30 August 2012

Scottish Bridges: 46. Crathie Suspension Bridge (revisited)

Continuing west (and upstream) along the River Dee, the next bridge is the Crathie Suspension Bridge. This was one of the bridges I was most keen to see on this Scottish bridge tour. I had been to it previously many years ago, but before I was aware of its significance, and I also failed to take very good photographs.

It's such an interesting and significant bridge, that I'll repeat some of what I said last time I featured the bridge here.

The bridge spans 42m, and it is 4.3m wide. It was built in 1834 to carry carriages, although it is now only open to foot traffic. It was designed and built by John Justice Jr, one of a family of blacksmiths from Dundee. Five bridges are known to have been built by Justice, of which only three survive intact, with one derelict, and one, at Glen Clova, now gone.

In his paper on early Scottish suspension bridges, Ted Ruddock described the structures as "vernacular bridges", which hits precisely upon what interests me about them - the ingenuity and eccentricity of bridges designed by direct experimentation rather than from the custom and habit which hamstrings many professional bridge designers.

We were lucky enough to visit all four surviving Justice bridges on our trip, and they certainly are marvels.

The bridge at Crathie appears to have been originally built as a "cable-stayed" structure, although it was refurbished in the mid-1880s by Blaikie Bros (the firm also responsible for the bridge at Abergeldie Castle). It's believed that was when the suspension chains were added.

Along with its siblings, Crathie is one of the earliest stayed bridges to survive in the UK, and possibly in the world. It's incredible that it has lasted so well, as many early stayed designs were conspicuous failures, accounting for the form's widespread unpopularity before the mid-20th century.

At first sight, a number of features of the Crathie bridge suggest that it might have been the work of “proper” engineers. The A-frame towers have clean, strong lines not entirely unlike the suspension footbridges of the later 19th and early 20th centuries.

However, the closer you look, the more features appear that indicate a more idiosyncratic hand at work. The layout of the rod stays is one, as is the way in which the rods are connected together, with open-form “turnbuckle” connectors. The stays in the main span form a double-fan arrangement, with two stays coming together at each of two points on the towers.

From each point on the tower, only a single, larger stay continues on to the bridge anchorage. At the south end, even these two anchor stays connect together at a fork before a single rod is anchored.

Perhaps the oddest feature of the stay rods is that the part of the stays which passes through the towers is curved, with the curves extending beyond their points of support. Where ties are continuously curved over their support, you would expect them to sit on a curved saddle to eliminate bending in the rod. As it is, the rods are subject to considerable bending at their upper end, which must hugely reduce their effectiveness. What's even stranger is that the same arrangement has been replicated in the later suspension chain supports.

The suspension chains added during the bridge’s refurbishment are of very different form to the main stays, consisting of flat wrought iron links held together with pins in a manner not dissimilar to many other 19th century suspension bridges. It’s impossible to guess to what extent the two structural systems contribute to the bridge’s ability to bear loads – the stays are stiffer, but imperfectly engineered.

Both systems are abetted by further structural elements which can only be seen from the riverbank. These consist of small bowstring trusses spanning transversely, as well as larger ones spanning longitudinally. Despite their lack of triangulation, these must provide vital stiffness to the structure.

From below, you can also see lateral ties which run diagonally from the deck back to the abutments, on its west side (the right-hand side in the photo). I guess there may once also have been ties on the east side. In any event, the remaining ties are now loose and no longer helping to stabilise the bridge.

The profusion of structural rods, bars, and struts is not especially aesthetically pleasing, although I quite like its unembarrassed pragmatism. However, what I admire even more is the exceptional slenderness of all the various bridge components. With the exception of the tower legs, there are no metal parts more than an inch or two thick anywhere on the bridge. It seemed largely immune to wobble when we crossed it. Aside from cable-net structures, it’s hard to think of a contemporary footbridge equivalent, which suggests that modern designers may still have more to learn from the past.

Further information:

28 August 2012

Scottish Bridges: 45. Abergeldie Castle Bridge

This was once a very fine suspension footbridge in the classic style (as can be seen in an old photo), with similarities to the two Abernethy spans covered in the previous two posts. This one was designed by Blaikie Bros and built in 1885, and always lacked one significant design feature present in the Abernethy structures: stiffening deck trusses.

There are not many suspension footbridges left in the UK without such stiffening. John Hume’s classic paper on Scottish suspension bridges included a gazetteer of every such bridge known to the author, right up to and including the Forth Road Bridge. He identified a total of 14 unstiffened suspension footbridges in Scotland. I think the only one I’ve previously walked upon in Britain is the Sapper’s Bridge at Betws-y-Coed.

The stiffening truss normally also doubles as the bridge’s parapet, and as well as rendering the vertical deflection of the bridge less disconcerting for its users, it also helps to evenly distribute loads amongst the hangers and reduces the likelihood of damage to the structure.

A contemporary drawing of the bridge included in Civil Engineering Heritage shows the bridge as having two layers of suspension cables, one passing from the top of the tower to the top of the parapet at midspan, and a second passing from mid-way up the tower to the deck at midspan. Both cables are still present, and this is a very unusual feature of this bridge, one I can't immediately recall seeing anywhere else.

I don’t know whether the Abergeldie Castle bridge became derelict because of damage caused by the bridge’s users, or because the footpath became redundant leading to no maintenance, or a combination of the two. Today, it is a bridge to nowhere, for nobody. Gates discourage anyone from setting foot on the ruinous main span, but it’s straightforward to clamber around and stand on the tattered remnants of the deck. I wouldn’t recommend it, though, and the thought of falling onto scraps of rusty metal ought to deter most people from going anywhere near it.

Having degraded to its current parlous and perilous state, I’m unsure whether the bridge is in a process of gradual decline, or has reached a point of temporary if precarious stability. It is a protected Listed Building, but is on the Buildings at Risk Register. I imagine there’s no continuing need for it to be refurbished and can’t believe the cost of doing so could be justified. I’d be astonished if the deck is still there in ten year’s time, so if, like me, you have a soft spot for derelict bridges, I suggest you visit it while you still can.

Further information:

26 August 2012

Scottish Bridges: 44. Polhollick Bridge (revisited)

We drove west along the River Dee from Cambus O'May. Our next stop was Polhollick Bridge, near Ballater.

Like the one at Cambus O'May, this bridge was build by the firm of James Abernethy and Co, in 1892. I featured it here before, in February. Pre-dating Cambus O'May by 13 years, it is clearly a less visually attractive design. The towers are more crudely capped, and the "ladder rungs" separating the main cables are peculiar, although presumably not an original feature.

I reported previously that a budget has recently been agreed to refurbish the bridge. It certainly looks like it needs it.

Further information:

22 August 2012

Scottish Bridges: 43. Cambus O' May Bridge (revisited)

From Craigmin Bridge, we drove south to Deeside, before heading west to Braemar. The Dee Valley is particularly rich in fine bridges.

Our first stop was at the Cambus O'May bridge, which I discussed here in February, with a couple of old photographs. It was nice to have an opportunity to visit it again, and look at it more thoroughly.

You can find the basic facts at the February post, so I'll try not to repeat myself too much. It's a bridge by the firm of  James Abernethy & Co., dating from 1905, as was the bridge at Aberlour which we had visited the previous day.

The bridge was substantially rebuilt in 1988, with the original riveted deck trusses replaced in welded steel, and a staircase at the north end removed. The riveted lattice towers seem to be unaltered.

The towers sit slightly outside the bridge deck, so that the main cables are slightly off vertical. One consequence of this is that the hangers are also slightly inclined. However, where they are fixed into the deck, they are bolted vertically through the top flange of the parapet trusses. The result is that the hangers are slightly curved, which I can't imagine does them any good.

The replacement of the deck with welded trusses hasn't done the bridge any harm visually, in my view.

Further information:

21 August 2012

Scottish Bridges: 42. Craigmin Bridge (revisited)

From Elgin, we travelled east to Buckie.

I can't recall when I first visited Craigmin Bridge, but as my previous post makes clear, it was a very hard bridge to photograph. I had hoped that a return visit might get much better images, but as it turned out, poor light and the continued growth of vegetation meant that proved very difficult. So I apologise for the quality of some of these images - they are by far the best of a fairly limited set!

If you're visiting the bridge, which is on private land (the Letterfourie Estate), the best "official" approach is probably to park in nearby Drybridge and walk from there. Scottish rambling rights permit you to walk through fields, but not to park on private land nor to walk through farm yards. I would recommend being cautious and polite, and this will apply to some other bridges on private land that I'll feature later.

I also suggest going well-prepared with a good map, as there's absolutely no sign of the bridge's presence until you're pretty much right on top of it, hidden deep within a wood. The "paths" down below the bridge are also steep and dangerous, so take care.

The bridge has to be one of Scotland's most-neglected architectural treasures. Unsignposted, unheralded, largely unknown, there's no other structure quite like it anywhere else in the UK.

It has a single large masonry arch at a low level. On the north-west corner of the bridge, a doorway opens into the abutment of this arch, which contains a small room. Two further arches sit above the lower arch, springing from its crown. Above the crown, where the two arches come together, there is another room hidden behind walls which are themselves pierced by further arches. At roadway level, there are also arched features in the bridge's parapets.

It's a bizarre arrangement, and a shame that the conditions made it impossible to get a good photograph showing the scale and layout of the bridge properly. The photo at Google maps below is perhaps the best you'll get online, and there are some on the Scottish Highland Bridges site which are pretty good too.

Further information: