30 August 2013

Cumbria Bridges: 4. CKPR Bridge 67

Carrying on along the trail from Keswick to Threlkeld, the next bridge after Bridge 66 is ... Bridge 67.

This bridge is of the same basic form, an inverted bowstring truss, with the same X-bracing on the elevation. This one is a single span of 90 feet, with a slight skew. There is a short flat slab span on the approach at the east end of the bridge.

Like Bridge 66, it has been strengthened with the addition of a single plate girder between the trusses, which can be seen more clearly on these photographs. Cross-bracing connects the two trusses below the soffit of the central girder, while towards the ends of the span the trusses are connected directly to the girder with short stubs.

I'll come back to another bridge later on in this set which shows the details even more clearly.

Further information:

29 August 2013

Cumbria Bridges: 3. CKPR Bridge 66

The Cockermouth, Keswick and Penrith railway line (CKPR) was designed by the famous (or infamous) Thomas Bouch, an engineer later to become notorious following the collapse of his Tay Railway Bridge. His bridges on the Keswick to Threlkeld section of line have had a happier fate, with eight splendid metal structures surviving to the present day, some decades after the railway line was closed to traffic.

I didn't have time to visit all eight bridges, but you can read more about them in Paul Dunkerley's Bowstrings over the Greta, which has a useful map, photographs, and further details. I've relied on it extensively in this and the forthcoming posts.

Bridge 66 is the only double-span bridge to survive, and comprises two spans of "inverted" bowstring truss construction, reportedly Bouch's preferred solution for this line, wherever clearance above the River Greta permitted its use. Nonetheless, he apparently only adopted this design for one other bridge, at Penicuik in Scotland.

The spans are of 57 feet and 89 feet, and the bridge originally carried only a single railway track. In recent years, like the others, the bridge has been adapted to carry a foot and cycle trail by replacement of the deck with a new timber walkway.

It is possible to pass over this bridge with no idea of what lies below, and indeed that's precisely what I did when I first cycled over it. However, it's well worth finding a way down to the river bank to see the bridge properly, as this bridge, along with three other inverted bowstrings on the line, is of a very rare type. My travelling companions called them the "upside-down" bridges, but this form is more economical than the conventional "upright" bowstring bridge, as the curved members are placed in tension and less material is therefore required to resist buckling.

The bridge has an interesting combination of robustness and elegance, and this isn't marred by the presence of a single plate girder in between the two trusses. This was added in 1931-32 to enhance its strength. It must have been a very difficult construction operation, although the photo here shows that a very neat slot was cut in the abutment to accommodate the girder bearing.

I wasn't able to get photographs that show this whole bridge clearly, but some of the other bridges on the route will hopefully show a few more details for the form.

Further information:

27 August 2013

Cumbria Bridges: 2. Greta Bridge

I've recently had the chance to visit a few interesting bridges in Cumbria, in the north-west of England, and will spend the next few posts reporting on them. I've previously featured two other Cumbrian bridges, the bridge at Aira Force waterfall, and the remarkable Fisherman's Bridge.

The majority of the bridges I'll feature over the next few posts form a set, as they all either span over or carry a foot / cycle trail running between Keswick and Threlkeld, which was formerly part of the Cockermouth, Keswick and Penrith Railway. This railway line was completed in 1864, primarily to transport minerals although also serving as a passenger line until its closure in 1972.

Greta Bridge is part of this set but also the odd-one-out, as it spans over the former railway line but was built much later, opening in 1977 as part of the A66 highway bypass to Keswick. It was designed by Scott Wilson Kirkpatrick and Partners, and built by Tarmac Construction. Its chief claim to fame is that in 1999, it was voted Britain's "best concrete engineering structure of the century" by members of the Concrete Society. It was claimed to be one of the first bridges to have been designed using finite-element analysis. New Civil Engineer's report on the award stated: "The structure, designed purely by engineers with no architectural input, is renowned for its aesthetic qualities which ensure it blends well with the surrounding Lake District countryside."

