23 December 2019

London Bridges: 54. Chiswick Park Footbridge

It has been a while since I've visited and reported on a recently-built bridge (May, since you ask, and before that, August 2018), so this visit to see Chiswick Park footbridge in London felt long overdue.

According to the Ian Visits blog, the bridge itself is also long overdue, with planning applications dating back to 2003. It forms part of a walking route connecting Chiswick business park to Chiswick Park tube station. The bridge finally opened to the public in January this year. It was designed by Expedition Engineering and Useful Studio, with the steelwork constructed by Severfield.

I believe it's only the second network arch bridge to be built in the UK, and the first such pedestrian bridge. The design didn't start life as a network arch structure, and has been through a lengthy evolution to get to what was eventually built.

Originally, proposals were for a truss bridge of some form, as per the 2003 and 2006 planning applications shown here:

The bridge's three spans were largely determined by clearances to road and rail routes below, and in these early designs it was indicated that the truss span over the railway would be fully enclosed to prevent risks from vandalism.

In the 2012 application (top diagram in the image above), the design had become three steel bowstring arches, each of increasing span and height from west to east, with vertical hangers supporting the deck. Tall mesh parapets were indicated above the railway, eliminating the need for full enclosure.

The overall form of the bridge changed very little thereafter. The shape of the arches was the result of a form-finding exercise, to maximise the visual slenderness of the arch. I'm not clear how that will have worked, as the critical bending arrangement for an arch of this sort is usually with only half the span loaded with pedestrians.

That scenario is often more onerous in design than the full span loading which produces the greatest axial load in the arch. In a conventional bowstring arch, the stiffness of either the arch or the deck (or both) is required to resist this half-span bending.

In any event, during design development the bowstring arch was found to perform badly under dynamic pedestrian loading, and was amended to a network arch in the 2015 planning application (middle diagram above).

This is many times stiffer than the previous design, raising the bridge's vulnerable natural frequencies, and eliminating or mitigating the dynamic problems. Bending moments in the arch due to asymmetrical loading arrangements are also greatly reduced.

I believe the previous design incorporated a concrete deck - adoption of the network arch also allowed a lighter all-steel deck to be used, minimising the weight required and making craneage of the spans into place easier.

The final change is indicated in the 2017 planning application (bottom diagram above), and indicates that the design team had failed in their desire to persuade Network Rail that a mesh parapet would be sufficient above their railway line.

The railway authority is never noted for its flexibility when there is a rulebook that can consulted, so the final introduction of a solid (imperforate) parapet screen above the railway tracks is unsurprising. It is at least largely disguised by being hidden behind the facing mesh.

The bridge's spans are 37.0m, 40.7m and 44.4m, totalling 122.1m. The arches and deck are connected integrally to the two intermediate piers, with bearings allowing thermal articulation at each end. All the structural steelwork is weathering steel, with stainless steel parapets and hanger cables. The decking is floored in timber planks.

The articulation is interesting, as conventional wisdom would be that arches of this type should sit on bearings at all points, allowing the tie girder connnecting the ends of each arch span to expand freely. This allows it to take up a full tension balancing the compression in the arch, and allowing the hanger network to interact efficiently with the main steelwork.

In this instance, the V-shaped piers are sufficiently flexible longitudinally that they will offer only limited restraint to the arch thrust, and the network on the central arch will still work although I'd guess with slightly reduced effectiveness.

The most striking aspect of the bridge is the effort that has been expended to make its main elements slender, with cruciform sections for the arch and piers, and a simple stiffened steel plate for the deck. In addition to being slender, all parts are visible for inspection and maintenance, unlikely the closed box sections often seen in footbridges.

What is also very much apparent is an impressive attention to detail. Wherever possible, connections are kept simple, with welding used extensively instead of bolting. The edges of the deck are made clear and sharp in profile, and the hanger and parapet connections are well-detailed and as minimal as possible.

