Puentes de la autopista de Madrid a La Coruña

Abstract

The fortunate circumstance that an urgent plan was approved to transform the road to La Coruna into a motor road, over a distance which was already being converted into a double lane road, led to the standardised design of the ten viaducts over the motor road, between Las Rozas and Villalba. The consequence of the above requirements, and much careful thought, are the viaducts which now emerge from constrasting landscape of plain and mountain, between Madrid and the nearby Sierra. The road ascends, following the natural topography and the positioning of the villages, and also taking advantage of parts of old Roman roads, cattle lanes, and later roads built in the times of the Austrias, the Bourbons and for the use of horse carriages. Actually, by a logical coincidence, the viaducts are sited at the cross roads of this old road pattern. The traffic and the geotechnical conditions are fairly similar for all the viaducts, and this factor was reinforced by the present tendency to standardise the design of Spanish motor roads. But in contrast to this there is the need to attend to the particular requirements of each case: a consideration that perhaps has deserved even more care than the first one. As a result of these contrasting conditions, the underpasses, three of the motor way and another of the railway, have been standardised: the same design has been used on two occasions (prestressed slabs at Pinar de Las Rozas and at Penascales), and in addition, this design could be repeated for any other motor road or for any other two level crossing, when the lower one is a railway or secondary road. On the other hand, on the overpasses standardisation has not been possible: althought the basic pattern has been the same in all the structures, the detailed construction is different in each. The overpasses are more important projects than the underpasses, and in addition, their detailed adaptation to the site has a greater influence on the total project. However, this influence must be somewhat reduced, since in our case we were dealing with a dual lane road, which was transformed into a motor road whilst in course of construction. Actually, the final result is not too disappointing, since the main advantage is to have a series of underpasses that meet the requirements of this kind of motor road. On the other hand, the number, and the varied circumstances affecting the overpasses do not make it possible to attain this aim. Even so the design calculations for these structures are now computer standardised. This reduces to a minimum the tedious calculations, and also makes it possible to obtain the most precisely suitable variants. For those exceptional cases where this is not possible, we utilised reduced scale structural models. Thus the engineer can concentrate on his main function of designing, especially so in the case of one of the structures that is most susceptible to this, due to the rich, untapped, possibilities of prestressed concrete. It is not by chance that in this medium it is possible to attain spans and slenderness ratios that are nearly twice as large as in reinforced concrete. Hence prestressed concrete is more apt to meet the requirements for dintel depth and maximum bridge opening. This type of viaduct for motor road crossings, which is the most recent development in bridge evolution, demands the highest possible technical refinement, since it is the most advanced form of structure, and its own structural efficiency determines how far it influences the road over which it runs. In this sense, there is a spirit of cooperation between the engineer and the road user, expressed in the serene balance of the stressed structure. This sense of peace is specially necessary to the modern man, who is so highly pressed, not only by road traffic, but generally by the deeper conflicts in the flow of life.