Gem Bridge, Dartmoor National Park

Structural Steel Design Awards 2014 - Commendation

Gem Bridge is part of the 26km long Drakes Trail that connects Plymouth to Tavistock as part of the National Cycleway Network.

The scheme received £600,000 of European funding from the Cross Channel Cycle Project and is also part of wider initiatives to establish better cycling tourism links on both sides of the English Channel.

As the cycleway follows the track bed of a disused GWR Plymouth to Tavistock railway line, Gem Bridge is a replacement of the original Brunel Viaduct that was demolished in the 1960s.

Located on a highly sensitive site in Dartmoor National Park, it was vital that the structure was sympathetic to its surroundings as well as being constructed with minimal environmental impact. The choice of structural steelwork was fundamental to achieving both of these objectives.

The bridge is 200m long and is elevated 24m above a valley floor. It has five spans and comprises of an elegant open steel truss fabricated from hollow sections, with each span having a graceful arched profile. It has a light and open appearance allowing it to easily blend into the wooded area.

The deck is formed from precast panels attached to the top chord of the truss. Viewing galleries are provided at each of the four intermediate piers to give refuge to users.

A steel truss superstructure was chosen as it could be built quickly, with minimal temporary works and fewer on site deliveries than other options.

Buildability was a key factor for this project given its remote location, with access only via the Drakes Trail cycleway for the new structure, the cranes and other plant and equipment.

Comprehensive planning and engagement was required from an early stage to ensure the structure could be constructed. As a result, some of the embankment had to be graded to enable the crane to reach the bottom of the steeply sloping valley.

The structural design of the truss was optimised, not only to minimise material quantities, but also to improve constructability and appearance. Member sizes were also optimised to reduce steel weight and lessen the visual impact. Splice locations were also carefully selected in conjunction with the construction team to minimise the number of lifting operations.

Lifting operations were also constrained by the maximum crane size that could access and operate within the site. The structural design was undertaken with respect to the agreed lifting sequence so all temporary cases and locked in effects were fully accounted for.

Tema fabricated a total of 15 x 15m long steel truss sections in its Cardiff works before being transported to site. Each section had to fit within a 5mm tolerance when assembled into a full span on the valley floor before being lifted into position. The southern 30m span was first to be installed followed by the northern 30m and 40m sections then the central 40m and 60m spans.

Gem Bridge opened in September 2012, providing walkers and cyclists with panoramic views over the steeply sided Walkham Valley and wider Dartmoor countryside.

Architect	Devon County Council
Structural Engineer	Ramboll
Steelwork Contractor	Tema Engineering Ltd
Main Contractor	Dawnus Construction Ltd
Client	Devon County Council

Judges' comment

This is a simple, yet elegant, replacement for a previous historic structure which respects the environment and the heritage of the site. The bridge carries pedestrians and cyclists and spans a deep valley. Site access for construction presented considerable difficulties, which were overcome with careful planning and ingenuity.

The result is a bridge which the public enjoys and of which the client is proud.