Scottish Bridges: 39. Ghurka Bridge, Aberlour

A short distance to the south of Aberlour's Victoria Bridge lies another suspension footbridge, although very different in both scale and form.

I only knew of the bridge thanks to an entry at the excellent Bridgemeister website, which reports that it was built by Ghurka troops temporarily stationed nearby, as a replacement for an old railway bridge. The masonry abutments from the railway bridge have been retained.

The bridge spans a minor watercourse which empties into the River Spey nearby, and is very basic in nature. Steel portal towers support steel spiral strand rope. Steel rod hangers carry a timber deck. The saddles for the main cables are exceptionally simplistic (pictured, right).

The main cables are anchored by wrapping them around anchor pins, and then clamping the steel rope together (pictured, left). This is a straightforward, military-style detail, quite different to what you would see on a more conventional, civilian bridge. It does not lend itself to easy adjustment.

Most light suspension footbridges use trussed parapets to provide stiffness to the deck, as is the case at the Victoria Bridge. The Ghurka Bridge is unstiffened, with only slender longitudinal timber beams below the deck spanning between the hangers.

The result is unsurprising: it deflects significantly under load, and is easy to excite into movement simply by jumping up and down. Indeed, this is so much the case that I was afraid I might break it! You can get some idea of how much it moves by watching somebody run across it, although I think the video doesn't really make clear quite how shaky the bridge is.

I have to say, I really don't understand how this design was ever deemed acceptable on such a readily accessible public footpath.

Further information:

• Google maps / Bing maps
• Bridgemeister

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)

Footbridge 2011: papers roundup pt 1

Okay, I thought I would attempt a quick round-up of some of the papers and presentations from Footbridge 2011 which particularly caught my eye. Apologies to anyone whose presentation I saw and found interesting, but have missed out, I only have so much time so will have to be quite selective!

In addition to the three keynotes previously mentioned, I enjoyed Henryk Zobel's Contemporary structural solutions of timber pedestrian bridges, which offered a nice survey of the range of timber designs currently in use. One that struck me as especially attractive was the treetop walkwway in Tharandt, Germany, pictured right (click any image for a larger version). This 118m long curved, stress-laminated timber structure was explained further in its own paper, although I missed the presentation.

There were relatively few other presentations to suggest any acceleration of the use of timber footbridges, although the Margaretengürtel design in Vienna was presented (see my previous post for details), and another presentation addressed some of the reasons why timber bridges remain unpopular in the UK. Client inertia appeared to be the deciding factor, although I know from my own experience as a designer that client concerns over vandalism, fire damage and general durability are often hard to refute. Having said that, at one time, the UK was claimed as home to the longest timber arch footbridge in Europe (the Middlewood Way Bridge near Macclesfield, a 50m span built in 1992), although that claim has certainly now been overtaken by bridges over the River Lora, near Florence (72m span) and in Rimini, Italy (92m span).

One aspect of the conference which I thought was quite unfortunate was the separation of most of the footbridge dynamics papers into a separate strand, which meant that those of us interested in more general design case studies or topics probably saw very little about dynamics at all. The ghettoisation of the dynamics specialists did mean that the entirely non-technical could avoid the subject, but runs the risk of re-opening a gulf between academia and practising engineers which had been temporarily narrowed in the post-Millennium Bridge years.

Nonetheless, some of the dynamics presentations which I did see were quite staggeringly esoteric, and it was tempting to wonder to what extent they were researching areas which were likely to have real practical relevance.

One dynamics paper which clearly was highly relevant, was Aleksandar Pavic's keynote Vertical crowd dynamic action on footbridges: Review of design guidelines and their application. Pavic's paper reviews five current published methods of dealing with this problem (see table, left), provides worked example calculations for a simple structure (itself very useful, given the ambiguities present in some of these documents), and observes that they can lead to very different results, with serious implications both for economy of design and the potential for error.

Two of the guidelines, HIVOSS and SETRA, are available online. It's notable that when Eurocodes 0 and 1 were published, a methodology for dynamic analysis of footbridges was conspicuous by its absence, including the load models which had been expected to appear (and which are given in the 2005 fib document Guidelines for the design of footbridges). Some of this is because the state-of-the-art in this area has been a constantly moving target, but I think it's a shame that Eurocratic deadlines for publication of the Eurocodes were allowed to take precedence over the presentation of standards which would assist rather than confuse designers. For anyone who doesn't follow the footbridge dynamics literature, it must be a real struggle working out how to proceed.

I saw two presentations where weathering steel was treated in very different ways. Martin Knight and Simon Fryer presented Combining engineering and aesthetics: The Town Centre Link, London, which must be one of the largest scale footbridges shown, at some 135m long and 12m wide. It's enormous weathering steel Vierendeel trusses (pictured, right) are detailed to avoid water traps, and are protected against the risk of graffiti by their height above the rail station platforms below, and by the use of full-height glazing on their inner face.

In contrast, Xavier Font discussed the lovely Can Gili Footbridge (pictured left), which I have covered on this blog before. Here, the pedestrians are kept away from direct contact with the rusty weathering steel by an internal guardrail. However, the potential for graffiti clearly exists, and I understand graffiti is now present. The absorbent surface patina on weathering steel makes graffiti hard to clean off, and only a fresh blast cleaning can reliably remove it (in the UK, this is covered by Highways Agency standard BD 7/01). In addition to the cost, I guess that will affect the uniformity of colour of the weathering steel patina, which may be undesirable. I greatly admire the appearance of weathering steel bridges, but as with the timber designs already mentioned, clients may have to adopt a more flexible attitude to the issues of durability and vandalism.

I was also impressed by a couple of weathering steel footbridges presented by Mario Guisasola, but would like to prepare a full post on them some day, as they were easily amongst the best designs presented at the conference.

Okay, that's all I have time for now, I will say more on the conference presentations later.