23 December 2018

Design published for new Polcevera viaduct

Details have been made public for the new Polcevera viaduct in Genoa, Italy, the proposed replacement for the bridge which collapsed in August this year killing 43 people.

The bridge design is credited to architect Renzo Piano, who offered his services free-of-charge in the aftermath of the August disaster. I imagine a structural engineer is involved, but I haven't found out who they are yet.

The €202 million construction contract has been awarded to Pergenova, a joint venture of Italian contractors Salini Impregilo and Fincantieri Infrastructure. They state that the new bridge will be complete in 12 months from the site becoming available, which will be when demolition of the existing structure finishes in March 2019.

The new structure is to be an elegant and, I think, consciously unspectacular steel viaduct, some 1.1km long, comprising twenty spans mostly 50m long, with some 100m long where required to span the railway and river. The reinforced concrete bridge piers are elliptical in section.

The design is sober and straightforward. The only nod towards the site's tragic history comes with the lighting columns positioned above each bridge pier. These extend in height and support a number of solar-powered lanterns, reportedly one for each victim of the bridge collapse.

I do wonder about the width of the piers - in some of the renderings, the deck is shown as much wider than its supports, which for me looks visually unsatisfactory, potentially unbalanced if only one side of the roadway is heavily loaded.

Of course, there will be no issue if the bridge deck is heavy enough, and Genoa is not in one of the more seismically active areas of Italy, classed as having a peak ground acceleration between 0.05g and 0.15g, for a 50-year return period, and from what I can see, at the lower end of this scale even for a longer 475-year period.

The Piano design was chosen against one other competing contractor, Cimolai, who submitted four proposals, three of them designed by Santiago Calatrava. The proposals featured a girder viaduct with a series of 125m spans; a cable-stayed viaduct with 140m spans; and a 550m-span tied arch bridge, all pictured below. You can find more detail in Cimolai's video and in a gallery on the Domus website.

It's no surprise that Cimolai were unsuccessful: which public authority would wish to run the risk of an over-budget fiasco at this particular site, as so often seems to accompany anything Calatrava designs? It's also apparent that any of these solutions would take longer to build than the Pergenova proposal.

Visually, two of the three lack a sense of lateral stability, and all of them are visually over-complicated and inappropriately dramatic. The arch design, with its awkward stiffening truss at deck level, is particularly poor.

Cimolai's non-Calatrava option (no designer appears to be credited), is worse still:

Meanwhile, some 20 people are reportedly under investigation on suspicion of involuntary manslaughter in connection with the original bridge failure.

25 August 2018

London bridges series: 52. London Wall Place Highwalks

Look, up in the sky! Is it a bridge? Is it a walkway? Is it a skywalk? No, apparently it's the return of London's pedways.

The original pedway scheme was the idea of London's town planners in the 1950s and 1960s, an attempt to elevate pedestrians above increasingly car-dominated city streets. Several new buildings were required to include provision for pedway access at first-floor level, but few of the pedways  planned were actually built.

They were perhaps most fully realised in the vicinity of the Barbican development, spanning across roads, and connecting the new Barbican cultural centre to the residential blocks which surrounded it.

However, with most foot journeys originating and ending at ground level, pedways were often as much of an inconvenience as a boon, and the idea soon died away. Despite their lack of success, pedways continue to exert a strong fascination for design and architecture critics and bloggers, and I've included various relevant links below providing more information.

With the construction of a massive new office development at London Wall Place, the idea has been revived. This was an original pedway location, so the new development is essentially reinstating what had once been there, albeit with a 21st century sense of style.

The new walkways span one busy road (and others that are less busy), and at least do serve a useful function in connecting into the Barbican's pedways, which do still form a useful connection. However, the walk below the pedways at ground level also an attractive route with extensive public realm enhancements in the St Alphage Garden section.

The walkways were designed by the team also responsible for the office building, Make Architects and WSP. Spacehub contributed to the landscape architecture.

The walkways are accessible from several staircases, and there's a new lift, although that wasn't in operation when I visited. The detailing on the main staircase is particularly nice.

In two places where the walkways span across roads, they are cantilevered from the office buildings, with the aid of stainless steel masts and stay bars. The walkway is suspended in an attractively-shaped weathering steel trough.

I found the mast-and-stay system visually awkward - it gives something of the impression that these are independent structures, although they obviously depend on the buildings for support. There's something about the arrangement that is out of place with the rest of the scheme.

The "centrepiece" of the walkway is another trough structure which snakes across the public space, skirting the ruins of an old church. This structure is a continuous beam cantilevering from each end, and it gives the impression that it's floating in space, thanks to the lack of intermediate supports.

It would have been so easy simply to have a multi-span walkway across this stretch, in the same style as other spans, but it's definitely this curved span that puts the whole scheme at a higher level of quality.

The various spans have spaces for seating, ensuring the walkways feel part of the public realm as a place to stop and breathe rather than just another way of traversing the city as rapidly as possible.

The whole scheme has been very well designed, and is a great addition to this one small corner of central London. I certainly can't see it inspiring yet more elevated pedways - the city streets are already too constrained and difficult an environment, but it's good to see it resurrected here.

Further information:

15 August 2018

Collapse of the Polcevera Viaduct

I'm sure most of my readers will have seen yesterday's tragic news that the Polcevera Viaduct in Genoa, Italy, collapsed, with at least 39 fatalities reported. At the time of writing, rescue and recovery efforts are ongoing. A state of emergency has been declared in the local region.

The 1.1km long viaduct carried a major toll road through Genoa, and was a key connection in the route from central and southern Italy to the south coast of France. It was completed in 1967 to a design by the famous Italian engineer Riccardo Morandi, one of a series of innovative concrete stayed bridges that he created starting with the Lake Maracaibo bridge in 1962.

These were distinguished by the use of very simple stay arrangements, using prestressed concrete stays rather than the steel cables already in wide use at the time (e.g. Strömsund Bridge in Sweden, 1955, and the Nordbrücke in Düsseldorf, 1957).

Eduardo Torroja's Tempul Aqueduct, built in 1926, is one of the few predecessors, although Torroja used concrete only as a protective material, it was not prestressed. Concrete-encased stays were also later used on the Prins Willem-Alexanderbrug in the Netherlands (1972) and the Metten Danube Bridge (1981). Morandi's tower arrangement was also used (without the concrete stays) on the Chaco Corrientes Bridge in Argentina (1973).

For more on Morandi's bridges and their relatives, see Walter Podolny's 1973 paper Cable-stayed Bridges of Prestressed Concrete.

Morandi's designs, although highly innovative, were a dead end. They required extensive temporary works to support the bridge deck until the stays were completed.

At Polcevera, temporary prestressing was used in the cantilevering deck sections, only to be removed once the stays had been installed. For his bridge at Wadi el Kuf, in Libya (1972), an array of temporary stay cables was used to support the longer spans, with all these cables then removed, rather than left in place as the permanent support system, which was the more logical and much more widespread solution. The Libyan bridge, incidentally, was recently closed for safety reasons.

Possibly relevant to the Genoa disaster, these designs also lack structural redundancy. The failure of any one key structural member of the bridge can lead to disproportionate collapse.

The Polcevera Viaduct, and its cousins, have been much admired by engineers and architects.

Michel Virlogeux, in his paper Bridges with Multiple Cable-stayed Spans, notes that the Lake Maracaibo Bridge was "much admired by architects who understand the evident flow of forces and who are sensitive to the impression of strength that emanates from the mass and shapes of the structure". Leonardo Fernandez Troyano described the same structure as "one of the great works in the recent history of bridges" in his book Bridge Engineering - A Global Perspective. In a 2010 paper summarising Morandi's work, Luca Sampo claimed that the Polcevera bridge's "technical features may still today be considered unsurpassed".

There is plenty of speculation on the internet regarding the cause of the collapse, which I won't repeat here.

The bridge's brand-new Wikipedia article is a pretty good source of information. There's a paper from 1995 which discusses previous remedial works to the bridge's main stays. Probably the best read is a contemporary article from 1968 with lots of construction drawings and photographs. All the images I've used here are taken from that article.

30 July 2018

"China's Unique Woven Timber Arch Bridges" by Zhou et al

I don't normally make a point of mentioning technical papers on this blog, but maybe it's something I should do from time to time. I have previously wondered about putting together a semi-regular roundup of papers that might be relevant or interesting to my readers. However, I don't want to make more work for myself, so this may only very rarely happen!

I did think that this paper was worth drawing to wider attention: "China's unique woven timber arch bridges" (Zhou, Leng, Zhou, Chun, Hassanein and Zhong, Proc. ICE - Civil Engineering, August 2018).

This gives an overview of timber bridges in China of a type that dates back over 1000 years. I first properly encountered them in Ronald Knapp and Chester Ong's excellent book Chinese Bridges, which presents several bridges from the Zheijang and Fujian regions. The design and construction of these bridges is considered important enough for them to be included on UNESCO's Intangible Cultural Heritage List since 2009. A historic example, the Rainbow Bridge, is illustrated on the Song Dynasty painting Along the River During the Qingming Festival (~1085-1145), pictured above.