It's the sort of bridge that any designer would be proud of, and especially impressive given that this is predominantly a structure used to get rapidly from A to B, rather than a destination in its own right. it has been shortlisted for an IStructE award, been a finalist in the CE Awards, and won two ICE awards. I'm a little surprised that it hasn't been more widely rewarded, to be honest.

The bridge is not completely without its flaws. Site constraints mean that although at its western end the bridge connects directly with a podium deck level in the business park, at the eastern end the approach is via steps and a lift.

I imagine mobility-impaired users are crossing their fingers in the hope that the lift will be better maintained than is often the case. It is at least attractively detailed in keeping with the rest of the bridge.

Also on the eastern approach I noticed a sign that recommends no cycling, perhaps inevitable given the steps and lift, but also advises users to "Walk With Care" due to gaps in the decking.

The timber decking is visually attractive, but perhaps some users with high heels have found it a problem. In any event, I suspect problems with the decking won't end there.

The timber slats are raised above the bridge deck, the upper surface of which is a flat steel plate. Rainwater drains through the slats, and flows along the deck (which I am told is waterproofed), before spilling straight to the ground at the ends via cut-outs in the deck plate. The whole arrangement is an inspection and maintenance liability - nobody will lift the decking to properly inspect underneath, and it's easy to imagine dirt and detritus leading to trapped water over time.

Despite these oddities, the Chiswick Park Footbridge is a very impressive feat of design and construction and well worth a visit.

Further information:

26 October 2019

"Tower Bridge: 1894 to date. Operations Manual"

Haynes Publishing must be best known for their car and motorcycle maintenance manuals, but they have increasingly branched out into other territories, with recent publications including "The Human DNA Manual" and the "Milky Way Owner's Workshop Manual". In the areas of architecture and infrastructure they have published Manuals for "London Underground", "The Great Pyramid", "Hadrian's Wall" and now "Tower Bridge" (188pp, 2019, ISBN 978-1-78521-649-7).

It is, of course, Tower Bridge's 125th anniversary this year, and this new book by engineer John Smith joins books by Kenneth Powell and Harry Cory Wright published to mark the occasion. A comparison against the Powell book is inevitable, and although there is plenty of overlap between the two, there are some very clear differences.

Powell's book has, on the whole, the better photographs, and is a much easier read for a non-engineer, with much more detail on the context and a strong narrative surrounding those who designed and built the structure. As befits its publication by Haynes, Smith's book has far more detail on the construction work, the bridge components, and its operating technology.

The early sections of the book give a fairly comprehensive account of the somewhat tortuous process by which the bridge was eventually conceived, including the sometimes ingenious and sometimes monstrous alternative designs put forward.

The real dive into detail begins in the third chapter, documenting the eight separate contracts which were let for construction of the bridge, dividing up the works required for the piers and abutments, approach structures, metal superstructure, masonry superstructure, hydraulic machinery, paving and lighting. No client today would take this approach, retaining the entire liability for integrating a complex construction process on their own, but when the bridge was built there would have been no single contractor with the capability to do it all.

It's interesting here to see the extent to which the contract conditions used in the 1890s are very similar to those still in widespread use at the end of the 20th century. Extracts from the very first contract (for the piers and abutments) make this clear: the power of the resident engineer, payment retention, liquidated damages etc. The unrealistic timescales demanded by the client, and unrealistic prices submitted to win the work, also remain familiar today.

The core of the book consists of four chapters which itemise every single element of the bridge, describing them in exquisite detail and explaining just how every piece fits together. At times, the level of detail presented, with dimensions, plate thicknesses, etc, is numbing rather than interesting. For the engineering reader, there are several interesting extracts from drawings included, and the comprehensive nature of the text does mean that there appear to be no significant details left unmentioned.

There are many aspects of the bridge explained here which are essentially absent from the account in Powell's book. One example is the presence of stiffening girders concealed within the balustrades of the southern span, which ensure that water pipes carried across this span were protected against excessive movement. Another is the explanation of the arrangement of the high-level footways, the suspension bridge ties which pass through these, and the additional suspension cable added in 1960 to relieve the footway girders of the weight of those ties. These elements of the bridge are not immediately apparent to the casual visitor, but Smith's text, photographs and drawings make everything clear.

The book contains one excellent cutaway diagram showing how the components of the bridge fit together, and it's a shame there weren't more. My over-riding impression, after reading this book, is quite how complex Tower Bridge really is, and how well it merits this wealth of information. It really is an engineering masterpiece, whatever anyone may think of its architectural merits.

The book concludes with biographies of the main participants in the bridge's design and construction, and a detailed timeline of alterations and maintenance work in the period from 1894 to date. One of three appendices gives a detailed breakdown of the author's calculations of loads and forces in the bridge's key structural elements.

I couldn't, with any honesty, recommend this book to anyone who is not an engineer, but it is so detailed that it will probably remain a key reference work for Tower Bridge for the indefinite future. It is clear, thorough (sometimes too much so!) and well-illustrated throughout.

13 October 2019

Lancashire Bridges: 8. Seven-arch Bridge, Rivington

This is an interesting bridge that I stumbled across on a visit to Rivington Terraced Gardens, near Bolton in Lancashire.

The Gardens were built over a two decade-period starting in 1905, for industrialist William Hesketh Lever, also known as Viscount Leverhulme. Lever made his fortune with his family firm Lever Bros (later to become Unilever), selling Sunlight Soap. He had made a home, the Bungalow, at Rivington, and the surrounding estate was landscaped by Thomas Mawson, who later became the first President of the Institute of Landscape Architects.

Most of the Gardens then fell into ruin, but a Heritage Trust has been busy in recent years clearing vegetation and conserving the site's historic structures (there are eleven Grade II Listed Buildings within the site).

The Seven Arched Bridge carries a footpath over an access track, and was reportedly inspired by a bridge that Lever had seen while visiting Africa.

Historic England lists the bridge as part of a group of structures including a staircase, walls and a summer house, all of which proceed up the hill from the bridge. All these structures are built in a common style using thin slabs of gritstone.

Historic England's listing suggests that the design for the bridge was based on a bridge in Nigeria. Lever did source palm oil for his soap from the British colonies of West Africa, but he is also known to have visited Belgian Congo (now the Democratic Republic of Congo) in 1911 to set up palm oil plantations.

There is an image of a bridge in Albertville (now Kalemie, in the D.R.C.) from circa 1930 which might be relevant - I've certainly not found anything else online (there are further images of the same bridge on these pages).

In any event, this is not an elegant bridge, but it is interesting for its unusual architectural style, and the way its robust mass is built up from fine detail.

Further information:

06 October 2019

"An Encyclopaedia of British Bridges" by David McFetrich

David McFetrich's "An Encyclopaedia of British Bridges" (Pen and Sword Books, ISBN 978-1-52675-295-6, 2019, 444pp) is the 2nd edition (with a very slight change in title) of a volume previously published in 2010.

It updates and expands its predecessor with one-third more structures discussed across 1,600 individual entries (previously 1,350), and a page-count increased by a quarter. Size does matter in an effort like this - it can never possibly be comprehensive, but an already impressive reference work has been made significantly more valuable.

The additions are from all periods of history, bridges both ancient and modern, and many of them are structures I'd never heard of. Picking the letter K at random, the bridges added are Karlsruhe Friendship Bridge, various Kew Gardens bridges, Kildrummy Castle Bridge, Glasgow's Kingston Bridge, and Knostrop Weir Footbridge. One of these is an inexplicable omission from the previous volume, and the others are all worthy inclusions.

The core of the book remains a well-illustrated alphabetised compendium of notable bridges (I should declare an interest here, as some images in the new volume have been provided by the Happy Pontist). Descriptions vary in detail but always convey the core facts and usually offer interesting information or context. Some entries have been expanded from the previous edition. The Encyclopaedia is frequently my first point of reference when investigating British bridges, and helpfully includes cross-references to other sources and a thorough bibliography.

The book is topped-and-tailed with a brief history of Britain's transport infrastructure, details of how bridges work, a fine glossary, lists of record-breaking bridges and a very helpful geographic index. A length "miscellany" puts the bridges in many different contexts, covering not just "timber bridges" or "suspension bridges" but less obvious subjects such as "tea house bridges", "relocated bridges", "finback bridges" and "ugly bridges".

Every time I open the book, I discover something new, and I imagine most readers will find the same. If you already own the first edition, it may be difficult to justify this new one, unless your interest in the subject is serious. If you don't, and you are at all interested in British bridges, I think this book is indispensable. If you are involved in the bridge design or engineering community, you may even find some of your own bridges here - I certainly did!

An Encyclopaedia of British Bridges is currently available at a discounted price of £54 (postage free in the UK) from the publisher, and also on Kindle via Amazon.

03 October 2019

Iceland bridges: 6. Hvítá bridge

This is the last bridge I'm going to feature from my Iceland trip, and it's the best.

When celebrating its 90th anniversary in 2002, the Association of Chartered Engineers in Iceland designated this bridge the most notable achievement of the third decade of the 20th century, the only bridge to make their list.

The bridge was built in 1928 by the national highway authority to a design by their engineer Árni Pálsson - it was one of the first projects in his career there, he went on to become their chief engineer in 1947.

The structure is 106m long, with two 51m span concrete arches spanning the river Hvítá (the "white river"). This structural form was chosen on cost grounds in preference to a two-span steel girder bridge or a one-span suspension bridge.

The structure carries the road Hvítárvallavegur between Hvítárvalla and Ferjukots. As you can see from the photo, this is a fairly rough highway, as with many in the country.

Prior to construction of the bridge, a ferry crossed the river, but this was unreliable when the river flow was high. Efforts to build a bridge began with surveys in 1910, and drawings were prepared in 1922, six years before construction eventually started.

The bridge would remain the main route from south-to-north in western Iceland until a bridge was completed downstream at Bogarnes in 1981.

The structure is instantly impressive, as attractive as many better-known concrete arches built in mainland Europe in this period. The 3m wide bridge was designed to carry a 6-tonne truck, or a uniform load of 400 kg per square metre (roughly 4 kPa), a similar load to what a pedestrian bridge would be designed for today.

The arch is very slender at its thinnest points, but unlike the broadly contemporaneous deck-stiffened arches of Robert Maillart (starting with the Flienglibach Bridge in 1923), it does not take its stiffness from the road deck.

The bridge draws its strength from the shaping of the arch - its connection to the deck at the middle of each span, and the thickening of the arch towards each support. This could have led to an ungainly appearance, but the sinuous profile of the upper arch surface combines well with the elliptical profile of the underside.

The set-back of the vertical support struts from the edges of the arch and deck also contribute to a fine appearance, emphasising the profile of the arch.

There are many more interesting bridges in Iceland, I only had time to visit a handful. Hopefully I'll get the chance to see more on a future trip!

Further information:

01 October 2019

Iceland bridges: 5. Jökulsárlón Bridge

This must be another one of the most-seen bridges in Iceland. It spans the outfall river from the Jökulsárlón glacier lagoon, and carries the island's ring road, route R1. You can't drive along the south coast of Iceland without eventually passing over this bridge.

The hengibrú (suspension bridge) was built in 1966-7, and has a main span of 108m. A ferry operated here from 1932, but before that the river was very difficult to cross.

I believe this was one of the last of a series of suspension bridges built in Iceland starting in 1945, and there are obvious similarities to the bridge over Jökulsá á Fjöllum that I featured previously, even though that is 20 years older.

When the bridge was built, the glacier Breiðamerkurjökull extended much closer to the highway. The glacier lagoon has grown steadily as the glacier has retreated, some 5.6km in the last century. This location, hugely popular with tourists, will at some point likely become the mouth of a new fjord, with the extent depending on how successfully global warming is tackled. Although efforts have been made to protect the foundations of the bridge against scour, it's lifetime may be limited.

