Keywords: bridges, highway, dynamic analysis, prestressed concrete, rivers, code.

Dynamic response has long been recognized as one of the significant factors affecting the service life and safety of bridge structure. Even though considerable research, both analytical and experimental, has been devoted to dynamic behavior, the identification and extent of the controlling parameters that govern dynamic response have still not been clearly identified. On the other hand, under the current design loads, dynamic effects taken into account by increasing the static load by an impact factor, I, which is a function only of the bridge loaded length and does not distinguish between simple and continuous bridges. Also the important factors affecting the bridge dynamic response such as the vehicle speed, the vehicle weight and the pavement surface roughness are not considered by these codes. Therefore, the need of studying the actual dynamic behavior of highway bridges under traffic loads to evaluate the accuracy of the impact factors given by codes is strongly arises. Much of previous research works concerned with evaluating the dynamic response of single span bridges. The primary objective of this research is to develop a finite element bridge model that can accurately predict the static and dynamic responses of multispan highway bridges. Hence, to understand the dynamic behavior of this type of roadway bridges under traffic loads, two study cases of roadway prestressed concrete bridges are analyzed and their static and dynamic responses are investigated. The highway live loads and the impact formula given by the American bridge design code are considered in this study. The accurate dynamic factors estimated from the static and dynamic analysis of the investigated bridges are plotted against the approximate values of the impact factor given by the American bridge design code.

In this work, a study has been made to get a complete view on the dynamic behavior of multispan prestressed concrete bridges under the effect of traffic loads. A finite element modeling for the problem is developed and a computer program is constructed and verified. Two practical cases of continuous prestressed concrete highway bridges are analyzed and investigated. The AASHTO HS20-44 modified design truck is used in the analysis. Exact values for the dynamic effect factors for the considered two cases are calculated and plotted against the corresponding impact factors given by the American highway bridge design code. The response parameters of interest in this study are the deflection, the moment and the dynamic effect factor. Some important conclusions and recommendations for the highway bridge designers are obtained from the present work and can be summarized as follows:

1. The maximum dynamic responses for highway bridges are increased with the increase of vehicle speeds.
2. The dynamic effect is very small and can be neglected in designing the multispan prestressed concrete highway bridges when vehicle speeds less than 40 km/h are expected.
3. The empirical values for the impact factor given by the American design code are recommended only for multispan bridges with expected speed range from 40 to 70 km/h.
4. For multispan highway bridges on main roads where high vehicle speeds are expected, the dynamic effect must be increased to a range from 1.5 to 2.5 times the impact factor given by the American specifications.
5. For multispan highway bridges, if the ratio of span lengths of two adjacent spans is large (greater than or equal two), the dynamic effect can be neglected for the support moments between them.
6. The dynamic effect factors calculated for deflections are larger than those for moments.

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