22 April 2018

Yorkshire Bridges: 22. Leeds Dock Footbridges, Leeds

These two bridges weren't on my "to-do list" in Leeds, but I spotted them from nearby and took the chance to have a quick look while there.

There are two of these little bascule bridges spanning Leeds Dock (formerly Clarence Dock) and one of its side arms, part of a redevelopment completed in 2004. The bridges were designed by Buro Happold, with mechanical engineering by Bennett Associates for contractor Spencer.

Both bridges are stiffened steel u-frame structures. One is a twin-bascule span, with mechnically driven pistons which interlock the two sections at the central joint. The other is a single leaf bascule with a short cantilever approach span - the opening leaf rests on the fixed section via a half-joint.

Both structures are deeper at the hinged end, or, to be more precise, deepest directly above the hydraulic rams that open the spans.

The bridges are not spectacular, but are certainly quite nice.

Further information:

21 April 2018

Budapest bridge design competition winner announced

In August 2017, I posted the 17-strong shortlist for a bridge design competition in Budapest, Hungary. The contest was seeking a design for a major new road bridge over the River Danube, to provide a long-planned transport connection on the south side of the city.

The competition winner was announced yesterday, and is a twin-mast cable-stayed bridge designed by UN Studio and Buro Happold. So far, I've not found much detail on the internet (and it's all in Hungarian), but I guess the span is around 250-300m.

The most obvious thing about the bridge (remarked on in several of the Hungarian news features), is its striking resemblance to the Erasmus Bridge in Rotterdam, also designed by UN Studio (but with different engineers). I guess if plagiarism is the highest form of flattery, plagiarising your own ideas shows who you respect the most.

It's basically two Erasmus bridges joined together, and has a figurative resemblance to two people kneeling down facing each other. The cranked tower legs are connected to massive edge girders in the back-spans.

Other than the doubling up, the other obvious difference to the Erasmus Bridge is that the towers are significantly chunkier in Budapest, which I guess is a consequence of carrying a heavier carriageway (the Danube Bridge contest specified four highway and two tramway lanes).

The Erasmus Bridge design was problematic - expensive to build; and the two edge girders were to a great extent over-sized fascias, sized for visual rather than structural effect. It was also greatly complicated by the details required to transfer axial loads between the main-span and back-span girders, as these are offset from one another. From what can be seen in these images, the same problem exists in the Budapest design.

There are two joint second-prize winners, Lavigne et Chéron Architectes, Bureau d’Etude Greisch, Közkekedés Engineers, Geovil; and Leonhardt Andrä und Partner, Beratende Ingenieure, Zaha Hadid Architects, WERNER Consult, Smoltzcyk and Partner. I've only found an image of the second of those:

This twin-arch design has the kind of gargantuan disregard for context that you would expect from ZHA. I'm struggling to understand why the judges would so highly praise a concept which puts a support pedestal slap-bang in the middle of the river, when this is so easily avoidable. The "hangers" connecting the arch to the deck are also so large that they give the impression they are holding up the arch, rather than suspending the deck.

Three of the entrants were were selected for runners-up prizes:
  • Knight Architects & Ove Arup and Partners
  • Pont-terv Mérnöki Tervező és Tanácsadó Zrt
  • Speciálterv Építőmérnöki Kft
I'm hoping that more of the entries will be made public, as it will be interesting to put the winners in context, and understand what made them the stand-out choices.

19 April 2018

Yorkshire Bridges: 21. Knostrop Footbridge, Leeds

This is the newest of all the structures I saw on my trip to Leeds, having only opened to the public in October 2017. It carries a branch of the Trans Pennine Trail cycle/footpath across the River Aire, and it was installed as part of works for the first phase of the Leeds Flood Alleviation Scheme. The bridge was designed by Knight Architects and Mott MacDonald, and built by contractor BAM Nuttall.

