Scottish Bridges: 23. Aberfeldy Footbridge

Things seemed to be looking up. It had rained in Dunblane, but less so in Callander, and by the time we got to Aberfeldy things seemed to be improving.

We were stopping in Aberfeldy to see the Aberfeldy Footbridge. Built in 1992 to connect two parts of a golf course over the River Tay, this was the world's first all-plastic footbridge, and is probably still the largest such structure. It was a pioneering piece of engineering technology, and we were interested to see how it looked, and how it had performed after 20 years of life.

The bridge is a three-span cable-stay structure, with spans of 25m, 63m, and 25m. The span requirements were driven by river flood conditions. The towers are 17m tall A-frames, giving an inclined cable arrangement which helps stabilise the bridge deck laterally.

All main elements of the bridge were built in fibre-reinforced plastic. Glass-reinforced plastic, in the form of multi-cellular "planks" was used for the deck and the towers. The parapets are made from pultruded GRP sections. The cables are Kevlar, an aramid fibre, inside a protective plastic coating.

When built, the extensive use of reinforced plastic was promoted not just for its expected high durability, but also for its lightness of weight. This allowed the bridge to be erected by a relatively small team, using relatively straightforward plant, and no craneage. Indeed, the bridge was so lightweight that concrete ballast had to be hidden inside the deck to prevent any risk of wind-induced uplift. It's reported that the structural elements weigh in total just under 15 tonnes, which is very impressive.

 In 1997, the bridge was strengthened by the addition of bonded GRP plates, following damage caused by taking an unauthorised vehicle over it. The bridge was originally designed for a uniform pedestrian load of 3.5kN/sq.m and had never been designed to carry the intense local loads from any size of vehicle.

One of the difficulties with GRP bridges is that they do tend to be assembled from a kit of parts, as at Aberfeldy - standard panels and sections bolted or glued together to form the final cross-section. The deck edge beams, for example are made from five standard square sections glued to each other. This considerably limits the scope for aesthetic consideration - GRP bridges tend to look simplistic and blocky, like something that could be made from a child's modelling toy.

The Aberfeldy footbridge doesn't suffer too greatly from this problem, and to a large extent I think that's due to careful consideration of the overall form, with the gently curved deck and simple tower profiles drawing the viewer's attention. Close-up, some details appear clumsy, but little more so than on a timber bridge.

The bridge appears to have weathered well. A paper by Stratford (presented at Structural Faults and Repair this year) provides a detailed assessment, and notes issues such as impact damage to the deck and parapets. I didn't notice those, but did observe that the bridge has provided a very attractive platform for moss and lichen. I doubt that they cause any damage. Some deterioration of the resin surface of parapet sections, exposing the internal fibres, has also been reported.

The bridge parapets have clearly not been as successful, mainly due to a fairly simplistic fixing detail at their base where they are chased through the deck and secured in place by dowels. They rattle as the bridge moves, and can be shaken very noticeably by hand. Here's a (very) short video:

The bridge deck itself is very easy to excite into vibration. This is one of the biggest disadvantages of a lightweight structure, and the ballast on Aberfeldy Footbridge is nowhere near enough to prevent it (the design live load is roughly three times the design dead load). As a private structure, the bridge was never designed to satisfy normal pedestrian bridge vibration criteria, but I was still surprised to see how flexible the deck is. A single person can excite quite large amplitudes of vibration.

The first vertical frequency of the main span has been measured previously, with values of 1.59 Hz (1995), 1.52 Hz (2000) and 1.49 Hz (2011) reported in various papers. A damping ratio of between 0.84% and 0.4% has also been recorded. My little smartphone makes no claim to be an accurate vibration monitor, but I used it to take my own readings, the output of which is pictured below (as with all images, click for a larger version). This confirms a vertical frequency of roughly 1.5 Hz.

As a highly innovative prototype structure, it's remarkable that there is actually so little wrong with the Aberfeldy bridge. It's startling to realise that two decades after its construction, it remains unparalleled in its extensive use of reinforced plastic materials over such a long span. And finally, it's pleasing to see that all that has been achieved with a bridge that doesn't look too bad, either.

Further information:

• Google maps / Bing maps
• Structurae
• RCAHMS
• NGCC
• A Reinforced Plastic Footbridge, Aberfeldy, UK (Harvey, Structural Engineering International, 1993)
• Aberfeldy Bridge - an advanced textile reinforced footbridge (Burgoyne and Head, TechTextil Symposium, Frankfurt, 1993)
• The design, construction and in-service peformance of the all-composite Aberfeldy Footbridge (Cadei and Stratford, Advanced Polymer Composites for Structural Applications in Construction, 2000)
• A critical analysis of the Aberfeldy Footbridge, Scotland (Skinner, University of Bath, 2009)
• The condition of the Aberfeldy Footbridge after 20 years in service (Stratford, Structural Faults and Repair, 2012)
• An Encyclopaedia of Britain's Bridges (McFetrich, 2010)
• Civil Engineering Heritage: Scotland Lowlands and Borders (Paxton and Shipway, 2007)