23 July 2017

Yorkshire Bridges: 12. Cobweb Bridge, Sheffield

Next stop, Sheffield, where I visited several bridges recently.

The Cobweb Bridge was top of my list, I'd wanted to see this bridge for quite some time.

Built in 2003, it forms a key link in the city's Five Weirs Walk. It allows walkers to negotiate two obstacles at the same time: the River Don, and the Wicker Arches railway viaduct (officially the Victoria Viaduct). The Cobweb Bridge spans the river while passing below the Grade II* railway bridge.

The bridge was designed by Sheffield City Council's in-house structures team, and built by Thyssen. It's a lovely idea: a slender platform suspended from the rail viaduct vault by a spider's web of steel cables. Lurking above it at the crown of the arch are giant metal spiders, which house lighting units.

I saw it on a grey and drizzly day, which didn't help, but I found the bridge a little disappointing in reality.

It is S-shaped in plan, with two arced segments at each end connected by a long straight walkway in between. It makes for an interesting series of perspectives, although the main length of the walkway is very tunnel-like.

At one end, the walkway is supported on a set of support legs, designed entirely without visual finesse. At the other end, it spans from the riverside onto an intermediate suspended frame. In places, the geometry of the bridge and its parapets seems slightly mis-aligned.

The support cables are slender steel wires anchored into the stonework of the viaduct via proprietary Cintec anchors. The cables are arranged with a degree of triangulation to eliminate sway, and further stiffness is provided via tie-down cables inclined below the deck. These incorporate turnbuckles, which will have allowed both sets of cables to be stressed against each other. I didn't observe any significant vibration, indicating the design to be effective.

The major problem with the bridge relates to its troubled history, not to its original design. After opening, the bridge suffered repeatedly from vandalism, and deterioration of the original flooring, which appears to have been timber with a red-coloured anti-slip covering.

The parapets as originally designed featured longitudinal steel wires as the infill, and the cleats which supported these are still visible today. You can see how the bridge once looked by following the Wikipedia link below. However, the wires were repeatedly vandalised and stolen, eventually leading to closure of the bridge and a refurbishment programme in 2012. The wires were replaced with a tougher metal mesh, and the timber floor with grey FRP panels.

These appear to be vandal-resistant, but the changes have dramatically downgraded the appearance of the bridge, from something that was once lightweight and open to something more greatly resembling an industrial cage.

It remains a remarkable and interesting structure, and an imaginative way to carry a footpath past obstacles with difficult issues relating to physical geometry, asset ownership and heritage value. I can't think of anything else in the UK with a similar structural form.

It's a tremendous shame to see it in its current state.

Further information:

20 July 2017

London Bridges: 50. King's Bridge

My last post discussed the Somers Town Bridge, a high quality new pedestrian and cycle bridge providing access into the King's Cross redevelopment site in London.

This time, I'm covering the nearby King's Bridge, a designer highway bridge providing access to the same area.

The bridge spans Regent's Canal, and was designed by Knight Architects with Arup (initial design) and Peter Brett Associates (contractor's designer). It was built by BAM Nuttall, with precast elements by ABM Precast (prestressing by CCL).

The bridge deck comprises thirteen 26m long precast beams spanning 23m, which unusually for a highway bridge in the UK are post-tensioned rather than pre-tensioned. This appears to have been driven by the required site levels, resulting in a very limited construction depth between the highway and the canal clearance envelope.

What I like about this bridge is that it shows how easy it can be to create something substantially better than the norm. The bevelled edges to the abutments and edge beam are enough to create a picture-frame effect, all crisp and clean geometry. Unsightly stringcourses are eliminated, the bridge is made to appear sculptural, monolithic, rather than the assemblage of pieces that it really is.

Attention to detail includes the ribbed finish to the underside of the bridge deck and to the faces of the abutment. As well as ensuring a consistent appearance, this eliminates the large concrete surfaces which might otherwise attract graffiti. It's a small but significant signal to towpath and canal users that somebody actually cares about their environment.

The need for quality is evident from a short walk around the area. A set of astroturfed benches spills down to the canal right next to the bridge, and on the opposite towpath there's a canalside cafe. It's a popular place to stop and sit, at least in good weather. I believe a further crossing is planned at low level to connect to the two towpaths in the future.

Another positive feature on the bridge is the use of non-standard parapets. I'm not sure what the source of these is: are they custom-designed or off-the-shelf? Whatever the case, they're not the bog-standard type.

The metal mesh screening emphasises that they are part of the urban realm, not just an edge barrier. If I have a criticism, it's simply that the geometry of the mesh is rather boxy, making the parapets visually more prominent than they maybe need to be.

I think there's an entire book to be written on the subject of bridge parapets. They are one of the most highly visible elements of a bridge, yet on too many occasions an afterthought.

As with the Somers Town Bridge, what strikes me most about King's Bridge is the contrast between its careful and un-showy appearance, and the amount of work the design team have put in. It takes effort to make something look so effortless.

Further information:

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