Approaching this along the path from the nearby Thwaite Mills, my first thought is what a real oddity this bridge is. It feels a little like a space-age intruder in a semi-urban wilderness: super-sleek architectural design, the sort of thing that would feel right at a visitor "destination" but feels much more of a surprise on this out-of-the-way pathway. I found it disconcerting, at least to begin with.

The 70m long bridge spans across the new Knostrop weir, a series of three inflatable weir structures intended to be adjustable in times of flood. The weirs (and associated fish passes) are staggered in plan and are separated by slender concrete walls, which also form the supports for the bridge.

There seems to be nothing technical published online about the bridge design, but there are some images of construction at the website of steelwork contractor SH Structures, which provide a little illumination.

The YouTube video above is worth a look first, as the early sequences include some overhead views of the bridge layout.

The bridge crosses the river at what looks like roughly a 45-degree skew, and sits on a series of support legs which run parallel to the river (hence at 45 degrees to the bridge), plus a square abutment at one end. The bridge is straight in plan, but widens out with curved standing areas above each support position.

In cross-section, the bridge is a shallow multi-cellular steel box girder, with a smoothly curved soffit plate. The intersection of the plan geometry with the curved underside leads to a distinctive "scalloped" profile along both edges of the bridge.

The bridge legs are formed from simple steel plates, each only 50mm thick, which must be amongst the most slender footbridge supports anywhere in the UK. It's not quite down to Ney and Partners standards, but it's quite remarkable nonetheless. The legs are attractively shaped as well, holding the bridge delicately to one side of the weir so that bridge users get a good view of the site's main attraction.

The construction photos indicate that the pier legs also form diaphragm plates within the box girders, and that each box span is bolted through the diaphragm. I guess a cover plate was then welded across the finished joints.

Presumably the "leaf" piers are thin enough simply to flex under thermal expansion and contraction, although I think the effects must be complex due to the high skew. They must also be short enough to be safe against buckling against vertical load, so a careful balance of stiffness is required.

The parapets comprise stainless steel ribs with timber leaning rails. I like the simplicity of the main parapet section, but the upper rail, presumably to provide standard cycle parapet height, looks a little like an afterthought.

Nonetheless, this is a very high-quality bridge to find in such an unexpected place.

Further information:

17 April 2018

Yorkshire Bridges: 20. Urn Farm Bridge, Leeds

From Lofthouse Interchange, I continued north towards Leeds.

The next bridge I visited was the Urn Farm Bridge, which today carries the footpath Orion Walk across the M621 motorway. It was built as an accommodation bridge as part of the original M1 motorway circa 1967.

This is a three-hinged reinforced concrete arch, spanning 213 feet (65m) between springings and 320 feet (98m) long in total. The upper side spans are supported on half-joints. A photograph of the bridge under construction can be seen at the Motorway Archive online.

It is a moderately familiar form of construction, with other examples including Needle Eye Bridge, also on the M1, and Swanscombe Cutting Bridge. However, I doubt there are more than half-a-dozen in the UK. I always admire their clarity and elegance.

This example has been slightly marred by the addition of anti-climb guards to the legs, but it's still a lovely structure, a fine gateway on the motorway approach to Leeds, or a place for walkers to pause and admire the view.

Further information:

16 April 2018

Yorkshire Bridges: 19. Lofthouse Interchange

From Stanley Ferry, I was heading north towards Leeds. My next stop was the Lofthouse Interchange, the magnificent junction between the M1 and M62 motorways, which can be viewed from Long Thorpe Lane on its northern side, for anyone so inclined.

The Lofthouse Interchange may be magnificent, but it's also notorious. Built in 1967, it is a three-level interchange, with the M62 crossing above the M1, and an 800-foot diameter roundabout built above both motorways to accommodate all interchanging traffic. It looks fantastic from above, but it creates a terrible bottleneck, due to the conflict between traffic streams entering the roundabout.

In 1999, major improvements were made by the introduction of new link roads connecting the western arm of the M62 to the northern arm of the M1. However, many conflicting movements remain on the roundabout, and it is known both for congestion and for collisions.

