29 April 2018

Yorkshire Bridges: 25. Centenary Footbridge, Leeds

Completed in 1992, the Centenary Bridge was claimed to be the first new crossing of the River Aire in Leeds for over 100 years.

It was planned and promoted by the Leeds Development Corporation, and engineered by Ove Arup and Partners. The contractor was Kier, with Billington Structures fabricating the steel elements and C.V. Buchan precasting the concrete.

The site was (and is) asymmetrical, with a relatively narrow approach on the north bank of the river, and a wider open space on the south. The resulting design is an asymmetrical cable-stayed footbridge.

The A-frame mast is 18.3m tall, with two reinforced concrete legs supported at their base on bearings. The bridge deck has an 11.8m backspan and 43.3m main span, and is a steel half-through girder bridge, with Vierendeel edge girders formed from closed hollow sections.

A short article in the Arup Journal describes the bridge but doesn't explain its articulation. For an asymmetrical cable-stayed bridge of this type, there is an out-of-balance thrust in the bridge deck, acting towards the mast from the main span. This can be accommodated at the mast, but in this instance appears to be resisted at the back-span abutment, where the horizontal component of force in the deck is balanced by the force in the back-stays. Net vertical forces must be carried by the foundations.

The bridge will expand and contract under temperature from the fixed support at the abutments, which is presumably why the concrete mast has bearings at its base. The mast was precast, so a simple support detail at the base may also have been desirable for that reason.

The bridge girders are not very good looking, and doubly so due to the accumulation of dirt on these and other parts of the bridge.

The parapets use horizontal stainless steel wires, and are an attempt to lighten the general appearance of the bridge.

As with many urban bridges, there were a number of "love locks" attached to the parapet when I visited, although the local council clearly does not want them on the bridge.

This is not, in my view, a great bridge design. It's easy to see why the designers put the tall mast in the open space on the south bank, but the mast and associated steps and ramps for this end of the bridge dominate this space so that much of the openness has been lost.

The tower is angled at a very slight angle away from the river, and I think a vertical tower would have looked better in this close proximity to surrounding buildings.

Further information:

26 April 2018

Yorkshire Bridges: 24. Crown Point Bridge, Leeds

This is the next bridge across the River Aire in Leeds, proceeding west from Knight's Way Footbridge.

Crown Point Bridge was built in 1842, and is Listed Grade II. The Listing states that it was designed by George Leather, but an ICE obituary attributes the bridge to George's son, John Wignall Leather. The pair played a key part in development in the Leeds area at the time, developing the Aire and Calder Navigation, and a series of bridges, including Monk Bridge (1827), Victoria Bridge (1839) and the Stanley Ferry Aqueduct (1839).

The Crown Point Bridge was constructed to provide a connection to new areas of Leeds then undergoing development, and was authorised by an Act of Parliament in 1840. In its original form, it comprised ten parallel cast iron arch ribs, spanning 120 feet.

The cast iron elements were supplied by Booth and Co., of Sheffield's Park Ironworks, at a cost of £8,750. The total bridge construction cost was £36,000, including approach spans. Initially the bridge was tolled, although the toll was removed in 1868.

The bridge is ornate in the extreme, but it remains elegant due to the basic clarity of the different pieces - the arch ribs, spandrel X-bracing, beams and parapets.

In 1989, the bridge was assessed as unsuitable for heavy modern traffic, and Leeds City Council designed a strengthening and widening scheme, completed in 1995. The basics of the scheme can be seen from below the bridge: the two edge arch ribs on each edge of the bridge have been retained and relocated, and twelve new steel arches have been inserted in between.

The cost of the scheme was approximately £2.2m. The existing foundations were widened and strengthened with the insertion of new mini-piles. The arch ribs on the eastern face were extended in length by replacing one 7.5m segment with new longer cast iron segments: this is visible in some of the photographs, although it's certainly not immediately obvious.

The bridge alterations won a City of Leeds Award for Architecture (1996) and were Commended in the ICE Yorkshire Association Awards (1997). Photographs taken during construction can be found at the Richard's Bridges website link below, the Richard in question being J Richard Kay, former Chief Bridge Engineer for Leeds City Council.

Further information:

24 April 2018

Yorkshire Bridges: 23. Knight's Way Footbridge, Leeds

From Leeds Dock, I returned to the River Aire, heading west.

Knight's Way Footbridge was built as part of the Clarence Dock redevelopment in 2007, and is a two span bridge across the River Aire, designed by Buro Happold.

It belongs to a family of crane-like cable-stayed bridges, with other examples in the UK including Lockmeadow Footbridge (1999), Lune Millennium Bridge (2001), Newport City Footbridge (2006), and the Eureka Skyway (2011).

The bridge deck is partially supported by the cables, which carry the forces up to the tip of each pylon, where they are balanced by the compression in the pylon and tension in the overhead cables. The effect is that most of the load on the bridge, including its self-weight, is carried to the foundation on the central island in the river.

The whole bridge will have a tendency to tip over when only one of the two spans is loaded. One way to resist this effect would be to have cables running to ground anchorages at one or both ends of the bridge (the solution adopted for the Newport City Footbridge).

The other, used here, is for the out-of-balance overturning to be resisted by the bearings at the two ends of the bridge, implying that these are probably uplift-resisting bearings. This places a higher bending demand on the deck, losing much of the efficiency that a cable stayed bridge design would normally have.

As with many white-painted bridges, this one is affected by a considerably build-up of grime. Cleaning bridges is rarely a high priority. I have no idea whether this bridge is the responsibility of the city council or the developer, but it often seems to be the case that "landmark" bridges built as part of an exciting new development prove to be a maintenance burden, if their maintenance is remembered at all.

The bridge deck has upstand girders on either side, which I would expect to spoil the appearance of what should be a lightweight, transparent form of structure. The floor plate is about half-way up the girders, and I guess a series of crossbeams are hidden below. The alternative solution, a box girder below floor level, would have given a more slender appearance, but would have been more expensive.

The depth of the girders is not out of proportion to the bridge span (especially given the bending required to prevent overturning of the masts), but seen while standing on the bridge, this arrangement gives the parapets a tacked-on appearance, rather than integrated.

Further information:

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: