Bridge Over the Water Storage of the Špilje Hydroelectric Power Plant
The bridge over the Špilje HydroelectricPower Plant water storage is the first application of pre-cast slabs in a composite and prestressed structure with a large span.
The "Melnik bridge" over the Špilje hydroelectric power plant water storage is on the road linking Skoplje with Debar and further on with Ohrid.
The bridge consists of two statically distinct structures: continuous girder 68 + 83 +83 + 68 m, on the straight section of the bridge, and a continuous girder 2 x 33 m in a R= 50 m curve. The piers of the bridge on open-pit foundations were designed as reinforced concrete panels between 35 and 45 m high.
With the aim of reducing the time of construction to a minimum, a composite prestressed structure with pre-cast reinforced concrete slabs was designed for the first section. It was the first time this construction technology was used in our country for a bridge of such spans. The steel structure of the bridge was assembled on the sides and then pulled into place over the piers and temporary steel supports. Site connections were performed using high grade prestressed bolts. The reinforced concrete slabs, dimensions approximately 2 x 10 m and of an average thickness of 18 cm, were cast at the same time as the execution of the steel structure, and then, after the steel structure was erected, put in place by a crane and a cart specifically made for the purpose. The slabs featured an appropriate number of openings which were later concreted to achieve a composite action. The transverse expansion joints between the slabs are 19 cm wide. After inserting prestressing cables and welding the connections, the expansion joints were grouted in situ.
In order to achieve optimum stress distribution for the dead load, the central supports of the bridge were lowered by 45 and 63 cm using hydraulic presses.
The bridge differs from bridges with pre-cast slabs previously executed in Europe also because the composite concrete slabs carry the largest part of the dead load and not only the live load. This is achieved by having the structure rest on one temporary support in each field during erection, the laying of the slabs, expansion joint grouting and prestressing. During the trial loading, the bridge demonstrated exceptional elastic features and a high elasticity module. The plastic deformation was insignificant.
The advantage of this system of construction is the great reduction of concrete shrinkage and creep and the possibility to shorten the total building time. Bridges of this type and size can be built during a single building season, provided that all the participants in the construction meet their deadlines. On the other hand, their construction calls for greater expertise, careful work, and, most importantly, a good organization of work.
The second part of Melnik bridge, in the R=50 m curve, is an interesting structure in its own right. This part was designed a composite continuous girder, with the slab cast in situ. The cross section of the bridge is box-type, and it is prestressed in the central support zone. Due the considerable overhangs of the main girder and the great torsional influence of dead and live load, a relatively complex statical analysis of the main girder was performed.
The bridge was completed and opened for traffic in 1972.