Further information:

29 September 2019

Iceland Bridges: 4. Jökulsá á Dal Canyon Bridge

There are many arch bridges in Iceland, but this is probably one of the more unusual ones.

Built in 1994, this bridge is 125.5m long, with a main span of 70m. The steel-concrete composite road deck is supported on the arch via slender piers at 14m spacing.

The bridge was designed by Línuhönnun Consulting Engineers, who became part of EFLA Consulting Engineers in 2008. Swiss engineer Christian Menn was involved as a consultant.

The bridge is unusual for the arch being of composite construction, with a concrete slab supported on two steel box girders, and for its angular form. In the UK, we'd describe it as a "thrupenny-bit" profile. This solution was chosen over girder and framed options for aesthetic reasons, although studies showed a steel frame bridge to be slightly less expensive.

The composite form was chosen to eliminate the need for falsework as far as possible. The steel girders were erected first, and used to support 150mm thick prefabricated concrete panels. A further layer of in-situ concrete was then poured to create an arch 300mm thick in total. The width of the arch varies from 4.4m at the crown to 6.4m at its springings.

The bending stiffness of the arch and deck are similar, so in the finished bridge, they both resist asymmetrical bending equally.

The construction sequence had to be considered very carefully to ensure that the very slender arch remained stable at all stages - the construction photo below (taken from a technical paper describing the bridge's design and construction) shows quite how slender it appeared.

Further information:

26 September 2019

Iceland bridges: 3. Suspension bridge over Jökulsá á Fjöllum on Route 1

My journey took me east from the previous two bridges, following the Route 1 highway.

Iceland is well-supplied with large rivers, carrying meltwater from icecaps and glaciers. The Jökulsá á Fjöllum river appears wide but relatively unspectacular. However, the volume of water is substantial, as can be seen around 20 km to the north where the river spills over the enormous Dettifoss, reportedly Europe's largest waterfall.

Before there was a bridge here, the river could only be crossed by a ferry. The bridge was built in 1947, one of a number of suspension bridges completed within a 12 year period from 1945 to 1957, following Iceland's independence from Denmark.

The bridge is 171m long, with a main span 102m long and 3.7m wide. The steel ropes were supplied by British Ropes Ltd, and the steelwork was supplied and erected by Dorman Long.

The Icelandic roads authority have been planning a new bridge a little to the south of the existing structure, on the grounds that the existing bridge requires both speed and weight restrictions (lorries are forbidden by signs from travelling in convoy across the bridge). The new structure is proposed as a 5-span concrete box girder bridge, 230m long. Construction was due to start in 2015, but evidently it has been delayed.

Further information:

24 September 2019

Iceland Bridges: 2. Road bridge over Skjálfandafljót at Fosshóll on Route 1

This bridge was built across the Skjálfandafljót river in 1972, replacing an older truss bridge dating from 1930. The older bridge (and the remains of its 19th-century predecessor) can be seen in the photo at the end of this post.

Today, this structure carries Route 1, the main Icelandic ring road. Like many bridges in the country, it is only a single lane wide, although reportedly the national highway authority is considering building a new 2-lane bridge immediately to the north of this span.

As with many bridges in Iceland, it can best be described as pragmatic. The ladder-like inclined legs allow the main bridge girders to be more economical in size.

Further information:

21 September 2019

Iceland Bridges: 1. Former road bridge over Skjálfandafljót at Fosshóll

I visited Iceland earlier this year, and stopped briefly at a few bridges while there.

This first structure was previously a highway bridge spanning the River Skjálfandafljót. It carried Iceland's main ring road (Route 1) until a new bridge was built in 1972. The older bridge has been retained today as a pedestrian bridge.

The steel truss bridge was built in 1930, and replaced a previous wooden bridge dating from 1882-83. You can see the remains of the older bridge in the photos.

The bridge was refurbished between 1999 and 2000.

This must be one of the most-seen footbridges in Iceland, not because it is of any great interest itself, but because it is just downstream of the spectacular waterfall, Goðafoss. It provides a pedestrian link between car parking areas on each side of the river.

Further information:

27 August 2019

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: