Merseyside Bridges: 13. Bradley Swing Bridge

I crossed this bridge en route to its much bigger and better known neighbour, the Sankey Viaduct. It is a rod-stayed pedestrian bridge spanning the Sankey Canal, and although there may have been several bridges like this in the canal's heyday, I believe this is the only one of this type that is left.

The Canal dates all the way back to 1757, but Historic England suggest that the Grade II Listed bridge dates from around 1857. The Listing states that the turning gear and pivot remain in place, although clearly the bridge is no longer operational, and the canal reaches a dead-end a short distance to the north of here.

If the 1857 date is correct, it must be one of the oldest surviving stayed bridges in England (there are certainly older examples in Scotland). It's not clear how much of the bridge is original - Historic England date the parapets to the 20th century, and there are turnbuckles in the main rods which are clearly an alteration.

The bridge is currently painted black and white but was previously painted green, as can be seen in photos at the Towpath Talk website, and on Wikimedia Commons, so the repainting is fairly recent.

The bridge's most unusual feature is the way in which the main span stays split into two. The stays are flattened locally to allow a pin to pass through. Combined with the bending of the rod over the narrow width of the cast iron posts, this is not an arrangement which could carry substantial loads, it would too easily be prone to fracture.

At deck level, there is a short linking piece connecting the main stay to the floor beams, which don't look original to me.

Further information:

• Google maps
• British Listed Buildings
• Canal Plan

Merseyside Bridges: 12. Sankey Viaduct

Time for a couple more "Merseyside" bridges (using the regional term in a broad sense, before the pedants write in, again).

Completed in 1830, Sankey Viaduct has been described as "the earliest major railway viaduct in the world". Protected by Grade I Listed Building status since 1966, it still carries trains today.

The nine-arch viaduct was built as part of George Stephenson's Liverpool and Manchester Railway, to carry the line over a valley containing both the Sankey Brook and the Sankey Canal. The latter is now defunct, and was infilled at this location in 2002, so the viaduct now spans the Brook and a public footpath.

The viaduct is reported to have been designed by Stephenson's assistant Thomas Longridge Gooch, with William Allcard acting as resident engineer. Both men had worked with Stephenson for several years, and although Gooch is often described as Stephenson's draughtsman, he would in modern terms be called an engineer. Some sources cite Allcard as the main designer.

In 1825, Stephenson had been temporarily displaced as the railway's engineer, and John and George Rennie proposed a seven-arch viaduct 273 yards long. Once reappointed, Stephenson initially put forward a 20-arch brick viaduct, which was rejected by the railway company's directors. Describing his first design, Stephenson wrote to his son, Robert:

I have drawen a plan on the gothick principal there will be 20 arches of 40 feet span it will be quite a novel[ty] in England as there will be a flat arch sprung between the centre of the tops of the gothick and so on it has a fine appearance in the plans.

The Viaduct was only necessary at all because the Sankey Brook Navigation Company refused any obstruction to tall sailboats passing along their canal. Compare this old image of the viaduct with how the valley looks today.

The viaduct is a brick structure with sandstone facing on the two elevations. The piers are generously tapered and robust in appearance. Below ground, they sit on sandstone foundation blocks, which are in turn supported on driven timber piles.

The arches are semi-circular, each spanning 15.2m (50 ft). The keystone is prominent, projecting not just below the elevation, but below the entire width of the arch barrel. The underside of the arch is substantially covered in calcite staining, and in need of at least a clean if not more thorough refurbishment.

New overhead electrification portals were added in 2015; this seems to have been done with some sensitivity, choosing the positions carefully and only with small visible protrusions above the cornice line.

Looking up at the spandrel walls, occasional openings can be seen on one or other side of the central pilaster. I wondered whether these indicated the bridge to be of hollow-spandrel construction, with a series of internal spandrel walls. I found the planning consent application for the overhead electrification online, showing this guess to be correct, see the drawing extract below.

Further reading:

• Google maps
• Wikipedia
• Structurae
• Engineering Timelines
• British Listed Buildings
• Transport Trust
• Sankey Canal and Restoration Society
• British Railway Bridges and Viaducts (Smith, 1994)
• Civil Engineering Heritage: Northern England (Rennison, 1996)
• An Encyclopaedia of Britain's Bridges (McFetrich, 2010)
• Aqueducts and Viaducts of Britain (Owens, 2019)

"The Mersey Gateway - A Bridge To Prosperity"

It's not unusual for major bridge projects to mark their completion with a souvenir booklet of some kind. I've recently got hold of three examples where they have gone a bit further and an entire book has been published to celebrate the occasion.

The first of the three is Mersey Gateway – A Bridge to Prosperity (118pp, 2018), published by the project's client and bridge owner Halton Borough Council. The Mersey Gateway connects the towns of Runcorn and Widnes across the River Mersey and Manchester Ship Canal, providing a new route for highway traffic, bypassing the Silver Jubilee Bridge built in 1961.

The new river crossing is a three-tower cable-stayed bridge with an unusual configuration, having a central tower visibly shorter than the two other towers. The new structure has been built to provide improved traffic capacity, reduce reliance on ageing infrastructure, and improve travel times not just locally but for nearby areas, particularly Liverpool and its surroundings.

The book concentrates on the client's view of the scheme. There is very little information on the engineering or architectural design of the bridge, not even an explanation for why the central tower is shorter, to give just one example. However, the book makes up for this with its strong coverage of the bridge's economic and social benefits.

The structure of the book is not chronological, but instead sets out to grab attention from the start. A description of the project's history and development doesn't start until page 77. Instead, the first chapter documents key features of the construction phase: the temporary trestle bridge, foundation cofferdams, the moveable scaffold systems (MSS), deck construction, cable stays, traffic management, and the opening ceremony. It's a well-chosen selection of highlights from the project's construction timeline.

There are short interviews with two people involved in the project: tower crane operator Peter McDonough and visitor centre volunteer Evelyn Edwards. I'd like to have read more of these, as my own experience of major projects is that a huge number of people contribute, and they often have plenty to say.

The second chapter emphasises community and people, including education outreach, local training and apprenticeships, recognition of women in engineering, community volunteers, and also the work of the Mersey Gateway Environmental Trust. This really shows what matters to the project client on a project such as this: it's not just all about quantities of steel or traffic statistics.

The third section, on design, environmental assessment and other aspects of the development phase, is weak by comparison – perhaps it's because I'm a designer by trade, but I feel this was a lost opportunity to tell this part of the story. It reads pretty much as if nobody involved in producing the book had actually spoken to anyone involved in design. The weakness continues into the fourth and final section, which attempts to credit the main project participants but which I think will leave most readers little wiser about what some of those involved actually did.

This reflects the nature of the book as the celebratory record of a very substantial achievement: Halton is a small local authority now responsible for two of the UK's largest bridges – they are right to be proud and to emphasise the benefits of the scheme beyond the bridge as a structure. This also means that the book (like most souvenir publications) has a uniformly optimistic, positive narrative – if anything went wrong along the way, if difficult challenges were overcome, those stories are absent.

The bridge's graphic layout is excellent, and there is a wide range of excellent photographs of the bridge and its construction (taken by David Hunter, and some of them reproduced here). The author or authors are uncredited, but the writing is very clear, and particularly good at explaining engineering and construction issues in lay terms – much better at this than I would normally expect.

It isn't the easiest book to get hold of – I wasn't able to locate anyone selling it online, and I got my copy by contacting Curiosity Bookshop in Runcorn directly – if they still have it in stock, they can take an order over the phone and post the book out (it costs £9.99 plus postage).

Further reading:

• The Mersey Gateway - Lasting Infrastructure Connecting Societies (Ramboll, 2017)
• Official website for the Mersey Gateway
• Project photos by David Hunter

Mersey Gateway preferred bidder announced

A preferred bidder has been announced for the £2bn Mersey Gateway scheme in North West England. The Merseylink consortium (Kier, FCC, Samsung) now enters final financial negotiations with the promoter, Halton Borough Council, leading to contract award later this year.

 

While this is good news for anyone who like big bridges and to see people employed welding together lumps of steel and pouring tons of concrete into holes, I am more interested in what it means for the design of the bridge.

The original design was by Gifford (now Ramboll) with Knight Architects, and was used to secure planning consent prior to inviting construction tenders. Their design featured three mono-tower masts supporting the deck via "harp"-layout cable-stays, i.e. an arrangement of parallel cables. This is visually very attractive, but the low slope of cables near the bridge towers mean the deck is not well-supported in these areas. The design also allows for essentially a single plane of cables along the bridge deck centreline (actually, two planes very close to one another), which is visually very open and legible from all perspectives. This is the original design:

For multi-span cable-stayed bridges (with more than two towers), a key challenge is how to stabilise the spans, as they are far less stiff than a bridge with only one or two towers. The original Mersey Gateway design addressed this issue using a stiff steel truss deck, pictured in cross-section below (an excerpt from the planning drawings), which was conveniently deep enough to also accommodate future light rail tracks below the road deck (an idea later dropped from the scheme requirements).

A deck of this type would be lighter than a concrete alternative, and subject to reduced wind load. However, the cost of steel fabrication and site assembly would be high.

I mentioned the bridge in May 2012, noting that the promoter was sensibly varying the planning conditions, to give tendering contractors flexibility to reduce costs. I commented that "the structure which will be built is unlikely to retain either the harp cable layout or the truss deck, although I imagine the single plane of cables has a fair chance of surviving."

It's always nice to be proven right, as this is precisely what has happened in the preferred bidder's design, which I understand is by Flint and Neill. The cable arrangement is replaced with a single-plane "fan"-layout which is structurally much stiffer, and the single plane will be less expensive to install than two closely-spaced planes. This is the proposed design:

In place of the steel truss, the new design uses a conventional post-tensioned concrete box girder, which judging from the video will be cast in-situ in sections. The image below is taken from the project's new publicity video and shows the use of internal steel frames to transfer load between the girder webs and the cable-stays:

I imagine the deck will be shallower overall than the original truss design, which will make up for any heaviness of the solid concrete. I also expect that it makes the transition to approach viaducts very straightforward, as the same box girder cross-section can be applied consistently.

There is one significant visual disadvantage to the preferred bidder's proposal, which comes in the relationship between the tower above and below deck. This is a common problem with large cable-stayed bridges, where a slender tower above deck often gives way to an extremely stocky tower below deck. The Dartford Crossing is a particularly egregious example of how bad this can look. The Millau Viaduct shows how it can be done well.

In the Knight / Gifford design, the truss deck could be pierced, so that the bridge mast and piers form one continuous column passing through the roadway. A similar approach was used on Stonecutters Bridge in Hong Kong, to pick just one example. This is clearly impossible with a concrete box girder, and so the contractor's alternative design features piers below-deck which are much bulkier than the tower above, and appearing doubly so because of the large crossheads used on the two taller piers to provide torsional restraint to the whole deck.

Despite this, I think the promoter's decision to allow a certain range of variation from the original design was clearly the right approach. We will have to wait until later in the year to see whether it has realised the cost savings they expected.