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How Moxon gave the Cairngorms’ Gairnshiel Bridge hidden strength

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Words:
Andy Pearson

Some complex engineering lies behind the slender new river crossing, a robust but sympathetic design that sits elegantly in its context

Moxon Architects’ new Gairnshiel Bridge in Scotland’s Cairngorms National Park provides a crucial north-south link between Deeside and Speyside and diverts traffic from the original river crossing. That will continue to be used by non-motorised traffic. How did the practice’s involvement stack up? Ben Addy, managing director at Moxon Architects, explains.

Why was the new bridge needed?

The old 1752 bridge is Category A-listed, and crosses the River Gairn, a tributary to the Dee. It was commissioned by Major William Caulfeild, an officer in the British Army, as part of a military road to link Deeside and Speyside following the second Jacobite uprising.

Locally it is a significant structure. It is quite diminutive, with a spare, elegant humpback arch form. It has beautiful ends that taper off into the landscape making a very fine looking structure.

The bridge was never designed for vehicles other than horses but was used by Highland tourist buses that would occasionally get grounded on the top of the arch. Its narrow width and tight approach meant it was constantly hit by vehicles, damaging the parapet and resulting in frequent bridge closures which entailed a 58km detour.

Spanning the River Gairn, a tributary to the Dee, linking Deeside and Speyside and responding to the landscape forms of the glen.
Spanning the River Gairn, a tributary to the Dee, linking Deeside and Speyside and responding to the landscape forms of the glen. Credit: Simon Kennedy

How did Moxon Architects become involved with this project?

Our introduction was unusual. Usually we’ll team up with an engineer to pursue an opportunity, but this time a local councillor approached us. We have an office in Aberdeenshire. Knowing that we’ve been involved in bridge design since the practice was established in 2004, she asked us: ‘What can I do?’.

That was in 2014, after which followed seven years of lobbying. She did the political part. With our background in bridge design, we did a feasibility study pro bono. This proposed bypassing the existing crossing with a new two-lane bridge capable of carrying large goods vehicles. We also provided some approximate ideas of cost based on our bridge design experience.

We proposed five or six alternative alignments, some immediately either side of the existing bridge and others further downstream. In the end, it was the downstream alignment with the best fit for the landscape that was chosen.

Following the initial concept sketches we were commissioned by Aberdeenshire Council to develop the design and make the Planning Application, which was then determined by the Cairngorms National Park Authority. 

How did you set about developing the design for a new bridge?

As architects we were obviously thinking about what a bridge would look like here but the initial discussions with Aberdeenshire Council along with representatives from Transport Scotland and the Cairngorms National Park Authority and other stakeholders were as much about provision of the highway either side of the river as the geometry and construction of the bridge structure.

We were also involved in the environmental and landscape discussions because the project included embankments and tie-ins to the highway which are elevated above the flood plain.

One key stakeholder was the bridges department at Aberdeenshire Council. They quickly became involved so we did a pro bono concept design with their team.

  • The 1752 Gairnshiel Bridge is Category A-listed.
    The 1752 Gairnshiel Bridge is Category A-listed. Credit: Simon Kennedy
  • Moxon’s new Gairnshiel Bridge with the original behind in Scotland's Cairngorms National Park.
    Moxon’s new Gairnshiel Bridge with the original behind in Scotland's Cairngorms National Park. Credit: Simon Kennedy
  • Two trapezoidal steel box girders in weathering steel support the new bridge.
    Two trapezoidal steel box girders in weathering steel support the new bridge. Credit: Simon Kennedy
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Did Moxon Architects have a vision for what the new bridge might look like?

The aesthetic and what it looks like in the wider context of the glen were very important. That is why it is not even remotely typical of equivalent, functional structures of a similar span.

The original bridge was built at a constriction where the bedrock is close to the surface. We’re only a few hundred metres downstream, but we’re on the flood plain so the river is wider with a different morphology.

The new bridge responds to three key considerations: the river flood levels, which required a single clear span with an aperture no smaller than that of the original; the low alignment of the highway, which required the bridge to have a slender profile in elevation; and the desire to create a sculptural yet robust form that responds to the natural formation of the glen and complements the adjacent 18th-century structure.

How is the bridge deck supported?

While the bridge is neither humpback nor of structural masonry construction, there are aspects of the original, such as the way it tapers at the ends and its slender arch midspan, that we felt it was important to emulate in the new bridge.

Two trapezoidal steel box girders in weathering steel support the new bridge. Over time, these will gradually change colour to a muted, dark brown. The deep part of the beam is underneath the centreline of each carriageway, which is where it really matters, while the outer web of each girder is angled so it appears to be very thin at the outer edge to emulate the slenderness of the original bridge.

  • River Gairn is wider at this point than under the original humpback bridge, as it spreads into a flood plain.
    River Gairn is wider at this point than under the original humpback bridge, as it spreads into a flood plain. Credit: Simon Kennedy
  • Precast concrete parapets are faced in locally sourced granite.
    Precast concrete parapets are faced in locally sourced granite. Credit: Simon Kennedy
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How are the parapets attached?

Admittedly, the thin outer edge of the box-girder creates some complex engineering connections with the precast parapet, so that is boot-shaped in section and incorporates starter-bars to tie it into the deck reinforcement. It is a bit of a tricky detail, but we were doing something very similar on HS2 viaducts.

The precast concrete parapets are faced in locally sourced granite, as are the concrete abutment wing walls, which taper down to gently merge into the landscape as a companion to the original granite bridge.

Huge credit goes to Aberdeenshire Council’s bridges department for seeing the value in these decisions and holding on to them.

Did Moxon Architects have much involvement in the final design?

As it happened we were a victim of our own success. Everyone bought into the project and the decision makers at the council’s transport committee decided they’d pursue the scheme based on our concept design.

Our concept was tendered for the full structural design. Structural engineer Arcadis won the job and developed the scheme with us in an oversight role, directly appointed by Aberdeenshire Council. When it came to procurement our involvement continued in much the same way with the contractor, Wills Bros Civil Engineering. The bridge opened to the public on 5 October.

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