TWO CASE STUDIES |
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In the middle of the 1980s the Danish Government decided to construct a permanent road and railway crossing of the Storebælt (figure 1). For this purpose, a public company charged with the design, the financing and the construction of the works - the A/S Storebæltforbindelsen - was established. The crossing - 18 km long - connects the islands of Funen and Zealand passing through the island of Sprogø, located in the middle of the straits between Knudshoved and Holsskov. It therefore comprises the West Bridge between Funen and Sprogø, and the East Bridge between Sprogø and Zealand (see figures 1 and 2). The two parts of the crossing are remarkably different, since the seabed has a different profile. The western crossing is characterised by low depths - 25 m - and therefore two flanked road and railway concrete bridges have been planned. The eastern crossing is marked by great depth and it is used as a channel for heavy international sea traffic; hence, a bridge with a large suspended free span 1.624 m. wide and 64 m. over the sea level at the mid span has been planned. The bridge will only be used for the road traffic while rail traffic will be served by two twin tunnels of 8 Km length, located between 10 and 40 m under the sea bed. The works of the West Bridge and of the tunnel began in 1987 and their completion is planed in 1995. For the East Bridge, the international tender was issued in 1990 and the opening of the traffic road is planned for 1999. Further data on the overall project are provided in Working Paper 14. The client divided the works of the East Bridge in four parts: superstructure and substructures of the approaches and of the main span. For each of the four elements, A/S Storebæltforbindelsen issued an invitation to tender based on a basic design. The competitors for each of the four tenders, selected in a pre-tender phase, were free to propose alternative solutions to the basic project. Two types of bid were then allowed:
For the superstructure of the viaduct approaches and of the main span (whose global worth amounts to 6 billions of Danish crowns), a consortium that joined CMF and the technical services of Iritecna, in association with the engineering company Steinman Parsons, submitted its bids proposing several alternative solutions for the basic designs of the approaches and the main span superstructure. For the approach viaducts, in particular, CMF proposed the two following major variations of the basic project:
The A/S Storebæltforbindelsen selected the CMF project and accepted this last variation of the basic design for the two parts of the superstructure. In fact, this solution allowed a significant cost reduction of the overall costs: the increased length of the spans of the approach viaducts reduce the numbers of necessary piers, cutting in this way the global cost of the works.
The second case study concerns the bridge over the Grand Canal Maritime at La Havre in France. At the beginning of 1993, the Conseil Général de la Seine Maritime issued an invitation to tender for the construction of a 1.4km and 9300 tonnes weight bridge over the Grand Canal Maritime. The project started in March of the same year at a value of 110m Francs, and finished at the end of 1994. There are several analogies between the procurement process in the Storebælt and the in the Grand Canal Maritime bridges. The administration consultant Scetaroute proposed three basic alternative designs; a limited number of preselected groupments d’entreprises (consortia) submitted their bids with the possibility of proposing variations to the basic designs. As in the Storebælt project, the design of the bridge on which the construction companies competed was modifiable. Respecting some standards imposed by the maître d’oeuvre Scetaroute, every participant could modify part of the basic design in order to better its performance and to obtain a cost reduction. As in the former case, the variations could be minor if the one of the three basic solutions was just partially modified, or major if the new design was radically alternative to the one proposed by the maître d’oeuvre. The administration evaluated the bids on the basis of several criteria: the economic bid, the programme of the works, and the engineering quality of the proposed variations to the basic design. Unlike the former case, the bid had to be for the overall project, and not only a portion of it. A consortium composed of four Italian companies (Torno, Maltauro, Pizzarotti and Cimolai) and one French company (Nord France) won the tender: just like in the former case, an important element for winning the tender has been a partial modification of one of the basic designs. This specified a 20 concrete pile viaduct, in which the two central ones were at the distance of 180m. The canal was wider and so it was necessary to fix the two central piers in the water (see figure 3). That made necessary the construction of expensive defensive works, to protect the two central piers from the water current. Cimolai Spa proposed a different solution which made it possible to fix the bridge directly on the land and not in the water (compare figures 3 and figure 4): this was made possible by constructing a steel "V" (see figure 5) - a rare technical solution indeed - that took the place of the planned central piers.
The variation, conceived by the engineers of the Italian firm and further developed by an anglo-french engineering company EEG (Europe Etudes GECTI) obtained a cost reduction for the whole project, since it avoided all the costs related to the defensive works, and also achieved a better functionality of the bridge by widening its main span.
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