Nearly 100 of these woven timber arch bridges survive. Several have fallen victim to disaster through fire or flooding, including at least one of the bridges featured in Knapp's book. However, the construction skills have undergone a revival, such that some of these bridges have since been rebuilt. Indeed, the paper lists some 19 woven arch bridges which have been rebuilt or newly built since 1999.

The essence of these bridges is the structural form of a woven polygonal arch, which is described in detail in the paper, including several construction photographs. It consists of two sets of arch members which alternate across the width of the bridge, so that there are two superimposed polygons. These are locked together by transverse timbers, creating a triangulated system which in one way behaves not like an arch, but like a beam. However, it must also behave as an arch, as the main timbers are carefully butted together to be able to transmit axial load.

The paper in the ICE Proceedings is a short (6 pages) but very clear and useful introduction to these amazing bridges, and definitely worth a read if you have access to it. In case anyone would like to learn more about the woven arch bridges, I've collected a set of links to more detailed technical papers at the bottom of this page.

It wasn't until several centuries later that a similar bridge design was developed in Europe by Leonardo da Vinci. His design is discussed in a 2004 paper by Ceraldi and Ermolli, which compares da Vinci's design to the earlier Chinese bridges. Da Vinci's solution does not use the butted timbers, and is an open frame rather than having many alternating arches all immediately adjacent to each other.

Further information:

24 July 2018

Awards shortlists announced

The shortlist has been announced for the Structural Steel Design Awards 2018. Bridges projects on the shortlist include:
There are also several bridges featured in the shortlist for the British Construction Industry Awards 2018:
  • Chapel Street Bridge upgrade, Salford
  • Highbury Corner Bridge replacement
  • M4 River Usk Bridge Strengthening and Rehabilitation
  • Mersey Gateway Bridge 
  • Queensferry Crossing
  • Somers Town Bridge
  • The Ordsall Chord
The BCIAs also shortlist Leeds Flood Alleviation Scheme and London Wall Place, both of which have interesting bridges as part of the wider projects. There may be others, it's not always easy to tell from the scheme names.

I've included links to structures that I've previously featured here. I will be reporting on the London Wall Place walkways soon as well. Hopefully I can visit some of the others some day.

Winners for both the above awards schemes will be announced in October.

Finalists have also been announced for the European Steel Bridge Awards, with a winner to be announced in September:
  • The Railway Bridge line Hohenau-Prerov, Czech Republic
  • Rethebrücke, Germany
  • Loftnesbrui, Norway
  • Årstabron, Sweden
  • Parkbrug Spoor Noord, Belgium
  • Footbridge for Pedestrians, Cycles and Reduced Mobility, Luxembourg
  • Jungle Pedestrian Bridge, Norway

"Samuel Brown and Union Chain Bridge" by Miller and Jones

Union Chain Bridge is the oldest suspension bridge in the world which still carries vehicular traffic. Opened in 1820 to a design by Captain Samuel Brown, it is remarkable that it has survived so long, especially considering that so many of Brown's other bridges failed early in their lives. The bridge will reach its bicentenary in two years time, and the publication of this book is therefore timely.

Samuel Brown and Union Chain Bridge (Friends of the Union Chain Bridge, 306pp, ISBN 978-1-5272-1616-7) is the end result of extensive research by architect Gordon Miller, commenced in the early 1970s. Most of the content has not seen print before, although Miller published a paper in the ICE Proceedings in 2006 which summarised some of the Union Bridge story. Sadly, Miller passed away in February this year after completing this book.

The first, and lengthiest, section of the book explores Captain Samuel Brown's career, first as a naval officer and then as a pioneering supplier of iron chains. Initially, he developed chains for use in the navy as anchor cables and rigging. In 1813, he built an experimental 100-foot long suspension bridge at his Millwall chain factory, which was visited by eminent engineers such as Rennie and Telford. This led directly to the use of chains to suspend the Union Bridge, and soon to many more structures, principally built in the period from 1820 to 1832, with Brown's last bridge built in 1834. 

Miller's book recounts far more detail on Brown's many bridges than has been published anywhere before, drawing in depth on the surviving archive papers. However, this lengthy section of the book is not well structured, with no subheadings to help identify individual structures, and a number of digressions. The material is also in some cases curiously incomplete - the best known previous survey of Brown's works was in Emory Kemp's 1977 paper, which although briefer than this new book included some details which Miller omits. A useful chronology of Brown's work can be found at Engineering Timelines.

Comparing Miller's account against other books and papers relating to Brown, I'm left with as many questions as answers. Some other accounts report that Brown won the Union Bridge contract in competition with North Shields chain-maker Robert Flinn; elsewhere this is said to relate to a proposal for Norham Bridge circa 1817-1818. Miller's book does nothing to clarify the matter, with Flinn barely mentioned at all and no reference to any competition.

There are also unfortunate errors, such as citing Brunel as the designer of the Menai Suspension Bridge. These are not errors of knowledge, but can be put down to the absence of any kind of editor or proof-reader. This also explains the poor and inconsistent structure to the book. There is no kind of referencing throughout, and coupled with the other flaws, I have to say that for any serious student of bridge engineering history, this makes the book very difficult to trust. It is a terrible missed opportunity.

The lack of referencing means that previous documentation on Brown and his works is largely ignored: Kemp's paper is barely acknowledged, Day's papers are ignored entirely, as is Paxton's paper, which is a shame as it includes a useful numerical appraisal of the strength of Brown's chain designs, putting his practice into the context of what his contemporaries were doing.

The core of the book is an account of the Union Chain Bridge itself, its design, construction and opening as a toll bridge. Much of this is drawn from the records of the bridge trust, and it is thorough and informative. As is the case throughout the book, a great deal of contemporary record material is reproduced directly, including correspondence, images of the original bridge drawings, and Brown's 1817 patent. It's thorough, but sometimes a mixed bag, with many pages given over to exact reproductions of primary sources.

There is also a great deal of information on the bridge's history post-construction. This section includes an excellent set of sketches and details which illustrate how the bridge deck was modified on numerous occasions. Many of these were drawn up at the time of the bridge's major refurbishment in 1974.

Correspondence between the bridge trust and their consulting engineer at various stages is included, reporting continuing doubts about the strength of the bridge which had never really gone away since its original construction. The bridge's south anchorage was strengthened at the time of construction, and the north anchorage in 1902. The saddle details were a perennial problem: at the outset, a roller arrangement was provided to address thermal expansion, but the chains were allowed to rub on the edge of their support, and the hidden nature of the saddles inevitably led to progression of unseen corrosion at various stages.

I think that anyone looking for an in-depth understanding of 19th century bridge engineering history in Britain, or anyone with a deep interest in the history of suspension bridges, will regard this book as an essential purchase. However, it is grievously let down by the lack of any intervention from an editor, and the lack of proper referencing. For any more casual reader, the article at Engineering Timelines is a good enough introduction to the subject, and I can't recommend this book to them.

Further reading:

22 July 2018

Welsh Bridges: 16. Footbridge at Devil's Bridge

For the last in this series of posts on bridges in Wales, I travelled down the valley below Devil's Bridge, following the Devil's Bridge Falls woodland walk.

At the lowest point of the walk, a small wrought iron footbridge carries visitors across the River Mynach before they climb back uphill to the main road.

The footbridge has been Grade II Listed since 2005. A plaque on the bridge states "Aberystwyth Foundry 1867. Thos Stooke Engineer". There doesn't seem to be any further information on who Stooke was, or which foundry built the metalwork.

The bridge is has two highly arched truss girders, each in double-Warren truss configuration. The upper and lower chords of the trusses are quite slender. More modern anti-slip flooring has been added at some stage, I'd guess mainly for durability reasons.

It's a pleasant bridge but the real attraction here is the scenery, with a cascade of waterfalls totalling 91m in height.

Further information:

18 July 2018

Welsh Bridges: 15. Devil's Bridge

So, Devil's Bridge, we meet again.

Well this is a different Devil's Bridge to last time. Two down, many more still to go!

Devil's Bridge (or in the local tongue, Pontarfynach) is one of the better known tourist attractions in the Aberystwyth area. Anyone can cross the bridge, as it carries a public road, but to see it properly requires payment to access private land. In addition to the bridge, this gives access to a very scenic woodland walk, and is well worth the price of admission.

The legend that gives this bridge its name is the same or similar to most other Devil's Bridge tales: an old woman's cow somehow crossed over the river Mynach, and she couldn't get it back. The Devil offered to build a bridge in return for the first living soul to pass across it, and the old woman agreed. She threw some bread across the bridge, and her dog ran after it. The Devil, having expected a higher price, had to be satisfied with the dog.

What I learn from this legend is that the old woman was pretty smart to give up the dog in return for getting her cow back. It must have been a very impressive cow to have jumped across the River Mynach gorge before the first bridge was built.

There are three bridges here, each one built above its predecessors. It goes one better than Rumbling Bridge, in Scotland, in that respect.

The lowest span is medieval, generally thought to date back at least to 1188, and comprises a pointed masonry arch sitting astride a remarkably deep cleft in the local rock, which contains the River Mynach (Afon Mynach). The most widely-held view seems to be that it was built by the monks of Strata Florida Abbey.

In Gwyndaf Breese's book on Welsh bridges, he suggests that the bridge reported by a traveller in 1188 was a "rickety wooden bridge", and that the stone span may have been built a century later.

Other than for its age and situation, it is relatively unremarkable. The arch barrel consists of thin bands of stone, springing directly from the rock. The profile of the barrel is noticeably distorted and unsymmetrical.

Breese quotes Jervoise in suggesting that the lowest bridge was widened or rebuilt at some stage. Jervoise's comment was that pointed arches were not in use in the 12th century, and the span must therefore have been a later reconstruction. I'm no expert but I don't think that is conclusive: the pointed spans in the medieval Exe Bridge are believed to be original, so why not here?

In 1753, a second bridge was constructed, a segmental stone arch spanning 32 feet. This bridge was later modified, with the height of its spandrel walls increased in 1814 to reduce the steepness of the highway approaches. It looks like you can see evidence of this in the banding of the horizontal stones in the spandrel walls. The ornate cast iron parapets were added at this time.

Buttresses at either ends of this span appear to have been added later. At one end of the bridge, the arch appears to spring from a higher point than at the other. Instead, it seems that the original springing is hidden within the masonry buttress, which continues under the arch barrel.

The third bridge was built in 1901 to a design by the County Surveyor Roderick Lloyd. Masonry abutments were built up to support steel lattice girders spanning 60 feet and carrying a 20-foot wide roadway.

This was substantially modified in 1971, when the castellated plate girders visible today were inserted along with a concrete deck slab. It looks to me that the parapets were designed to retain the appearance of the lattice girders, but clearly what's there now could not have been the original spanning trusses as there are no upper or lower chord members.

The newer girders are supported at one end on the abutment of the middle bridge, and at the other end on steel portal frames carrying the load to either side of that span. This was obviously an unfortunate period in the life of Devil's Bridge: the position, appearance and level of the new girders were all entirely unsympathetic to this span's predecessors.

Further information:

15 July 2018

Welsh Bridges: 14. Llandeilo Bridge

Completed in 1848, the mighty Llandeilo Bridge is one of the largest masonry arch spans in the United Kingdom. By my count it takes third place behind Grosvenor Bridge (61m / 200 ft, 1832) and Ballochmyle Viaduct (55m / 181 ft, 1848), making it the longest masonry span in Wales. (I'm happy to be told otherwise if there are any bridge spods out there somewhere.)

The Grade II* Listed bridge is said to span either 143 ft or 145 ft (take your pick!), and to be 26 ft or 10.1m wide (again, you choose!).

However tall and proud it may stand today, it had a difficult beginning.

The first bridge on this site was a seven-span arch bridge, which partially collapsed in 1795. The failed centre spans were replaced with a timber structure.

According to some accounts, the bridge was replaced in the early 1800s by a narrow three-arched bridge, which proved too narrow for traffic, although not all histories seem to agree (for the most thorough story of the bridge's past, see Llandeilo Past and Present).

County bridge surveyor William Williams was appointed to design a replacement in 1843, estimating the cost of his design to be £10,000. Builder Morgan Morgan was appointed on a contract price of £5,870.

The cash ran out while Morgan was still constructing the bridge foundations, and work was further set back when a flood destroyed part of the works. Edward Haycock took over the scheme, completing the bridge in 1848 with a total expense of £22,000.

Despite these difficulties, it was a huge engineering achievement. The causeway to the south is a substantial structure in its own right, giving a total length of 111m for both causeway and arch. The town of Llandeilo sits some height above the River Towy and its flood plain, and a lengthy ramp was required to allow traffic to enter the town at a suitable level.

The causeway is pierced by a smaller cattle creep, span which like the main span has an elliptical profile. On the main arch, this choice was driven by the span dimensions and height of the roadway. The cattle creep arch is described elsewhere as a flood arch, but I doubt it adds greatly to the bridge's flood capacity.

The main arch is described as having long, thin voussoirs, but as can be seen close up, they are in fact made up of short(ish) stones with thin ashlar joints.

Looking at the bridge today, it's impossible not to be impressed by the sheer ambition of this small town and those involved in building the bridge. It's difficult to know whether they could have fully appreciated the nature of the task they were taking on. It's interesting to think what would have happened if the entire project had been abandoned after Morgan's failure.

Further information: