02 May 2009

Forthside Bridge opens

The £6.5m Forthside Footbridge in Stirling, Scotland, has opened today, some twelve months behind schedule.

The footbridge (pictured right) crosses Stirling Railway Station, and provides access between the new Forthside development and the town centre.

Designed by Wilkinson Eyre and Gifford, and built by Edmund Nuttall, the new bridge comprises two 88m span inverted Fink trusses, in an unusual arrangement that makes the bridge significantly different to the only other major bridge of the same form, the Royal Victoria Dock footbridge, not to mention unbuilt designs such as Flint and Neill / Exploration Architecture's Perpignan Bridge (shown below in colour) or Mott MacDonald / Knight Architects' River Douglas Bridge (shown below in black-and-white).



The trusses are assymetrical, being higher at one end than at the other, and each truss decreases in height in the opposite direction (see extract from planning drawing on the right), such that the bridge deck is supported on each edge by a propped cantilever - but with the cantilevers in opposing directions such that there is presumably a marked tendency in the deck to twist (dealt with structurally by making the deck a flattened box girder). The inverted Fink truss is inherently inefficient, and the assymetric arrangement will have added to that, as does the fact that the trusses aren't in a vertical plane, but incline outwards from the deck at an angle which warps along the deck by 35°.

All this adds up to a bridge which is exceptionally complicated geometrically. The justification for this comes from the wish to cradle the walkway in a manner which visually echoes the pedestrian desire lines (which are at an angle to the bridge span). From the photos and images currently available, I think it's an exciting design, taking a simple structural idea and playing with it in a very interesting way. It's especially nice to see such an innovative bridge resulting from the much-maligned Design-and-Build approach, often seen as a source of poor design quality.

The bridge is 113m long overall, so for those who pay attention to such things that's a cost of £57,500 per metre length. Assuming the bridge to be typically about 4m wide (actually it varies, but the clear useable width at midspan is certainly less than 4m - see planning drawings), that's a cost of over £14,000 per square metre of deck. This is high even by the usual standards of landmark bridges (a range of £4,000 to £7,000 per square metre was about right just a couple of years ago), so Stirling Council could have had an attractive, albeit less remarkable, structure for far less.

The delays completing the bridge have been attributed by Nuttall to the complex stressing sequence required, as well as the need to verify cable connectors supplied without proper testing. The difficulties of building such a highly assymetrical structure were alluded to in a technical paper by the designers [PDF].

Overall, it's one of my favourite Wilkinson Eyre designs of recent times, and likely to be a strong contender for various design awards over the next year or two.

Updated 4 May 2009: added credit to Explorations Architecture for Perpignan Bridge, and higher-resolution image

11 comments:

Anonymous said...

Dear Happy Pontist

Its great to have some costs per m2 rate for bridges of high aesthetic value.

Anonymous said...

Dear Unhappy Pontist

Shame it wasn't a simply supported beam that costs £1/m2. I think that would be the only thing that would actually make you happy.

Bridges are more than just functional - shame you cannot appreciate this.

Also - to credit Wilkinson Eyre at the end, when this is clearly an engineering led design is quite unbelievable.

The Happy Pontist said...

Crikey, I call it an "exciting" design and you think I'm unhappy about it?

The point about money is not related to the aesthetic merit of the bridge or otherwise (I would hope this would be fairly clear to anyone who's actually read this blog regularly - I suggest you read my posts on bridge criticism to get a broader picture of my opinion).

However, it is entirely relevant to whether or not the client is getting value-for-money. I'm sure they're getting value from this bridge. But for the money paid? They could have had a high-quality landmark bridge for far less, and I think that's something their ratepayers may find interesting. Are designers not to be concerned with value-for-money in their pursuit of aesthetic extravagance? In assuming that I am only in favour of a least-cost solution, you are wilfully misinterpreting my views.

As for "crediting" Wilkinson Eyre - both designers were credited. However, my point at the end is to make a brief comparison with other WE designs, some of their recent designs (Salford, tensegrity, Paradise Street), being considerably less successful to my eyes. I guess it could be of interest to attempt a comparison of Gifford's designs some time.

As for the design being "engineering-led" ... the main feature of the design, the asymmetric supports and the warping of the cable plane, seem to me to be clearly architecturally led, a thought that is supported by the designers' joint paper, which makes clear that this decision is for visual and experiential reasons to reflect the pedestrian desire line.

The Happy Pontist said...

PS: In case that last response seems unnecessarily crabby, I would welcome more serious comment on the merits of the Forthside design, on the relevance of cost to an evaluation of a landmark structure, and to the extent that designers receive credit or otherwise.

I am conscious that what little architectural criticism of landmark bridges exists is almost entirely from architectural writing perspective (see numerous coffee table books), and have made considerable effort here previously to identify precedents for an "engineering-led" criticism.

Rather than sniping at each other (as is all too easy on the internet, particularly when we're anonymous), I would be keen for some more constructive discussion. If the designers themselves should happen to read this and care to contribute, that would be particularly of interest (I am aware that designers of several of the bridges featured on this blog also read it)!

Anonymous said...

Dear Happy Pontist

I would appreciate some comment in due course as to the extent of wind or pedestrian induced vibrations experienced with the Forthside bridge.

I note in the paper (linked) it mentions a fundamental torsional frequecy of 0.92Hz. There is no guidance in BS5400: 2006 Annex B about limits for the torsional frequency. Do other designers of slender bridges have some practicle limit they would suggest? Obviously if the deck is essentially moving vertically in the torsional mode then the vertical vibration criteria of BS5400 should apply.

Aesthetically it is difficult to make trussed tructures "harmonious" because of the angularity and diverse directions close to the observer(pedestrian on the deck), but I think the designer's have had a fair go at acheiving this.

I don't know anything of the local setting, but I believe it is a good thing to make some sort of emotional connection with it when designing these tyes of structures, even if it is not the most structurally efficent or lowest cost solution.

Anonymous said...

It is true that most people, whether client or public, forget that a bridge has run over programme the day it is opened, and forget that it is expensive within a very short while. But a bridge which is banal or ugly or in conflict with its context will be evident for every day it is used. Wilkinson Eyre and Gifford's bridge at Stirling may be unnecessarily flamboyant for a railway overbridge, but it will be a delight to use and to look at for ever.

The Happy Pontist said...

Taking the comment on vibration first ... in the UK, wind-induced vibration is covered by BD 49/01, which includes a number of checks on vortex excitation and flutter which relate to the fundamental torsional frequency. For its effect, if any, on pedestrian-induced wobble, I can't comment.

I am not entirely convinced about the need for trussed bridges to be visually harmonious. I recently had a debate with a planning authority about the exact angles for truss diagonals to look best, and the main challenge was getting them to recognise that nobody actually sees the bridge as drawn on elevation, and that in any perspective view, the angles are seen differently (also because visual focus in real life is different from when looking at a drawing - we only rarely take in "the whole" in the same way).

I agree with the other comment that programme delays and cost over-runs are rapidly (and sometimes rightly) forgotten. But I also think too little attention is given to value-for-money when taxpayers' cash is spent on regeneration projects - very little research is done to demonstrate that the economic regeneration benefit achieved justifies the outlay. Of course, the improvements to visual amenity and the creation of beauty or spectacle may justify expenditure in their own right!

Anonymous said...
This comment has been removed by a blog administrator.
The Happy Pontist said...

Previous comment removed because the language is close to libellous. I would be happy to include a revised version with the language reconsidered!

neil craig said...

Pontist I do think the bridge is beautiful (otherwise I wouldn't have checked it out here).

But it is also obviously very expensive & I do not think, if I was a ratepayer, I would be overly happy. It pedestrian crosses 3 separate obstacles - 2 sections of railway & 1 road - which clearly could have been crossed by arches at minimal cost.

With the new Forth crossing at £2,600 million when the previous one cost £19.5m (£329m in modern terms)I would prefer if our political masters had concern for value for money, rather than the opposite.

Good engineering includes efficient use of resources. I wish good politics did too.

David Baillie said...

Have a look at it now, a complete eyesore and falling to bits.

What a disaster of a project