18 July 2017

London Bridges: 49. Somers Town Bridge

I recently went exploring in London and found two interesting new bridges.

The first, Somers Town Bridge, is new to the point of not being quite finished: it's open to the public but I think some snagging remains. It carries a new footpath across Regent's Canal, a few blocks north of King's Cross station. The whole area was once derelict but is now seeing glossy new development spreading across the landscape like some kind of fast-multiplying spores.

This is the territory of a massive Google office, of over-priced hipster food joints, of aspirational bright-young-things living in the overpainted shells of former gas holders. There's the smell of money everywhere, and where there is new money, there have to be new bridges, connecting each new mushroom patch together.

Somers Town Bridge currently links a fairly circuitous pathway between Camley Street and the Granary Square plaza. It will become more useful soon when the Gasholders residential blocks are open, along with Heatherwick Studio's urban chic shopping mall, the Coal Drops Yard. These are all elements in the Kings Cross Development Partnership's wider regeneration scheme.

For now, the new footbridge is only open to the public between 6am and 9pm each day. This is an initial measure because of security concerns.

It has been designed by Moxon Architects and Arup, and spans 38m across the canal. The 55-tonne bridge was built by main contractor Carillion and steelwork subcontractor SH Structures.

Structurally, it is a bathtub form, or to be technical, a half-through U-frame girder bridge. This form of structure gives a shallow construction depth (from floor level to soffit). The upper flanges of the girders are stainless steel, tilted at 15 degrees to be perpendicular to the inclined bathtub web plates (and, presumably, to reduce the risk of creating a flat ledge). The walkway floor forms the bottom flange, and is painted structural steel, along with the webs.

This is an unusual combination of materials, with stainless steel welded directly to ordinary structural steel. Wherever these two metals are in direct contact, there is a tendency for greater corrosion to occur at the interface, as stainless steel is the more noble material. This has been addressed on the Somers Town Bridge by simply overpainting the boundary between the two types of steel. This detail can't be seen as it's tucked away on the underside of the top flange. It's made clear on one of the planning consent drawings, however.

Why choose stainless steel for the top flange at all? There are certainly aesthetic benefits on an element which may be prone to being trodden on, but the main reason seems to be to allow the stainless steel parapet to be welded rather than bolted on. This is driven by the form of the parapet, which consists of a flat top rail and vertical flats at 100mm centres.

To bolt several hundred verticals onto the main girders would have been absurd, so the only two options are either to weld every vertical, or to introduce an intermediate lower parapet rail, which would in turn be bolted to the girders via intermittent posts or stiffeners. I guess this latter option is not compatible with the design's minimalist aesthetic, and having taken the decision to weld the vertical slats onto the girder, using a stainless steel top flange and tucking the bimetallic junction underneath does make some kind of sense.

The planning documents state the stainless steel material as Duplex 2205, which is a popular choice for bridgeworks and should have good strength, toughness and corrosion resistance.

The floor plate is 15mm thick, and the walkway is 3.5m wide. The floor plate sounds pretty thin, but is not especially remarkable - a suitable point of comparison is the standard Network Rail footbridge, which can span up to 28m, with a 2m wide walkway, and which uses only a 10mm thick floor plate, with the U-frame stiffeners at significantly greater spacing as well. Although the floor plate is slender, the two stainless steel top flanges are each 45mm thick.

The depth of the girders varies, increasing from 0.4m at the abutments to 1.1m at midspan. The parapets, which are also in stainless steel , are vertical and a constant height of 1.4m (1.1m to handrail). Because the edge girders are inclined at 15 degrees to vertical, the result is a constantly changing relationship between the girder top flange and the balustrade elements, with L-shaped slats being tall and narrow at the ends of the bridge and short and wide in the middle of the bridge.

The planning documents show the bridge as being painted in a dark "anthracite" colour, with a gritted epoxy resin floor similar to the resin-bound gravel walkway approaches. The proposal was to continue the resin finish up the inside of the webs, but this has not been undertaken in practice, with the painted finish used on both faces of the edge girders.

There are a number of particularly interesting features to the bridge. The first took some time for me to figure out: all the parapet support slats are at an angle to the main girders. I struggled to see the point of this arrangement, wondering if it was to create a particular louvred view, opening views from some angles and obscuring them from others.

The answer to the puzzle can be found on the underside of the bridge: the bathtub lateral stiffeners (the U-frames) are also placed at an angle. The bridge itself crosses the canal at a significant angle, and everything is skewed accordingly. This means that the U-frame stiffeners are parallel to the skewed bridge abutments (and canal edges) rather than perpendicular to the deck, and the same is true of the parapet slats.

It makes sense but still feels quite peculiar when encountered crossing the bridge. An advantage of the detailing is that it orients and opens up views when looking along the canal. If the U-frames had been perpendicular to the bridge axis, they would merge together visually into an undistinguished mass, making the bridge deck look significantly deeper than it really is. With the skewed arrangement it's easier to see the true depth of the bridge, and its visual impact is diminished, although only a little as the visual merge effect is still very much apparent when viewing the bridge from any other perspective.

The dark paint colour is appropriate to the canal environment, reminiscent of the black paint used on many historic canal structures, but it does lead to a degree of visual "murk". The texture of the U-frame ribs adds visual interest, but the dark colour reduces visual differentiation between ribs and bathtub plates. Some of my photos are misleading here, as I've adjusted the contrast to make the form of structure clearer.

Another prominent feature is a series of three panels at the centre of the bridge, adorned only with the number "34B". This is the official Regent's Canal bridge reference, of course, but this chunky block at midspan seems at odds with the desire to express a visual lightness.

Again, the planning submission explains the reason, which is that the bridge is physically light enough to require the presence of a tuned mass damper (TMD) at midspan to counteract pedestrian-induced vibration. In the planning drawings, only a single bay is occupied by the TMD, which is a cast steel element suspended on springs. In the bridge as-built, three bays are occupied, and the TMD is hidden behind a cover plate.

The bridge has to do more than just span across a canal; it is also bridging two very different environments: the redevelopment to the east, and Camley Street Nature Park to the west. Between the two, there is over a 4m change in level, and to address this there is a lengthy curved ramp at the western end. Building this has taken a considerable bite out of the nature park, and I gather that either a visitor centre will be relocated or some replanting will take place in this zone.

There is a slightly mannered physical transition between the two environments as well: heading downhill and westward from the bridge, the stainless steel balustrades give way to combined timber and stainless steel guardrails, with a graduated transition between the two. At the bottom of this approach path there is a bollard to block vehicle access.

At the upper end of the bridge, to the east, a second bollard is located on the bridge deck, and this serves as a support for two swing gates used to provide night-time security.

Given its scale and location, this is a remarkably interesting bridge. I think it is a little coy in nature, adopting some very sophisticated detailing in the service of a simple yet forced modesty. It's certainly a far better choice for the location than a more "showy" structure, but it's interesting to see quite how much work has gone into producing something so carefully restrained.

Further information:

15 July 2017

Zombie bridge awaits decapitation

Remember the Garden Bridge, the weed-capped insult to honest procurement, the unlamented jolly folly in the middle of London's River Thames, a celebrity-garlanded monument to starry-eyed foolishness?

You may recall that London's mayor, Sadiq Khan, struck the project a mortal blow when he withdrew his support for the scheme on 28th April. Khan was responding to Margaret Hodge's project review, which had already assaulted the victim with a series of timely and well-deserved knife wounds.

So why is the Garden Bridge still in the news now? It seems the body was not properly buried, but has emerged from the grave and is shambling onwards, a terrifying vision of a zombie bridge. What animates this supposedly lifeless corpse?

The power of celebrity clearly remains strong, and retains its power to utterly and completely cloud critical thinking. Architect Richard Rogers recently offered the project extensive praise in the Evening Standard, the official journal of the Garden Bridge Fan Club.

The speciousness of Rogers' arguments is apparent from the outset. He opens his article by observing: "The River Thames should be London's greatest asset but for centuries it was a barrier rather than a connection", and commenting on how the better parts of the Thames river banks are those that act as public promenades. It is these very same promenades that the Garden Bridge would obliterate at its chosen location, and the over-tall bridge would have been as much a barrier as a connection, destroying fine views along the river. The celebrity friends and hangers-on can sprinkle the zombie bridge with perfume, but it will not obscure the rotten stench of the undead.

Petitioners against the project have discovered that Khan did more than withdraw financial guarantees for future bridge maintenance, he revoked a series of previous mayoral decisions which had been issued in its favour. This removes not only financial support, but policy support, essentially preventing the London Assembly or Transport for London continuing to support the private Garden Bridge Trust in pursuing the scheme.

Given this, what is surprising is that the other public bodies involved in the project, such as London Borough of Lambeth, have remained silent, and not also made public an intention to no longer engage with the Garden Bridge Trust. It's remarkable that the Trust itself, which was barely a going concern several months ago, has not made public any plans to wind itself up, settling its debts (such as to the disappointed main contractor) and returning whatever funds it is able to.

And it's amazing that there is still complete silence from all involved on who is to blame for the whole fiasco (especially the shady design procurement processes), and what consequences will fall upon them.

Previous posts: Garden Bridge saga

30 June 2017

Tintagel Castle Footbridge submitted for Planning Consent

An application has been made for Planning Consent for a new footbridge at Tintagel Castle in Cornwall.

The last time I featured this project was to discuss the six shortlisted competition entries back in December 2015. In March 2016, the winner was announced as Ney and Partners with William Matthews Associates. The scheme is for a new bridge to take visitors onto the Tintagel Castle promontory, a beautiful and deeply historic site. The bridge will provide access for the mobility-impaired for the first time.

You can find the full planning application online, but I've extracted some of the pertinent material to share here.

The bridge gives the appearance of being a very slender arch structure, but in fact it is formed of two giant steel cantilevers, each shaped with a parabolic curve in elevation. In theory, this means that the lower rib carries a constant force when the bridge is subject to a uniform load, allowing for an efficient use of structural steel. In practice, things are never so simple.

The lower and upper ribs each comprise two weathering steel fabricated box girders. These span 66.7m in total. The cantilevers are not quite symmetrical.

The upper ribs are parallel, with the 3.0m wide structure supporting a 2.5m wide walkway. At midspan, these ribs are a mere 175mm deep, impressively slender by any standard. The lower ribs converge towards their foundations, and are also exceptionally small, being only 140mm deep over most of their length.

The structure's strength comes from the depth of the twin cantilevers, which reaches 4.4m near the supports. The upper and lower ribs are connected by what the designers refer to as a "Thomas Telford" detail, for reasons which should be obvious. These spandrel lattices are formed from solid stainless steel bars varying in cross-section from 30mm square to 65mm square.

The structural dimensions illustrate a peculiar talent that Ney and Partners seem to have for exploiting design standards to their absolute limit, and creating structures of astonishing slenderness. The total weight of the steel structure is stated as 66 tonnes, which is quite amazing for this span. It's no surprise given the slenderness to read that the bridge's first natural frequency is 1.6 Hz (well into the vulnerable area for pedestrian excitation).

The bridge deck consists of slates placed on edge in a sand bedding layer, carried on stainless steel pans supported between the primary structural ribs. The pans have drainage scuppers in the soffit, and there is an air gap between the deck pans and the main ribs to ensure the weathering steel can weather properly.

The bridge balustrades consist of stainless steel bars supporting oak handrails, 1.3m high in total.

The bridge foundations are proposed as rock anchors for both the compression (lower rib) and tension (upper rib) elements, with additional rock anchors used to stabilise the exposed cliff faces.

The gap in the middle of the bridge is nominally 42 mm, reducing to 5 mm under maximum temperatures, and increasing to 85 mm under minimum temperatures. There's clearly a degree of controversy to this particular detail, given the risk of a trip hazard or simply the discomfort caused to visitors already made anxious by height and exposure.

At competition stage I observed that significant differential deflections could also be expected when one cantilever was loaded more than the other (by pedestrians or by wind), but the planning submission makes clear that the two cantilevers are in fact connected by shear pins, in a similar manner to a twin-bascule bridge.

I think the poetic idea behind the gap justifies the problems that it creates: the intention is to make intensely apparent the sensation of stepping from the present into the past, of the division between the Tintagel Castle and the mundane world.

The designers have also sought to address the other major objection raised at competition stage, which was to the adoption of weathering steel at an exposed coastal site, where the wind will blow salt spray high above the sea. They have instituted a series of corrosion tests on steel plates exposed at the project site, and the planning submission documents make clear that if these are unsuccessful, the weathering steel will simply be substituted with conventional painted structural steel.

However, there seems to be little acknowledgement of the bimetallic corrosion issue created by the use of so much stainless steel and weathering steel connected together. This combination will tend to lead to accelerated corrosion of the weathering steel at connection points, especially if moisture and salts are present.

Results of the on-site salt spray corrosion tests were due to be completed in June 2017 so it would be very interesting to see the results.

An article in The Guardian focuses on what appears to be increasing opposition to the entire idea of a bridge and captures some of the key issues. If you visit the planning consent website, it's clear there are numerous objectors.

One that's particularly worth reading is from Cornish bard, Bert Biscoe, arguing that whatever the merits of the particular bridge design, they cannot outweigh the damage that will be caused to a site of major archaeological importance. The argument is not about the physical impact of the bridge, but about the very desire of the site's custodian, English Heritage, to increase visitor numbers in such a sensitive site. This is an argument about the merits of preservation over the merits of public access - it is intrinsically anti-populist, but perhaps necessary.

I have been to Tintagel and my initial feeling about the bridge was that the improved accessibility would be very welcome. The promontory is currently accessed via a low-level bridge and a series of awkward steps, which are very difficult for some visitors to traverse. The planning submission notes that some 15% of visitors who buy a ticket for the Castle never actually make it up the existing steps onto the promontory.

However, the bridge is no panacea for this, as there will still be areas which are only accessible via steps or very narrow paths. It's therefore legitimate to consider whether the adverse impacts of such a major intervention are justified by the benefits.

I will be very surprised if the bridge fails this initial planning consent hurdle. However, Scheduled Monument consent will also be required, and I expect opponents of the scheme will petition central government to call in the entire planning application for further review, such is the sensitivity of the site

 I think the designers involved have done an excellent job in addressing the site constraints, within the limits of their brief, and this will be a very interesting project to follow, especially if it proceeds all the way to be built.

27 June 2017

Footbridge Awards 2017

The shortlist has been announced for the triennial Footbridge Awards. Winners will be announced on 6th September at the Footbridge 2017 conference in Berlin.

I've posted links below to sites where you can find more information about the entries. I've only visited two, Merchant Square Footbridge and Greenwich Reach Swing Bridge. Most of the others are new to me. In general, there are some interesting bridges here, please share opinions via the comments if you want to!

Short span
Medium span
Long span

21 June 2017

Unbuilt bridges of London

I've been thinking for some time about writing a few articles about unbuilt bridges, but it turns out there's so much good material elsewhere on the internet that I'm better off directing you appropriately.

The best online resource for this is the "Ian Visits" blog, which has uncovered a cornucopia of unbuilt London architecture, including bridges.

A feature on Victorian railway stations that tried to span the Thames discusses unbuilt railway bridges (and stations) at Pimlico (pictured) and Waterloo. These early proposals came to nothing, and it's only with the recent remodelling of Blackfriars Railway Bridge that a railway station has finally been extended to span both banks of the River Thames.

The Victorians were also responsible for an 1852 proposal to run a railway viaduct not across the Thames, but along it. This structure was proposed to run between Westminster and London Bridge, and can be seen as a forerunner of what is now the District Line, built in a  tunnel below the north bank of the Thames.

At the end of the 18th century, a number of proposals were invited for the replacement of Old London Bridge. The scheme eventually selected would not be completed until 1831, but some of the alternative designs are interesting. Thomas Telford proposed a 600-foot span cast iron arch (pictured), which would have been a tremendous technological achievement if built. Creating suitable foundations to withstand the arch thrust in London Clay would have been a significant challenge! "Ian Visits" also discusses George Dance's proposal for two parallel bridges.

Thomas Dunn was the engineer behind a proposal for a remarkably modern looking iron footbridge at Ludgate Hill in 1863. Before traffic signals brought order to the city streets, Dunn's design was for a walkway-in-the-air, allowing pedestrians to cross a busy road junction without the inconvenience of having to cross overcrowded and dangerous highways.

Over the years, there have been various proposals for a new bridge across the Thames near St Paul's Cathedral (see my previous posts for some examples). "Ian Visits" documents a 1909 proposal for a road bridge, and the amusing 1997 design from FAT for a kitsch garden bridge in tribute to the late Princess of Diana (pictured). This one was ahead of its time, I think, the market for ironic monuments to celebrity has surely expanded further over the last two decades.

There have been plenty of proposals for inhabited bridges across the Thames over the years (again, some of which I've covered in the past). It's a perennial theme from incurable nostalgicists blithely unconcerned with the visual impact of what they design. One such proposal was the Crystal Span Bridge, a 1963 proposal to replace the Vauxhall Bridge with a seven storey monstrosity complete with gallery, roof garden and ice rink.

Further curiosities include a proposed elevated viaduct along Oxford Street, to allow the area below to be pedestrianised, and a high-speed travelator for London Bridge.

What unites most of these unbuilt bridges is simple: we can be glad they were never built, given the number of potential eyesores avoided!

See also Unbuilt London: Bridges To Nowhere And Mad Masterplans at the Londonist.

12 June 2017

London Bridges: 48. Vauxhall Bridge

This is the third and last in this latest series of posts on the bridges of London.

Vauxhall Bridge sits on the site of a pre-Roman structure within the River Thames, only discovered in 1993 when shifts in the riverbed exposed its wooden piles. From that structure's demise until 1816, the only crossing of the river at this point was via ferry.

In 1806, engineer Ralph Dodd campaigned to build a new bridge at the Vauxhall site, leading three years later to the passing of an Act of Parliament and the creation of the Vauxhall Bridge Company. Dodd's 13-arch bridge design was, however, rapidly dropped in favour of a 7-span stone arch design by John Rennie. This then proved too expensive, and approval was obtained via a further Act of Parliament to construct an iron bridge instead. Rennie proposed an 11-span iron bridge, but a 9-span cast iron proposal from Samuel Bentham was preferred.

In its turn, Bentham's design also fell out of favour, and the bridge eventually completed in 1816 comprised nine cast iron arches supported on stone piers, designed by James Walker. This bridge proved highly successful, and was eventually taken into public ownership in 1879. However, by this time, the bridge foundations were in increasingly poor condition, and in 1895, permission was granted to replace the structure.

The new bridge also went through several incarnations before being built. Alexander Binnie's initial steel design was not popular, and he then proposed a 5-span granite-clad concrete structure. Work began on this, but problems during construction of the foundations led to a change in plan: it was decided that the concrete bridge was too heavy. The granite-clad concrete foundations and piers were completed, but Binnie and Maurice Fitzmaurice designed a steel arch superstructure instead, which was finished in 1906. This is the bridge that can be seen today.

The new bridge attracted further controversy even before it was complete. Complaints about the engineer-led design led to a decision to install statues on the bridge piers, four on each side of the bridge. The downstream statues by Alfred Drury represent the themes of Science, Fine Arts, Local Government and Education. On the upstream side, statues by Frederick Pomeroy illustrate Agriculture, Architecture, Engineering and Pottery.

Many users of the bridge will not even realise the statues are there: they face outwards to the river. They are visible from the river bank, but for a close-up view you either need to get into a boat or lean over the side of the bridge.

The bridge originally had wrought iron railings on each side, but these were replaced in 1973 with the squat parapets seen today. These are at least shorter than the original railings, which were some 8 feet tall! This was before the bridge achieved Grade II* Listed status, which only came in 2008, as part of a group of London bridges.

Apart from the parapets, Vauxhall Bridge is an attractive structure when viewed from the river. I think a large reason for this lies in the paintwork, which has been arranged to highlight a hierarchy of elements, with the riveted steel arch ribs in yellow, spandrel stanchions in white, and red, white, blue and yellow all used in different parts of the stringcourse and parapets.

There are thirteen arch ribs in each span, and the span dimensions vary from 45.6m in the centre span, via 44.0m in the intermediate spans to 39.8m in the end spans.

What struck me most about the bridge was its sheer width. It accommodates two footways, a two-lane cycleway, a bus lane, and a four-lane highway.

This is a major traffic artery, and there are further major highways running along the river at both ends of the bridge.

The result is a bridge that makes it easy cross the river, but which also plays its part in creating division, as it's almost easier to cross the river from north to south than it is to cross the bridge from east to west. Seen from the river, it's like a giant staple connecting the two river banks. Seen from above, it's like a giant pair of scissors, helping to subdivide London into smaller and more navigable city blocks.

Further reading