Greta Bridge has four spans for a total length of 220m, and carries a dual two-lane carriageway. It is a concrete box girder of varying depth, superelevated to suit the highway curvature. The boxes are trapezoidal in cross-section. It's the lower, inclined part of the webs which give this bridge its unique character, as in combination with the curved soffits to the girders, they create a lovely series of "scalloped" surfaces defining the lower edge of each span. It's rare to see a concrete box girder bridge which is so visually successful.

It isn't only the box girders that provide the charm to this bridge. The concrete piers are rectangular in cross-section, but indented with a series of dished "flutes" on their long faces. These provide some texture to break up what would otherwise be a very large, flat concrete area, they emphasise the verticality of the piers, and the curved profile to the flutes complements the curved elevation of each girder span.

I personally think it's silly to claim that this was the best concrete bridge in Britain of the 20th century, but it's a very charming structure, and certainly an unusually high quality design for its type.

Further information:

22 August 2013

Bridges news roundup

Time for a few quick links to bridge-related news from elsewhere. I've been out visiting some bridges myself, so hopefully will start blogging those over the next few weeks.

Narrow Water bridge plan is put on hold
Considering all the fuss around the cancellation of the River Wear crossing, you may be forgiven if you're unaware of yet another "iconic" bridge scheme which has hit the skids for very similar reasons. This is a project for a new bridge between the Republic of Ireland and Northern Ireland, and in an odd parallel to River Wear, it is also a cable-stayed bridge with no back-stays, although nowhere near as adventurous as in Sunderland. The scheme had a reported €17.4m budget, but tenders have come in the range €26m to €40m, indicating a staggering failure to anticipate the realities of building this type of bridge (the disparity between tender prices is also quite remarkable). The most obvious comparison is Calatrava's Puente del Alamillo, which experienced such significant cost escalation that only one of two planned spans was built. What with this and Sunderland, the cost estimating profession should be hanging their heads in shame.

Parkland bridge to be designed without controversial arch
Funny news from Dallas - a hospital link bridge is to be redesigned without its supporting arch, because it's US$5m cheaper. So what on earth was the arch doing in the first place? This idea that major structural elements are only merited as decoration is quite absurd, but sadly not atypical of the American approach to bridge design.

Linking up London: sensational new bridges will create new communities and riverside homes
You can always rely on Homes and Property for the best summaries of bridge news. Surprisingly informative.

Scale Lane Bridge, Hull – review
The Guardian's architecture critic Rowan Moore is a happy pontist. Hopefully, I'll get a chance to visit this, the world's first ride-along pinball flipper bridge, some time soon.

Victorian Footbridge Restored To Former Glory
Canford Suspension Bridge, in Dorset, has had a makeover which sounds basically like a complete reconstruction.

Thousands watch Whangarei bridge opening
Something of a British success story for this new opening bridge in New Zealand, with both Knight Architects and Eadon Consulting involved. There was also a very informative article about the project in New Civil Engineer recently, but I can't find it online.

Rhyl Pedestrian and Cycle bridge July Update
An aerial video shows the Foryd Harbour bridge taking its final shape, giving a pretty good idea of the final appearance.

Consol bridge soars, sways 100 feet above the valley floor
A very generous donation has given the Boy Scouts of America a spectacular new suspension footbridge, where people can walk on the cables as well as on the main deck. There's a video on YouTube, and a nice article in Wire Rope News.

18 August 2013

Bristol Bridges: 2. Pero's Bridge

I've badged this and the previous post as "Bristol Bridges", as if you're going to get a whole series of bridges in the area that I've visited. Well, actually only the two, for now anyway. But I do plan to make a return visit to the area and cover a few more.

Pero's Bridge is a relatively recent addition to Bristol's scenery, having opened in 1999. It spans an inlet of Bristol Harbour, and undeniably improves pedestrian access in a very busy part of the city. Designed by Arup with artist Eilis O'Connell, it features an 11-metre long bascule span, flanked by two fixed spans.

Visually, the most obvious feature of the bridge is the two large "trumpets" forming the rear half of the lifting deck. These provide a counterweight mass to balance the main deck, so that minimum force is required to open or close it.

The bridge is unusual in being a modern implementation of the rolling bascule, which rocks back onto its supports like the exaggerated motion of a rocking chair. Teeth are provided on the support beams to ensure it does not move out of tolerance when it does so. Bridges of this type were once common: I've visited those at Queensferry and Liverpool previously. One advantage of the form is that the centre of gravity of the rolling span can be positioned so that it lies to the span-side of the centre of rotation when the bridge is down, but so that it moves to the rear-side of the centre of rotation when the bridge is up; in both cases, it holds the bridge secure in the rest position.

The bridge is operated by a hydraulic piston below deck level, which remains above the water level at all times (the level being constant in Bristol Harbour). In the photograph on the right, it can be seen that the deck is a box girder with strutted cantilevers. Much of the detailing below deck level is visually awkward, particularly the main pier supports for the roll path, including their uneven relationship with the other pier.

I find the bridge unconvincing overall. It's a reasonable engineering solution, but the counterweight trumpets seem awkward to me. To some extent their shape is compelled by the engineering demands - the bulk of the weight has to be high up so as to balance the main deck properly. Other elements of the bridge are over-fussy, such as the parapets, which with their complex array of posts, bars, rails, and mesh panels, offer little in the way of elegance.

The bridge is named for an African slave, Pero Jones, brought to Bristol in 1783 by a local merchant.

Further information:

15 August 2013

Bristol Bridges: 1. Clevedon Pier

The pier at Clevedon is one of my favourite bridges to nowhere. Built in 1869, its elegant iron arches make me think of the Pont des Arts in Paris. There are eight spans, each 100 feet in length, comprising a wrought-iron superstructure supported on cast iron screw piles. It seems impossible that such spindly supports could both carry crowds of Victorian pleasure-seekers and withstand the force of storm-tossed waves.

However, other than various minor alterations, they did indeed withstand them, and the collapse in 1970 of the seventh and eight spans from the shore was man-made rather than due to the forces of nature. The bridge was being load-tested for insurance reasons, with water tanks providing a load of 2 kPa, and while the first six spans passed the test, the seventh failed, dragging the eighth down with it. This seems a very low load capacity when you consider that a modern pedestrian bridge would be designed for 5 kPa loading, plus a safety factor (so, typically 7.5 kPa), but this simply highlights the conservatism of current codes.

Another aspect of the pier which would not comply with current codes is the presence all along both edges of continuous bench seating. This is prohibited on any modern footbridge because a child can stand on the seating and no longer has safe containment. At Clevedon, a sign simply asks visitors to supervise their children; it should also be noted that the pier is a private space, with a toll paid for access, rather than a public “crossing”.

When built, the pier took advantage of the availability of “top hat” section Barlow rails, surplus stock from the South Wales railway. These were riveted back to back to form columns and longitudinal arches, supporting wrought iron plate girders along the edge of the deck. They're easily visible in the photograph on the right, showing the original pier construction.

Although the reconstructed spans match the general elevation of the original spans, both the girders and support framework are in modern welded steel, and to my eye look out of place on a Grade I Listed structure. If you compare the photograph on the left with the previous one, you can see some of the differences: the column and arch sections are cleaner, with no rivets and less interesting shapes.

Another photograph of the original construction shows further differences. The trestle supports have different horizontals at mid-height, with a straight bar on the new sections (above left) in place of lenticular struts on the old sections (right). The new trestles have a horizontal cross-beam at the top, while the old sections are arched at the top. Coupled with the loss of detail in the main deck girders, I find the pier reconstruction disappointing in its lack of adherence to the original construction. You wouldn't realise this from most websites or texts, which give the impression the pier was rebuilt exactly in its original form.

