Welsh Bridges: 1. Swansea Sail Bridge

I visited this bridge back in July 2008.

It opened in June 2003, and was designed by Flint and Neill with Wilkinson Eyre. The main contractor was Balfour Beatty, with the bridge structure fabricated by Rowecord. It was funded by the Welsh Development Agency as part of a docklands-type regeneration scheme in the vicinity of Swansea's marina area, connecting the city centre to a development area to the east. It cost approximately £3m to build.


The bridge spans 140m across the River Tawe, supported from a single central mast. The deck is boomerang shaped in plan, and supported on its inner edge by cable stays in a semi-fan arrangement.

Torsion in the deck is accomodated by an 800mm deep steel box girder spine along the inner edge, which in turn supports aluminium deck planks from steel outriggers (see cross-section). The deck varies in width from 4.5m to 5.5m. The parapets are stainless steel.

The mast is a 44m tall variable section steel box, inclined at 10° to the vertical to optimise the balance of deck loads and self weight and hence minimise bending moments. Its shape is complex - the face nearest the deck is cut from a curved steel plate, while all other faces are flat. It varies in section from triangular at the very top, through a set of near-triangular and near-diamond shaped irregular pentagons, to an 8-sided shape towards the base. I can't quite decide whether it looks too fussy, or just right.

There is a considerable torsion in the deck, resisted at the abutments by pairs of bearings, one below the deck, and one above, acting like a gigantic clamp to prevent twisting. It is resisted at the central mast by a horizontal thrust bearing, which creates a reaction forming a couple with the cable supports.

In any bridge supported only on one side, torsion is a key issue, and there are a number of ways of dealing with it. If the girder has a larger curvature in plan, then the bridge deck can become stable by virtue of the multiple points of support - only three are required for global stability. In such a case, torsion in each part of the deck can be resisted by bending moments in other sections, although a stiff torsion beam is still required to prevent excessive deformation.

The position of the cable supports relative to the shear centre of the deck can also be used to reduce torsion. On the Sail Bridge, the deck curvature is limited, and the line of the cables actually passes to the "wrong side" of the shear centre, increasing rather than relieving the twisting effect. Awkward torsion restraints at the abutments are therefore probably unavoidable, and although they look odd on drawings of the bridge, they are essentially invisible to the end user.

As designed, the bridge has a lateral natural frequency close to that vulnerable to the "wobbly bridge" effect, and so there are tuned mass dampers hidden below the decking. Wind tunnel tests also showed the bridge to be vulnerable to aerodynamic excitation from vortex shedding, which has been addressed by the inclusion of fairings to the deck section, visible in the photograph.

The bridge was erected in segments on a scaffold. Erection by cantilevering from the support is normally an advantage of the cable-stayed form, avoiding the expense of temporary supports. However, in this case, it was thought that the deflections at each cantilever stage would be hard to predict, given the twists involved, and hence erection on falsework was preferred.

The bridge is well-detailed, with the two different forms of parapet on the inner and outer edges complementing each other well. It suits its visual context well, being prominent but dignified, and not out of scale with the urban landscape. Its resemblance to a sail is serendipitous rather than forced.

Further information

• Google maps / Bing maps
• Structurae
• Corus Construction
• A critical analysis of Wilkinson Eyre's Sail Bridge, Swansea [PDF]
• Design and construction of the Sail Bridge, Swansea, UK, Bridge Engineering, ICE, December 2005
• Footbridges, Baus & Schlaich, Birkhauser Verlag, 2008

Is this the UK's first prestressed granite bridge?

Nobody seems to have made much of a fuss over this, but I believe this new footbridge may be the UK's first prestressed granite bridge.


It was installed in December as part of a redevelopment of the Town Centre Gardens in Stevenage. A deteriorating timber footbridge had to be replaced, and the chosen solution was a 14.35m long, 0.25m thick prestressed granite bridge, designed and built by the German firm Kusser Aicha Granitewerk [PDF].

The bridge weighs 22 tonnes, and has a remarkably shallow span-to-depth ratio of 57. I've posted previously about this type of bridge, including a discussion of the various design issues and structural behaviour.

Bridges news roundup

This time, I'm offering a "footbridges only" news roundup ...

Archbishops first to step onto MediaCityUK bridge
They will pray for the bridge's users, although hopefully that will be unnecessary. It's odd to see a news feature like this which only credits the bridge engineer and not the architect.

Plans for Perth Connect2 bridge resuscitated after agreement is reached with landowner
Tay Bridge likely to proceed following resolution of issues with one of the affected landowners.

Have our design or no bridge at all, says Bradford council
An angry mob protests against "biased" presentation of the options to a town meeting. No pitchforks wielded, apparently.

After a 70-year wait, the plans for a new bridge in the town were unanimously approved last night
It's nice to see it's still possible to develop bridge proposals with consensus rather than vociferous dispute.

North Sheen Railway Footbridge Competition

This is something of a mickey-mouse contest, but in case any of the heavyweight bridge designers who read this blog want to pit their wits against an army of schoolchildren and pensioners, you should pay a visit to http://rpnkconservatives.co.uk/

The site, in Richmond, Surrey, has an existing level crossing, and it is proposed to build a new footbridge to prevent children and others from taking risks crossing the railway. A standard Network Rail design has been proposed, in its rabbit-cage variant.

This is a half-through girder bridge with two metal girders doubling as parapets. It has a mesh cage across the top which is either there to prevent people dropping concrete blocks on trains, or to create a climbing frame for foolhardy yobs (take your pick). Why they want the cage I don't know - there are plenty of similar bridges without one, and I would always advocating retrofitting one in response to a real problem rather than anticipating one that may never occur.

It's clear from the planning drawing that the site is highly constrained, with even the standard design not quite fitting into the available property boundary. However, it's an awful bridge design (see picture, below) and it's great to see residents agitating for something better.


The bridge has already received planning consent, albeit seemingly against the advice of the local planning officer.

So, the local Conservative Association has invited entries to a design contest, by 22nd April, to see if anyone can come up with something better. Anyone can enter, there are no constraints on what information is to be submitted (although I'm not sure whether the back of a fag packet can be emailed), there's no prize, and the winner is not guaranteed any subsequent design work. Designs will be exhibited locally and a public vote will help choose the winner.

I think it could be fun. Instead of a cage for the bridge users, how about tall posts from which wealthy financiers could be suspended? That might do well in the popular vote, at least.

Bridges news roundup

Corporation Street Footbridge to be repaired
I visited this bridge last year, and noted that it was certainly a bit shabby below the internal deck, but didn't think it was in a dangerous condition.

Transportation officials back deck truss bridge
No great surprise to see that the Columbia River Crossing is likely to go with the cheapest of three options currently on the table. There are still plenty of ways they can refine it aesthetically. See previous post.

Bridge too car
Nobody drove across when I visited the Constitution Bridge in Venice, but it's a surprise to see a city offical quoted as saying "it was never intended to take the weight of a car". Maybe not, but unless bollards prevent access, it should have been designed for a point load at least equal to a car tyre, and for a global load many times in excess of the weight of the car.

Three UK transporter bridges eye World Heritage status
Not to mention the other members of the World Association of Transporter Bridges, too. The locals in Middlesbrough and Newport seem supportive, although there's radio silence from the most neglected of the trio, in Warrington.

Celebrating bridge firm’s global reach
History project examines the bridges built by Patent Shaft Steelworks.

The stone bridges of Epirus
I spotted this via the arches blog, several interesting historic stone arch bridges in Greece.

Rivers - Friend and Foe
The latest episode of the BBC's interesting and often spectacular documentary, Human Planet, included footage of the living bridges of Cherrapunjee, in Meghalaya, India. I featured them here before - bridges built over decades by gradually training aerial tree roots across a river. The programme showed one bridge grower at work, teaching his daughter his skills, and will be online at BBC I-Player for another week yet (probably available in the UK only).

Bridge competition debris part 23: Poole Harbour Crossing: Commended

Okay, this will be my final roundup of the entries to this contest for now. These are the ones which were "Commended", not "Finalists", "Shortlisted", or "Highly Commended", but also not "also-rans". There are what feels like another 369 entries in the last category, but it will take me a while to get all those ready to post.

