Precast Arch Bridges
Where headroom permits, arch-shaped precast bridge units provide an aesthetically pleasing alternative to rectangular portal bridges. The units typically rest on strip foundations. Depending on the span and loading, the arch may be a single unit or consist of a pair of units linked together at the crown of the arch. Typical applications are road, rail and pedestrian bridges, culverts and subways.
They have been used to form the soffits of cut-and-cover bridges and tunnels, with fill material being placed over the units up to the required level. The approach has been used for construction over railway lines because of the very limited track possession time required for installation.
Due to the arrangement of the structural elements a wide range of geometric configurations can be achieved to provide a varied selection of clearance envelopes using a limited stock of casting moulds. Depending on location, use, ground conditions and depth of cover, a structure comprising two, three or four elements can be used to provide economically viable for spans between 2.5m and about 20m, with internal heights up to 8m. The simple structural form means that structures can be installed in a comparatively short time. Quality can be maintained and easily controlled, thus ensuring the long-term durability of the structure.
Units will usually be 2m to 4m long, between 180mm and 300mm thick and weigh up to 34 tonnes. In the arrangement in which two units are used with the joint at the crown of the arch, the units can be erected in a staggered fashion, thus avoiding temporary propping. In the case where three units form the arch, the wall units are free standing and support the curved roof units, again avoiding the need for temporary works. Using these installation techniques, 30 to 40 units can be installed during a working day. Because of the speed of erection, the route below can be returned to use quickly with the minimum of disruption.
There are more than 60 precast concrete arch structures in the UK and Ireland, having spans in the range 3-20m and lengths from 2.5-361m. The Concrete Bridge Development Group has published Technical Guide No. 12, Precast concrete arch structures: A state-of-the-art report, which is based on a report originally prepared for the Highways Agency (now Highways England). The Guide provides guidance on good practice based on the experience of the industry suppliers. It is not intended to be a detailed design guide but to provide guidance for experienced structural engineers and advisers on the use of such structures.