As with Southport Pier, after the load testing collapse the local council at Clevedon proposed to demolish the pier, but public outcry eventually won a reprieve. Some of the funding for rebuilding, and further refurbishment, was provided by many individual donations, commemorated with plaques inlaid into the deck planks and mounted on the rear face of the benches. This crowd-funding was supplemented by grants, and I think it's a great way of raising money and demonstrating popular support for a project.

Although this is a pier, not a bridge, it clearly has many similarities. I wanted to feature it here because it's such a supremely elegant structure, with a slenderness and loveliness of form that few such structures can match. Poet Sir John Betjeman described it as "the most beautiful pier in England", and it recently won the "Pier of the Year Award" for 2013. Money is currently being raised to fund construction of a new visitors centre.

Further information:

14 August 2013

IStructE awards shortlist announced

Okay, this news is a few days old now, but ... the Institution of Structural Engineers have announced the shortlists for their 2013 Structural Awards. The award winners will be announced on 15th November.

They have a number of categories, but two are relevant here:

Highway or Railway Bridge Structures
King Abdul Aziz Flyover
Taizhou Bridge
Walton Bridge

Surely an easy winner for the pioneering Taizhou Bridge (pictured below)?

Pedestrian Bridges
Anaklia-Ganmukhuri Pedestrian Bridge
Castle Green Bridge
Olympic Park Footbridge
Pembroke College Footbridge
Teign Crossing Cycle Bridge

A couple of these were new to me. I'd seen the lovely little Castle Green bridge before, but really it's exaggerating to call it a bridge at all. The Pembroke College bridge is a charming little curio, but my vote would go to the Anaklia-Ganmukhuri Pedestrian Bridge (pictured below), proof that timber can be exploited in ways most engineers would never consider.

04 August 2013

River Wear design airbrushed from history

You could be forgiven for thinking that somebody was trying to airbrush the ill-fated New Wear Crossing out of history.

First, the project website www.newsunderlandbridge.com was removed, along with many detailed project documents which, to me at least, remain quite interesting. If you want to know about how Sunderland Council developed the cost estimate for the bridge or analysed the benefits it would bring, you can forget it.

The bridge has also become a ghost on lead designer Techniker's website, removed from their project list and from their news/blog entries. There are no longer any links given to their ICE lecture on the bridge, or to a selection of design drawings. A few ectoplasmic traces remain, in the form of the Stitching Structures lecture, and Google still provides access to both the ICE lecture and some project details, even if the direct links have been edited out.

Of the others involved in the project, Roughan O'Donovan, responsible for much of the detailed design work, make no mention of it on their website, while only Hewson Consulting Engineers, who also provided significant design expertise, seem unembarrassed enough to still feature the scheme on their website. You can also, if you wish, still read about the design's "Very Good" CEEQUAL rating, awarded for sustainability which seems somewhat daft in light of the design's grossly excessive material demands.

I feel sure I had read of plans for an IStructE regional lecture about the project, but can find no sign of it on the IStructE events website.

I think this is all a shame, as whatever the scheme's merits or lack thereof, it was a breathtakingly bold design, and should form a useful case study for students of bridge architecture, engineering and procurement for many years.

There has been little in the way of public analysis of what went wrong, with Sunderland Council clearly determined to bury the whole fiasco as quickly as possible, building a new bridge proposal upon the grave as quickly as they can, bound as they are by a commitment to spend the central government money already granted to them.

One group of gravediggers has been firmly rebuffed, with local Tories refused permission to ask the obvious questions, such as "so, just how much would it have cost?" and "How much money has been wasted?" This may seem somewhat ironic to locals, who will doubtless recall that local Tory leader Robert Oliver was one of the bridge scheme's most enthusiastic promoters. I don't know enough about what has gone on behind the scenes to be able to judge, but given what has happened on several other ultimately unproductive bridge design competitions, I would think that an unwillingness of politicians to put their grandiose ambition aside and to seek out properly critical advice has possibly played a very real part in events.

Update 15th August: Thank you to Matthew Wells for advising that his IStructE evening lecture on the bridge will take place in Manchester on 11th November.