Manchester Bridges: 22. Spinningfields Footbridge

This is the second of two new footbridges over the River Irwell, connecting central Manchester to neighbouring Salford. I covered the other one in the previous post.

Neither bridge could be justified on a cost-benefit analysis of journey time saved. Greengate Footbridge is particularly absurd in that respect, being only 5m away from an existing foot-accessible bridge. The £1.5m Spinningfields Footbridge is a little better, shaving maybe 3 or 4 minutes off a journey walking across the nearest existing bridges (Bridge Street to the north New Quay Street to the south). However, both bridges act as gateways, as ways to improve the permeability of the urban fabric, enhancing the feeling of accessibility as much as the reality of it.

Greengate Bridge's minimalist appearance shows the structural engineer in full retreat, a functionary whose role is quite literally to support the architectural vision with as much discretion as can be mustered. Sited between Greengate and Spinningfields, Santiago Calatrava's Trinity Footbridge is the exact opposite, the structural engineer as flamboyant showman, the centre of attention.

Spinningfields Footbridge lies somewhere between the two extremes. The structural engineering takes centre stage, but not at the visual expense of everything else in the immediate vicinity. It's a very sophisticated piece of engineering, but retains a sense of modesty and a great deal of elegance. I reckon it's the finest contemporary footbridge in Manchester, and one of the best in Britain.

Designed by Ramboll (based on appearance, their former Whitbybird team rather than their former Gifford team), this is a design that distils many years' bridge design experience, matching a simple and beautiful overall concept to a super-careful attention to detail. It was built by Eric Wright.

The River Irwell at this point has high banks and so far as I can tell is not regularly navigated, nor prone to flooding. This gives designers the opportunity to "hide" all the bridge structure below deck level, something that is not always an option. Ramboll have used that as a cue for a lenticular form, deepest in the middle where the highest bending forces are found.

I described a generally similar bridge in Manchester two posts back, and discussed how the form can be seen in several different ways, one being that it is an "under-spanned" self-anchored suspension bridge. This is like a normal suspension bridge but with the suspending cable below the bridge deck rather than above it. Instead of being hung below the cable, the deck sits above it on struts. Instead of the main cable's tension force being anchored into the ground, it is anchored to the bridge deck itself, which therefore must resist compression. It's a design which can sit much more lightly on the ground than the normal suspension bridge.

Bridges of this type can be prone to torsional flexibility, a tendency to twist. I've seen several designs where the deck and cable are connected by a triangulated structure to create a skeletal "torsion box" which can resist this twisting – indeed, the Leech Street Footbridge is of that type. The Spinningfields bridge is considerably more adventurous.

The key feature of the bridge is that it is curved in plan, as is the supporting cable, although it follows a different line. The two are connected by individual struts, with no triangulation. The struts themselves are of folded steel plates, giving them necessary stiffness, and this is cleverly integrated with folded-plate deck bracing. There are clear echoes here of the ribbing on Lune Millennium Bridge (by the same designers), which I featured here not long ago. I have to say that I find it startling that so trimmed-down a structure is stable, and would need to see the engineering drawings to really understand how it works.

The Y-shaped parapets and the knife-edge fascia are reminiscent of the Chelsea Bridge Wharf Link Bridge, yet another Whitbybird / Ramboll design, and in my view are very attractive. Lighting is incorporated into the handrail, and there's further lighting below the deck level to illuminate the bridge structure at night. The triangular fascia does a great job in making the edge of the deck look extremely slender. The decking is permeable aluminium sheeting, allowing rainfall to drain freely through to the river.

I found vibration to be quite perceptible, although the amplitude is low enough not to cause any distress. Feedback on the Skyscrapercity website indicates I'm not alone in noticeing the movement.

A seating bench is provided on the deck, which I guess may make it a popular sandwich or smoking spot for workers from nearby offices. The deck is up to 4.5m wide, which is pleasingly generous.

Overall, I think this a real gem of a bridge, structurally daring yet in no way ostentatious, and one which deserves wide recognition.

