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.