31 July 2012

Scottish Bridges: 34. Island Bank Road Footbridges, Inverness

Heading further south along the east bank of the River Ness, we found two tiny little footbridges which span a drainage channel onto part of the Ness Islands. They weren't among the bridges we were looking for, but the parapets are so cute that I had to include them here anyway. Both bridges were installed in 2007.

There's very little I can say about these - I found almost no information online other than the date of construction, and structurally they are extremely simple single span beam bridges.

It's always a pleasure to see a designer and their client who don't feel the need to kow-tow to the idiotic monotony of codes and standards. The bridge parapets don't comply with normal design requirements (specifically, BS 7818, Specification for pedestrian restraint systems in metal), insofar as they are climbable and the openings considerably exceed the standard 100mm maximum. But if they did comply, what would be the point? There's an unprotected river bank a few metres away which is a far greater hazard.

If only all clients were as sensible.

Further information:

30 July 2012

Bridges news roundup

A few bridge-related links and stories from the last few weeks:

Torrens footbridge design unveiled
Winner announced in AUS$40m pedestrian bridge contest in Adelaide, Australia.

Leith Walk ‘Green Bridge’ / biomorphis
A proposal for an elevated "High Line" style garden walkway in the city of Edinburgh.

Wupper-Bridge Opladen / Ağırbaş & Wienstroer
I like this interesting little German footbridge.

Footbridge will link city centre to nature reserve
Proposals for a £0.25m footbridge in Gloucester are kicked off with a "young engineers" design competition by the Institution of Structural Engineers.

Shoreham footbridge to go ahead despite £9.8m cost
Ouch. Previous budget £5.8m.

LEGOs Hack Bridge in Germany
Delightful decoration of a bridge in Wupperthal.

Pennington Road Footbridge / Softroom
Timber footbridge over canal in Liverpool. I visited it previously.

Recession shatters plans for £5m glass Heatherwick bridge
He may have triumphed with a rather nifty Olympic "cauldron", but Heatherwick's all-glass footbridge at Kings Cross, London, has fallen victim to its price-tag. £5m is rather a lot for a 16m long bridge.

29 July 2012

Scottish Bridges: 33. Infirmary Bridge, Inverness

The next bridge upstream from the Ness Bridge is a sibling to the Greig Street Bridge, built in the same year (1881), designed by the same engineer, but with a number of subtle differences.

The design is credited to C R Manners, and a plate on the bridge credits W Smith and Sons, Ness Ironworks, although I note that Civil Engineering Heritage credits Rose Street Foundry for both bridges.

The main span is 61.0m, marginally shorter than at Greig Street, while the two back spans are significantly shorter, at 12.8m instead of 20.4m. I can't say that they look particularly different as a result. The main cable passes slightly higher above deck level, but that's about it.

Both bridges have slightly tapered, banded cast iron piers. The towers have a similar, but less dense, lattice than at Greig Street, and the arched tower portal members are both less sturdy and less ornate.

As at Greig Street, the main cables open out horizontally towards their ends, meaning that the hangers are at a progressively more shallow inclination. All the hangers incline inwards from the main cable towards the deck, which is different to Greig Street, where the hangers near midspan actually incline outwards.

The deck trusses are similar to its sibling, although appear to be original rather than having been replaced in welded construction.

As at Greig Street and some other Inverness bridges, there's a low-height railing to provide pedestrian containment.

The most significant difference between the two bridges is that the Infirmary Bridge features tie-down cables below deck adjacent to the piers. I presume these were added at some stage to reduce horizontal and/or vertical movement of the deck.

It's not by any means a great bridge, but it's a slight cut above many of the similar pedestrian suspension bridges which were built in the early 20th century. I think it's aesthetically superior to the example at Pitlochry visited earlier the same day. We visited a few more later on the trip, so I'll compare those later.

Further information:

26 July 2012

Scottish Bridges: 32. Ness Bridge, Inverness

Heading south along the River Ness, the next bridge after Greig Street is the Ness Bridge. A series of wooden bridges first occupied this site. A seven-span masonry arch viaduct was built here in 1685, but washed away in a flood in 1849. It was replaced for £26,000 by an asymmetric suspension bridge, designed by J.M. Rendel, with a massive masonry tower at one end. This was thought to be less susceptible to flood damage, but the towers were so massive that there was barely any room for vehicles to pass through their portals. Consequently, in 1961, the bridge was replaced again, at a cost of £250,000.

The current bridge is a three-span prestressed concrete structure. Two 19m side spans frame a central span of 36.6m. The central span in turn comprises an 18.3m "drop-in" section, supported on two 9.15m cantilevers. The designer was Sir M. MacDonald & Partners (now part of Mott MacDonald), while the contractor was Duncan Logan Ltd.

The bridge's abutments are faced with granite from Shap in Cumbria, and the piers with granite from Corrennie in Aberdeen.

The concrete elevations have been bush-hammered, presumably to give a more even appearance.

It's a very fine example of this type of bridge. The shallow curvature of the soffit is such that the presence of the drop-in span is well disguised, and I find it visually very attractive.

A lighting system was added to the bridge in 2008, and is simple but effective. We saw the bridge on an overcast evening with a heavy-flowing river; I imagine it looks splendid in clearer weather and with calmer, more reflective water.

Further information:

24 July 2012

Scottish Bridges: 31. Greig Street Bridge, Inverness

At last, we had reached the end of our first day's tour of Scottish bridges - Inverness. However, although this was the end of the journey, Inverness itself held several more bridges that we wanted to see (and many more that we wouldn't have time to get anywhere near).

Heading south from the town centre, along the River Ness, the first bridge that we visited was the Greig Street Suspension Bridge. Built in 1881 for £1400, it is credited to the engineer C R Manners, and the local Rose Street Foundry. It's main span is 61.3m, and the side spans are each 20.4m. The piers below tower level are cast iron.

The bridge deck is stiffened by warren trusses on either side, with separate railings used for pedestrian containment. The form of the railings is shared by other Inverness bridges. The deck is described as iron in a number of places, but it seems rather obvious from the photograph on the right that it has been reconstructed in modern steel at some time. It's known that the cables were renewed in 1952, and the anchorages in 1989, but I can't find any information on when the deck was replaced. John Hume's photographs of the bridge taken in the 1970s, available on the RCAHMS website, show a different detail for the truss web members, so it has happened since then. Notwithstanding these substantial elements of replacement material, the bridge remains a Listed Building.

The pylons employ latticework which is much denser than normally seen on pedestrian suspension bridges from the same era. The ornate archways below the tower crossbeams are particularly attractive. As you'll see in a couple of posts time, there is a very similar bridge further upstream over the River Ness. However, I think the Greig Street structure is marginally the better of the two.

The positioning of the main cables is interesting, as they splay out noticeably towards the anchorages. The hangers have a small inclination near the towers, becoming progressively shallower near the anchorages. You can see quite how shallow they become in one of the photos above, at the top right. Visually, it helps to open up the entrance to the bridge, giving it a more "welcoming" appearance. I haven't seen it in such a pronounced way on similar bridges.

One oddity of the hanger arrangement is that they actually reverse inclination at midspan. Everywhere else, they incline outward, but in this location they incline inwards, requiring the use of short extension stubs below the main parapet support members. I'm unclear exactly why they do this, unless it enhances lateral stability. The bridge certainly didn't strike me as prone to much pedestrian-induced movement.

The bridge is a key Inverness landmark, both an important pedestrian route, and part of a riverside promenade which offers views of local buildings, and of the hills beyond.

22 July 2012

Scottish Bridges: 30. Aultnaslanach Viaduct

From Tomatin, we drove north towards Inverness. Our Scottish bridge tour's first day was drawing to a close, although Inverness itself had at least half-a-dozen more bridges lying in wait.

The railway to Inverness passes over a small stream on the Aultnaslanach Viaduct. Opened in 1897, this was built as an all timber trestle structure, supposedly because of difficulties with foundations for a heavier bridge. The ground is certainly quite boggy here, and it was difficult to walk even the short distance from a nearby minor road across to the bridge.

I visited this bridge several years ago, after first reading about it in Gillian Nelson's excellent Highland Bridges. It's a very rare survivor, as most timber railway bridges from the Victorian age were rebuilt long ago, most obviously Brunel's timber viaducts in Cornwall. Several still survive in Wales, although they are slowly falling prey to "progress" (e.g. Pont Briwet, approved for demolition earlier this year despite its protected  heritage status).

The bridge's five spans are each about 20 feet long. The four trestles each have four vertical posts, and two raking posts, supplemented by raking outriggers, and they are further connected to the longitudinal beams with knee-braces, resulting in a distinctive series of octagonal bays when viewed on elevation. The trestle posts are fixed to timber piles penetrating 25 feet through a layer of peat to underlying gravels. Like the Broomhill Bridge visited earlier, it's a primitive design, robust and delightfully un-pretty. There's an excellent archive drawing to be found online which shows the bridge's layout in detail.

The bridge was found in the mid-1990s to be suffering from wet rot, with a number of timber samples showing moisture at levels well above the 20% rotting threshold. Actual removal of timbers from the bridge failed to confirm this finding, and a structural assessment found the trestles to have sufficient strength for the railway loads applied. Indeed, monitoring of the bridge while trains passed showed only minimal deflections, less than 2mm at the top of the timber posts. At that time, it was proposed to remove individual timbers thought to be suffering most from rot, and to replace them.

Sadly, the powers-that-be decided that repair was not the best long-term answer, and a £2.6m project was undertaken to "refurbish" the bridge in 2002. By this time, it was reported that rot was causing some timber members to suffer from up to 40% section loss, and a 40mph passenger traffic speed restriction had been in place for several years. The work essentially consisted of gutting the bridge, replacing its previous corrugated metal deck with a conventional steel girder railway bridge, and supporting this on concrete support piers. An image of the bridge under reconstruction shows what was done. Clearly, whatever foundation problems had troubled the original builders were no match for the modern railway engineers. The new bridge was inserted in the middle of the existing bridge, allowing the vast majority of the timber frame to be left in place surrounding it. A German firm was brought in to inject the timber with a new chemical preservative treatment.

The new bridge piers were positioned midway along the 2nd and 4th spans of the bridge. While this eliminated the need to modify the main trestles, it has the unfortunate effect of ruining the bridge's appearance in elevation. The timber elements are essentially now redundant, and it's hard not to imagine that deprived of their load-bearing function, they will receive less maintenance in the future than if they still carried the trains.

The project won a series of awards, most notably a National Railway Heritage Award, a "conservation" commendation from the Saltire Awards, a British Construction Industry Award, and it was a finalist in the Prime Minister's Better Public Building Awards. From one point of view, it is ingenious how they have hidden the new bridge in the belly of the old, thus preserving as much of the original fabric as possible.

From another perspective, this was a catastrophic intervention, turning the historic timber structure from a heroic survivor into a mere decoration. Looking at it, I find it very hard to believe it could not have continued to carry railway vehicles, with the original timbers replaced individually where necessary. It looks to me like the railway saw an opportunity to be rid of a long-term liability, short-term expediency triumphing over any serious desire to preserve an exceptionally rare historic structure.

Further information:

19 July 2012

Scottish Bridges: 29. A9 Viaduct, Tomatin

The A9 highway which used to run over the Findhorn Bridge now runs immediately north of the Findhorn Railway Viaduct.

 The A9 highway viaduct was completed in 1976. Finding information on the bridge online has been almost impossible, but an old issue of The Highway Engineer reveals that it cost about £2.5m, and employs about 1,440 tonnes of weathering steel in its eight girders. The consultant engineer was W A Fairhurst and Partners, the main contractor was Whatlings Ltd, and the steelwork fabricator was Fairfield Mabey.

It was the second weathering steel bridge to be built in Scotland (the first was in 1973 at Seabegs near Stirling). Fairhurst designed three other bridges in the same material on the A9, at Slochd Beag, Nairn and Dulnain.

There's quite a nice picture of the bridge in the Corus publication Weathering Steel Bridges. This does indicate some rust staining of the piers where water is running off the girders. The bridge has apparently also suffered problems with its deck drainage, which was originally via gully pots into down pipes which discharged 150mm below the girder soffits - wind blown spray back onto the girders allowed chloride-contaminated water to damage the girder bottom flange. Water also became trapped near bearing stiffeners, causing further corrosion damage.

As with its neighbouring railway viaduct, it's remarkable how modern the A9 bridge looks. Not only are the deck lines very clean and straightforward, but the bridge piers have also been very well shaped. They are in the form of reinforced concrete twin-legged hammerheads, with both the legs and crosshead heavily chamfered, presumably to give an impression of slenderness.

Frederick Gottemoeller's excellent book on the aesthetics of simple highway bridges, Bridgescape, recommends avoiding hammerhead piers, as the end of the crosshead can become visually prominent, and "visually attach" to the superstructure, making the bridge seem thicker overall. This is less of an issue for tall piers, such as those at Tomatin.

Overall, this is an excellent bridge. We (the bridge design community) are still designing far worse structures today, some 36 years on.

Further information:

18 July 2012

Pontist critiqued (again)

A short interruption to the ongoing series of Scottish bridge posts, if I may.

Every so often, I get the impression that people are treating this blog with undue seriousness.

You can find references to it on Wikipedia, as if it were an authoritative source of information. There are links to it from the websites of some bridge designers, presumably on those happy occasions when I've found something good to say about their designs. The blog has even been cited in at least one conference paper. Until now, my personal favourite was when this blog's critique of the River Wear Bridge drew a direct response from the designer.

Here are two more examples that I've only just spotted.

The first comes in a discussion paper by the researcher Professor Sohei Matsuno. Earlier this year, I posted a fairly detailed commentary on the collapse of the Kutai Kartanegara bridge. Prof. Matsuno evidently found it sufficiently interested that he has drafted a detailed comparison of his own theory of the bridge collapse against what I cobbled together from various news stories and Google (if the link doesn't work, you can hopefully read it via Google's quick preview). I get the impression that it's a self-conscious sparring bout, a dress-rehearsal for a discussion yet to be had with the official collapse investigators.

I'd highly recommend it to anyone with a keen interest in bridge failures. The technical detail is fascinating, and Prof. Matsuno does a fine job in elucidating the difference between the forensic engineering approach (to identify a single key cause of failure, and identify supporting factors from a technical perspective), and my more generalist approach, which regards the specific engineering issue as secondary to commercial and behavioural issues - the latter are generally the key areas requiring change if safety is to be improved.

There are a few points where he perhaps reads more into what I wrote than was ever intended (particularly on factors of safety, where stating that they help safeguard against design and construction errors is not the same thing as stating that they are intended to do so - they are not). Nonetheless, it's an excellent read, as Prof. Matsuno clearly has a sense of humour and a rare ability to combine academic rigour with a more personal perspective.

The second example is the preface to a recent book by Haig Beck and Jackie Cooper about Brisbane's Kurilpa Bridge. I haven't read the book, but the preface can be found on books.google.com. I discussed the bridge in June 2009, and Beck and Cooper have kindly used that blog post as the pivot around which their entire preface circulates. They note that my blog post very clearly exposes some of the heuristics (rules of thumb) which apply both to the decisions made by bridge designers, and also to those who attempt to aesthetically evaluate the results. They say:
"The identification of such heuristics raises the questions: How did these heuristics come about? Why do they persist? Are they valid?"
These are good questions, and ones that I've touched on several times in the past. Whether "design critics" are well-qualified to answer them is another matter. It's certainly true that there are certain axiomatic assumptions that are rarely challenged within the core bridge design community.

At some point, I'll buy the book and review it here, and address their arguments properly. For now, it's interesting to note how a quick blog post thrown together in an hour or two can offer a starting point for something far more serious.

Tomorrow, I'll be back on with the Scottish bridges ...

17 July 2012

Scottish Bridges: 28. Findhorn Viaduct, Tomatin

The village of Tomatin is dominated by the presence of two massive viaducts which cross the valley of the River Findhorn nearby. This photo shows them both visible from the nearby Findhorn Bridge.

The first of these is a railway viaduct, variously described as the Tomatin Railway Viaduct or the Findhorn Viaduct. I've gone with the latter as that is after all what the bridge was called on an original drawing from 1890.

The bridge opened in 1897 as part of the Highland Railway. The engineer for the railway was Murdoch Paterson, and the bridge engineer was John Fowler. The bridge's nine spans total 405m in length, and the viaduct is up to 44m tall above ground level.

The bridge deck comprises two "double-Warren" steel trusses, which sweep across the valley on a gentle but dramatic curve. John Fowler, of course, is best known as the designer of the Forth Railway Bridge, and the Findhorn Viaduct has several similarities, both in the use of the double-Warren arrangement (two superimposed sets of "V"s making up an "X" arrangement), and the use of tapered stone masonry piers. The approach viaducts at the Forth have both these features.

In November 1898, the Inverness Courier described the bridge as follows:
"The Viaduct across Findhorn Valley and river, of which it commands magnificent views, is 1335 feet in length, and its maximum height is 140 feet. It has a rise of 1 in 60 from north to south. The superstructure is of steel, resting on native granite piers, of massive and towering proportions, which have their foundations on immense blocks of concrete, those in the river bed being also supported by deeply-driven wood piles. The Viaduct has a graceful curve of a half-mile radius".
I think it's a remarkably elegant bridge. The tapering piers may have a structural rationale (the weight of the pier itself increases vertical loads at the bottom, which should therefore be larger in cross-section if it is to carry equal stress to the top), but are also visually very attractive, helping to emphasise the bridge's height. The uniform linearity of the deck makes it feel less heavy overall than a haunched design would. It's a remarkably modern-looking design.

In fact, I think it's one of Britain's finest metal railway viaducts.

Further information:

15 July 2012

Scottish Bridges: 27. Findhorn Bridge

Our first day's journey through the bridges of Scotland continued to head north, towards Inverness, where we would rest. Our next stop was the tiny hamlet of Tomatin, home to three interesting bridges. Sadly, we didn't have time to visit the nearby bucket bridge at Shennachie, that will have to be one for a future trip.

Findhorn Bridge was built from 1924 to 1926, and carried the old A9 highway for two spans over the River Findhorn. An inscription on the bridge states it was built as a replacement for a Thomas Telford structure of 1833, although it is likely that Telford's bridge had in fact been lost earlier. The bridge was built by Sir Robert McAlpine and Sons for £36,120.

The bridge was designed by Owen Williams, with Maxwell Ayrton as consulting architect. The two worked together on a number of bridges, and Findhorn Bridge is generally thought to be one of the most successful examples. On earlier schemes, such as the now-demolished Lea Valley Bridge, Ayrton's main contribution was to add monumental towers at the ends of the bridge, structurally irrelevant but marking the bridge as a gateway structure. For the Findhorn Bridge, the choice of a structural form which could be monumental in its own right, led to a happier collaboration.

So far as I am aware, there's nothing quite like Findhorn Bridge anywhere else. It has two 29m spans, each broadly of the Vierendeel truss type, in massive reinforced concrete. That in itself isn't completely without precedent. What is unusual is the architectural treatment of the bridge.

The openings in a conventional Vierendeel truss are rectangular, sometimes with filleted corners. Where applied, the fillet enhances the strength of joints between the vertical and horizontal truss members, the joints being subjected to particularly high stresses in a Vierendeel design. In the Findhorn Bridge, the upper fillets have essentially been massively extended until a semi-octagonal opening is formed, instead of a rectangular one.

The effect is less like a bridge truss, and more like some kind of gothic arcade. The bridge elevation could be the facade of an architectural folly, particularly with the hulking overlook towers at each end. The central "tower" incorporates rectangular view-ports with a distinct resemblance to the arrowslits on a medieval castle. The overall impression is of sullen impregnability, a barely-pierced grey rampart. Stand here and prepare to repel invaders.

Nonetheless, the bridge has a considerable portion of charm. Much of this comes from the shaping and detailing of the various elements. For example, the outer face of the parapet wall is inclined inwards, keeping it visually distinct from the main truss elements. The vertical members have a chevron groove at their top which enhances the visual impression of it being a colonnade rather than a truss. The crossbeams below the deck are fish-bellied, reflecting the way the designer presumably understood the stresses to be distributed.

The trapezoidal opening in the central bridge pier is echoed by the shape of the legs recessed into the two end abutments. I'm far from sure how the bridge is supposed to articulate - the recessed legs, which narrow towards their base, suggest that under thermal expansion, the end legs are intended to flex while the central pier is a fixed point.

One nice feature on the bridge is the presence of "inscriptions" cast into the concrete surface. One, near the end of the bridge, credits "E. Owen Williams", something I'm sure many modern bridge designers would like to see emulated. Another, near the middle, describes the purpose of the bridge and offers more detailed credits to the various parties involved.

A short distance to the east of the bridge lie three flood arches. The stonemasonry suggests that these are part of the original Telford construction. On the south face, the road was widened in reinforced concrete by Owen Williams, and the result is a nice instance of old-meeting-new with neither suffering.

Further information: