Bridge Across the River Sava at Orašje
The bridge across the River Sava at Orašje features one of the largest continuous composite structure spans and is the first example of a composite upper and lower slabs of the main girder in the world.
At the time of its designing the conditions for the construction of steel bridges were relatively unfavorable. In some instances, even when large bridges were in question a steel structure would be more expensive than a prestressed concrete structure. These circumstances dictated the designing of a steel and concrete structure with the maximum possible utilization of the latter.
Apart from a concrete slab as the carriageway structure the composit effect which is fully active in the field, a lower concrete slab has also been envisaged in the central supports zone resulting in a specific form of the cross section with two concrete slabs and two steel webs.
The main bridge structure is a continuous girder over three spans 84+ 134 + 85 m of varying height, designed as a composite prestressed structure. The carriageway is composit with the main and transverse girders. The maximum slab thickness is 20 cm. The height of the web is 2350 mm at the beginning of the bridge, 5500 mm above the central support, and 3000 mm at the midpoint of the bridge. The transverse girders make an integrated whole with the vertical stiffeners, forming semi-frames capable of transmitting loads and stiffening the lower compression flange in the zone where there is no lower concrete slab.
The composite girder is specific in that, as mentioned, the lower concrete slab has been placed in the support zone. The addition of this slab slightly increases the internal forces due to the dead load. In addition to savings in steel, the concrete slab also considerably enhances the overall stiffness of the structure, since the cross section of the girder along one part of the bridge is of a box type.
In elaborating the detailed design very much attention was devoted to the analysis of the main girder. The very nature of this structure necessitated a quite thorough study of the composite prestresssed main girder. In addition to that the complexity of analysis was influenced by the method of erection, concrete casting and the fact that there were sections with two concrete slabs.
Cantilever erection in the central span of the bridge was performed. The steel girder was gradually erected in stages proceeding from both ends parallel with the concreting of a part of the deck and prestressing by cables. The total stress level results from the superimposition of different stresses experienced by the structure.
The bridge was completed and opened for traffic in 1968.
The bridge was demolished in 1992, during the war operations in the territory of the former Yugoslavia.