29 January 2017

"Bridges of Paris" by Michael Saint James

This book offers a big contrast to The Glow of Paris, reviewed in my last post, even though ostensibly its subject is the same.

Both books are hefty coffee table photographic surveys of the river bridges of central Paris. Glow squeezes 35 bridges into 208 pages, while Michael Saint James's Bridges of Paris fits 37 bridges into 280 pages (Citron Bay Press, 280pp, 2015).

The photographs in Glow are all taken at night, in black-and-white, and in a fine tradition of photography which takes as its subject landscape, architecture, and light. The Bridges of Paris is more varied in approach and style.

Most photos are taken in daytime, some on short exposure, and others on long exposure, traffic and pedestrians becoming only a blurred presence. Most photos are reproduced in full-page size, but there are also plenty of smaller photos, taking in not just overviews of the bridges but also details such as sculpture or love-locks.

Each bridge is accompanied by an explanatory text giving key facts (dates and dimensions) as well as something of each structure's history. There are satellite maps to locate the bridges, and a very nice introduction with several reproductions of historical paintings of key bridges.

However, the main attraction to this book is its focus on the bridges not only as beautiful architecture, but as lived spaces. Most of the photographs include people using the bridges, people looking at the bridges, people looking out from the bridges. In several photographs you'd hardly know there was a bridge there at all: they focus on how they provide public spaces, occupied by artists, street traders and performers.

The quality of the photography is probably more uneven than in Glow, but there are some stunning photographs in Bridges of Paris, and the sheer variety shows how each bridge's character changes with season, time of day and weather.

There is more information on the book at its dedicated website. US readers can pick the book up fairly easily from amazon.com (currently at a significantly discounted price). Readers elsewhere may find it more difficult to get hold of, but for any admirer of Paris and its bridges, I think it's certainly worth the effort - it's an excellent book.

26 January 2017

"The Glow of Paris - The Bridges of Paris at Night" by Gary Zuercher

Having spent the last few posts on a whistlestop tour of some of the bridges of Paris, I'm going to round things off by featuring a few particularly relevant books which I consulted.

Gary Zuercher's The Glow of Paris - The Bridges of Paris at Night (Marcorp Editions, 208pp, 2015) [amazon.co.uk] is a photographic survey of the 35 bridges which span the Seine between the points where it is crossed by the Boulevard Périphérique.

It's a serious coffee-table tome, 30cm square, with all the photographs reproduced at least full page size. They are all in black-and-white and all photographed at night.

The photos have generally been shot on a long exposure, which gives a landscape quality to the bridges, softening the sky and river and eliminating fast traffic and pedestrians. Indeed, the images are posed to remove signs of life as far as possible, to focus on the structures and their context. This is clearly a conscious choice, and while it ensures bridges are depicted clearly and beautifully, it does divorce them from the cityscape as a lived space rather than merely a collection of stone and steel.

There is a good mixture of scales, from wide open shots depicting bridges in their setting, through to close-ups of architectural details. Throughout, the emphasis is on the interaction of form and light.

The book includes maps to help the reader understand where each bridge is, and each bridge is accompanied by typically a couple of pages text, describing the bridge's history and any interesting stories about it. These are aimed at the general reader, but are informative enough for all but the most serious bridge enthusiast. I referred to them quite a bit while preparing my Paris blog posts, although I did have to look elsewhere for more detail. The text isn't referenced, but it's simply not that kind of book!

I very much enjoyed the book and am really glad to have it on my shelves now. Taken together, the bridges cover a long and varied period of history, with numerous different materials, technologies and architectural styles. The photographs are excellent, and the book is recommended for admirers of photography, Paris, bridges, or all three.

There's an interview with Gary Zuercher about the book online, and you can find out more about it at his website, including many more sample images than I've had space for here.

24 January 2017

French Bridges: 19. Passerelle Simone-de-Beauvoir, Paris

This is the last in my current series of Parisian bridges. I started with the oldest bridge over the Seine in central Paris, the Pont Neuf, and I'll finish with the newest, the Passerelle Simone-de-Beauvoir.

