A typology of spans
Seemingly effortless, Moxon’s glass and steel viaduct around Taunton Castle belies the complexity of its construction, writes Felix Mara
Photography by Simon Kennedy
You could apply that quintessentially Victorian adage ‘horses sweat, men perspire and ladies glow’ to structural engineering – and to bridges in particular. Rogers, Stirk Harbour + Partners’ 8 Chifley Square in Sydney (AJ 15.11.12), designed with the Lippmann Partnership, sweats mega-buckets, but the majestic horizontal suspension structure of Foster + Partners’ Millennium Bridge perspires as it stretches across the Thames, whereas Moxon Architects’ Castle Green Bridge in Taunton’s Goodland Gardens, which opened in September, declines to unfurl its fan.
Truth be told, Moxon’s bridge carries a lighter burden and is essentially a 37m ramp rising 1.8m from the sylvan Goodland Gardens to Castle Green, the town’s historic core. It should however be seen as part of a sequence which will include the Mill Stream Bridge and Tone Bridge to the north when completed. Like the Moat Bridge, these will replace non-DDA-compliant structures and the sequence will provide a coherent and memorable route across the site, both passing and acting as a contemporary foil to Taunton’s medieval castle.
Moxon conceived the bridges as a typology of spans, each with very different structural principles: a viaduct in the case of the Castle Green Bridge, a simply supported beam in for the Mill Stream Bridge and a tied arch to the River Tone Bridge. Their contexts are also very different: a Scheduled Ancient Monument in the case of the Castle Green Bridge, landscaped parkland for the Mill Stream Bridge and a river in the case of the Tone Bridge. The essential point is that they have a shared language of detailed design.
The architecture was unusually centre-stage for an infrastructure project. Nevertheless the bridge was subject to engineering scrutiny, which emphasised logistics, efficiency and rational construction. Its concrete deck spans between four irregularly spaced circular piers located according to ground bearing capacity and archaeological constraints. The alignment of engineering concerns with the pursuit of a consistent architectural language can be seen in the repetition of elements such as the stainless steel plates used to clamp the low-iron toughened glass balustrades and laminated glass deck planks.
Both are fixed with countersunk snake eye-headed bolts, also used to connect the lighting bollards’ components, which replicate the forms of the parapet end guards.
Moxon, also the designer of Taunton’s Third Way Bridge that completed last year, is one of several architectural practices with a convincing portfolio of bridge projects, including Wilkinson Eyre and Clash Architects. These firms bring unique architectural skills to this work, seen, for example, in the use of false perspective in the Castle Green Bridge, which tapers from 5 to 2.6m in width to increase or diminish expected travel distances. A meticulous note on Moxon’s drawings states that no horizontal paving grout lines should be in the same line within five rows of each other. This refinement extends to practical features, such as drainage gaps between glass decking planks and the strips of grey slip-resistant fritting on them.
Each balustrade panel – and this is super-clever, though also complicated – is clamped by two stainless steel strips, 90mm apart, exposing the glass panels and the frosted strips at their base. The edge of the deck thus reads as a lightweight translucent volume and, with LED strip lighting in the cavity between the tapered concrete slab and the glass deck, this structure glows, literally as well as metaphorically.
Start on site January 2012
Completion September 2012
Bridge deck area 123m2
Form of contract NEC
Total cost £350,000
Cost per square metre £2,845
Client Project Taunton
Structural engineer Flint & Neill
Lighting design Moxon
Landscape architect LDA Design
Bridge deck area project manager Peter Brett Associates
Main contractor Britannia Construction
CAD software used Autocad, 3DSMax, Nodle, bespoke FE analysis software
Glass deck supplier Romag
LED lighting units supplier Surelight
Glass parapet Taunton Fabrications