Derbyshire Bridges: 3. Hope Catenary Footbridge

This bridge carries what is now a public footpath across the Breedon Cement works railway line in Hope. See my previous post for some background on the railway line.

I haven't been able to work out whether this bridge, built in 1974, replaced an earlier bridge or perhaps a foot level crossing. Its construction may have been necessitated by the widening of the railway from one track to two tracks at this time.

It was the first stress ribbon bridge to be built in the United Kingdom, and I believe it remained the only one until the completion of the Kent Messenger Millennium Bridge in 2001.

The 1974 footbridge was designed by Oscar Faber and Partners (later merged into Faber Maunsell, now part of AECOM), and both A.A.W. Butler and Geoff Hope claim credit for the engineering design. It was built by A. Monk and Company.

Butler had worked on the Elephant and Castle Shopping Centre and the M1 Calder Bridge while at Gifford and Partners, before joining Faber in 1968. Bond had designed the Windsor End Footbridge for Buckinghamshire County Council, and worked on refurbishment of the suspension system for Marlow Bridge. He had joined Faber by 1969 and then worked for a time in Nigeria before returning to the UK to work on bridge designs for the Hope cement works (this one, and also the subject of my next post). Bond's potted autobiography is a hugely entertaining read, revealing that Bond's engineering career was essentially a mistaken diversion for a man clearly with wider interests to pursue.

It isn't immediately clear why Butler and Bond opted for a stress ribbon design. In his 1977 paper describing the bridge, Butler suggests that appearance was of "paramount" importance in a sensitive landscape, and it is worth bearing in mind that the site was originally much more open in appearance than it is today. The cutting slopes rise around 6 metres above the level of the railway track, and rock is present close to the top of the cutting.

Butler indicates that interference with rail traffic during bridge construction was a further concern. The bridge spans 34m, is 1.8m wide, and is 0.16m thick (increasing to 0.38m thick over a short length towards the abutments). However, during preliminary design the design team considerd both an in-situ ribbon slab and one made of precast segments. The in-situ variant was chosen on cost grounds, but it's hard to see how it can have been less disruptive to the railway!

The superstructure of any stress ribbon bridge is economic in its use of materials (here it is just the deck slab, 12 prestressing cables, and steel parapets). The bridge hangs like a catenary and the cables are employed both to carry tension to the foundations and to pre-compress the deck concrete. The concrete provides the walking surface, protects the cables against corrosion, and ensures the bridge is sufficiently stiff under loads.

Although the bridge deck is economic, the tension loads inherent in the bridge form can result in expensive foundations. The tension is partly resisted by four ground anchors at each end of the bridge, but the designers struggled to provide sufficient restraint in this way, and opted to also install concrete struts which pass down the cutting slope and below the railway. In the final design as built, the struts, in conjunction with the mass of the abutments, are sufficient to resist all dead loads and the ground anchors are only required to resist the effects of live loads.

The in-situ concrete ribbon was constructed on timber formwork supported on scaffolding, with steel beam supports over the railway to minimise disruption. I imagine that very few stress ribbon bridges have been built in this way.

In 2002, the bridge was used as a test site for bridge dynamic measurement. This recorded a first vertical natural frequency of 2.44 Hz, and a modal damping ratio of approximately 0.5%. Vibration was certainly perceptible when I crossed the bridge, especially when I asked a companion to jump up and down. Perhaps this is the reason why signs restrict the bridge to no more than 20 persons at a time, although I doubt it ever encounters that many at one time (Butler's paper mentions a design live load of 6kN/m2, which is clearly far more than 20 persons).

The bridge was very difficult for me to photograph due to tree growth, so I can recommend Geoff Bond's website for photographs taken shortly after construction.

This is not by any means a significant bridge in the context of international stress ribbon bridges. Within the British context, the main reason for taking note is that its very rarity shows how difficult it can be for engineers to find opportunities to depart from the tried-and-tested. It's always nice to find the instances where engineers did indeed escape from the conventional rut.

Further information:

• Google maps
• An Encyclopaedia of British Bridges (McFetrich, 2019)
• Modal testing of a 34m catenary footbridge (Pavic and Reynolds, Proceedings of SPIE - The International Society for Optical Engineering, 2002)
• Stressed Ribbon Footbridge in the Peak District (Butler, The Highway Engineer, 1977)

Kent Bridges: 2. Kent Messenger Millennium Bridge

Walk north in Maidstone from Lockmeadow Footbridge along the river Medway, and you'll come to the first of the town's two other millennial footbridges. This pair provide entry across the River Medway into the Whatman Field Park, and are named the Kent Messenger Millennium Bridge and the Whatman Field Downstream Bridge. These other bridges were both designed and built by a single team, as part of the Millennium Commission funded development of Whatman Park, one of the main sections of Maidstone's Millennium Riverside Park. This post will cover the Kent Messenger bridge, with the Downstream bridge to follow later.

In a paper on the two bridges, their architect Cezary Bednarski notes that they were the only millennium bridges in Britain not to be the result of a design contest. I'm not convinced about that, but Maidstone Borough Council certainly got good value, with both bridges being interesting and innovative structures, and both going on to win awards.

At the time the bridges were designed, Bednarski was a director of Studio E Architects, although he started a new firm Studio Bednarski a few months after the Kent Messenger bridge opened in July 2001. His engineering partners for the design were Strasky, Husty and Partners, and Flint and Neill. The main contractor was Balfour Beatty.

I'm not sure what the Kent Messenger bridge cost: Structurae and fib's Guidelines for the design of footbridges both report a figure of £1.76m, which works out at about £5.6k per square metre of deck, but Bednarski's paper states the cost to have been £2.9k per square metre.

The bridge is a very unusual structural form - the world's first cranked stress ribbon bridge. Perhaps it's still the only one. A stress ribbon bridge is essentially a catenary structure which has been stiffened by prestressing against itself. Depending on the specific geometry, it can be thought of as a reverse arch, where the prestress provides a broadly uniform upwards force along the structure's length, locking the structure in place. I've covered one such structure here before, the Punt da Suransuns, in Switzerland.

The stress ribbon bridge's generally low load-bearing capacity, together with the sagging geometry, mean that it's a structural form generally limited to footbridges. Even so, it's rarely an optimal form. What is gained in material economy in the deck (Kent Messenger bridge's 3.1m wide deck is only 0.29m deep) is generally lost in the cost of massive foundations, and the foundations for the Kent Messenger bridge are massive indeed, as can be seen in a diagram at Strasky's website. The sagging geometry is also often hard to reconcile with the need to provide a gentle slope, suitable for access by the mobility-impaired. Keeping the slope shallow enough can result in very high horizontal forces, and still larger foundations.

So, what does "cranked" mean, and why is it so unusual? Essentially, it means that the bridge is kinked in plan. The Kent Messenger bridge is 101.5m long in total, with two spans of 37.5m and 49.5m respectively. At the central support, the deck changes direction in plan by 25°. To make that possible, a large horizontal restraint is called for, as the tension forces in the two spans don't balance.

The nearby Downstream bridge (which I'll cover in the last post in this short series) was also originally proposed as a cranked stressed ribbon, with the resultant force at the "crank" restrained by a tie. It was eventually built in a much different form, but the Kent Messenger bridge retained the crank, with the resultant force restrained by a strut. The strut is angled downwards according to the vector sum of horizontal and vertical reactions, and conveniently doubles as a staircase, a very effective solution.

Unfortunately, it doesn't quite work perfectly under all load cases, and a very slender stainless steel member had to be incorporated below the support position, acting in many cases as a tie to prevent the deck rising, and in some cases as a second strut. It could only have been omitted by providing the staircase with much enhanced bending capacity, but I think it's a shame it was included, as the bridge would be far more impressive without it.

The bridge deck itself consists of 3m long precast concrete segments composite with an in-situ infill deck slab. The segments are supported on a set of four bearing and four prestressing cables cast into the infill.

Drainage grilles are located along the centreline of the bridge (and can be seen on the image on the right), breaking up the structure's monolithic appearance and allowing the river to be seen through them. These alternate with lights embedded in the deck (see below left). These are the same kind of lights you would find on an airport taxiway, a rather idiosyncratic choice.

The bridge parapet incorporates a custom-designed stainless steel mesh, which is now marketed as the "Medway" mesh by Potter and Soar.

As with the Lockmeadow footbridge, the Kent Messenger Millennium Bridge wasn't shown to its best on a dull, overcast day. Given that stress ribbon bridges are often chosen for their lightweight, slender appearance, I was surprised to find this bridge offering an almost brutalist statement. The cranked geometry was clearly an attempt to push at the boundaries of structural design - there are several other forms which would have served as well, perhaps incorporating some of the length eastern approach ramp into the bridge structure. The staircase strut is a great design feature, although it's a shame about the presence of the central tie, as the bridge would have had a far more startling visual tension without it.

The overall geometry aside, it's a plain, functional, rather minimalist structure, and none the worse for any of that. Compared to the tangled cat's cradles of two of its contemporary Millennium Bridges (at Bankside and Hungerford), it deploys its structural ambition to a more self-effacing end. The views off the structure are kept free of cables and struts, and there's little more to see below. I can't say I found it at all an exciting bridge to visit, but I guess that's part of the intention, and it's good to see a strong design that doesn't take its lead from Calatrava.

Further information:

• Structurae
• Google maps
• Kent Messenger Millennium Bridge, Maidstone, UK (C Bednarski, Footbridge 2002)
• Kent Messenger Millennium Bridge, Maidstone (Concrete magazine, Nov/Dec 2003)
• Stress ribbon and cable supported pedestrian bridges
• Footbridges
• Masterpieces: Bridge architecture + design

Swiss Bridges: 6. Pùnt da Suransuns

Throughout our second day of the bridge study tour, we were accompanied by Jürg Conzett. In the afternoon, we visited two of his bridges, both part of the Via Spluga hiking trail in Via Mala gorge near Thusis.

The gorge itself is absolutely stunning. At several points, you can look 80m down and struggle to see the water below in a dark crevice as little as 1m wide. And looking up there are tall cliffs for another 100m or more. Where it widens out, it's crossed by bridges including Christian Menn's Great Viamala Bridge.

We walked only a relatively short length of the hiking trail, under the Menn bridge to first see the Pùnt da Suransuns, a remarkable stressed ribbon footbridge spanning 40m across the river. I'm not entirely a fan of the stressed ribbon bridge - while they can be beautifully slender structures, the way they droop often looks unhappy - see the Maldonado Bridge in Uruguay for an example of this. They rarely meet the engineering or administrative constraints of most sites - the sag leads to slopes greater than are desirable for many users, and for the same reason they don't work where there is limited headroom below (which is the case for most footbridges). They are also almost never the most economic solution, with the costs of expensive foundation anchorages far outweighing the material savings to be made in the main bridge deck.

Suransuns, however, is a perfect example of just how a stressed ribbon bridge can work well. The river valley sides have plenty of stable rock, required for an efficient anchorage design, and the setting demands as minimal an intervention as possible. And Suransuns must be amongst the most minimal of stressed ribbon bridges you could get.

The bridge comprises granite planks, 60mm thick, 250mm wide, and 1100mm long, which sit directly on stainless steel strips only 15mm thick, and are held apart by 3mm thick aluminium inserts. That's basically the entire structural system of the bridge, and it is beautifully complemented by ultra-minimal steel handrails supported on 16mm diameter vertical rods.

Clearly, the location allowed Conzett to break many of the rules which normally bind footbridge design: minimum widths; maximum gradients; strength of parapets; susceptibility to vibration. However, it's the way he responded to some of the challenges of stressed ribbon design which was particularly impressive. Bending of the deck slab at its support abutments is a key consideration in design, which Conzett dealt with using a seemingly simple "leaf-spring" arrangement, just using more of the main stainless steel strips locally.

The elegance of the bridge's minimal silhouette is matched by the simplicity of the engineering, and for me, this was one of the best bridges we saw. Conzett, Bronzini and Gartmann are designing a very similar bridge (albeit multi-span) at Gemeinde Windisch, and it will be interesting to see how it compares.

Further information:

• entry at structurae
• location on Google Maps (satellite photo predates bridge construction!)
• Pùnt da Suransuns Pedestrian Bridge, Switzerland (paper in Structural Engineering International, 2000/2)
• Via Spluga (Italian & German only)
• Matthew Well's book 30 Bridges
• Mike Schlaich and Ursula Baus's book Footbridges (link is to German edition, also available in English)