A design competition was organised in 1998 for a new crossing of the Seine, halfway between the existing Pont de Bercy and Pont de Tolbiac. The new bridge would link the Parc de Bercy with the National Library of France, as well as the riverbanks, which are here somewhat isolated from the rest of the cityscape by busy highways. Both park and library are like large plateaus situated well above river bank level.

Five teams were invited to submit designs: Tadao Ando / Setec TPI; SEEE / Patrick Berger; Marc Mimram; Arup / RFR / Chris Wilkinson; and Deitmar Feichtinger / RFR. In March 1999, the partnership of Feichtinger and RFR were declared the winner.

The winning proposal connects the riverbanks at lower and upper levels and avoids any supports in the river. The structure is therefore not also the latest bridge across the Seine, but also easily the longest span, with a 194m span between piers, and a total length of 304m. The deck is a generous 12m wide in total, although over most of its length this is split at different levels, so for practical purposes it's only 6m wide. The bridge was completed in 2006 at a total cost of 21 million euros.

By any standard, the bridge is enormous. I think it's particularly instructive to compare it to Patrick Berger's proposal, which was for a variant on a conventional suspension / stress ribbon bridge. This would have required substantially less material over the river, giving a lighter and less redundant appearance, but would have needed enormous anchorage structures well beyond the ends of the bridge itself.

Feichtinger's design (I'm going to call it his for the sake of shorthand, although clearly it was the result of a team effort) can be conceptualised structurally in several different ways. Think of a lenticular truss (like the Royal Albert Bridge, or the Maryhill House Footbridge). This can be thought of as a truss, with upper and lower chords carrying compression and tension, as well as shear forces by virtue of their inclination. It can also, however, be seen as an underspanned self-anchored suspension bridge, where a suspension cable below-deck is anchored not into the ground, but against the deck, which provides a balancing thrust.

The Passerelle Simone-de-Beauvoir is essentially the same type of bridge, except that the ends of the main span members, instead of meeting at a point where the forces balance, have been displaced vertically. The result of this is that although the horizontal forces from the suspension element and the compression arch element are broadly in balance, they are applied eccentrically, creating large overturning moments at both riverbank supports.

The overturning is restrained by a coupled foundation: the tension member passes vertically over a saddle, and into a deep anchor foundation. The saddle is balanced by an inclined leg, which connects directly to the ends of the main arch element. At this point, in a perfect world the horizontal forces would balance precisely. However, the suspension cable is significantly longer than the arch, and the forces do not balance, so there is a further inclined leg below the arch to carry the vertical loads from the bridge into the ground, via a massive deep caisson foundation. Taken together, the upper and lower inclined legs can be nicknamed the "boomerang".

For a pure lenticular truss of the same span, the foundation loads are vastly smaller, as the vertical load on the two foundations is equal to a half-share of the vertical weight of the bridge and whatever it carries. For the Passerelle Simone-de-Beauvoir, the main foundations must support the same vertical load, but must also react against the anchorage force, so the foundations are much more heavily loaded. However, the benefit is obvious: a much shallower structural form and one which better ties together the various ground levels. A lenticular truss of 194m span would sit ridiculously high in the air and look completely inappropriate.

The observant reader will, of course, have noted that the central part of the bridge is indeed a conventional lenticular truss, with upper and lower balancing elements coming together at a point. Indeed, this part of the bridge was fabricated precisely in this way: as an independent structure, fabricated elsewhere, assembled into its whole, and shipped up river ready for erection. In this respect, the structure can be seen as different structural form: the Gerber beam.

This concept was named after Gottfried Heinrich Gerber, whose idea was to take a continuous beam and insert hinges at the points of inflection in the diagram of bending moments (the points of theoretically zero moment). This transforms a structure which is complex to analyse into one which is simple to analyse, as well as eliminating troublesome side-effects of bending continuity such as vulnerability to foundation settlements. However, it retains the main advantage of continuity, which is reduced bending moments relative to simply-supported bridges. The exact position of the hinges is to some extent arbitrary, as the position of the theoretical points of inflection depends on the nature and position of loads applied to the span.

The central lenticular truss of the Passerelle is 106m long, and is structurally highly-efficient, just like the drop-in span of a Gerber beam. Here, the drop-in section is held aloft by cantilever structures either side, so another way to conceptualise this bridge structure is as an unbalanced cantilever bridge, like taking a single span of the Forth Railway Bridge and splitting it off from the rest by taking a giant machete down the centreline of the two towers. It's easy to image how much work would be required to haul back the severed towers and stop them from from simply toppling into the Forth Estuary, and that gives a good idea for how hard the Passerelle Simone-de-Beauvoir is working towards the same end.

One issue for the designers will have been how to combine the catenary and arch elements in a structurally and visually sensible manner. Catenary and suspension bridges typically employ circular cables as their main tension elements, but the Passerelle instead uses grade 355 steel plate as the tension elements, which although structurally inefficient makes visual sense.

These plates are 100mm thick at midspan, and 150mm over the anchor saddles, where they split into three bands before being anchored into the foundations. The plates are 1m wide, so each one can carry up to about 5000 tonnes without breaking. Each catenary can therefore carry the weight of roughly 15 fully-laden jumbo jets, which gives some idea quite how hard the bridge and its foundations are working. For comparison, this is a total tension load of roughly five times the total cable forces on London's Millennium Bridge, which is both shorter and narrower and lacks the arch element.

The tension plates are separated from the arches by bundles of four steel rods, which the architect nicknamed "obelisks". Compare the much smaller Maryhill House Footbridge, which has triangular struts in a similar arrangement, with the "pointy" end of the strut on the tension member and the thick end on the compression member. This creates a semi-Vierendeel arrangement, using the bending stiffness of the connection to the thicker compression element to prevent longitudinal movement and hence ensure the lenticular truss behaves as a fully integrated deep beam.

The bridge has two end spans across the adjacent highways which are more straightforward fishbelly trusses. These sit on the abutments at one end, and at the other are connected onto the main bridge saddle zones. If you compare the various photos, you'll see that the detailing of these areas is quite tricky, and the designers have generally done a very good job in keeping everything as simple as they can.

As with other long-span footbridges, the Passerelle Simone-de-Beauvoir could have been vulnerable to vibration. I didn't spot them on my visit, but there are tuned mass dampers below the decks to address flexural and torsional movements, and viscous dampers at the ends of the bridge to reduce longitudinal movement. Extensive crowd testing was undertaken to ensure these would perform correctly, and the bridge was also tested under static loads by filling 550 water tanks laid out along the length of the deck.

The layout of the bridge is not entirely as rational as it first appears. Although the upper deck surface follows precisely the contours of the structural form, the lower deck does not. At its ends, it sits at a higher level than the arch elements, a departure from the competition-winning scheme. Presumably, it just didn't prove possible to get the structural requirements properly matched against the topography of the site.

The bridge reminds me of the great wooden rollercoasters of the USA. As you walk onto the upper deck from the end, you at first see a wide expanse of timber and can't at first see the rest of the bridge. As you go further, great sweeping waves of bridge gradually rise into view. The change in perception of the structure is charming, and more so once you discover the ramps which lead both downwards and off the bridge and also down below the central crown.

Here, you find a sheltered and very different space, somewhere you can pause, rest, meet people, or contemplate the view. It's a shame that there's no seating at either level, as this is a bridge which is clearly as much about the creation of generous public space as it is about getting from A to B. It's a space which invites alternative uses, and has been used on several occasions for art events.

For me, the most interesting thing about the Passerelle Simone-de-Beauvoir is the relationship between its lightness and its massiveness. It's a structural design intended to maximise efficiency over a long span - the words "slender", "lightness" and "ribbon-like" are used in the architect's book describing the structure. I certainly admire the bridge's generosity of space, but it's this which for me also makes the bridge appear massive and heavy, something that I think is simply unavoidable for such a wide structure. However efficient the design, there's an accumulation of surface and metalwork which creates an enormous, unavoidably massive presence within the river surroundings.

Further information:

22 January 2017

French Bridges: 18. Pont de Bercy, Paris

Ok, this is the penultimate bridge I'll cover in this series of posts on the bridges of Paris. There are many more fabulous bridges in this city, but I'm sticking to those where I have some reasonable photos.

The Pont de Bercy crosses the River Seine to the south of Paris city centre, near the gargantuan Ministry of Finance building and the horrible multi-sports stadium.

As with so many Paris bridges, there is a bit of history.

The first bridge here was a toll bridge which replaced a ferry in 1832. This was a three-span chain suspension bridge designed by Bayard de la Vingtrie and Fortune de Vergès, who have also been credited with the 1828 suspension bridge at the site which is now the Pont d'Arcole (although I've also seen that bridge credited to the better known Marc Seguin). Several images of the bridge are available online.

In 1864, the suspension bridge was replaced by a stone bridge, designed by Edmond Jules Féline-Romany, which is at the heart of the structure seen today. This bridge was widened in 1904, and an arcade built on top to support a Metro line. The bridge in these various stages of development can again be seen online, and my photo on the left shows a view which has not changed since the 1904 widening. The arcade gave the bridge something of the appearance of the Pont du Gard.

Curiously, the bridge has 5 shallow masonry arches, however, sources from the date of its construction refer only to 3 arches. I haven't found any reference to the bridge being lengthened at any stage in its history.

In 1986 a decision was taken to widen the bridge again, this time symmetrically so that multiple lanes of road traffic could pass on both sides of the Metro arcade, not just to one side. A design competition was held, and the winning design essentially replicated the original facade on the outside of a series of new reinforced concrete arches. The stone facade is an exact match for the opposite side of the bridge, but is in new masonry. The widening was completed in 1992.

If you look closely underneath the bridge arches, you can see the concrete section, the widening for the Metro arcade, and the original stone arch barrel. Above pier level, there is a gap between the reinforced concrete and the original masonry, ensuring that the new structure is structurally independent.

The arcade is an unusual feature, and ensures this is an exceptionally well provided bridge for different forms of transport - there are two wide roadways, footways at the edges, the metro line, and a cycle path running beneath the central arcade.

At the end of the bridge, the Metro line is carried on a metal truss structure over adjacent roadways. As can be seen in the final two photos below, this leads to an interesting contrast: the main arcade consists of parallel stone arches supporting a metal bridge deck, whereas the end sections consist of parallel metal trusses supporting brick jack arch deck construction. The relationship of metal and stone is reversed.

This is a fascinating bridge, which I passed by quite quickly but which I expect could repay considerably closer attention.

Further information:

19 January 2017

French Bridges: 17. Viaduc des Arts, Paris

Okay, three more Paris bridges to go and then I'll stop.

For this one, it's time to move away from the river to find something very different.

The Viaduc des Arts is an old railway viaduct which has been repurposed into a high-level, landscaped walkway. Just like New York's High Line, except that the Viaduc des Arts was completed in 1994, while the High Line first opened in 2009. The Paris greenway was its inspiration.

The 1.5km long viaduct was originally built as part of the Paris-Bastille to Marles-en-Brie railway line in 1859, and was closed to rail traffic in 1969. The refurbished structure now forms part of a longer route normally called the Promenade plantée, which connects the Paris city centre beyond the Boulevard Périphérique ring road to the Bois de Vincennes park.

The viaduct refurbishment was planned by architect Patrick Berger, and quite literally works on two levels. On top of the viaduct there is a walkway with extensive planting, a very pleasant promenade affording view down into and across adjacent streets. The walkway opens out where existing flat spans have been refurbished to create more open spaces with views in all directions. Up here, you are away from the noise of the city streets below. Stairs and lifts provide access at intervals.

Below, the brick arches have been turned into 64 glass-fronted tenanted spaces, primarily occupied by art galleries and craft studios (hence the new name of the viaduct). The structure has also been extensively cleaned. The occupation of the spaces is nothing new: they were occupied when trains still ran here, but what is new is the provision of a consistent theme and identity.

I think this is the key to the viaduct's successful new life. The walkway is pleasant, but the tenanted spaces presumably generate cash to maintain the viaduct as well as contributing more generally to the local economy and community. The Semaest organisation ensures the viaduct and its occupants are managed and promoted, and I think this level of positive, purposeful activity is key.

I can think of plenty of places where old railway arches are occupied by successful businesses but which lack this sense of community, visual identity, and promotional effort. There's a fine article in the Boston Globe (link below) which goes into more detail on this, and the lessons that other cities might learn.

Further information: