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Over Bridge
River Severn, west of Gloucester, UK
associated engineer
Thomas Telford
date  July 1826 - 1828
era  Georgian  |  category  Bridge  |  reference  SO815196
ICE reference number  HEW 148
For almost a century and a half, Telford’s Over Bridge carried the main road over the Maisemore Channel of the River Severn at Gloucester in the west of England. The single-span elliptical stone arch has segmental voussoirs, which give it an attractive chamfered appearance. It is a scheduled ancient monument, now reserved for foot traffic.
The Severn Estuary has the second highest tidal range in the world. Even as far upstream as Gloucester the river is turbulent, with a surge wave known as the Severn Bore occurring at high tides.
A 13th century stone bridge stood here the construction of the bridge by Thomas Telford (1757-1834). The earlier one consisted of eight arches of 3.4m to 6.1m span between substantial piers 2.7m to 6.1m wide and carried a narrow roadway of up to 4.6m in width. The piers occupied some 44 percent of the space between the abutments, restricting the river’s flow. Perhaps as a result of past collapses, a section of it had been rebuilt in brick. At times of high water or flood, the arches could be "quite impassable, or extremely dangerous", according to Telford.
In 1825, the Gloucester authorities asked Telford to design a new bridge to be located to the north of the existing one. He prepared two single-span options — a cast iron arch of 51.8m, and a stone arch of 45.7m span. The town’s magistrates preferred the stone bridge.
Telford wanted to give his structure a better chance of withstanding the Severn’s floodwaters, allowing them to flow through the arch rather than sweeping it away. In this he was influenced by the work of French structural engineer and architect Jean-Rodolphe Perronet’s (1708-94) five-arch bridge across the River Seine at Neuilly in western Paris (completed 1774, replaced 1942).
Telford used a complex edge shape for Over Bridge’s arch on both faces: upstream and downstream. The main soffit is constructed to an elliptical curve, spanning 45.7m with a rise of 10.7m and maximum width of 8.3m. However, the voussoirs follow segmental curves that begin 6.7m above the main springing, and rise only 4m to the centre.
The funnelled shape produced under the arch reduces its obstruction to water flow, and this form is sometimes called cornes de vache, literally 'cow horns'. Over Bridge is the largest bridge in Britain to be built with this feature.
The abutment foundations were sunk inside cofferdams of timber piles and puddled clay. Pits were excavated to 8.2m on the west bank and 10m on the east, at which depths 11.2m x 12.2m platforms of rubble stone topped with pine sleepers and beech planking were constructed to support the first stone courses. The bridge’s 2.1m thick wing walls are founded 2.4-3m deep, directly onto the ground without foundation piles or platforms.
In constructing the arch, Telford reduced its overall weight by using four interior longitudinal walls, each 610mm thick, to support the roadway, presumably with voids between. The spandrel walls of the arch are 1.1m thick.
The crown of the arch fell 51mm when the centring formwork was removed and a further 203mm following movement of the east wing walls. It is Telford’s only admitted error as a bridge designer and one that he much regretted. He blamed his own "injudicious parsimony of omitting the piling and platform" (for the wing walls) and "now mention this as a useful caution to practical engineers".
Telford estimated that if the east wing walls’ foundations had been adequate, the total deformation would have been only 76mm — it was expected that any masonry arch would settle slightly after the centring was struck.
Over Bridge and its approaches cost £43,500 to construct. They carry a 5.2m roadway and two 1.2m footpaths, and proved well able to handle increasing traffic loads. The original medieval bridge was removed but its foundations (SO816194) remain in the river bed.
Until the Severn Road Bridge at Aust opened in 1966, Telford’s structure was the furthest downstream bridge crossing of the Severn. In March 1968, it was designated a scheduled ancient monument (the citation was amended in October 1997).
Over Bridge carried the A40 trunk road until 1975, when the modern bridge (SO816196) to the north opened. The bypass route became the A40 and the Georgian bridge was saved from demolition to be retained as a footbridge.
Resident engineer: Thomas Fletcher
Contractor: John Cargill
Research: ECPK
bibliography
"Two Masonry Bridges: II. Telford’s Bridge at Over" by J. Heyman and B.D. Threlfall, Proceedings of the ICE, Vol.52, pp.319-330, London, November 1972
"Life of Thomas Telford written by himself; containing a descriptive" by Thomas Telford, John Rickman
http://transportheritage.com
www.historicengland.org.uk
www.ice.org
www.severn-bore.co.uk
reference sources   CEH W&WBDCE1
Location

Over Bridge