While scanning all these in, one thing that really struck me is how presentation standards have changed. The artist's impression was very much the norm when this competition was held in 1997, but it's hard to escape the thought that the skills are largely lost and that you'd be laughed at without computer visualisation today. In part, that's a good thing, as it allows the viewer to more rapidly understand from a variety of different perspectives how a structure is layed out in space. However, I wonder if it also induces a certain laziness in the viewer, who must be less inclined to apply their own imagination to interpreting the image, and hence more likely to be seduced by what they see rather than forming their own understanding.

Commended
Parkman / Carlos Fernandez Casado / Fairhurst Design Group

It's always a joy reading jury reports from bridge design competitions. With one breath they'll decry a design for lacking drama, and in the next, as with this bridge, advise that it's "perhaps not calm enough for a low-key mudflat site". That's not an unfair judgement - there's simply no pressing reason for the structural complications of this design at this site - but must leave competitors tearing their hair out wondering what the judges really wanted.

Whitby & Bird

This is a structurally logical modernisation of the traditional arch viaduct, but it doesn't really seem to work visually, particularly because I understand the V-shaped supports to be built in brick, which would be quite a challenge.

Render Palmer & Tritton / Nicholas Grimshaw & Partners

One of three entries by the same team. The other two were highly commended for their "elegant minimalism" (see previous post), but I prefer this one, a composite steel truss on trestle legs. It's fairly straightforward to build, structurally efficient, and can be made to look quite attractive.

Dorset Engineering Consultancy

Beaten by more stylish designs, this effort points up the perennial question of why exactly so many bridge clients are desperately seeking novelty. Sure, there's a need to draw attention to yourself, particularly for a small town trying to promote an exciting image in the hope it will trigger investment, but there's also a need to balance the budget.

Harris & Sutherland / Jean Muller International / Charles Lavigne

Another weird criticism from the judges - it "is not structurally explicit - is it acting as an arch or not?" Why the fear of structural androgyny? There are umpteen other truss arches around which seem quite happy acting as a bit of both. I don't think it's a beautiful design but the specific criticism seems unfair.

Harris & Sutherland / Jean Muller International / Charles Lavigne

Perhaps it's the very competition format which drives the desire for novelty. A number of logical, uncontrived cable-stay bridges were entered, which although inappropriate for a scheme where there was no need at all for long spans, were nonetheless modest, reasonable solutions. But after you've seen six or seven similar designs all of which reflect the engineering rationale, it's easy to imagine how the sheer repetition will encourage enthusiasm for the one design which is unusual, even where it has no need to be. In this respect, the design competition is inferior to optioneering via the traditional one-consultant feasibility study, where the various options would be treated more even-handedly.

Bridge competition debris part 22: Poole Harbour Crossing: Highly Commended

Five entries from three design teams were Highly Commended in the Poole Harbour Crossing competition.

Render Palmer & Tritton / Nicholas Grimshaw & Partners

The slender trestle-like supports suggest otherwise, but this is a post-tensioned concrete bridge. The judges felt it had the civilised mood of a Victorian pier, but lacked drama.

Render Palmer & Tritton / Nicholas Grimshaw & Partners

This was described as "a very attractive proposal ... graceful and elegant", but I can't see it myself. The fanned support legs are like an attempt to make a Brunel timber viaduct out of knitting needles.

Halcrow / Walther Mory Maier / Cezary Bednarski (Studio E)

Compared to many other designs that Bednarski has been involved in, this is a very straitlaced bridge. It's not so modest as to be austere, but nearly so.

Sir Norman Foster & Partners / Ove Arup

The judges commended this bridge for making "so many of the other entries look contrived and overdesigned", but it's no surprise to see simplicity commended without being rewarded.

Sir Norman Foster & Partners / Ove Arup

Apparently, this second Foster/Arup design only just missed out on being shortlisted, although from this image at least I think it looks pretty awful, with no real cohesion between the Y-shaped piers and the deck structure. Compare the same design team's subsequent design for the Millennium Bridge, where a very similar pier design was rotated 90° and deployed with far greater success.