Scottish Bridges: 26. Carr Bridge

From Broomhill Bridge, we drove back towards the A9. On the way, we stopped in the tiny village of Carrbridge. The name is a clue: this is a village of a thousand picture postcards and holiday snaps, all of the same landmark.

The Carr Bridge itself dates from 1717. It was built by a stonemason, John Niccelsone, reportedly for the sum of £100.

Supposedly, it was built as a "funeral bridge", to allow coffins to be carried to a burial ground. I have to say I think that is somewhat unlikely. This is the oldest bridge over the River Dulnain at this location, and it seems hard to believe that a bridge wouldn't have had commerce and general transportation as its main objective.

Only the masonry arch ring now survives, with any parapets and surfacing long gone. What remains is just over 2 metres wide, not that we were foolhardy enough to measure it. My companion was happy to chance the span, but I took the side of prudence. An information sign at the bridge includes a sketch showing how the bridge may once have looked.

The bridge's remains are particularly picturesque, both for the slenderness of the arch, and for its semi-circular profile.

Further information:

• Google maps / Bing maps
• RCAHMS
• Highland Council Historic Environment Record
• Scottish Highland Bridges
• Highland Bridges (Nelson, 1990)
• Civil Engineering Heritage - Scotland Highlands and Islands (Paxton and Shipway, 2007)

Scottish Bridges: 25. Broomhill Bridge

The first day of our Scottish bridge trip was still fairly young. From Dunblane, we had been to Callander, then Aberfeldy and Pitlochry. We then drove further north, up the A9, past Aviemore, and on to Speyside. The next bridge on our list was Broomhill Bridge, described in John Hume's seminal 1977 book 'The Industrial Archaeology of Scotland' as "the finest wooden bridge surviving in Scotland".

Built in 1894 to replace an earlier bridge which had washed away, the bridge carries a minor road over 15 spans across the River Spey. The engineer was John MacKenzie and the contractor was Charles MacKay. It's remarkable that an all-timber structure has survived this flood-prone river for so long, although the bridge was partly rebuilt in 1987.

The bridge has an interesting combination of superimposed structural systems.

The trestle piers are fairly straightforward - each has five posts, with the outer ones raked for stability and to resist water flow. Overall stability is assisted further by cross-brace members. Planking prevents debris getting trapped between the legs, and metal covers protect against debris impact on the upstream posts.

The piers are topped with timber crossheads which support the five main longitudinal timber beams of the deck. Over the piers, the five main beams are doubled, cantilever-style, so that the effective span is reduced.

For the main river spans, a third doubler beam is added, and the whole thing supplemented by triangular king post trusses on the outer faces of the bridge. These are built in line with the timber parapets, and essentially serve only to suspend a further point of support for the span.

The central truss post holds an underslung crossbeam which passes the full width of the deck. Thrusts from the main truss members are resisted by a simple iron "bowstring" bar.

The bridge is clearly an antique, and much of its charm comes from the "thrown-together" appearance of its various components. In some ways it has more in common with American timber bridges, or even Bhutanese timber cantilever spans, than anything else to be found in Britain. Equally clearly, it was a carefully thought out design. I doubt whether an equivalent in steel would have survived so well.

One thing that is particularly pleasing to see is how little altered the bridge has been. Council engineers have resisted the temptation to bring it in line with modern standards (which, in particular, would prohibit the use of main structural trusses as part of the bridge parapet, in case they are hit by vehicles, bringing down entire spans).

While this may just seem like common sense, such pragmatism is often in short supply amongst the jobsworths of the engineering profession. In about 5 bridges time, I'll be featuring another Scottish timber bridge which has fared far less well at the hands of modern engineers.

Further information:

• Google maps
• RCAHMS
• Engineering Timelines
• British Listed Buildings
• Highland Council Historic Environment Record
• Scottish Highland Bridges
• Highland Bridges (Nelson, 1990)
• An Encyclopaedia of Britain's Bridges (McFetrich, 2010)

Scottish Bridges: 24. Port-Na-Craig Suspension Bridge, Pitlochry

We left Aberfeldy, and went north to Pitlochry, to stop at a pedestrian suspension bridge which crosses the River Tummel.