Highways England are consulting on a scheme to improve the junction, although there's no information yet on what this may actually involve. The best long-term solution would be to completely separate all slip road movements, but that is likely to be both hugely expensive and hugely disruptive during construction.

One casualty of any radical change could be the junction's distinctive "banana piers". Judging from the degraded state of the concrete on these, that may not be such a bad thing.

The designers of the Lofthouse Interchange were looking to solve two structural engineering problems, at a time when computer structural analysis was not as ubiquitous as it is today. The first was the concern that mineworkings in the area could lead to settlement of the bridges.

The common solution to this at the time was to introduce as much articulation as possible, so, for example, using a series of simply supported spans rather than continuous beams. The second concern for the engineers was thermal expansion and contraction of the curved bridge decks on the tall support piers.

The issue is not entirely clear: for short simply-supported spans, sliding could be accommodated by bearings on the top of each pier, and the piers designed accordingly. I think the issue here is that the designers wanted the decks to be connected together so that there was only one expansion joint at the end of each bridge: the decks are therefore connected to the pier via fixed bearings (permitting rotation under settlement), all expanding from one abutment.

In any event, the "banana piers" were the solution. The "banana" element is hinged at its base, and supported from a cruciform-section concrete strut hinged top and bottom. The effect of this is that the bridge deck can expand along the direction of its curvature while both deck and pier remain stable. It is a simplistic, statically determinate solution, which today would probably be dealt with by designing the piers to be flexible enough to accommodate thermal movement.

The outcome is one of those highly distinctive structural solutions that the Yorkshire motorways are filled with (see also past posts on Droppingwell Footbridge, Smithy Wood Footbridge, Needle Eye Bridge etc).

Further information:

14 April 2018

Upper Orwell Crossings project: was procurement fair?

When it was first announced back in August 2016, I labelled Suffolk County Council's Upper Orwell Crossing project "one of the worst bridge design competitions to be organised in the UK for quite some time".

Working with RIBA's competitions office, SCC had set up a contest to select an architectural partner for their previously-appointed consulting engineer, WSP.

The prequalification conditions were set in such a way as to prevent smaller practices from entering (although this was later changed, after complaints), and engineers, however creative, were excluded, unless they were also registered as an architect. The contest was in essence a beauty parade to set up an arranged marriage between the winner and WSP, but was formatted as a design contest, with each competitor preparing designs to be judged, but not necessarily selected for further development.

Probably the contest's worst feature was that the evaluation was to be made on a quality/price split, with 60% of marks for quality of the design, and 40% on the basis of a lump sum fee quotation. This gave SCC some assurance that they would not be screwed in post-contest negotiations over the architect's fee, but it set up the possibility that a poor designer could be selected on the basis of being cheaper than better contestants, or that a good designer could lose out due to a slightly higher fee.

It was far from clear what SCC were really seeking, but the whole competition seemed to fly in the face of best practice.

My comments were echoed by others, with several criticisms reported in the Architects' Journal the following month. Much of the criticism focused on the arranged marriage element, which is not the way most successful collaborative teams normally come together.

The competition organisers selected five competitors in October 2016: Foster + Partners, WilkinsonEyre, Knight Architects, Marc Mimram, and a team of Adamson Associates / Ney and Partners / William Matthews Associates (WMA). The observant reader will note that three of the five (Foster, Mimram and Ney) have substantial engineering expertise of their own. At this time I commented: "The financial element will work against certain entrants, I think, as my experience is that architects' fee levels can vary significantly."

In March 2017, the winner was announced as Foster + Partners, which was interesting as they were one of the competitors I would have expected to have been disadvantaged by the price element of the evaluation. I doubt that many clients select Foster because they are cheaper than the alternatives.

Local MP Ben Gummer was quoted:
"The fact that we will have what will be a globally recognised bridge of beauty will say something powerful about our town's ambition and our place not just in our county, or our region, or our country, but in the world."
I expressed the feeling that perhaps Suffolk were being over-ambitious, and I also noted the way that what had supposedly been a contest to choose a design partner had subtly slipped into being a contest to identify the best design.

And that was that, until the Architects' Journal returned to the fray a couple of days ago with an exhaustive investigation of the Upper Orwell Crossing procurement process.

Now, I must say I don't really trust the AJ on bridge procurement. They have played a key role in exposing the shambles of London's ill-fated Garden Bridge, but they have also manufactured some completely nonsensical controversy with regard to the Canary Wharf to Rotherhithe crossing.

Nonetheless, their take on the Upper Orwell Crossings does not make for happy reading. I'll pick out only a couple of the key points.

First, the AJ draws attention to the scoring of submissions by the judges, and they include an image of the scoring sheets in their article. What jumps out here is that judges Michael Hopkins and Patty Hopkins each scored the Foster + Partners submission 10/10 not only on every criterion, but for all three of the bridge spans presented (there were three spans required, A, B and C). For those who don't follow architecture closely, Michael and Patty are husband and wife. Michael Hopkins was, of course, also one of Foster's close early associates, both having worked together on the Willis Building in Ipswich, Suffolk.

Here are the evaluation criteria set out in the original invitation to tender document (which is helpfully archived online):

What is notable here, in the quality marking, is that 50/60 of the marks are for the quality of the designs submitted, and only 10/60 for "method of working", despite this supposedly being a contest to select a design partner, not a design.

As did all the contestants, Foster + Partners submitted two designs for each of the two main spans. Looking at the score sheets provided by the AJ in their article, Michael Hopkins did not even bother to mark the second design (it appears that all the other judges did). Patty Hopkins did mark it slightly lower than the first design.

Some of  the other judges appeared more critical. In comparison to the two Hopkins judges, Kevin Drain, the lead WSP engineer responsible for the project and who is presumably now working with the architect to take the project forward, awarded Foster's designs 102 out of the maximum possible 120 marks. Jonathan McDowell, the member of the jury with perhaps the greatest experience as a bridge architect, gave only 79 out of 120.

Here is the jury panel as originally announced in the tender invitation:

The AJ remarks on the notable absence of Patty Hopkins from that list.

The designs of the other competitors have not been made public, so it's impossible to comment on their relative quality. I note that the Foster design for the main highway bridge (all the images used in this post) shares some distinctive design elements with their Chateau Margaux Winery, tree-like supports designed for a small building roof and here adapted for a much heavier highway bridge. It will be interesting to see whether the competition-phase design is taken forward.

Second, this extract from the timeline put together by the AJ makes interesting reading:

The original plan was for a winner to be announced in December 2016. By the time of shortlisting, the timetable had slipped, and the announcement was then scheduled for January 2017. The timeline shows that instead, Suffolk sought "financial clarifications" from bidders, delaying the announcement until March.

The AJ reports that the amendments to tender prices may not in fact have been legally appropriate: the "restricted procedure" (shortlisting followed by tender submission) under EU law does not normally permit post-tender negotiations. It is intended to be used by clients where their requirements are clear, such that bidders can price the scope of work fairly.

The AJ has uncovered the range of prices submitted, with the lowest being £537,202, and the next lowest £1.1m. The highest was around £2.63m. This seems an extraordinary range of prices (even in light of the range of bidders involved), and suggests that the scope for the architectural services was not clear at all.

The winning competitor, Foster, is reported to have submitted a price around £1.4m. According to the AJ, the scoring system used to compare prices was that the lowest bidder received 100 points for price; with others receiving 100 minus one point for each percentage point by which their price exceeded the cheapest. Under this system, unless prices are close, most bidders (including Foster) would have received negative scores for the price component of their bid.

However, in February 2017, Foster wrote to Suffolk proposing a reduced fee of £845,000. The following month, they were declared the winner.

The overall impression created is clearly not good. No doubt the process has been entirely above board. The same, of course, was claimed in the Garden Bridge's notoriously skewed procurement which appointed Heatherwick Studio instead of seemingly better-qualified competitors. On that occasion, the winning designer's fee was more than three times as high as either of the other bidders, yet they alone were given the opportunity to reduce their price and ended up being awarded the job. It's hard not to see the distinct similarity in the Upper Orwell Crossing procurement.

I don't have any reason to think that the judging was unfair: the two Hopkins partners were perfectly entitled to show enthusiasm for whatever they thought was best. They were not the entire judging panel.

However, the root of the mess clearly lies in the way the original tender evaluation scoring was set up, which would be shame had it not been utterly predictable. Set up to give the impression that price was secondary to quality, the scoring regime instead made certain that price was all, and eliminated the possibility of making a sensible judgement primarily on quality.

As always, the project will be one to watch as it moves forward. I very much doubt that the designs shown at competition stage will be what is built, and in the absence of any meaningful cost or buildability evaluation, it's entirely possible they are beyond what Suffolk County Council can actually afford.

Yorkshire Bridges: 18. Stanley Ferry Aqueduct

I recently had some time on a trip to Leeds to see a few bridges, both in the city and nearby.

The first bridge I visited was the Stanley Ferry Aqueduct, which is claimed to be the largest cast iron aqueduct in the world, and possibly the first iron aqueduct to have been supported from suspension hangers. It is both a Scheduled Monument and also Grade I Listed. This photo makes it look much smaller than it really is:

Built in 1839, the aqueduct carries the Aire and Calder Navigation, a canal, across the River Calder. The structure was designed by engineer George Leather (probably with his son John Wignall Leather), and comprises a cast iron trough suspended via wrought iron rods from two cast iron arches.

Leather had originally developed a multi-span design, which met with the approval of Thomas Telford, but it was eventually decided to build a single span structure, to reduce any hindrance to water flow in the river. As built, the arches span 47.2m across the river, although the canal trough is longer at around 50m. The trough is 7.3m wide and 2.6m deep, containing some 955 tonnes of water. This phenomenal load is carried on a series of cast iron cross-frames, supported from the hanger rods.

None of this can be seen directly, as it is all hidden behind decorative colonnaded fascia panels. The bridge abutments are also hidden, disguised behind faux-Greek pavilions (you can only see the tops of these in some of my photos). The support arrangement is show in this diagram (taken from Broad's paper, see link below):

The two arches are also cast iron, each cast in seven segments, with each segment having four Vierendeel-type openings. The arch ribs taper from 1.83m deep at the crown to 3.02m deep at the supports. In contrast to the rest of the bridge, the form of the arch is surprisingly modern, compare for example the Taunton River Tone bridge.

The bridge was extensively refurbished circa 1986. Impact to the sides of the trough from large canal barges was repaired using the Metalock process. The hangers were in some cases highly corroded, and found to be carrying uneven loads (ranging from 5 to 25 tonnes), so several hangers were replaced, and all were re-tensioned. The bridge was completely repainted.

It's a magnificent structure, but not an easy one to see well. It can be viewed from a road bridge to the west, or from a field and canal towpath to the east. Closer viewpoints on its west side are within private property, and I could not get access.

If you want to see how the bridge looks from close-up, there are some good photos at the Stanley History website linked below.

Immediately to the east of the bridge, a second aqueduct was built in 1981. This is a huge, deep prestressed concrete trough structure. It resembles a dam more closely than a bridge.

East of that there is a "trash screen footbridge", which provides access along the canalside but also helps to trap the great piles of debris which wash down the Calder. All three bridges have surprisingly little clearance to the river below. When the Calder floods, the aqueduct is nearly entirely submerged.

It's a shame Stanley Ferry Aqueduct isn't more accessible for visitors: it is a one-of-a-kind structure. The Canal and River Trust held an open day at their adjacent workshops back in 2016, from where great views are possible, and perhaps they will do so again.

Further information: