01 March 2015

Tower Bridge ... some musings

I recently posted about Tower Bridge in London, and observed that it is one of the world’s most unusual suspension bridges.

Most suspension bridges have a suspension cable or chain from which the deck is hung. Tower Bridge, unusually, is suspended from a suspension member made up of riveted steel plates (pictured).

Additionally, most suspension bridges have the deck stiffened with trusses or girders to prevent excessive deflections. On Tower Bridge, the suspension system is itself stiffened, comprising trussed elements in the form of an inverted three-hinged arch.

Both these features are unusual, and I wondered whether the latter one is in fact unique.

Since my post, suspension bridge expert Bridgemeister has introduced me to a number of other bridges which share either or both of these features, and I thought I'd cover some of them here.

I already knew of one other suspension bridge which has riveted steel plates as its suspension "cable", the Grunwaldzki bridge in Wrocław, which I visited in 2011. This bridge was built in 1947 and it's really difficult to understand why plates (pictured, left) were used instead of cables.

Bridgemeister also highlights the Trukhanov Island footbridge in Ukraine; the Lahn River Bridge at Nassau in Germany (pictured right); and a hybrid bridge over the Salzach between Laufen in Germany and Oberndorf in Austria.

Of those the 75m span Lahn River Bridge can be counted twice, both for its 2005 incarnation and its 1926 predecessor (this is the one in the photo). It is a thoroughly delightful structure, especially the detailing of its towers. It deserves to be much better known.

The plated suspension elements on these bridges are in some cases very different from the riveted plates of the Tower Bridge. The welded steel flats on the Lahn bridge are simple and elegant by comparison, yet the bridge still retains a great deal of character, mainly thanks to the marvellous towers.

Bridges with a trussed suspension "cable" are also hard to find, and often not well known. The 1895 Mill Creek Park, pictured left is a good example, but others are more questionable.

An 1889 suspension bridge in Rome has only minimal stiffening, while several examples were either never built (Gustav Lindenthal's spectacular Hudson River design in New York, pictured right) or have been dismantled (St Louis's Grand Avenue bridge, and Lindenthal's Seventh Street Bridge and Point Bridge in Pittsburgh).

The Kindee bridge at Ellenborough in Australia adopts broadly the same form as Tower Bridge, a three-hinged arch formed from two crescent trusses. It's a glorious monument to engineering idiosyncrasy.

The 1947 Lumberville-Raven Rock bridge (pictured left) has two-hinged stiffening trusses, and is another triumph of engineering. In this arrangement, there are actually two suspension cables one each side of the bridge, separated by an arrangement of lightweight trussed steel ties and struts. These are held together with a fascinating assemblage of custom-made clamps and connecting nodes. It's like a modern-day Schlaich Bergermann design transported back in time.

Another example of the genre can be found at the former John Roebling offices in Trenton, New Jersey, providing a high-level walkway connecting two buildings (pictured right). This shares the Lumberville bridge's two-hinged truss arrangement, although it has had to be further stiffened with tie-down stays below the deck.

It's not hard to see why both variants on the normal suspension bridge have found little favour. The plated "cable" solution is a step up from historic chain bridges, being less vulnerable to single-point failure than chains. However, it simply can't compete with any kind of wire cable, as wires are inherently stronger than plain structural steel, and don't need to be peppered with rivet or bolt holes, which further reduce the capacity. Wires are lighter and much easier to install. However, I do like the Lahn River Bridge and wonder whether there are not some nice bridges still to be designed along similar lines.

As for trussing, again, this is not an efficient means of construction – the trussed "cable" requires expensive temporary support while it is assembled. It's much easier to install a relatively lightweight cable first and use that to provide all the necessary support while the stiff bridge deck is assembled.

Picture sources: Bridgemeister (Lumberville-Raven Rock, Trenton); out of copyright (Hudson River, Lahn River); rg998 at Wikipedia (Mill Creek Park); the author (Grunwaldzki, Tower Bridge).

23 February 2015

Nine Elms Pimlico Bridge Design Competition entries published

O frabjous day! Callooh! Callay! A gallery of the entries for the Nine Elms Pimlico bridge design competition has been published today, and my, what an entertaining box of delights there is to behold.

As with so many bridge design competitions (see past blog posts ad nauseam), I'm left wondering exactly what motivated some of the entrants to this one. This is a serious competition for a genuinely useful river crossing which appears already to have much of its funding in place. However, quite a number of the entrants seem to have put forward entries which nobody could even contemplate building, gaudy scribbles which could be held up only by a wing, a prayer and the magic of Photoshop. Why would anyone put so much effort into something which clearly will go straight into the discard pile?

Of the more sensible entries, it's interesting to consider the many different ways in which the bridge designs have been depicted, from razor-sharp photomontage through to whimsical artists impressions, even one that depicts a 21st century bridge being used by 19th century Londoners.

With 74 entries on display, it seems foolhardy to attempt any proper consideration of their merits. I reckon there are at most a couple of dozen which have a serious prospect of further consideration by the competition jury panel. And how could anyone top this ultra-quick skewering?

So, with the aim of avoiding all that hard work on my part, what's the reaction on Twitter?

20 February 2015

London Bridges: 40. Tower Bridge

I've featured many London bridges on this blog, but until now, not the city's most famous span. It is a bridge about which very little needs to be said here, as its history and engineering are well recorded elsewhere. Its iconic status is never in doubt, indeed it defines the very term in a manner which most other city bridge builders can only aspire to.

Of all the bridges across the River Thames, it is a special structure for many reasons. Until the building of the Dartford Crossing in 1991, it was the most easternmost bridge across the river, and due to the demands of navigation, it remains the only Thames bridge which is required to open to river traffic.

I find its structural form particularly intriguing, as it is probably a unique variation on the many clever options which have been used to adapt the basic suspension bridge design. Essentially it comprises two back-to-back suspension bridges, with the twin towers and their aerial walkways providing a central anchor block. This is not an entirely unusual arrangement, as there are other, much larger suspension bridges with a shared central anchorage, but surely none where the anchorage is connected at height to allow space for a twin-bascule bridge below.

It's unusual among suspension bridges because the main suspension element is a series of riveted metal plates rather than a wire cable, although this is certainly not unique, and I've previously visited a bridge in Wroclaw which adopted the same approach much closer to the present day. It's also unusual in that deck deflections are minimised by employing a truss to stiffen the suspension element rather than the deck itself, an arrangement very rarely employed due to the cost of erecting the large suspension element. Is it in fact unique?

Possibly the thing I find most interesting about Tower Bridge is not what it is, but what it might have been. It is the result of a series of bridge design competitions, which saw a tremendous variety of proposals put forward, most of them extremely bizarre in appearance even in comparison to what was actually built, which is itself pretty odd. However, these will have to be a story for another time!

Further information:

17 February 2015

Bath Quays Bridge Design Competition

Bath and North East Somerset Council (BANES) have announced a competition to design a new £2.5m pedestrian bridge over the River Avon in Bath.

The bridge will span approximately 60m and is to be approximately 5m wide. It is intended to help spur regeneration of a new riverside quarter in the city, and the intention is for a contract for bridge construction to be awarded in 2016.

I was particularly interested to see that BANES's competition brief claims to be "strongly guided by" IABSE's Guidelines for Design Competitions for Bridges, which I have discussed here before. It seems a pretty dubious claim in some respects.

The Bath Quays contest is currently requesting prequalification submissions, which will be used to shortlist 5 or 6 entrants for the main competition stage.

The prequal criteria include a number of standard local government requirements on equal opportunities, health and safety, management systems etc, intended to weed out those firms who might struggle to deliver the project, but with the possible effect of also ruling out a number of smaller firms who might be able to deliver interesting and creative proposals.

The main prequal criteria consider previous experience, both in terms of individual CVs and previous projects. Again, this may have the effect of discouraging new entrants to the bridge competition arena, but that depends on how the applications are evaluated. Interestingly, BANES proposes to identify a longlist of potential competitors, and leave the final shortlisting to their jury panel, which I think is a very positive move. It's a good jury panel as well, with a couple of highly competent engineers on board.

There are two main areas where the competition seems to me to be inequitable. The first is in the reward. Teams who prequalify have eight weeks in which to prepare their designs, which is a generously long time for such a little bridge, and receive in return only a £5,000 honorarium, which is frankly derisory. Assuming a typical architect / engineer partnership, that's only £2.5k each, which is no reward at all for the work which will be undertaken and from which BANES will benefit. No doubt many experienced bridge designers will grizzle about it but enter all the same. There's not even a prize for the winning designer to help them recover some of their entry costs: the only reward is the promise of the main design contract, which for a £2.5m bridge will be at a level where the profit margin will struggle to offset the time and money expended in getting that far.

And make no mistake, BANES are expecting quite a lot of effort: 3d visualisations, both architectural and engineering drawings, a construction methodology, a project programme, and a construction cost estimate "based on a preliminary bill of quantities and unit rates to be provided by the competition entrant". In total, that's easily £25k worth of work from each entrant, largely unrewarded, and it's hard to see that the construction cost estimates will be of any quality.

The evaluation method for the main contest stage is equally odd. Entrants are to provide both their design and also a detailed commercial submission, including a lump sum design fee, design programme, schedule of fee rates, activity schedule and partially completed contract. The quality and commercial submissions will be evaluated, weighted, and combined to give a final score, raising the possibility that the poorest design could still win if the accompanying commercial offer is sufficiently cheap!

No designer will be able to make a sensible judgement of their final fee based solely on concept stage design development, and this aspect of the contest goes quite squarely against the IABSE guidelines, which suggest that if a fee proposal must be provided at competition stage, it should be in a sealed envelope, opened only after the winning design is chosen. I imagine BANES are concerned not to have to negotiate an acceptable fee with the winning designer after they have won the contest and hence achieved a strong bargaining position. But I struggle to see that this aspect of the competition evaluation will provide a positive influence on the selection and subsequent development of a high quality bridge for Bath.

10 February 2015

London Bridges: 39. Merchant Square Footbridge

The bridge at Merchant Square, Paddington, is the latest addition to a peculiar assemblage of spans built across a little-used canal basin in an attempt to flag up a developer's site as somehow far more interesting than it really is.

It's a replacement for the defunct Helix Bridge, another odd structure which rotated corkscrew-style while retracting (or advancing) across a narrow canal throat, but which had long since ceased to work. The ability to work was not a practical handicap for the Helix, as it spanned across the entrance to the extreme end of the canal basin, a body of water which has for some time been more of an ornamental pond than a useful harbour for canal boats. However, its function was less as a useful moveable bridge and more as a piece of kinetic sculpture, and the new Merchant Square Footbridge adheres to the same brief.

Given that the bridge's raison d'être is to open and close in a spectacular, unusual yet elegant manner, it is perhaps unfair to have visited it at a time when it remained resolutely closed (it opens every Friday at noon). However, this is how it will be experienced for the vast majority of its lifetime, so it is at least not unrepresentative. Indeed, the canal basin is, at present, physically barred to canal traffic by the floating equivalent of bollards, so there isn't even the chance of an errant barge passing up to it and demanding that the bridge be lifted to permit passage.

From the video showing the bridge being raised and lowered, this is clearly an imaginative and highly unorthodox design. The bridge deck comprises five separate fingers connected only at a common axle, which are arranged so that each rises by a different degree, creating, with an admirable sense of the poetic, the impression of a Japanese hand fan. The structural engineering would appear reasonably straightforward, but I imagine the mechanical engineering to have been a real challenge.

In its quiescent state the bridge retains a degree of visual interest, principally due to the manner in which the five bascule counterweights have been raised above the ground. This is a significant gesture, with the designer clearly recognising that the bridge must remain interesting even when unaroused. The counterweights are arrayed in manner which I find reminiscent of the Sydney Opera House, a group of heavyweight steel shells or petals which hint at the bridge's occasional party trick. Each counterweight is labelled clearly with its weight in tonnes, a charming idea.

The bridge is generally well detailed. The forward bearings are compact and fitted snugly, and the deck and parapets have been assembled with some precision. There are interesting details, such as on the stainless steel mounting plates for the parapets. These are cut square to the bridge span, as is the obvious thing to do, except where they pass over the canal edge, where they have been cut parallel to the canal and hence at an angle to the (skew) span.

