, may render inadequate human walking conditions. There is a tendency of modern structural engineering towards slender members, due to more efficient materials and more powerful analysis tools. Here, we study these effects via a theoretical-numerical approach by an approximated model derived by Rayleigh's Method. The model is a concrete slab walkway under pretension supporting. We suppose the original design have provided for natural frequencies away from the excitation frequency. Nevertheless, the presence of large axial compressive force will reduce the beam stiffness and natural frequencies wich may lead to unexpected potentially dangerous resonance states.

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