31 July 2017

Yorkshire Bridges: 16. Pond's Forge Footbridge, Sheffield

Okay. I think we all need to draw a deep breath before looking at this utterly baffling bridge.

This little curiosity is just next door to the Park Square Bridge. It forms one of two approach ramps to a footbridge over the Park Square roundabout, with the other ramp and main structure being much less interesting steel box girder spans. This ramp provides a main access route into the Pond's Forge International Sports Centre.

My only source of information on this structure comes from an ICE leaflet titled "Sheffield: A Civilised Place". This states that the footbridge was required by the client to be "interesting and imaginative", against which criteria it must be considered a great success. If anyone out there can share more information, please do!

At first glance, it seems like some weird preying-mantis version of a cable-stayed bridge, but I suspect this is not correct. At one end of the bridge there is an elevated tower, with two spindly arms cranked towards the span as if holding it up, but it seems unlikely this element can carry any significant load, in the absence of back-stays. The cables themselves are very slender, and they connect not to the bridge deck, but to the upper ends of parapet posts (again, with nothing to balance any horizontal force in the wires). I think these parts are essentially decorative, or perhaps helping slightly to resist twisting of the walkway.

I wondered whether the bridge might be half-through U-frame structure, with the perforated parapet plates acting as the webs, but note that there is no upper compression member, and that these plates are discontinuous before the upper end of the span: the main part of the bridge has parapet plates and posts inclined inwards, but towards this end of the span there are vertical walls contiguous with the mast pieces.

The parapet posts are almost grotesquely over-sized, and are aided/abetted by horizontal parapet rails which seem quite superfluous. My sense is that that parapet posts, which continue under the deck to form clamp-shaped support frames, are simply a series of transverse frames holding up both deck and balustrades.

Every second parapet post is connected at its upper end via two diagonal wires to the lower end of intermediate posts. These positions are then wired to nodes below the deck. I wondered if this might form some kind of stiffening truss, but there are no longitudinal chords. Perhaps they restrain the parapet posts against longitudinal movement, but the parapet panels are quite capable of doing that. Again, I'm left unable to decide whether they help a little to restrain twisting, or are simply decorative.

At the opposite end of the bridge from the "support" tower is a second tower, supporting what looks like an empty signboard. This is connected by wires to a baffling collection of bits and pieces below the bridge deck, but I think all this is doing nothing more than holding up the tower and its (empty) signboard.

The parapet "clamp" frames sit on a single tubular member running below deck, with a combined relationship something like a spine and rib cage. So far as I can tell, this single tube, dwarfed by the massive but seemingly over-wrought metalwork above it, is a simple beam supporting the main span and resisting both bending and torsional effects. This tube is connected to support frames at either end and hence to ground.

I think I have rarely encountered such a confused, confusing shambles of a structure. It is "interesting" in the way that a car-crash is interesting: it's difficult to look away from the sheer horror. It is "imaginative" in the way that a David Smith sculpture is imaginative: a creative juxtaposition of geometries in three dimensions. But a sculpture's primary function is visual, not structural.

In principle, I'm all in favour of whimsy in bridge design. Bridges, especially on a small scale, do not all have to be po-faced and technologically constrained. I'd quite like to like this bridge.

But I don't.

Further information:

30 July 2017

Yorkshire Bridges: 15. Park Square Bridge, Sheffield

Park Square Bridge is one of several bridges built for the introduction of Sheffield's Supertram system, completed in 1994/5. This tram system's three branch lines converge in a delta junction just to the west of Park Square, and this bridge carries the western branch above Park Square roundabout and up onto Commercial Street (hence the bridge's other name, Commercial Street Bridge).

Alongside other major Supertram structures, the bridge was designed by Sheffield City Council's in-house team, Design and Building Services. That seems barely imaginable today, it's hard to think of a UK local authority with the capability to design such a major structure any more.

The bridge spans 75m (with two smaller approach spans making a total length of over 100m), and was built by Balfour Beatty, with steelwork by Watson Steel (now part of Severfield).

The main bridge span is a bowstring arch. Inclined steel arch ribs are braced overhead and tied with steel girders, which form the edge members in a ladder-deck arrangement. The bridge deck is a 250mm thick reinforced concrete slab composite with the ladder-deck crossbeams. The deck carries a non-structural concrete slab, containing the tram rails, drainage channels and stray current protection system. The deck is suspended from the arch by 60mm diameter steel hangers.

Although the three spans are each simply-supported, the structural deck slab is continuous, to avoid leaking joints, and a paper on the bridge design states that there are "concrete hinges" in the slab above the piers.

The superstructure sits on reinforced concrete piers and abutments, with concrete blockwork cladding, and these are supported on piles down to bedrock. The design language is a little post-modern, I think, with an elaboration of detail and a type of mock-classical architecture that seems at odds with the high-tech or modernist sensibility of most bridges of recent decades.

The bridge span was driven by the highway layout and the presence of a large culvert below ground. However, Supertram planners were also keen to have a "gateway" structure at this prominent position. A cable-stayed alternative was considered but rejected. The choice of a bowstring arch allowed a low construction depth to be employed, minimising the length of approach ramps.

The designers sought to maximise maintainability by avoiding closed structural sections, so the arch rib is a channel section with the open face downwards (similar to another bowstring arch the other side of the Pennines, in Salford).

According to the paper, "careful treatment" was given by the designers to the parapets and to the anti-climb blisters on the arches. The latter are pretty horrible looking, but not enormously so given the scale and clarity of the rest of the bridge.

The arch ribs are pin-ended, which I find really quite odd. Are there other modern bowstring bridges with this detail? The design paper offers no explanation for why this choice was made. It isn't visually incongruous, but I think hinges should generally be avoided in main members of bridges unless there's a very good reason.

Behind the hinges, there are thrust blocks comprising a group of vertical steel stiffening ribs, which transfer the arch thrust into the end node of the tie girders. As you can see in the photograph, they double as a handy waste receptacle.

On the whole, I think this a well-designed bridge, generally with clarity of form and well-proportioned. It is, however, let down by some of the detailing, and not only the impromptu waste-bins, the lumpen parapets or the blisters.

There is a pastel colour scheme used to distinguish between a hierarchy of components, both the hanger assemblies and also the connection of these to the tie girders, which for my taste has been over-elaborated. The use of this number of pale colours is not very successful, as they are simply impossible to distinguish under most lighting conditions. A smaller number of complementary colours would have been enough.

The bridge was the winner of an ICE (Yorkshire) Award in 1994/1995, and Commended in the National Steel Design Awards.

Further information:

27 July 2017

Yorkshire Bridges: 14. Blonk Bridge, Sheffield

The next bridge upstream from Smithfield Bridge is Blonk Bridge, which carries Blonk Street across the River Don.

This is a three-span masonry arch bridge, completed in 1828 by Doncaster-based architects Woodhead and Hurst, the contractor being a Mr Birchett. It cost £250 to build.

The spans are elliptical in shape, or possibly three-centered. The masonry looks well detailed, with the facing voussoirs tying into the main spandrel stone courses, although the geometry is such that the spandrel courses are all of a different height.

The masonry above the stringcourse seems to be of a different type and construction, perhaps associated with bridge alterations undertaken in 1913 (I can't find any details of what was altered online).

The most attractive feature on the bridge is its downstream parapet, which is a decorative cast iron balustrade. It looks pleasant from the outside, and is very nicely painted on the inner face. The casting is "one-sided" which looks decidedly odd from close at hand.

The upstream parapet is in reinforced concrete, and quite out of character with the rest of the structure. There were plans to replace this with a metal parapet back in 2007, but evidently nothing came of it.

The bridge was Listed Grade II in the early 1970s.

Further information:

25 July 2017

Yorkshire Bridges: 13. Smithfield Bridge, Sheffield

Heading upstream along the River Don, from the Cobweb Bridge, you pass under the busy A61 road bridge, and come to a modern footbridge, which appears to be called the Smithfield Bridge after the site of the former Smithfield Market car park.

The new bridge spans 40m, and carries a 2m wide footway, connecting the river banks at high level (above flood defences), and inter-connecting a number of pedestrian routes including the Five Weirs riverside walk. Completed in July 2010, it was designed and built by CTS Bridges, and is a type of bridge they've supplied at several locations across the UK.

Structurally, the bridge is not especially remarkable. It consists of two lightly inclined painted structural steel tubular arches with Vierendeel infill. The parapets are perforated steel plate, which is not very attractive but probably a smart move given the nearby Cobweb Bridge's history of parapet vandalism.

There are two aspects to the bridge that I felt made it worthwhile featuring here.

The first is an observation. The bridge as built is not the bridge that received planning permission, and it would be interesting to learn quite how the original plan was substituted for something much cheaper.

The planning application for the bridge describes and shows a more attractive and generous structure, with a 3m wide rather than 2m wide footway, in the form of a "butterfly arch", described as "both striking in appearance and economical in construction". Evidently, it was not economical enough for somebody involved in the project.

Here is just one of the visualisation images taken from the planning application:

The planning application also shows and describes considerably more attractive parapets, "incorporating stainless steel woven wire infill panels".

The second point of interest is a very odd steel bar which appears to be either suspended from or propping up one end of the bridge. I spent some time on site trying to work out what this was for, without success, but the planning consent documents explain the purpose.

The bridge at this end is slightly lower, and although the bridge deck is above the river flood level, there is a concern that flood-borne debris could strike the bridge. The bridge alignment is skewed across the river, so debris hitting the bridge here could easily become trapped with nowhere to go, impeding river flow or in the worst case pushing the bridge off its supports.

The peculiar bar is therefore a debris fender, designed to channel debris away from the corner and back into the river flow.

It certainly isn't a pretty addition to the bridge, but presumably there was little choice, as the only other option would have been to raise the bridge, with an impact on ground levels, ramps and possibly adjacent buildings.

It would, however, have looked much better.

According to the Yorkshire Post, the bridge was funded by Yorkshire Forward, the Environment Agency, and the European Regional Development Fund, so it seems unlikely that the choice of the cheaper bridge solution can be laid at the door of a penny-pinching developer. Perhaps it owed more to a general tightening of budgets in the wake of the 2008 recession.

Further information:

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: