Reza Shahinpar - Project Engineering Manager of Urmia Lake Causeway Bridge, Sadra Co, Urmia, Iran,
farshad barmaki - Sama technical and vocational training college,Islamic Azad University,Urmia branch,Urmia,Iran

This arch bridge is the central part of the Urmia Lake Bridge in North West part of Iran. The bridge spans 100 meters and accommodates both road and railway traffic. A single un-ballasted railway track passes through arches and the outer cantilever parts are intended for double lane road traffic in each direction. The main load carrying structure is made of steel box sections. A monolith 100 x 30 m free sliding concrete slab transfers traffic loads to the steel structure through 134 numbers of buffer bearings. The dynamic behavior of the bridge has been investigated through some sensitivity analyses in different loading conditions and speeds. Dynamic amplification factors have been calculated directly from the results. To scrutinize the effects of bearings’ stiffness on dynamic response of the bridge, flexible and stiffer bearings were used to simulate bearings action for the events that the maintenance periods are overlooked. Finally, the effect of added mass, which is normally considered by bridge designers for the sake of stiffeners and connectors, was investigated. According to the results; high number of mode shapes and very short time steps should be considered for very accurate results. Dynamic Amplification Factors of the bridge lay within a limited range when they are calculated in different ways. The bridge is sensitive to stiffness of bearings and internal forces are increased insignificantly but, damping effect is not relevant. There is a limitation for added mass to consider the results reliable otherwise; the maximum values of quantities will be missed in the design.