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In this paper, the dynamic characteristics of two highway bridges have been extracted via performing several operational modal tests. The tests have been performed during the normal traffic passage and ambient condition. The first bridge studied here, the "Ziar-1" Bridge built in 2003s in the road between the cities of Isfahan and Ziar, is a two span reinforced concrete with cast in place slab-on-rectangular girder superstructure system with the total length of 33 m. The deck is simply supported over a wall type middle pier and two side abutments with closed type. The superstructure is supported by the piers and abutments through elastomeric bearings. The second bridge, the "Ziar-2" Bridge built in 2012s as the new line of that road, is a two span reinforced concrete with in-site slab-on-precast girder superstructure system with similar length of the first bridge. The deck of this bridge is continuous without middle or side expansion joints and is supported over a pier bent consisting of a bent cap and three circular columns. The bridges, which pass over the "Zayandeh-Rood" river, are structurally separate and a 50cm distance has been provided between them. Both the bridges have similar geometry but different boundary conditions. The results of the modal tests have been also compared with the results of supporting finite element models of the bridges and the effects of boundary conditions on the dynamic characteristics of the bridges have been investigated. At the time of the first phase experiments, the river was dry due to drought in recent years and the level of underground water was sufficiently deep. It is predicted that presence of water current around bridge piers, in which the superstructure have monolithic connection with piers and side abutments, may change the dynamic properties of superstructure. In order to investigate to what extent the presence of water current in the river may affect the dynamic characteristics of the bridges; additional modal test has been performed on the second bridge, in which its deck is continuously connected to the supporting elements. On the other hand, the seasonal effects of the water current in the river on the dynamic characteristics of one of the bridges, as representative of reinforced concrete slab-on-girder integral bridges, have been also studied. In order to realize to what extent the pier-superstructure connection in this bridge is monolithic, the actual displacement pattern of the measurement points on the superstructure at the pier location has been closely measured and compared with corresponding results of the finite element model of the bridge. The results show that, as far as the boundary conditions of the deck is concerned, it has significant effect on the dynamic behavior and corresponding natural frequencies of the superstructure, especially in the first bending and the first torsional modes. Also, it was found that the presence of water current around the piers of the bridge leads to slightly increase in the natural frequency and the associated damping values of the bridge.

Keywords: Dynamic characteristics, ambient vibration test, Boundary conditions, concrete bridges

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