31 May 2022

Garret Hostel Bridge, Cambridge

I was in Cambridge earlier this year. Unfortunately, there was no public access to the city's most famous span, the Mathematical Bridge, but I did get the chance to visit two other interesting structures.

The Garret Hostel bridge is a modernist classic, completed in 1960 to replace William Chadwell Mylne's cast-iron arch (completed 1837), which had fractured due to settlement. That in turn was only the latest in a long line of bridges built across the River Cam at this site, the first supposedly a timber bridge from the late 15th century. One of the bridges, built in 1769 by James Essex, was named the Mathematical Bridge, similar in appearance to its more famous cousin a short distance along the river.

The cast iron bridge can be seen in an illustration in the collection of the National Trust. The "mathematical bridge" of 1769 is depicted in an illustration on the Queens' College website.

The bridge is Listed Grade II, which is unusual for such a modern structure. It was reportedly designed by Timothy Morgan of the architects Guy Morgan and Partners, who died in the same year that his bridge was opened, according to the bridge's Listing details. According to Atlas Obscura, the architect was intead a Timothy Guy Morgan, an undergraduate at the local school of architecture. The National Trust Book of Bridges instead claims it was Guy Morgan, born 1902. I don't know which of these is correct! The bridge was built by J. L. Kier & Co, who are also cited as the engineer. That bit seems clear.

It is an immediately attractive bridge, from almost any angle, with its striking curved concrete underside and central crease-line. Getting up close and seeing how rough the bush-hammered concrete surface is just adds to its charm. The bronze handrails are also a lovely feature.

Light on the surface of the river reflects in patterns on the underside of the bridge.

The bridge has the visual form of a very shallow arch, albeit one that is steep to climb and clearly from the days before modern accessibility requirements! However, its true form is hidden within the stone-clad abutments at either end.

It is a post-tensioned concrete girder bridge, in a shallow portal frame arrangement supported on traditional concrete hinges at its west end, and on some form of metal bearings at the east end. The prestressing cables were tensioned from the west end only, after the concrete was cast, and if you look closely at my photos, you can see the manhole cover which provides access into the hidden jacking chamber.

It is relatively early for a post-tensioned bridge in the UK, completed 3-4 years after Cavendish Bridge in Derbyshire, and in the same year as structures such as Queen's Bridge, Perth, and Bridstow Bridge.

None of this is visually apparent, but fortunately details of the bridge were published in the book Modern British Bridges in 1965, and I've reproduced them here as they are quite informative.

The drawings show the soffit of the bridge to be parabolic in curvature, and make clear how the cross-section varies in depth from the crown to the supports. The prestressing cables are arranged to resist sagging at the crown, and hogging at the ends.

The superstructure is supported on huge reinforced concrete abutments, which sit on raked concrete-filled steel tubular piles.

As is often the case with bridges pushed to the limits of slenderness, the material is simply shifted from one place to another, in this case towards the ends and into the foundations. The span-depth ratio that results is 48:1 (85 ft span vs 1.75 ft depth at crown), which is visually attractive but not especially exotic.

The drawings make clear that the stone-clad abutments, which look so nice, are essentially fake. Their front faces incline backwards away from the river, perpendicular to the soffit of the bridge to give the visual impression that they resist the thrust of an arch, while in reality doing nothing of the sort.

My view is that if fakery is to be the approach taken in bridge design, this is a great example of how it should be done well. It's a beautiful bridge, and sits nicely amongst its surroundings.

Futher reading:

22 May 2022

"Thames Bridges" by David C. Ramzan

I do love a bridge book which takes a specific river as its focus, and Thames Bridges (Amberley Books, 96pp, 2022) is a nice addition to this genre.

As far as the Thames goes, it's a pretty crowded field already, with Crossing London's River (1972), Thames Bridges (1973), Thames Crossings: Bridges, Tunnels and Ferries (1981), Cross River Traffic (2005), Thames Bridges: Then and Now (2006), Thames Bridges: from Dartford to the Source (2007), London's Bridges: Crossing the Royal River (2009), Bridges: XXXIV Crossings of the Thames (2011), Crossing the River (2015), All the Thames Bridges from Source to Dartford (2019), Bridges over the River Thames: From the source to the Sea (2020) .... you get the idea! For collectors of bridge books, is there any need for another one about the bridges of the Thames?

Thames Bridges covers the entirety of the river from its source in Gloucestershire to its estuary, passing under over 200 bridges along the way. The book is extensively illustrated, with photographs on almost every page. For some of the minor bridges, the photograph sufficiently illustrates what they are, and their context. The images are a mixture of old and new, and I especially liked the inclusion of the older photographs - so much of the narrative relates to the history of the land, the river, and its crossings, that these help bring that tale to life.

Outside the heart of London, this is the river of Clark's Marlow Suspension Bridge, Brunel's Maidenhead Railway Bridge, the old and new bridges at Runnymede (Lutyens and Arup), Hampton Court Bridge, and of course the mighty Queen Elizabeth II Bridge. It is undeniably an important river, spanned by many undeniably important bridges.

Thames Bridges rarely wastes too many words on any span, and this is definitely not a book that can be considered an in-depth gazetteer. However, it's concisely written, with enough on each structure to grasp key facts, understand their significance, and relate them to the wider history of the area. There is plenty of history, and the book touches on Mesolithic settlements, Roman construction, as well as more modern attempts to reshape the landscape.

I particularly enjoyed the book's sense of pace, which remains unhurried throughout. I think it is best read in order, starting with relatively humble spans and a river that is little more than a stream, and seeing the images of bridges growing very slowly steadily in scale, with tales of increasingly impressive engineering achievement as the book draws you steadily downstream.

For those with a general interest in the Thames, and its history, I can certainly recommend Thames Bridges. It is an accessible, well-written survey. I think that those with greater knowledge of the Thames and its bridges should also enjoy it: it left me with a little bit of a desire to hunt out some maps of the Thames path, as this book would make an excellent companion to a river tour!


For some other examples of books in the bridges-by-river genre, here are some that I have reviewed previously:

11 May 2022

"An Encyclopaedia of World Bridges" by David McFetrich

This new book is the follow-up to the author's previous Encyclopaedia of British Bridges, which I reviewed in 2019 (and its predecessor, back in 2010).

It lists and briefly describes over 1200 bridges in over 170 different countries, and there truly is nothing else quite like it.

An Encyclopaedia of World Bridges (Pen and Sword Books, 352pp, 2022 - also available in ePub and Kindle format) starts with an introduction and useful glossary of terms, and finishes with 90 pages of Appendices (of which, more later). However the core of the book is an A-Z of bridges around the world: well-known, not-so-well-known, significant, and curious.

It's smaller in size, and shorter than its predecessor (British Bridges had 444 pages covering over 1600 bridges) but it's still a mammoth undertaking. I've pictured it as part of its family for scale.

The entry for each bridge has a paragraph giving key details, and every page in the main section is illustrated with colour photos, although less than half of the structures have an accompanying image. There is sufficient information in almost every case to answer key questions, and the internet will beckon if an entry particularly piques anyone's curiosity.

Nitpicking Pontists can go through the book looking for surprising omissions (and there are plenty - I will leave this as an exercise for readers to address through this blog's comments function, if they wish!) However, I found I discovered far more that was unknown to me than I felt was missing. There are bridges of every conceivable age, shape and size. The sheer variety gives the lie to the traditional idea that there are really only four types of bridge (beam, arch, suspension, and stayed), with plenty of bridges that defy these simple categorisations.

The Appendices include helpful indexes of bridges by country and according to key participants in construction, obstacle spanned etc. There is an excellent bibliography with over 325 entries (although sadly for true enthusiasts, the sources of information for each bridge are not linked to the bibiliography, as was the case in the British Bridges volume).

The most interesting Appendix offers no less than 76 lists of bridges by various categories, some obvious, some much less so. Here you will find lists of Inhabited Bridges, Highest Bridges, Monorail Bridges, Chain Suspension Bridges etc. But also the less obvious Copy Bridges, Bridges Stranded by Changes in the Course of Rivers, Natural Fibre Bridges, Pilgrim Routes over Bridges and many more. I found this a particularly intriguing section of the book, giving the reader a number of ways to engage with the topic other than simply flipping through from A to Z.

I can heartily recommend the book to anyone with an interest in the history or architecture of bridges. For the world tourist, it could easily have been titled 1001 Bridges to See Before You Die (and a few more), in the vein of the popular bucket-list books aimed at people who lack the time or opportunity to travel.

I must confess I have not actually read every page yet: but it will be sitting on my desk for the foreseeable future, a book to dip into repeatedly.

09 May 2022

Welsh Bridges: 23. Dernol Footbridge, River Wye

This was the third of three bridges that I tried to visit on the River Wye in Powys in August this year.

The first was barely a ghost, and the second a literal washout. Would it be third time lucky?

I parked up in a layby on the A470, north-east of the bridge, and followed a public right of way through the fields and downhill towards the site of the bridge.

A bridge! A palpable bridge!

Alan Crow's book Bridges on the River Wye indicates that a suspension footbridge was built here in 1975 by the Newbridge-on-Wye firm N.R. Hope. From the description, it is the same bridge that can be seen today, except that all or much of the fabric may have been renewed. Powys County Council tell me that the bridge was "refurbished" in 2018.

The bridge is of essentially the same design as the now-destroyed Cwmcoch footbridge, constructed by the same builder in 1967.

Timber towers sit on concrete footings, and support a steel tube or roller over which the main cables pass. Additional stay cables are attached to the rear face of the towers.

The main cables are anchored to steel cantilever beams at each end of the bridge. Three parapet wires on each side of the bridge are also tensioned against these cantilevers, and the tower stay cables are spliced into the lower of these parapet wires.

The bridge floor comprises three timber planks running longitudinally, sitting on transverse timber members. Every third cross-member also provides the support for braced parapet posts.

Below the bridge decking, six parallel wires run longitudinally, and these provide substantial support to the deck along most of its length, as there are no vertical hangers as you would expect on a conventional suspension bridge. The deck wires are anchored to steel cross-members attached to the main anchor stanchions.

The main suspension cables only directly support the middle part of the deck. The two cables pass below grooved longitudinal timber members on each edge of the bridge.

In the absence of the under-deck wires, this would be quite an unstable arrangement, as the central portion would "rock" longitudinally under load. The deck wires are therefore essential both to prevent that rocking, and to support the deck between the points where the main cables connect.

I've included a few more photographs to illustrate both the details and the setting of this attractive, economical bridge, and a video to show how much it moves under a single person load.

Further information:

08 May 2022

Welsh Bridges: 22. Cwmcoch Footbrige, River Wye

This is the next crossing of the River Wye upstream from  the Cwmbach Footbridge discussed in my previous post.

It was built in 1967 by N.R. Hope of Newbridge-on-Wye, a "swing" suspension bridge of total length around 53m. A wooden beam bridge had previously spanned the river here, before being destroyed in the 1960 floods.

If you were unaware that this bridge existed, you would never look for it. I parked in the entrance to a farm track (although I can't recommend this) on the A470, and walked downhill. There is also space to park on the minor road on the opposite side of the river.

And indeed ... there was no bridge to find. At least, not one that you can still cross.

Powys County Council have no record of the bridge, suggesting this may have been a private bridge as there is no recorded public right of way.

However, Alan Crow's book Bridges on the River Wye, indicates that the bridge was built by the council, after an application to the Ministry of Agriculture and Fisheries for a grant to build a vehicular bridge was rejected.

Considering that the remains of the bridge are still visible, attached to the western abutment, I guess that this bridge may have been destroyed in the floods of February 2020.

Peering closely at the debris of the bridge, it can be seen that there was a timber walkway, three planks wide, supported on timber cross-members. Below these are a set of six parallel wires, an arrangement similar to the Invermark Footbridge in Scotland. The remains of suspension towers and cables are also just about visible.

I have included a picture of this bridge from Crow's book, to show it as it once was.

Further information:

07 May 2022

Welsh Bridges: 21. Cwmbach Footbridge, River Wye

Back in August 2021 (yes, time has been moving slowly here at Pontist Towers), I had the chance to look for three interesting footbridges along the River Wye in Powys. I had discovered all three in Alan Crow's excellent 1995 book Bridges on the River Wye.

Unfortunately, my search was less successful than I had hoped!

To reach the first of these spans, I parked in a layby on the A470 (see Google Maps link below), then crossed the road and headed downhill towards the river. Slightly further south, a public right of way is marked on the OS map, but no bridge is shown.

Crow's book records that a "swing" bridge was built here in 1962, to replace a structure that had been destroyed by a flood in 1960. This is the term "swing" bridge in its colloquial sense, referring to a slightly unstable suspension bridge. It was demolished in 1991 on safety grounds, and a new bridge was installed in August 1994 by Tysons plc, of Liverpool.

This was the bridge I had come to see: an arched beam bridge constructed in laminated timber, described by Crow as "one of the most attractive small bridges on the River Wye".

Sadly, there was no bridge to be found. If you look very closely at my main photo, perhaps you can spot the abutments.

The concrete abutments are still there, as are the remnants of steel hinges attached to them. Originally, there was third hinge at the crown of the arch as well.

It's an attractive river setting, and it was disappointing there was no bridge.

Powys County Council have told me that the bridge was removed for safety reasons in 2013, "as it had reached the end of its working life". There are no plans to replace it. I imagine the public right of way saw very little use before anyway.

Since there was little else to see here, I have included an image from Alan Crow's book showing the previous bridges.

Further information: