Mountain Road Bridges

Abstract

The new layout of the road between Hirwaun and Abergavenny, in the Welsh district known as «Heads of the Valleys», has involved the construction of four new bridges, which because of the difficult terrain, were very carefully designed before the best solution could be discovered. Traditional methods would have required the use of much scaffolding and tall supports, owing to the depth of the gorges to be traversed. Hence progressive cantilevering from either side of the span was chosen. In three of the four bridges the final method was the one mentioned above. In the other, a treble portal frame, resting on two intermediate slender supports, hinged at their base, was preferred. The first three bridges are all of very similar construction: in each case an initial mass at the springer was concreted, to which additional sections, each 3 m long, were added, and attached by means of prestressed reinforcement. The fourth bridge was built with double T girders, cantilevered in 3 m sections. The 3 m spaces between the chords of parallel girders were concreted at the site. A set of four of these girders, and the slabs between the chords, constitute a box girders, and the depth of this structure decreases from 4.50 ms over the supports to 1.50 m at the crown. The mixed system of prefabricated units, and slabs, concreted at the site, was made possible because of the prestressing.