28 November 2013

Durham Bridges: 1. Kingsgate Bridge

The last few posts dealt with bridges on Teesside that I visited as part of an IABSE study tour. On the afternoon of the first day, we travelled a little further north, to Durham. Our first stop: Kingsgate Bridge.

Built in 1963, this footbridge was personally designed by Ove Arup, and is reported to have been one of his favourite designs. At the time, Arup was probably more involved in running his Partnership than in the specifics of individual projects, but for the Kingsgate Bridge he immersed himself in every detail.

Famously, the University in Durham had only £35,000 to spend on a new bridge linking their older buildings by Durham Cathedral to a new site on the opposite bank of the River Wear. They anticipated this would be enough only for a bridge at the river valley floor, some 17m below the tops of the valley slopes. Arup convinced them that the same money could pay for a 107m long bridge running at height, if the structure could be made efficient.

The engineering of the bridge displays a masterful balance of structural requirements with construction requirements. The bridge has only two main support points, each on a piled base. From these, V-shaped "fingers" carry the bridge deck, reducing the deck spans. The arrangement also allowed for a highly economic method of construction, eliminating the need to construct reinforced concrete in-situ above the river. Each half of the bridge was built parallel to its river bank. The pier supports conceal an internal cone, which allowed both 150-ton halves of the bridge to be rotated 90 degrees into their final position.

This is certainly not a unique construction method, but it is relatively rare and I'm not sure whether there were any precursors in the UK at this time.

At first sight, the bridge is not an immediately loveable design, notwithstanding its status as a Grade I Listed Building. The supports are stiff and angular, and the bridge deck, apparently envisaged as a "thin, taut, white band stretching horizontally across the valley", is blank and lumpen. As the river banks have become considerably overgrown, it's also almost impossible to see the entire bridge as the single composition which was originally built.

Even in detail, the bridge has its flaws, and I find the "fingertips" which connect the finger piers to the deck to be particularly awkward. There's little sense of how the forces are transmitted (the deck is in tension between the fingers, and that considerable force has to be transmitted through these tiny fingertips). There's no shaping of the concrete to respond to those forces either: the band-like concrete side-beams are the same depth at the ends as at the middle, although the stresses in each section are very different. Arup was certainly not a master of concrete on the level of Maillart, Torroja, Candela or their ilk.

Nonetheless, it's an admirable bridge in many ways. The detailing of the finger piers is interesting, minimising material while providing stiffness by means of a folded cross-section that constantly transforms from top to bottom. And the bronze expansion joints where the two bridge halves connect are quite exquisite. These allow horizontal movement while locking the bridge decks together vertically.

I am not generally a fan of "half-through" footbridges, where solid beams on either side of the walkway double as the parapets. Generally, visual transparency seems preferable. However, for a footbridge at height, as is the case here, solid parapets do offer a much greater sense of security for the bridge users.

Shortly after the bridge was built, Concrete Quarterly praised it for having a "subtlety without chi-chi or softness, but keeping in its fineness a certain strength of plane". Its angularity closely recalls the great Italian engineer Riccardo Morandi, whose structures are equally hard to love. Today, I think it would be improved considerably simply by cutting down a few trees and giving it the chance to be seen properly as a sculptural object within the landscape.

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20 November 2013

Teesside Bridges: 4. Infinity Bridge

Okay, it’s white elephant time.

A design competition was launched in 2003 for a new footbridge across the River Tees at Stockton. The intention, as in so many instances, was to use a landmark structure to signpost the local authority’s commitment to developing a new area, on the north bank of the Tees, and attract in outside investment. A budget for the bridge of £4.5m was stated.

The competition was won by Stephen Spence Associates with Expedition Engineering, who developed the concept for a twin arch bridge, with one arch twice the span of its neighbour. The asymmetric arrangement kept the central part of the river free from obstacles, this part of the river being used regularly for rowing training. There was to be some controversy over authorship of the design.

The bridge was completed in 2009, at a cost of £15m, an enormous increase over the original budget, and one which presumably involved the promoter having to plunge their hands deep into their pockets. This is above average for a landmark bridge, but not unreasonably so given the spans involved. The original budget was never enough for a truly iconic structure, and the Infinity Bridge is certainly that.

Visiting four years later, all that our tour group could find on the north bank was mud and grass – none of the hoped-for development has materialised, perhaps unsurprisingly given events in the wider economy. It’s in this sense that the bridge is a white elephant, and it remains to be seen whether the money was well spent.

Nonetheless, the bridge is both spectacular and elegant, and its architectural impact is largely a consequence of engineering requirements rather than the driver for the design. The “reflex curve”, or sag arch, which links the two main arches, originates as a gesture on a sketch. Expedition can make the claim that it is a functional element, as it serves to transfer bending between the two arch ribs, making them both much stiffer and improving behaviour overall. This is, however, probably rationalisation after the fact, a happy but not inevitable outcome.

Several things strike me about the structure. The deck is quite exceptionally slender, comprising a series of precast concrete slabs, prestressed together by the action of the arch’s horizontal bowstring cables.

There is very little stiffness in the deck, and although probably heavy enough to be difficult to excite, it still requires the presence of five enormous tuned mass dampers tucked below its soffit to ensure that vibrations are acceptable. The dampers are impressive but slightly odd – one above the south bank is set horizontally at a location where the deck has a longitudinal gradient, making it look as if the damper is peeling off the soffit and ready to fall.

The arch is tall yet narrow. Most designers, I think, would have adopted a wider arch to provide greater transverse stability. This narrowness is enhanced by the tripod form of each arch, where it bifurcates into two ribs at the central support, but comes to a single rib at the end support. The central support is therefore required to provide all the resistance to wind or eccentric loading. The effect is visually striking, but as an engineer I also found it a little disconcerting. It also means that the deck has to incorporate significant transfer elements to brace between the end supports of the arch (with an outward thrust) and the tie cables along the deck edges (with their inward tension).

The arches are formed from painted structural steel box sections, and generally are shaped attractively. In elevation, their shape was determined by form-finding, to minimise bending moments under dead load. However, the bracing between the twin arch ribs near the main support, which include huge circular sections resembling offcuts from a pipe factory, is much less attractive.

There’s also an odd disjuncture between the plain, minimal profile of the arch ribs, and the centre-pier struts which support them. Both above and below deck, these struts have a “fluted” form, which contrasts markedly with the arch ribs. I don’t particularly like the shape of these elements, although I do think that the high contrast was the visually appropriate decision. It is the shape of the arches including the reflex curve which needs to be highlighted and given visual continuity, even if this is structurally dishonest, in the sense that the primary flow of forces is from the arch rib down towards the point of support. I think this is therefore an instance where the popular maxim that form follows function would have it wrong.

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17 November 2013

Teesside Bridges: 3. A19 Tees Viaduct

By the time the Tees Viaduct opened in 1975, its older siblings (the Middlesbrough Transporter Bridge and the Newport Lift Bridge) must have been a major cause of traffic delays. It's no surprise that a fixed rather than opening bridge was built, necessitating a long, high-level structure. However, shipping on the Tees was already declining, and fifteen years after the viaduct was built, there would be no need for shipping clearance at all, with the Tees Barrage under construction and the Lift Bridge locked down.

The Tees Viaduct was a relatively unremarkable structure, comprising welded steel girders, a composite concrete deck slab, and reinforced concrete piers. The 117m main span contains a central section suspended on half-joints, which was typical of the time although would almost never be adopted today. The bridge totals 1.95km in length, of which 625m consists of fabricated plate girders, and the remainder is built from off-the-shelf Universal Beams.

Designed by Dobbie Sandford Fawcett and Partners, and built by Cementation Construction, the bridge would have been unremarkable if only it had performed well. However, the bridge performed badly almost from the date it was completed. The deck expansion joints leaked from an early stage, the bridge's roller bearings were not operating correctly, and cracking was observed in the concrete piers. These, and many other problems, led to the bridge undergoing major refurbishment just twelve years after it first opened, with the bulk of the work taking place between 1987 and 1990.

The leaking deck joints led to road salts contaminating the concrete bridge piers. These were the subject of a number of chloride extraction trials. The roller bearings were replaced, expansion joints renewed, and large areas of deck concrete, affected by faulty waterproofing, had to be repaired. A report in 1999 suggested that the original bridge construction cost had been £10m, but the cost of repairs totalled £25m.

The principal feature of interest on the bridge today, and probably a significant part of that refurbishment cost, is the presence of a glass-reinforced plastic enclosure which surrounds the main girders. This comprises Maunsell's patented "Advanced Composite Construction System", ACCS, later given the name "Caretaker". At the time it was installed on the A19 Tees Viaduct, it was being heavily promoted both as a way of protecting bridges against weathering and corrosion, and also of providing improved maintenance access, particularly at locations where such access might otherwise be difficult. The A19 viaduct crosses both a major river and also railway lines.

The only other major use of the system that I can think of, on approach roads to the Second Severn Crossing, had a similar motivation, but bridge enclosures have not proven popular.

The appearance is not as bad as I had though it might be, and the GRP panels seem to be durable so far. It doesn't look significantly worse than an unadorned plate girder bridge. I'm only guessing, but I presume the cost has been the major disincentive to wider adoption. I also wonder whether the system would have had more use if it wasn't shielded by patent.

Further information:

14 November 2013

Teesside Bridges: 2. Newport Lift Bridge

Like its neighbouring transporter bridge, the Tees Newport Lift Bridge is both an industrial relic and an icon for the area. The Transporter Bridge had been opened in 1911, and as with all bridges of its type it gave priority to shipping - in its "rest" state vessels can pass but vehicles cannot, and when in motion only a very limited number of vehicles can be carried at one time in one direction.

The lift bridge offered a considerable improvement for road users, giving priority to them insofar as traffic could cross while the bridge was at rest. It opened in 1934, built by Dorman Long to a design by Mott, Hay and Anderson, becoming at that time the first significant vertical lift bridge in Britain, and one of the largest in the world (at 82m, its main span was some way longer, for example, than Rotterdam's De Hef, built in 1927 and spanning 52m, which I also visited recently).

As traffic grew, even less frequent lifts of the bridge could lead to considerable disruption. The bridge lifted for the last time on 18 November 1990, remaining in its "down" position ever since.

On a dull rainy day, the bridge was impressive but also somewhat lumpen. The massive lift towers have a considerable presence but the bridge did not appear elegant. However, there are some nice photos online of the bridge at night (taken from both ground level and from tower top), which show how attractive it can appear.

It's remarkable how much of the bridge has been left intact. It's easy to imagine that most of the steelwork is redundant, and due to its complexity, prone to corrosion and expensive to paint. However, it's not just the towers that remain in place, but the control cabin at midspan, and most of the lifting machinery including cables, sheaves and counterweights.

I guess that perhaps the main span is still largely supported on its counterweights, with only a small dead load reaction and all the live load reaction carried down through the span-end bearings. That will certainly have been the arrangement during its working lifetime, and it may have been too expensive to change it when the bridge ceased operating.

Further information:

12 November 2013

Teesside Bridges: 1. Middlesbrough Transporter Bridge

I was very lucky recently to join a weekend study tour of the bridges and structures of north-east England, organised by the British Group of the International Association for Bridge and Structural Engineering (IABSE). Previous British IABSE study tours had taken participants to Switzerland and France. I had greatly enjoyed the Swiss trip (and missed out on France), and wondered whether north-east England could possibly be anywhere near as interesting or enjoyable.

I needn't have been concerned. While much of the pleasure of the tour was the chance to meet and spend time with fellow bridge designers, it soon became obvious that we were to visit some splendid and fascinating bridges.

The first stop on the trip was the Middlesbrough Transporter Bridge. This is one of only six transporter bridges worldwide which remain operational (the others are in Newport, Bilbao, Rochefort, Osten, and Rendsburg). We visited it while it was closed for a major refurbishment (structural steel repairs and repainting), but were lucky enough to get a guided tour to the top of the bridge from the contractor, and of the machine house by one of the bridge's electricians.

The bridge was opened in 1911, nearly four decades after Charles Smith first proposed the concept of an "aerial ferry" bridge. Smith's idea was taken up by French engineer Ferdinand Arnodin, who designed several transporter bridges. The bridge at Middlesbrough, however, was designed by Georges Imbault, of Cleveland Bridge & Engineering Co Limited.

Unfortunately, we didn't have time to photograph the bridge from a proper distance, but most of the website links at the end of this post have plenty of photos. Seen in profile, it's a particularly fine structure. The four towers support cantilevering trusses, joined at mid-river by a hinge, and held down behind the towers by cables anchored vertically to the ground. It's the hinge that makes the bridge particularly attractive, I think. Several of the other surviving examples, particularly those designed by Arnodin, are suspension bridges with stiffening trusses, lacking the simplicity of the Middlesbrough design.

As part of the bridge's refurbishment, a lift will be installed at the south end, allowing more regular public access to the walkway level. The staircase that we climbed was steep and, on a windy day, terrifying enough for a group of hardened bridge enthusiasts, let alone the general public.

The trip to the top of the bridge, and the opportunity to quiz one of the engineers working on the refurbishment, was a great start to the study tour. Much of the repair and repainting work at high level is being undertaken from the upper transporter carriage, a high-level platform which rolls along the support girders and from which the bridge gondola is hung. This is both safe and reduces greatly the amount of temporary containment required when removing existing paintwork. However, several other parts of the bridge can be reached only with the use of roped access.

We also had a very interesting look around the bridge's machine house. As this was not in operation, most of the protective covers for the machinery and electrical equipment had been removed. What you can see in the photos is therefore quite different to what would normally be visible. It was particularly interesting to observe the difference between the original control panel, with massive fuses and electrical contacts, and the modern push-button panel.


Further information: