Composite structures over the A-1 motorway

Structural typology: ROAD & HIGHWAY BRIDGES
Date: February, 2004
Owner: Ayuntamiento de Madrid.
Construction: DRAGADOS.
Scope: Detail Design and Technical Support

The two structures belong to the connection of Renault over the A-1 motorway and its left and right service lanes, alongside. On plan, this roundabout does not constitute a unique circular alignment but is composed of several tangent circular ones.
For each one of the two structures (E-3 and E-4), the chosen structural layout includes a continous beam of two spans and constant depth whose pier is based on the median of the A-1. The spans of the two bridges, as well as the typology of the foundations and of the pier on intermediate supports, are limited by the available space between carriage-ways.
The foundation of the piers are built by means of piles which were chosen due to the essential condition of minimizing space requirements, in order to affect the existing traffic the least possible.
The same reasoning, that is, to reduce traffic problems as much as possible throughout construction, was applied to project criteria for both bridges. Therefore, their final design consists of a composite deck whose beam, divided in four parts, was installed with provisional props, allowing its hoisting with cranes instead of centerings. Afterwards, the precast slabs were set in place and the upper concrete slab was cast.
Every single span of both bridge structures, E-3 and E-4, is of 45.00m. The deck of each structure is 21.50 m wide and carries four lanes of 3.50m, a hard shoulder of 5.00 m as well as sidewalks, bars and protective railings. As the maximum headroom is limited, total depth amounts to 1.60 m, divided into 1.25 m for the steel box and 0.25 m for the concrete slab. The cross section consists of three trapezoidal boxes with a width of 4.50 m a the top and 3.00 m at bottom. The cantilevers of the concrete slab are of 2.00 m. The upper slab is made up of reinforced precast concrete slabs.
The abutments of structure E-3 are built with shear walls of piles of 1.80 m diameter, separated 2.20 m between axes, and a lintel supporting the decks. The reason for the choice of these walls lies again in the lack of excavation space, as the operating service lanes are very close to the upper roadway branches.
In structure E-4, abutment 1 is of the same shear wall of piles-type as in structure E-3, while abutment 2 (side of San Chinarro) is a closed conventional one with wing-walls.
Moreover, for the final solution to these two composite structures, the future opening of a VAO-bus lane (exclusive lane for special bus services in Spain), in the median of the A-1, had to be taken into account. As the placement of the two piers of the bridges on this median is incompatible with this future bus lane, it has been foreseen that when the time has come for the latter, the bridge piers on the median will be substituted by a pair of piers, separated 10.60 m from each other, in order to allow the passing of a 9.00 m wide VAO-bus. Therefore, the future pier diaphragms will already be provided in the decks.

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