Built in 1913, this was the first of several suspension footbridges we'd visit over three days. It's also one of the longest spanning pedestrian bridges in Scotland. A plaque on the bridge states that it was "erected in memory of Lt. Col. George Glas Sandeman of Fonab, opened by the Marchioness of Tullibardine, on Empire Day 1913". Another plate notes that it was manufactured by the Lanarkshire Steel Co Ltd, using the Siemens Martin Process.

Lanarkshire Steel Co Ltd were based in Motherwell, and their name also appears on the Elvanfoot suspension bridge in Lanarkshire, which has a number of similarities, although with a significantly shorter span. The Elvanfoot bridge also bears the name "Rowell / London", and is very similar to yet another suspension bridge at Thames Ditton Island, which is known to have been designed and built by David Rowell and Co, so it may be that Rowell were also responsible for Pitlochry's bridge. All these bridges share similar lattice steel parapets, and lattice towers with an arched portal below cross-bracing, with similar finials atop the towers. I've featured a Rowell bridge here previously.

The Pitlochry bridge connects the main part of the town with Port-Na-Craig. It was originally built to replace a ferry crossing which dated back approximately eight centuries. Its span doesn't seem to be properly recorded anywhere but Hume (see Further Information, below), reports it as 250 feet (76m).

The latticework parapets are an economic part of this design. The lattice plates both provide the pedestrian containment as well as acting as a truss which stiffens the bridge deck against vertical displacement. As at the last bridge, the Aberfeldy Footbridge, I took a vibration reading on my smartphone:

This shows the vertical vibration only. The frequency is about 1.7-1.8 Hz, certainly well within the range which is easy to excite, but the acceleration is much less than was the case for Aberfeldy. The Pitlochry bridge is straightforward to excite, but not to a point where the vibrations are unacceptable.

A sign on the bridge reads "For the comfort and convenience of other users, please do not cycle or swing on the bridge". While the bridge is reasonably steady, I wonder how much it could sway sideways, if it were not for the presence of tie-back cables on each side. These horizontal stays are visible on the last photo above, and clearly greatly reduce lateral motion as well as damping bridge movement generally. I presume they were added in response to the bridge's behaviour, rather than being an original feature.

One peculiar feature of the bridge is a series of arch portals connecting the two parapets, but only towards the end of the bridge. I'm really not sure what purpose these serve. Did the bridge have some kind of covering when first built? I'd be interested to hear ideas as to what these portals are for.

It's also interesting to note the cable arrangement. There are two cables on each side of the deck, presumably because for this span, single cables of sufficient diameter were not available. One benefit of the use of two cables is that the clamp arrangement for the hangers is quite straightforward, as can be seen in the photo. I didn't note how the hangers are suspended from the clamps higher up the main cables, where the clamp is at an increasingly steep angle.

As can be seen in the photo, the European tradition of "love-locks" has spread to this bridge, although I only spotted two such locks.

Further information:

• Google maps
• RCAHMS
• British Listed Buildings
• Bridgemeister
• Scottish Suspension Bridges (Hume, Scottish Archaeological Forum, 1977)

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)

Scottish Bridges: 22. Bracklinn Falls Footbridge

From Dunblane, we drove to Callander. This town nestles among the hills and glens of the Trossachs, a particularly beautiful part of Scotland. Above the town, a river cascades down through a forested gorge, most spectacularly at the Bracklinn Falls.

The Falls are a fairly straightforward walk from a forest car park behind Callander Golf Course. The path previously crossed the gorge above the falls on a small metal bridge, but this was destroyed during a flood in 2004. A new bridge, designed and built by the marvellously monikered Strong Bridges, was completed in October 2010, at a cost of £110,000. In 2011, it was Highly Commended in the international Footbridge Awards.

Building a new bridge at this site must have been no mean feat. The waterfalls are dramatic, the gorge drops steeply below the site of the bridge. Access is via a forest track.

Accordingly, the bridge uses mainly materials that are widely available locally i.e. timber, and is in a blindingly simple structural form. Four 12m long Douglas Fir trunks form the main structural members in an A-frame truss arrangement. The central part of the bridge is roofed in copper sheeting, which helps obscure the presence of steel bracing at the truss crown.

The bridge was installed by winching it across on greased metal tracks supported on a temporary timber structure.

Before visiting the site, I'd seen pictures of the bridge and not thought it to be particularly successful aesthetically. The triangular truss shape seemed just too basic, too primitive. If the bridge were somewhere else, this might be true. Perching precariously above the Bracklinn Falls, however, it works extremely well, like a giant punctuation mark emphasising the centre of the Falls, and the experience of being able to stand at such a remarkable position.

It's one of those sites which is hard to do justice to with a photograph, hard to really get a good point-of-view that makes clear how the bridge relates to its context. So, in a first here at the Happy Pontist, here's a short video:

Even with the video, it's hard to really understand what a dramatic and beautiful site this is. To find out, you have to go there, and it's certainly worth doing that if you're anywhere near Callander.

Below the bridge, the gorge drops deeper than the photos here suggest. There had been heavy rainfall before we arrived, so the river was particularly full, and the view from the bridge was very impressive.

The tree-trunk bridge members gave a feeling of great security - unlike some of the bridges I'll cover later in this series, there wasn't a hint of vibration or insecurity.

The load from the bridge is carried by the massive main timbers, but their thrust is restrained by curved tie members below the deck. These are steel plates, two on each edge of the bridge, and shaped with a gentle scalloping between points of connection.

This is one of the bridge's more attractive features, the fact that the tie is curved rather than straight, visually shrinking upwards and away from the peril of the waterfall.

With a site like this, there are always a number of difficult issues to consider when evaluating a bridge.

If the bridge is visually successful, is it actually the case that the waterfall would render almost any bridge a success? I'd say no: the triangular form responds particularly well to the function of the bridge as a viewing gallery.

Would the natural scenery actually be better off without a bridge? Again, no: it allows access to views that would otherwise be impossible, and it doesn't diminish the views of the falls from other perspectives.

Would the same bridge look just as good in a different setting? Yet again, no: this bridge is clearly of its place, and might in fact look quite horrible in a number of other locations.

I believe the bridge has been a great success in attracting visitors, and it was certainly quite popular even on the rainy Saturday morning when I visited. It's great to see that a dramatic, structurally unusual landmark pedestrian bridge needn't be the stereotypical jumble of steel tubes and wires that is so often the case. This is a bridge that's highly appropriate to its site, was clearly economic to build, and well-deserves wider recognition.

Further information:

• Google maps
• Bracklinn Falls and the new bridge
• Callander Community Development Trust
• Scottish Access Technical Information Network
• copperconcept.org

Scottish Bridges: 21. Faery Bridge, Dunblane

This was the first bridge visited on my recent tour of Scottish bridges.

Conditions didn't bode well. Rain had been falling heavily since the previous day, and I was worried that I'd end up with poor photographs everywhere I went. As it turned out, they aren't too bad.

This reinforced concrete arch footbridge was built in 1911, and connects two areas of parkland across the Allan Water in the small Scottish town of Dunblane. It was designed and built by Considère and Partners, an early pioneer of reinforced concrete design in Britain. Reportedly, the name "Faery Bridge" is a corruption of "Ferro-bridge", from its use of "ferro-concrete".

The bridge spans 28.3m, and the deck is 1.2m wide.

The structure is in the form of a deck-stiffened arch, with a very slender arch member designed to carry only axial load, and a substantially thicker deck member which carries any bending from out-of-balance pedestrian loads. As well as the visual benefit of having a more slender arch, this design considerably reduces the cost of temporary scaffold centering, which only has to support the weight of the arch itself. The arch, once cast, then supports the weight of the remaining construction.

Perhaps the most famous example of this form in footbridge construction is Robert Maillart's 38m span Töss Footbridge, in Winterthur, Switzerland. Of course, Maillart's bridge wasn't built until 1933. This is something of a challenge to the normal historical perspective: Maillart is much-lauded as an innovator, and his bridges regarded in many cases as masterpieces.

The Dunblane bridge, meanwhile, is largely unknown and unheralded.

In their book on Scottish civil engineering heritage, Paxton and Shipway suggest that Maillart may have drawn on the example of Faery Bridge to influence his own work. Is it likely that he knew of the Dunblane bridge? The two authors don't offer any evidence, but Maillart's firm did apparently hold a license to Considère's patents in 1912, according to an advert in Schweizerische Bauzeitung. It's certainly not impossible, therefore, that Maillart had found out about the Dunblane bridge. During the 1920s, Maillart designed several deck-stiffened arches before completing the Töss bridge.

Maillart's use of this form has been much praised by David Billington in his various books, while acknowledging that the deck-stiffened idea itself dates back at least to the 19th century. Was Maillart merely plagiarising Considère?

So far as I can tell, Faery Bridge isn't even Listed, which would appear little short of criminal, if true. In fairness, it isn't quite as outstanding a bridge as Maillart's one, with the arching of the deck over each bay detracting from the simplicity of the whole. The arch itself is also less slender than Maillart would achieve. Even with a swollen river rushing below, I'd find it hard to describe the Dunblane span as particularly dramatic or beautiful. However, it does seem technically remarkable, and it's hard not to think how much better known it would be had it been built in a different setting, by a different engineer.

Further information:

• Google maps / Bing maps
• RCAHMS
• Civil Engineering Heritage: Scotland Lowlands and Borders (Paxton and Shipway, 2007)

Tips for pontists

Visiting 35 bridges in 3 days certainly brings home the importance of good preparation. Here are some thoughts on what the dedicated Pontist planning a bridge tour ought to take with them.

Maps
As I discovered in Scotland, some of the finest bridges can be quite tricky to find. I visited two in private forests, far from the main road, and several which were down obscure footpaths, not always visible on street maps. A combination of different maps was invaluable. Google's satellite views and driving directions are helpful, but proper Ordnance Survey maps were essential to locate some of the bridges. For historic bridges in Scotland, the RCAHMS website has coordinates for bridges and links to good quality OS maps at various scales. It might have been useful to have GPS to locate some of the sites.

Google street view is also invaluable for "casing the joint", particularly in working out where to park. For one bridge, I parked in what turned out to be somebody's back garden, which wasn't ideal. The ability to make a quick getaway is also essential in getting through a long list of bridges ...

Camera equipment
My camera broke down on the second day of my trip, leaving me reliant on my mobile phone and my travelling companion's camera. Their camera was hampered by a lack of space on the memory card. So, take a good camera, take a spare camera, take spare memory cards etc. I travelled with a laptop, so I backed up photos at the end of each day.

As for what makes a good camera for bridge photography, a wide-angle lens and good low-light capability are helpful, but portability also matters, as does the risk that an expensive camera might get dunked in a river.

Research
There's a lot to be said for knowing as little as possible about a bridge before visiting it, so it can come as a surprise. But I've had many occasions where I've visited and photographed a bridge, then learned more about it later and discovered that I never even bothered to look at a key feature. For the Scottish bridge trip, I did my homework, and it definitely helped. It also turned up links to other bridges which I was unaware of but which turned out to be well worth visiting.

Advance research also included working out which bridges were publicly accessible, and which were on private land. Scotland has a broadly-based right to roam which allows access to large areas of private property, but not to all. This bridge trip involved crossing both farmyards and private gardens, neither of which are included in the right to roam. The right also only covers only non-motorised use - there's no right to park a car on private property. For two of the sites, I wrote to the landowners and sought permission in advance. I will try to flag up any access issues when I post the individual bridges here, but do your own research if you are considering visiting any of these places.

Accelerometer
This has to be the latest gadget in any Pontist's kit bag. My tool of choice was the Accelerometer Toy app for an Android phone, but there are other apps including ones for the iPhone. With this, you can record bridge vibrations, and use the data to determine natural frequencies and possibly even coefficients of damping.

Waterproof clothing
I'm sure there are some places where this isn't necessary, but in Scotland? In early summer? Absolutely vital. Waterproof clothing isn't just about being proof against water - it also guards against stinging nettles, scratches from clambering through bushes etc.

Walking stick
On the last day of the trip, I found an abandoned walking stick below one of the bridges. I wish I'd had it for the whole trip. To get to several bridges, and to gain good viewpoints for photography, it's often necessary to clamber down steep and unstable slopes, climb walls, perch precariously at a river's edge etc. The walking stick really helped, and I'll be carrying it on future bridge trips for certain.

On that point, several of the bridges I'll be covered are actually quite dangerous to approach, particularly when trying to find a good place to take a photo. I'll try and highlight particular cases, but at the risk of being highly patronising, be careful out there!

Machete and/or axe
I've mentioned above the need to scramble through bushes and other undergrowth to get to bridges and viewpoints. A machete might be ridiculous, but a strong stick does help beat a path through dense vegetation. At some bridges, almost impossible to photograph due to trees and bushes, I wished I had an axe to clear a better view.

A companion
This particular trip would have been far poorer without a travelling companion. A fellow Pontist can help with navigation, but also with safety. There were several places it would have been much riskier getting to if travelling alone.