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Nov 01, 2023

Beam Architects’ perforated steel footbridge in Perthshire opens

The bridge was commissioned by The National Park Authority to re-establish a

The bridge was commissioned by The National Park Authority to re-establish a circular trail of 5km that runs on either side of a deep gorge to the remote but much-visited series of waterfalls.

While a rudimentary bridge had existed there since the mid-18th century, a more recent, long-standing steel bridge was destroyed by flooding in 2004 and subsequently its timber replacement needed to be removed in 2021 due to deterioration.

Source:Paul Saunders/See Loch Lomond

The new bridge has a 120-year design life and its design is low-cost and low-maintenance. It consists of a ‘walk-through’ structure constructed of perforated weathering steel, with longitudinal beams doubling as pedestrian parapets. GFRP decking and stainless-steel handrails complete the simple elemental composition.

Source:Paul Saunders/See Loch Lomond

Due to constrained access to the site, the bridge was constructed in modules and delivered to the site as 2.7m-long elements, which were erected with the aid of only a midi-excavator.

The perforated material reduces weight and adds transparency so that, seen from afar, the bridge appears semi-transparent and merges into the surrounding landscape.

Source:Paul Saunders/See Loch Lomond

The design is driven by the constraints of construction at such an inaccessible beauty spot. Conceived as a flat-packed kit of small parts to be brought to site in a series of prefabricated modules, the bridge was assembled on location next to the falls and pushed across the gorge on temporary rails.

As well as a pragmatic response to construction and budget, the design is simultaneously a careful architectural response to context. The site is dominated by the dramatic waterfalls cutting through rock, which attract tens of thousands of visitors annually. The bridge therefore provides a dual role as a crossing and a viewing platform from which to overlook the falls.

Simultaneously the structure is a central figure in landscape views, with a downstream right-angle bend in the river allowing a classic elevational view of the falls and the bridge. The discreet horizontal form of the design, and the tonal quality of the weathering steel structure are successful in melding the bridge into its surroundings. A key component in this integration is the use of perforated steel panels that render the upstands as virtually transparent in some views, with only the solid margins of the perforated material visible giving the appearance of a lightweight truss, rather than a solid girder.

Source:Paul Saunders/See Loch Lomond

The bridge cross-section is configured in response to the viewing-platform function, with the upstand girders inclined out from the deck to form an open V-shaped environment. The parapet beams migrate from the inside edge of the intermittent lateral U-frames at the base to the outside edge at the head, which maximises the incline. The incline encourages views downwards to the falls and provides an apparent spatial increase over and above the 2m functional width. The perforated panels are effectively and reassuringly opaque in longitudinal views along the deck, but significantly transparent in lateral views out from the deck. The aperture size and the percentage of open area of the panels are increased across the centre of the span, where forces are less, creating a triangular mid-span window to the falls.

The perforated panels are folded in a Z-profile, with the bottom flange creating a drip detail at the deck edge and the top flange forming a parapet head plate from which a stainless-steel handrail is bracketed. The top and bottom flanges are connected at each end with a radiused parapet panel that resolves the geometry into an attractive closed loop system. This feature creates a distinct threshold at each end of the crossing and deliberately codes the bridge as an independent structure laid lightly onto the landscape.James Marks, director, BEAM Architects

Source:Beam Architects

Start on site January 2023Completion February 2023Gross internal floor area 4m2Form of contract Design and Build – SBCCConstruction cost £200,000Construction cost per m2 £4,600Architect BEAM ArchitectsClient Loch Lomond & the Trossachs National Park AuthorityStructural engineer Cake EngineeringIndependent checking Format EngineersPrincipal designer Cake EngineeringCDM co-ordinator Cake EngineeringMain contractor Cake IndustriesInstallation BigCat ContractsCAD software used Solidworks

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TagsBEAM Architects Bridge Scotland