The alternating inclination of the parapet bars provides the necessary strength and stiffness against pedestrian loads without requiring the usual larger posts. This is a device I have seen and admired on previous (unbuilt) Knight Architects bridge proposals, but in practice it is not entirely successful, principally because the bars are much thicker than seems visually appropriate. I imagine a more transparent quality was sought, but not achieved. Other than the chunkiness, the parapets are well-detailed, with an attractive wooden lean rail at the top, incorporating hidden lighting on its underside.

The feature I found strangest about this bridge is quite how close it sits to the water's surface. This is, I suspect, a forced consequence of the overall design concept, which requires the bridge deck to support pedestrian crowd loading in its closed position as a simply supporting beam, and its self-weight as a cantilever when it opens. With the deck divided into five fingers, each has to be stiff enough to satisfy both functions, and I guess this has governed the depth of the structural members required. The upper level of the deck is governed by the canal-side levels, and the depth in which to accommodate the overall construction is such that proximity of the soffit to water is unavoidable. The relationship feels visually uncomfortable, to me at least.

Further information:

02 February 2015

Irish Bridges: 3. James Joyce Bridge, Dublin

Dublin is the lucky possessor of not one but two bridges by the renowned Spanish engineer, Santiago Calatrava. Both are highway bridges, spanning the River Liffey, built in steel and painted white. The James Joyce bridge is, by Calatrava’s flamboyant standards, an essentially modest design, completed in 2003. The other structure, the Samuel Beckett Bridge, is a case of extravagance at its most extravagant, a hugely gymnastic cable-stayed bridge with a passing resemblance to a harp, which is not only a piece of spectacularly ambitious sculpture, but adds to the spectacle by swinging open from time to time. On this very quick trip, I only had time to visit the simpler of the two bridges.

Designed in collaboration with Roughan O'Donovan, the James Joyce Bridge is one of Calatrava's better structures. It is relatively straightforward in conception, with two steel arches inclined outwards from a central highway, supporting the road and footways and helping define attractive and generous pedestrian spaces.

High strength Macalloy steel bars connect the deck to the arches, each hanger consisting of a pair of bars in a manner typical of Calatrava designs. The footways are supported on crossbeam cantilevers, and have a glass-block deck and glass-panel balustrades.

The overall form of the bridge is attractive from almost every perspective, but what I admire most about this bridge is the detailing, which has clearly been done with considerable care. It's a bridge simple in overall concept yet complex in the detail; I like it a lot.

Further information:

26 January 2015

Irish Bridges: 2. Millennium Bridge, Dublin

There's a website set up to document Lancaster’s Lune Millennium Bridge, built in 2001, which also attempted to chronicle a number of other Millennial Bridges built thanks to funding fever in Britain at the turn of the century. Some of these are significant and well known structures, such as the bridge between St Paul’s Cathedral and Bankside in London. Others are obscure, and of at best local significance, such as the Pennyferry Bridge in Durham.

Dublin's contribution to this array of structures unfortunately fell closer to the latter type than the former. It spans the River Liffey to the west of the Ha'penny Bridge, a river already blessed with a very large number of spans in much closer proximity than is the case in most other capital cities. Designed by Price and Myers with Howley Harrington Architects, it seems to have been drawn from a bottle of imagination which had already been thoroughly drained.

The bridge is a 41m long, 4m wide triangulated metal truss, deeper at its ends than at its middle, a form of structure which often seems clumsily industrial rather than lightweight and high-tech. That's no different here, and it's one of the least visually attractive of central Dublin’s bridges.

Remarkably, this seemingly prosaic design was the winner in a design contest with some 157 entries, and has since gone on to win multiple awards.

Perhaps this is a good thing, reminding us that designs don't have to be absurdly spectacular to merit selection. It's clearly trying to be slender and elegant, but one unfortunate consequence of slimming down all its constituent members is that they more closely resemble scaffold tubes.

The design seems to echo a number of other Liffey bridges by being arched, albeit extremely gently. In fact, it's a two-pinned portal bridge, with chunky portal legs hidden within "balcony" extensions to the river bank at either end, and supported on steel hinges.

I am torn between my initial impressions of this bridge, which are overwhelmingly negative, and a sense that it is perhaps in some way admirable in its restraint. Other opinions would be welcome!

Further information: