A number of recent research studies have provided insight into the seismic response characteristics of short-span overpass bridge systems. Application of system identification techniques to measured earthquake response data for this class of bridges has indicated that the bridge superstructure, abutments and approach embankment soil constitute a strongly coupled system. The dynamical behavior of the foundation and embankment soil have a first order influence on the dynamic response of the bridge superstructure. Analysis of measured strong motion response data has also indicated that localized nonlinear behavior of the embankment soil can result in significant nonlinear global behavior of the entire system, even when the bridge superstructure remains linear. The current paper presents the results of detailed numerical simulation studies of the dynamic response of a short-span overpass bridge system. Two distinctly different modeling approaches are investigated. The first approach utilizes simple reduced order “stick” model idealizations of the bridge, and the second approach utilizes a detailed, large scale, three dimensional finite element model. The detailed model includes a discretization of the soil embankments and a simple nonlinear material model is used to represent the hysteretic soil behavior. The sensitivity of bridge response to various parameters, such as deck skew, embankment soil stiffness and soil mass, stick model modal damping values, and soil nonlinearity has been investigated. Earthquake response predictions are performed with both model types and the response computations are compared to earthquake response data measurements. The ability of the models to accurately represent the bridge seismic response is discussed, and the two modeling approaches are compared and contrasted.
Although the potential for cumulative damage of structures during long duration earthquakes is generally recognized, most design codes do not explicitly takes into account the damage potential of such events. In this paper, a strain-based low-cycle fatigue model commonly used for the prediction of fatigue life in metals is adapted for cumulative damage assessment of structures under seismic conditions. By defining the number of load cycles in terms of the total plastic strain energy dissipated by the structure, the model is presented in a form capable of predicting the plastic strain energy capacity of the structure at the ultimate limit state. The plastic strain energy is expected to decrease rapidly with increased displacement in the small displacement range and to decrease gradually in a near linear manner with increased displacement in the large displacement range. The model is shown to calibrate reasonably well with small-scale aluminum cantilever specimens tested under large-amplitude reversed cyclic loading. At the ultimate limit state, the modified Park and Ang damage model may be considered as a linear approximation to the low-cycle fatigue model in the large displacement range.
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