Modeling and Analysis of a Steel Truss Railroad Bridge Traversed by Trains at Various Speeds

The dynamic analysis of a truss railroad bridge under a moving train is very complicated. The simplified model where an entire bridge is represented by a single beam of equivalent stiffness could be suitable to analyze a girder bridge, but is not adequate to analyze accurately truss bridges, which inherently consist of numerous structural components. Moreover, due to the repetitive nature of train loading on a railroad bridge, there is an excitation frequency as a function of speed associated with each moving train. If this loading (excitation) frequency coincides with the natural frequency of the bridge, the bridge response builds up continuously with time giving rise to the resonance phenomenon. It is important to avoid the condition of resonance in a bridge for the comfort of passengers and the safety and longevity of the bridge. The objective of this thesis research was to determine the static and dynamic responses of the Devon truss railroad bridge over Housatonic River in Milford, CT under moving trains. For this purpose, a three-dimensional finite element (FE) model the truss railroad bridge was constructed. The static analysis of the bridge was performed under its self-weight and the static train loads. The dynamic analysis consisted of (a) the modal analysis to determine the mode shapes and natural frequencies, and (b) the time history analysis to obtain the dynamic response of the bridge under moving trains. Mode shapes, natural frequencies, and dynamic displacements of the bridge under moving trains obtained from the field test data were compared with those from the FE model. Finally, the verified FE model was used to determine the safe train speeds to avoid resonance vibration of the bridge. The result from this study should help to address the rising concerns about the adequacy of old steel truss bridges for carrying trains with higher speeds than the allowable speed at present