Further information

• Google maps / Bing maps (both sites currently show images pre-dating bridge construction)
• Structurae
• Skyscrapercity (includes pre-construction visualisations, and construction photos)
• Manchester Confidential: New city centre footbridge lands
• Salford Online (installation video)
• Suave Aerial Photographers (great aerial photos of bridge installation)
• Salford City Council

Scottish Bridges: 41. Maryhill House Footbridge, Elgin

I wasn't entirely sure before visiting it whether this footbridge even existed. The main source of information I had was a reference in Gillian Nelson's excellent Highland Bridges book, which didn't have a picture but made the bridge sound worth a visit.

It spans the River Lossie, in Elgin, some way to the north of Craigellachie Bridge. We parked on Oldmills Road, from where a narrow passageway takes you down to the river, and this bridge. I don't think it has a formal name, so I've called it Maryhill House footbridge as that's how it's referenced in Nelson's book.

I love these little 19th century lenticular footbridges. They are the epitome of lightweight design, still a ready source of inspiration to a modern designer. I'd be pretty certain that this one was designed and built by the firm of Charles D. Young and Co., since it so closely matches the diagram given in their 1850 brochure:

Young's brochure describes the structure as follows:

The principle may be generally described as self-supporting; a strong wrought-iron beam on each side of the pathway, slightly arched, is separated by iron brackets from a tension-bar below, of corresponding strength, and deflected in a curve exactly the reverse of the former. Both of these, namely the beam above and the tension-bar below, join at each extreme, and are securely 'keyed' together; the effect of the principle being that the greater the pressure on the bridge, the firmer it becomes; the intervening brackets so acting as to set off the strain of the one against that of the other.

One great advantage of the Tension over the Suspension Bridge, is its requiring comparatively little mason-work or other outlay for abutments, as it may rest merely on the bank, or a facing composed of either stone or timber.

The brochure mentions an example at Hassendean Burn (near Hawick), built in connection with the now-gone Edinburgh and Hawick Railway. It suggests a price for their Tension Bridge of 20 to 25 shillings per linear foot of span, if for pedestrians and horses, or £3 to £6 for carriages and carts.

Clearly, their structural understanding wasn't quite right, as the bridge may become "firmer" under higher load, but it would also be more highly-stressed and hence liable to failure. From a modern perspective, it can be thought of as a lenticular truss, or as an underspanned self-anchored suspension bridge, where the deck sits above the main suspension cable, which is in turn anchored into the deck rather than into the ground.

I visited another of Young's lenticular bridges at Roxburgh Viaduct in 2010, which can be fairly confidently dated to 1850, so the Elgin bridge must be of similar vintage. There's also a very fine lenticular footbridge at Denham Court in Buckinghamshire, with the same triangular struts, variously attributed as built circa 1850 or 1870, and quite possibly by Young as well - I can't find a picture online, but there's one in ICE's London and the Thames Valley - Civil Engineering Heritage book.

I can find nothing to dislike about this bridge, it's a pure delight.

Further information:

• Google maps / Bing maps
• RCAHMS
• Scottish Highland Bridges
• Moray SMR
• A short treatise on the system of wire fencing, gates etc. as manufactured by Charles D. Young and Company (C.D. Young, 1850)
• Highland Bridges (Nelson, 1990)

London Bridges: 26. Thameslink Borough Market Viaduct

Okay, that was my trip to East London out of the way, now I'll cover a couple of bridges in central London that I visited on the same day. After that, I've got some bridges over in Kent, and then, who knows, maybe one day I will get back to posting more often. Currently, I don't have time to talk about the results of the Amsterdam Iconic Pedestrian Bridge competition, the recent much-delayed opening of Calatrava's Peace Bridge in Calgary, the only-a-little-delayed opening of Poole's Twin Sails Bridge, or even Knight Architects' spiffy new proposal for a bridge at Paddington Basin (see video) to replace the knackered Helix Bridge. You will just have to make do with those links ...

Anyway, the penultimate bridge from my recent London tour was the new railway viaduct over Borough High Street at London Bridge. This forms part of the massive Thameslink construction programme, which expands capacity for trains on the Thameslink route through central London. At London Bridge, it involves the construction of a new twin-track railway viaduct alongside the existing viaduct, wrecking a conservation area and a number of historically important buildings in the process. The existing viaduct is pictured on the right (looking south).

Most of the viaduct is a relatively conventional half-through girder structure, pictured on the left. It looks rather peculiar mainly because of the large curved panels attached to the main girders, which provide acoustic screening.

The most interesting part of the viaduct spans 72m over Borough High Street. This part of the bridge has been designed as a "gateway" feature, with a 6m deep lenticular truss supporting its southern elevation.

Normally, this type of truss is vertical, but here it is three-dimensional in form, with curved upper and lower chords offset from a main boom which connects to the bridge deck cross-girders. All the truss members are in tubular steel, and the diagonals are tapered in form. I can't think of anything quite like it anywhere else, although there is perhaps a bit of a nod to Brunel's Royal Albert Bridge.

The structural rationale isn't entirely clear, and I do wonder about the advisability of the curved bottom boom, which brings it below the level of the more robust bridge deck slab and hence makes it easily the most vulnerable part of the structure to vehicular impact. It looks like vehicles coming from the north can pass below the existing viaduct before they would hit the new truss structure.

The north edge of the new viaduct is supported from a more conventional plate girder, which is essentially invisible to the public. You can get a better idea of the overall structural form in the picture on the left, showing the bridge from below.

The bridge steelwork weighs about 1200 tonnes, and was launched longitudinally across the roadway. As can be seen in the video below, the bridge was assembled piecemeal on top of the main viaduct, with the main viaduct girders being used as the launching rails for the High Street span.

I find the design of the truss a little odd, and it does seem completely out of scale with its surroundings. Nonetheless, it's a very impressive feat of engineering.

Further information:

• Google maps / Bing maps
• Londonist
• Wikipedia
• Watson Steel
• Railway Strategies

West Country Bridges: 5. Royal Albert Bridge

What is there that I can sensibly say about Isambard Kingdom Brunel's greatest bridge (pictured, above left)?

First, the facts. The bridge was opened in May 1859, four months before Brunel's death. The two main spans of 138.7m are achieved using giant lenticular trusses in wrought iron. The upper chord of the truss is a single tube, oval in cross-section, 5.1m wide and 3.7m high. The lower chord consists of wrought iron chains, with vertical members carrying the loads from the deck into both chords. It's a design with split personalities - as well as the idea of the lenticular truss, you can think of it as a bowstring arch bridge, or as a self-anchored suspension bridge in which the main cable forces are anchored into an overhead strut rather than into the deck.

In this respect, the design was a development of Brunel's 1852 bridge at Chepstow, where again a simplified suspension arrangement relies on tubular strut for its anchorage (although at Chepstow, the strut was circular). Another predecessor was Brunel's 1849 bowstring arch at Windsor.

The Royal Albert Bridge also owed more than a little debt to Brunel's great friend and rival Robert Stephenson. His High Level Bridge in Newcastle was also completed in 1849, and has a series of bowstring arches carrying the roadway, with the railway on a separate deck above the arches. The idea of superimposed systems may have influenced the Royal Albert Bridge, but another Stephenson design was even more relevant, the Britannia Bridge.

Opened in 1850, the two main spans at Britannia were very similar to Royal Albert, at 140m. The same method of erection, by floating out and lifting vertically, was also used on both bridges. The Britannia Bridge (along with a related structure at Conwy) had pioneered the use of riveted wrought iron box girders, although used in a beam arrangement rather than as struts as in the more complex Brunel design. Stephenson had even suggested the use of an oval box girder at Britannia, although William Fairbairn's preference for a rectangular section proved more appropriate.

Brunel's Royal Albert Bridge was a more sophisticated design all round, but not of a type which would see much further use. The first major Warren truss bridge had been built in 1852, and lattice trusses such as the Runcorn Railway Bridge were soon to come into favour. Truss bridges were simpler to assemble than box girder designs, and used significantly less material. Nonetheless, Brunel's lenticular truss certainly didn't stand alone, with several other examples built by von Pauli and Lohse in Germany, and by Lindenthal and the Berlin Iron Bridge Company in America.

None of this is to detract from the Royal Albert Bridge. It may have been a technological dead end, but it remains a remarkable design and a spectacular structure.

I particularly like the texture of the main arch girders, with the riveted plate form very visible. The evidence of handicraft is a welcome contrast to the more impersonal surfaces that modern welded construction produces.

Perhaps less appealing are features which I presume to have resulted from strengthening works over the years. Chief amongst these are the diagonal bracing members immediately below the suspension chain, which aren't visible on older photos, and detract from the clarity of the structural system.

Further information:

• Google maps / Bing maps
• Wikipedia
• Structurae
• Engineering Timelines
• English Heritage
• royalalbertbridge.co.uk
• Royal Albert Bridge (Buxton-Smith, 2007, student paper)
• British Railway Bridges (Walters, 1963)
• British Railway Bridges and Viaducts (Smith, 1994)
• Civil Engineering Heritage: Southern England (Otter, 1994)
• Cornwall's Bridge and Viaduct Heritage (Kentley, 2005)
• An Encyclopaedia of Britain's Bridges (McFetrich, 2010)

Scottish Bridges: 16. Roxburgh Viaduct Footbridge

The last bridge from this recent visit is probably the least well known, not being such a historic site as at Gattonside or Dryburgh, but it's the most interesting structurally.

Roxburgh Viaduct (also called Teviot Viaduct) was built across the River Teviot by the North British Railway in 1850. It was designed by John Miller, who also designed the spectacular Ballochmyle Viaduct, which features the second longest masonry arch span in the UK. Roxburgh Viaduct, which no longer carries trains, is much less notable than that.


The Viaduct's four river piers are extended to one side to support a low-level footbridge, of a type which while supposedly once common, has very rarely survived.

The footbridge is 54m long, with three simply-supported spans of approximately 15m each. It is thought to date from the same time as the viaduct. Each span consists of a wrought-iron lenticular truss, 1.2m deep at midspan, with bars curving downwards below a very slender deck which curves upwards. Unlike many trusses, it is not fully triangulated, and relies on the V-shaped 'legs' to hold its shape.

Another way of thinking of it is that each span is a self-anchored suspension bridge, with the lower member being the suspension 'cable' (which would be above the deck in a conventional suspension bridge), restrained by the upper member at its ends.

The footbridge seems to have had very little alteration since its construction. The original parapet railings have been extended upwards in timber, presumably to give a greater sense of security, and infilled with a chain-link mesh (which strikes me as completely unnecessary at a rural site such as this). All the structural metalwork appears to be original.

The bridge has certainly suffered a little from age. This structural form has very limited torsional stiffness, and over time, the spans have twisted permanently, perhaps due to slip in the joints. It makes crossing the bridge a slightly unsteady experience, but not unsafe.

It's a type of bridge which is much under-used, although there are a number of very modern examples. Strasky's book on stress ribbon bridges features several, mostly in Japan, and a recent paper by Ruiz-Teran and Aparicio surveys a number of others. There's also the relatively modern "bridge to nowhere" over Biel Water at Belhaven, although I don’t know exactly when that was built.


Older examples include Robert Stevenson's proposal for an under-deck suspension bridge at Cramond from 1821 (although that was planned to be ground-anchored, rather than self-anchored), and the Micklewood Bridge of 1831, described in Charles Drewry's book on suspension bridges. There were also several examples in Europe, much discussed in Tom Peter's excellent book on Guillaume Henri Dufour.

It's hard to believe that a modern designer, constrained by modern design codes, could produce something as lightweight and charming as the Roxburgh footbridge, but if so, that's a shame. I hadn't even been aware of its existence until quite recently, but it's already one of my favourite historic bridges in the UK.

Further information:

• Google maps / Bing maps
• RCAHMS
• Forgotten Relics of an Enterprising Age
• A Heritage of Bridges between Edinburgh, Kelso and Berwick, Roland Paxton & Ted Ruddock, 1980
• Transitions in Engineering: Guillaume Henri Dufour and the Early 19th Century Cable Suspension Bridges, Tom F Peters, 1987
• Stress ribbon and cable-supported pedestrian bridges, Jiri Strasky, 2005
• Civil Engineering Heritage Scotland - Lowlands and Borders, Paxton & Shipway, 2007
• Developments in under-deck and combined cable-stayed bridges, Ruiz-Teran & Aparicio, in Bridge Engineering, 2010