23 August 2010

"Beautiful Railway Bridge of the Silvery Tay"

Having read John Rapley's biography of Tay Bridge designer Thomas Bouch earlier this year, I picked up Peter Lewis's "Beautiful Railway Bridge of the Silvery Tay" (ISBN 0752431609, Tempus Publishing / The History Press, 2004, 192pp) [amazon.co.uk] with interest.

The author, Dr Peter Lewis, is a senior lecturer in materials engineering at the Open University. He's also the author of a book (and paper [PDF]) on the collapse of Robert Stephenson's Dee Bridge, neither of which I've read. The fall of the Tay Bridge is a case study in the Lewis's OU Forensic Engineering course, and this book-length study gave Lewis the opportunity to explore it in great detail, as well as offering his own theory of why it collapsed.

The book's title is taken from William McGonagall's notorious poem. The poet ended his thoughts on the bridge disaster with the words: "For the stronger we our houses do build / The less chance we have of being killed", which, as a simplistic theory of why the bridge failed, is not far off the mark.

This isn't a book to read to discover much about the bridge's designer Thomas Bouch, or the background to the construction of the Tay Bridge. Rapley's biography already mentioned covers that territory more than adequately, and there are other books on the Tay Bridge collapse such as those by John Thomas and John Prebble (both out-of-print but readily available secondhand), or Andre Gren's "The Bridge Is Down!", although that appears to overlap considerably with Lewis in its reliance on the original eyewitness testimony.

Lewis's book places the focus on the engineering of the bridge, and as is perhaps proper for a forensic study, concentrates on what evidence has survived since the 1879 disaster. This largely consists of statements made by witnesses at the Court of Inquiry which examined the failure, as well as photographs of the bridge debris taken on behalf of the Court (all the photos used in this post are out-of-copyright images taken from Dundee library's website about the disaster).

The Court concluded that the bridge was badly designed, badly built and badly maintained, and all three conclusions are well supported by the evidence. Bouch and his assistants took only limited account of wind loading, with lower wind pressures used than those being applied by contemporary French and American engineers. The factor of safety against failure applied by Bouch was less than many of his peers, something that can only be justified where loads and materials are understood with higher than normal certainty, which was clearly not the case on the Tay Bridge, a lengthy estuarial crossing entering into new engineering territory.

Inadequate ground investigation led to the redesign of the planned masonry piers as trestle piers, with cast iron columns and wrought-iron bracing. A desire not to have to increase the foundation size meant that the inclined support buttresses normally used on trestle piers at the time (by Gustave Eiffel, amongst others) were not adopted.

Bouch's design also relied heavily on the attachment of the bracing to the columns using lugs cast as part of the column itself. These proved difficult to cast, and there was plenty of evidence of them being burned on to the column after casting, as well as of defects being disguised with beaumontage filler. Adding to the problems with the lugs, they were cast with tapered holes, which weren't subsequently drilled square, with the result that stresses were concentrated.

Workers on the bridge noted oscillations as trains passed, although the observations were never properly communicated to the design engineers. The pier bracing ties were tensioned using driven cotters to wedge two sections together, and maintenance work led to these being wedged in the loose rather than taut position, to prevent chattering. The result was that the ties carried less tension as time moved on, severely reducing the piers' ability to withstand wind load.

Lewis puts forward the theory that the bracing lugs failed due to fatigue. This idea relies partly on the reported oscillations, which would tend to produce cyclic stress, and also on close examination of distant photographs showing the fractured lugs. The images reproduced in the book are very difficult to interpret, however.

The fatigue theory is also presented in detail in Lewis and Reynolds' technical paper [PDF].

Some support for the concept of failure caused by dynamic oscillation was offered by Björn Åkesson in "Understanding Bridge Collapses". He noted that the choice of flat bars for the bracing diagonals to the bridge piers meant that they buckled under compression, such that the bars in tension had to carry a greater share of load (up to double). The use of stiffer sections for the bracing would have reduced the tension load on the bracing bars and hence on the lugs.

Åkesson also estimated the natural frequency of a bridge pier as 1 Hz (Martin and MacLeod, whose paper is discussed below, calculated 0.2 Hz, but it's not clear if this allows for the mass of the train). This could have left the piers vulnerable to excitation either by the wind (including vortex shedding effects from a train) or by lateral nosing effects as each axle passed over a vulnerable position (which would be enhanced by reported curvature in the track and in girders damaged during erection but incorporated into the final structure).

The fatigue theory has its critics. In 1995, Tom Martin and Iain MacLeod reviewed the Tay Bridge failure using modern 3d frame analysis software. Their own paper [PDF] explained failure purely in terms of equivalent static loads, and relied on the assumption that there would be small uplift at the column bases, considerably redistributing the forces in the bracing. Where their paper is particularly good is on the issues beyond the strength of the structure, the economic pressures that Bouch was under which may have led him to attempt a more efficient design than was justified by the state of knowledge at the time.

Martin and MacLeod published a further paper [PDF] in 2004, contesting the validity of Lewis's fatigue theory. Some of what they say strikes me as odd: they evaluate fatigue load due to wind, but ignore the possibility of fatigue caused by wobble excited by track defects. I'd think that neither theory is proveable beyond doubt, there can only be different levels of plausibility in the light of the lack of surviving evidence, and the unverifiable assumptions that a computer analysis must rely on.

More information on the competing failure theories can be found on Tom Martin's own website, as well as on an interactive site produced by Peter Lewis where you, too, can attempt to solve the 'mystery' of how the bridge fell.

Overall, I very much enjoyed "Beautiful Railway Bridge of the Silvery Tay". The presentation of the evidence is comprehensive, with many interesting anecdotes recounted by the eyewitnesses. The many different contributing factors to the collapse are generally well-explained and illustrated, although one or two more diagrams would have assisted.

For the modern designer, the question of which cause of failure is correct should be essentially an irrelevance. The statute of limitations has expired and there is no prospect of a victims' law-suit against a long-gone railway company. However, there are clearly many lessons to be learned which remain relevant today.

The somewhat intuitive approach to loads and factors of safety which Bouch could adopt has generally been superseded by the prescriptions of modern design standards. However, there are always matters of engineering judgement remaining, and the need to consider where the uncertainties in designs may lie. If loads today are generally more predictable, I suspect that hidden construction flaws and tolerance incompatibilities remain a potential cause of departure from the results of simplified analysis, ensuring the need to retain robustness beyond that calculated from the standards.

The need for those responsible for maintenance to properly understand how structures were designed to behave is also as important today as it was then. A greater involvement of builders and designers in the long-term maintenance of their creations would often still be of benefit.

The economic desire for ever-lighter structures which clearly drove Bouch is still strong today, and the Tay Bridge disaster emphasises the need for careful consideration of dynamic behaviour when structures are made more slender, and more vulnerable to excitation from unexpected sources.

19 August 2010

Scottish Bridges: 14. Dunglass Bridges

I chose to stop at Dunglass essentially by accident - driving up the A1 between Berwick and Edinburgh, I was mildly intrigued by a glimpse of three arched bridges in close proximity spanning the gorge of the Dunglass Burn (as in the photograph on the right).

There are actually five bridges here, although I didn't get to see the oldest, the so-called Old Bridge, built in the 17th century. That's a shame, because what I've read since then suggests it might be the most interesting, even though it has the smallest span (10.5m).

The next oldest bridge is the aptly named New Bridge, built in 1798 to a design by James Burn, a local builder and architect (although Historic Scotland attribute the design to his brother, George Burn). Spanning 25m, the arch rises 23m above the stream below. The crenelated parapet (pictured left - the bridge can be seen in elevation in the background of the next photo below) looks quite kitsch to modern eyes, but there are other interesting features, such as the facing voussoirs, some of which step out. Apparently there are arches and doorways within at least one of the abutments, but I didn't see them.

In 1836, a third bridge was built, Dunglass Viaduct (or "ECM8/109", as Network Rail would now have it), which originally carried the North British Railway. This was the largest and tallest structure yet, with five small spans (each 9m) and a large central span of 41m passing 33m above the watercourse (pictured right). It was designed by the NBR's engineers, Thomas Grainger and John Miller. All the bridges here (apart from the most recent), are Listed Buildings, generally Grade B, with only the railway viaduct achieving Grade A.

Nearly a century later, a new road bridge was built. The road had been designated as the A1 in 1921, and no doubt the New Bridge struggled to meet the demands on increasing traffic. In 1932, the first A1 bridge was completed (pictured left). It was designed by Blyth and Blyth and built by Crowley Russell and Co. It is a reinforced concrete arch structure, with five arch ribs. In 1987, it was apparently due to be demolished due to structural problems, but I guess demolition was avoided by building the fifth and final of these structures.

For me, the concrete arch bridge is visually the most interesting of these spans, especially when viewed from closer at hand, where the forest of columns (pictured right) seems to echo the forest of trees that surrounds it.

In 1932, Robert Maillart was building his splendid Rossgraben Bridge, having built Salginatobel two years earlier. In the same year, he completed deck-stiffened arches at Traubach and Bohlbach, dress rehearsals for his 1933 masterpiece at Schwandbach. But even his earlier bridges were more refined in form than this A1 bridge, with its clumsy detailing and awkward masonry parapet. It shows quite how far behind Maillart most reinforced concrete arch designers were, but I still enjoyed seeing the Dunglass bridge.

The final bridge carries the present A1, and is a steel composite girder structure, with two massive girders carried on 20m tall finger piers. I don't know exactly when it was built, but there was a technical paper published in 1997 concerning the launching method used for construction, so perhaps that gives an indication.

It's easily the least attractive of any of these bridges. It's a shame that they didn't build a fifth arch, but the economics of bridge construction have changed enormously since the first Dunglass bridge was built.

Further information:

18 August 2010

Northumbrian Bridges: 3. Heatherslaw Mill Bridge

The Union Bridge and Twizel Bridge are amongst the best known of Britain's historic bridges. Heatherslaw Mill Bridge is definitely not - I don't believe it features in any of the guides to British bridges that I covered recently, nor is it in "Civil Engineering Heritage: Northern England".

Nonetheless, it is a Grade II Listed Building, and a type of bridge that I always find interesting.

It provides road access across the River Till to a Victorian corn mill on the Ford & Etal Estate. It was built in 1877, supposedly as a temporary access in place of an existing river ford, by the Glasgow firm of A & J Main & Co. They were at one time better known for their wrought iron fence posts and gates, although they became increasingly involved in structural metalwork as time went on.

The bridge is described at SINE as consisting of iron with a steel lattice parapet, and at Scottish Ironwork as a wrought iron lattice girder.

The lattice girder has obvious attractions to the designer, in combining the functions of the parapet and support beam in one (this wouldn't be permitted on modern road bridge, due to the risk of vehicle impact leading to the collapse of the whole bridge). From one perspective, it overlaps multiple triangular trusses, allowing the shear force effects to be shared by smaller members which are easier to fabricate. Seen another way, it's a plate girder with the web perforated, allowing the largely redundant material in most girder webs to be pared away.

At Heatherslaw, the two sets of lattice members are noticeably eccentric to the girder centreline - I presume that any tendency of this to twist the main girder flanges in plan is reduced because the lattice members apply roughly equal opposing forces at any nodal point.

The bridge originally had a timber floor, which was replaced in 1951 with the War Surplus decking units now present. It's not a type of flooring I can recall seeing previously.

It's an unassuming bridge design, a reminder from the past that modesty can still be a virtue.

Further information:

17 August 2010

Northumbrian Bridges: 2. Twizel Bridge

Twizel Bridge sits alongside the A698 where it passes over the River Till, between Cornhill and Norham. The roadway which previously crossed over this narrow (4.6m wide) medaeval arch span has long since been rerouted onto a modern structure.

Along with the Devil's Bridge at Kirkby Lonsdale, Twizel Bridge is one of northern England's great medaeval spans. It's date of construction is not known for sure, although it was certainly there in 1513 when troops crossed it on their way to the battle at nearby Flodden.

It spans 27m, which was reportedly the largest stone arch span in England for three centuries. John Leland described it as "of stone one bow but greate and stronge".

What makes it particularly attractive are its five chamfered ribs, an element which make any masonry arch bridge much more interesting. The dentilated string course is also a nice feature.

Bob Robson's book, linked below, describes a number of repairs carried out to the bridge in the 1960s and 1970s, when it was clearly unsuitable for the traffic on it. Damage to the parapets required repair, a wing wall needed to be propped, and the spandrel walls were reinforced with tie rods (with their anchor plates reportedly hidden behind the masonry facing, although there are further tie rods visible in my photos which perhaps were added later by a less sympathetic engineer).

Further information:

16 August 2010

Northumbrian Bridges: 1. Union Chain Bridge

I recently visited the English/Scottish Borders area, and visited one or two interesting bridges while there, all historic structures.

I'm calling the first of these bridges a "Northumbrian" bridge, although it's every bit as much a "Scottish" bridge, given that it spans the River Tweed across the border between England and Scotland.

The Union Bridge was the first suspension bridge in Britain to carry vehicles, and when opened in 1820, it was the longest suspension bridge in the world. It cost £6,449 to build.

The deck spans 110m and is 5.5m wide. The bridge has three chains of iron links on each edge, spanning 129m. The chains are vertically above each other, with the links staggered so that the hangers are evenly spaced. The chains pass over a masonry pier at the Scottish end, while at the English end, an abutment is built into a cliff face, with the chains anchored by a masonry arch buried within the rock. The difference in spans is because the deck reaches ground some way in front of this abutment.

Its survival to the present day is remarkable. Its designer, Captain Samuel Brown, mainly got into suspension bridge works as a way of exploiting the iron chain technology he had developed while in the Navy, and his very slender, unstiffened bridge decks were prone to oscillation in the wind. Most of his bridges have long since been destroyed.

The Scottish tower looks particularly large and robust to my modern eyes, but that can be blamed not on Brown, but on John Rennie, who had helped the Berwick Turnpike Trustees to appoint Brown. Rennie described Brown's tower as "clumsy, ill arranged and overloaded with ornament", and suggested that the tower be redesigned as larger and with tapering sides.

The Union Bridge has been refurbished and strengthened on many occasions (the paper by Miller linked below gives ample details, as does Bob Robson's book), most obviously including the addition of a wire rope cable above the original three suspension chains.

Even then, the bridge's load capacity is very limited, and in addition to a 2-tonne weight limit, only one vehicle is allowed to cross at a time (this is probably to avoid the risk of damage to the cables, which are not protected by parapets of modern standard). Walking across it, the vibration from a large 4x4 was noticeable, and it was fairly responsive even to a bit of jumping up and down.

I especially like Robert Stevenson's contemporary description of the bridge in the Edinburgh Philosophical Journal (linked below):
"The general effect of the Union Bridge, which we have now endeavoured to describe, is interesting and curious; and such is the extent, and its light and elegant appearance, that it has not inaptly been compared to an inverted rainbow. Those who visit this undertaking, as affording much novelty to the scenery of this part of the banks of the Tweed, will not be disappointed in their expectations; while, in a national point of view, as a great improvement, it deserves the most particular consideration of the country at large."
Further information:

13 August 2010

Bridges news roundup

New bridge designs for Perth go on display
Neither the 126m arch or the underslung truss is particularly appealing or well-detailed, leaving voters with Hobson's choice (for full details of each design see the PDFs at the voting website). Given the span and general layout, I'm surprised a suspension bridge isn't offered as an option.

Columbia River Crossing design raises questions
Better safe than sorry, say people made nervous by an unusual "open web" highway bridge design in Washington, USA (pictured). So far as I can tell, the design is for a steel truss with concrete decks top and bottom. I like how the story misquotes a bridge engineer as suggesting that "the concept of using steel in conjunction with concrete is still new". The entire story is shot through with a deep fear of innovation - Japanese engineers are cited as saying they would never again like to go through a project where extensive testing, trialling and confidence-building is required. How else does innovation ever happen?

Base price set at $7.4 million for Peña Boulevard rail bridge
Calatrava's light rail bridge at Denver airport is predicted to cost up to US$60m. Mystery remains as to why they commissioned a design they can't afford. The US$7.4m is the price offered by the light rail system's design-build contractor to span the same gap. They have until January to decide whether they can afford the premium for the Calatrava design, which is a 189m span steel bowstring arch, rising 44m (pictured). I'd guess the D&B firm's proposal is something quite different!

Victoria Council votes to replace Johnson Street Bridge
I've covered this before: the choice between refurbishment of a historic bascule bridge, or its replacement with a new structure designed by Wilkinson Eyre (pictured). Following considerable further study and public consultation, Victoria's council has voted for the latter, but still has to win a referendum in November to be able to borrow the money required.

11 August 2010

Scottish Bridges: 13. Gogarburn Bridge

Here's another leftover, a couple of photos I dug out from last year, of Gogarburn Bridge, near Edinburgh.

This highway bridge provides slip road access off the A8 dual carriageway into the offices of the Royal Bank of Scotland (whose logo you can see dangling from the crown of the arch). It opened in March 2006, is 25m high, and spans 60m.

It shares its typology with a small number of other bridges, many of which I've covered here before:

All these bridges have a single steel arch spanning diagonally across the deck, with the cables arranged asymmetrically so that from one angle, they give the appearance of criss-crossing, in the manner of a network arch. At Hulme and Gogarburn, the bridges cross busy main roads, and the network arch appearance is therefore visible to many drivers every day. The same does not apply at Newport Street (above a railway) or Clyde Arc (above a river), and that makes me wonder whether the solution is visually appropriate at those sites.

There are also similarities to the Juscelino Kubitschek bridge in Brasilia, although that has a much more efficient (and less visually interesting) hanger layout.

There are a number of reasons why this type of bridge is structurally inefficient. The use of inclined hangers rather than vertical hangers induces axial forces into the deck, and because of the skew of the arch, this induces twisting of the deck in plan. Additional bearings are required to resist this, and Gogarburn Bridge also requires bracing to the hanger outrigger beams.

An arch subject to vertical load in its own plane is subject to very limited bending effects and hence can be made structurally efficient so long as any tendency to buckle laterally is prevented. At Gogarburn (and its siblings), the cables apply substantial asymmetrical forces horizontally to the arch, establishing large lateral bending moments which may control the design.

These inefficiencies are justified by the striking visual effect of the structure, which consists of its faux-network silhouette; the easy visual legibility of a single rather than double arch; and the visual interest created by a profile which varies constantly as you move around it.

At Gogarburn, I think the effect is justified, particularly by the desire of RBS to create a visual gateway to their headquarters site.

Gogarburn Bridge was designed by SKM Anthony Hunt (who also carried out the independent design check at Newport Street), and built by Watson Steel for Sir Robert McAlpine.

Further information:

10 August 2010

Manchester Bridges: 15. Exhibition Footbridge

I've mentioned this footbridge previously, but had the chance to return and photograph it again, so it's getting its own post this time. Unlike many of the bridges I feature here, it's certainly not because of its beauty.

The steel tubular truss bridge was built in 1985 as part of the works to create the G-Mex exhibition centre in what was once the Manchester Central railway terminal. It connects Deansgate Railway Station to G-Mex tramstop, although when I visited it was closed for repair work.

It's the sort of bridge which is very easy to imagine looking good on a drawing: simple, regular geometry; an interesting cross-section combining the pentagonal truss with the tubular "glazing"; and the thought that the bridge would be light and open in outlook.

The reality is a truss that would already have looked very "seventies" when built, wrapped around a horribly discoloured polycarbonate tube that has aged even less well. The brown tubular bookends that appear to support it are rather ghastly too.

The lessons for designers, I think, relate not only to the need to understand how a bridge will look in three dimensions, but also to plan to facilitate maintenance (such as cleaning, difficult here). Avoiding materials that don't cope well with a lack of maintenance should also be high up the agenda.

Further information:

08 August 2010

Coming soon

I'm just back from a short holiday and conscious that I didn't line up any posts here in advance to tide things over while away. So, here's a very quick update on what is coming soon:
  • a review of Peter Lewis's book "Beautiful Railway Bridge of the Silvery Tay"
  • a bridge in Manchester missing from my recent series of posts
  • a few bridges in Scotland

As ever, if you have bridge-related news that you think I might like to cover here, please get in touch!