29 November 2016

London Bridges: 43. Limehouse Basin Footbridge

While visiting the Rotherhithe Tunnel footbridge, I spotted this span through a gap between some buildings, and decided to take a closer look.

I don't know who designed or built this bridge, or when, and indeed that's probably for the best.

The bridge spans the canal right next to Limehouse Basin, just below the Commercial Road Locks. It provides a key link in the footway around the canal basin (now part of the Jubilee Greenway), spanning Regent's Canal.

It's a steel cable-stayed bridge, with a portal frame tower supporting the deck via an asymmetric fan-type cable layout. There are three pairs of main span cables, and one pair of back-stay cables. The bridge deck consists of steel edge girders supporting curved steel parapets and a timber plank deck.

At first sight, it's a slightly clunky but unobjectionable bridge, noticeable mainly for being awkward in height relative to the adjacent railway viaduct. There are two landings on the span which would have been better eliminated as they put ungainly kinks into the otherwise smoothly curving alignment of the main bridge steelwork.

Approaching more closely, two other features stood out for me.

The first is that the support arrangements are odd, with the deck bolted directly to steel supports at both the pylon and at the "tip" of the cable-stayed deck. With an asymmetric cable-stayed bridge, the normal arrangement is to fix the span at the position of the back-stays, and have an expansion joint and sliding bearings at the tip, so that the deck is loaded only in compression, and thermal expansion and contraction is catered for.

The second thing of note is that not only are all the support cables pretty slack, one has a pronounced kink in it, showing that it has failed inside a turnbuckle connection. Slack cables are not unusual on poorly designed short-span cable-stayed footbridges: it's often difficult to get sufficient weight in the bridge deck to ensure that all the cables remain tight. But it's unusual to find that none of the cables are tight, as this indicates that the entire deck is gaining no support from the cables.

On closer inspection, I spotted a half-joint in the main deck girders towards the tip end of the deck. This explained the fixed support at this end of the bridge, as any movement can take place in the half-joint instead. However, I was completely unprepared to then find a second half-joint on the other end of the span, closer to the mast, between the first and second pairs of stay cables.

This is a truly bizarre design. If the bridge deck has a genuine half-joint at both ends, then it presumably cannot transmit axial loads in either direction. However, a cable stayed bridge cannot function without axial loads in the deck, as these are necessary to balance the horizontal component of forces in the inclined cables.

The conclusion is therefore simple: none of the cables are in fact carrying any load from the deck, nor are they designed to do so. This accounts for their slackness. All four pairs of cables, and their supporting mast, are structurally redundant, just an affectation.

One positive thing about the slack cables is that this allows them to be threaded through the parapet railings in order to be connected directly to the deck stringers. Truly, this has to be one of the oddest arrangements I've seen on a bridge like this.

This is not the first time I've encountered "fake" cable-stay bridges (see, for example, this bridge in Salford). However, it is probably the most gratuitously stupid example that I've seen. There is, I think, sometimes a place for deceit in structural design – sometimes it may be necessary to give the visual impression of something rather than the reality because otherwise a structure may simply look wrong.

Here however, there is already a fine Victorian railway viaduct that can be admired – there was never any need for a short-span footbridge to try and compete for attention in this deeply ridiculous manner.

Further information:

27 November 2016

London Bridges: 42. Rotherhithe Tunnel Footbridge

This bridge has been in place since long before I started writing this blog, so it's perhaps a surprise that I've managed to cover 41 other bridges in London before finally getting round to this one.

This footbridge spans across the northern approach road to Rotherhithe Tunnel in east London, providing a direct route from St James Gardens at its north end to various streets and housing at its south end. The south end spills onto a piazza behind the horrible post-modern ventilation building which sits over the west portal of another tunnel, the Limehouse Link, completed in 1993.

The bridge connection is useful but not vital. There’s an alternative route a short distance to the east, but that involves crossing the Rotherhith Tunnel roadway at grade, so the bridge is a welcome alternative.

The footbridge was built for £550,000 in 1998, to a design by Nicholas Lacey and Partners with Whitby Bird and Engineers and WS Atkins. It was reportedly nicknamed the "butterfly bridge" at the time. Obviously something was in the air as a similar twin inclined arch bridge also nicknamed the "butterfly bridge" was built in Bedford the year before, with a different design team.

Whitby Bird and Lacey also collaborated on the St Saviour's Dock swing bridge around the same time, and there are definitely some similarities.

The Rotherhithe span comprises two tubular steel arches from which a steel bridge deck is suspended by stainless steel cables. Each arch rib has twin sets of cables, arranged to form a triangular arrangement when viewed along the axis of the bridge. This has two benefits: by doubling the number of cables, each cable can be smaller in size, which may be considered by some to be visually preferable; and in addition, the angle of the cables ensures that the arch rib is stable against out-of-plane buckling, allowing the arch to be more slender. Nonetheless, it's not an arrangement normally seen on bridges of this type. For a relatively small span such as this, it's unlikely that the arch will be greatly reduced in size, and the effect of having more cables can be visually confusing, counteracting any benefit from their slenderness.

The bridge deck itself consists of stiff frames spanning transversely between the points of cable attachment and carrying the main loads from the bridge deck. These frames project laterally beyond the area of the deck, because of the triangulated cable arrangement. The deck is then stiffened longitudinally by means of a cable truss, additional cables running below the deck connecting the lateral frames. The frames themselves incorporate steel tie bars as their lower element, a pointlessly expensive indulgence.

It's difficult not to conclude that the longitudinal cable below the deck provides little or no structural benefit, and that the profusion of wires and metal is more confusing than elegant. This proliferation of metalwork is the point of similarity with the St Saviour's Dock footbridge, and it seems regrettable now.

Other than the structure, the most notable feature of the bridge is the chainmail barrier on each side, which is there to stop objects being blown, knocked or dropped onto traffic below. Considering their size, these are reasonably unobtrusive. My photos make them look more transparent than they feel when seen in real life, at least when standing on the bridge.

Generally, the bridge seems to be in good condition, although the paving slabs are coming loose and one section of handrail has gone missing.

Overall, this is an interesting, but flawed, little bridge. I have the feeling it is not very well known at all, and would be interested to see how many readers were already familiar with it.


Further information:

22 November 2016

"La Résistance de Sites" by Alain Spielmann

Monographs published to document the work of specialist bridge architects are few and far between, so I was very interested to pick this one up, which covers the oeuvre of French architect Alain Spielmann. I've featured his work here once before, when I visited the Passerelle du Paillon in Nice.

"La Résistance des Sites" (Presses de Ponts, 208pp, 2013) [amazon.co.uk] opens with an essay (in English and French) in which the architect attempts to document his philosophy of "the resistance of sites". This is a difficult philosophy to summarise, but its essence is pragmatic and rooted in a clear design methodology, most of it related to gaining a thorough understanding of any specific site. For Spielmann, it's clear that views and perspectives are central, both on and off a bridge, so that design is predominantly about how a structure fits within a given visual context, and changes that context.

The bulk of the book depicts and describes 40 of Spielmann's bridges. dividing them into categories such as "blade bridges" (beams), "bow bridges" (arches), "landscape bridges" etc. The text for all these is in French only. His work has been wide ranging, with little preference for material or structural form. The book covers a period from 1984 to 2013, and I think much of Spielmann's work is rooted in the pre-Millennial phase of contemporary bridge architecture. This was a time when, for the most part, architects working on bridge design remained a little diffident in their approach, respectful of the engineer's traditional role as prime conceiver rather than challenging it. Architects often seemed to circulate around the engineering, searching for aspects to refine and enhance, or concepts to insert into the engineering whole which while bold didn't challenge the essentials of construction.

Some of Spielmann's bridges are excellent, such as the Viaduc de la Grande Ravine, the first bridge in the book. With its very shallow strut legs, this 288m long bridge spans heroically across a 170m deep gorge, and as well as being visually striking, it's highly unusual, a strange combination of beam and arch. I also particularly admire the Passerelle des Poètes, a suspension footbridge which is not entirely beautiful but has a certain je ne sais quoi to it, a slightly agricultural charm.

I also admire his several timber bridges, especially the sweet Passerelle de Preuilly in Auxerre. Timber remains so much under-exploited in contemporary bridge design that it's great to see these robustly attractive examples. However, not all of Spielmann's timber designs are a success, with the Pont de Chavanon let down by a central pier which looks very much like an afterthought.

Certain features recur throughout Speilmann's work. One is a taste for a carefully designed stringcourse, using a variety of added fascia elements to ensure the edge of the bridge is precise and consistent. A second is a liking for belvédères, with several bridges adapted either to provide specific viewing balconies, or otherwise centred around the needs of pedestrians as flâneurs. The Pont Pierre Brousse, for example, incorporates a lengthy shelter which turns the footway into an arcade, opening onto a balcony at midspan.

While almost all of Spielmann's bridge designs have a degree of interest, many are, in my view, aesthetic failures. In this group are structures such as the Viaduc de la Bidouze, with its ungainly brick pedestals clasping the bridge pylons, which themselves are compromised by unpleasant projections where the stay cables pass through, and the Passerelle du Pas-du-Lac, a cable-stayed footbridge propped up on one side by an enormous, lumpen steel strut.

Some designs pass well beyond simple impairment into a place from which I can only recoil in horror, including the Passerelle du port de Nanterre, a mess of completely incompatible structural elements, all crushed together so as to be fascinating solely in the manner of a car crash. The Pont sur l'Isère also reaches incredible heights of awfulness, with its portentous gathering together of cables at the tower top, like an angry concrete fisherman.

So I must be clear that this is not a book of magnificent designs, to inspire younger designers and to frequently raise from the coffee table in search of new insights. There certainly are some admirable designs here, but across the board I was left with quite mixed feelings. Many of the bridges are compromises at best, and it's not possible for me to tell where the fault lies - like many a bridge architect of an earlier era, Spielmann may well have been making the best of a bad job when confronted by engineers with their awkward ideas.

The final chapter of this book departs from the rest, showing Spielmann's work as a colourist, picking and choosing the paint schemes for refurbishments of existing bridges. These are mostly both interesting and attractive, and record an aspect of the architect's role that is often forgotten in the chase for expressive new structural form. Looking back through the rest of the book, I note that several of Spielmann's own designs show a sensitive and interesting use of colour.

So, it's still a book that I would recommend, perhaps to be taken with a critical eye and a questioning mind. Which designs work well, and which do not, and most importantly, why? It certainly encouraged me to think again about designs of my own which display elements of unfortunate compromise, and to consider how best to create the conditions for better work to arise in the future.

15 November 2016

"Brunel's Timber Bridges and Viaducts" by Brian Lewis

Here's a book that's perhaps only for the obsessives: Brunel's Timber Bridges and Viaducts, by Brian Lewis (Ian Allan Publishing, 2007, 144p, amazon.co.uk).

Isambard Kingdom Brunel is best known for his major works: Clifton Suspension Bridge, Royal Albert Bridge, SS Great Britain, and the Great Western Railway (GWR). Much of his other work is known only to those with a particular interest in engineering history.

I was certainly familiar with his timber viaducts for the South Devon Railway and Cornwall Railway, offshoots of the GWR: tall stone piers from which sprung outstretched fans of timber, an inverted forerunner of what is now the cable-stayed bridge. These were remarkable structures, built in timber to reduce initial cost and to allow railways to open to traffic more quickly, so that investments could be recouped. However, they were not long-lasting, generally being converted into metal girder viaducts, masonry arch viaducts, or demolished entirely.

However, timber was used much more widely than this on Brunel's railway lines, and this painstakingly thorough book attempts to describe every significant timber span on those projects. Today, not one of these bridges now survives.

The book opens with a lengthy discussion of how Brunel's timber bridges can be classified, well illustrated by Mike Jolly (who deserves a cover credit, given how much he contributes throughout the book). I found this thoroughly confusing, even on repeated reading, and as it forms key material for much of the book, I found I had to keep flipping back to this section to make sense of what follows. This is largely down to Lewis's classification system, which distinguishes between, for example, CLB2(E) structures and QTT1 bridges, rather than just fan-supported continuous laminated beams and queen post through trusses. Notwithstanding the confusion, it's a vital chapter, as most of the bridges used by Brunel and his assistants do fall into several key groups and then variations on each theme.

A second chapter discusses design, construction and maintenance. It's informative enough but sometimes lacking in depth, through no fault of the author. He tells us of William Bell's research into timber beam behaviour on Brunel's behalf, but there's not much to tell as none of the research actually survived. The discussion ranges from the use of iron castings and wrought iron straps to connect timber members, through alternative parapet details, load testing and wood preservation.

The majority of the chapters document different railway lines on which Brunel employed timber bridges, both as railway underbridges and overbridges: twelve main railway projects and several smaller or subsidiary schemes. The sheer volume in which timber construction was applied is startling, dozens if not hundreds of bridges of very different lengths and spans.

In addition to Mike Jolly's excellent diagrams, there are plenty of historic photographs, pictures, and, best of all, extracts from original construction drawings. These are, by far, my favourite thing here, many of them works of arts compared to the drawings used in modern construction, and it's a shame they haven't been reproduced at larger scale.

For me, the diagrams and drawings are the reason to buy this book, to dive into a plethora of timber detailing, "endless forms most beautiful and most wonderful" to borrow a quote from Charles Darwin. That may be apposite, as Lewis's book documents the generation and evolution of numerous related but divergent design solutions, each developed as a pragmatic response to the contingencies of construction and structural performance. Several of the bridges documented evolved even during their own lifetime, with additional strengthening members added in response to degradation.

Beyond the pictures, the text is in places so thorough it's like being repeatedly bludgeoned: for me, there's only so much railway history I can cope with in this level of detail, particularly when it mostly consists of "and then the broad gauge was replaced by narrow gauge" again, and again, and again. However, other readers may feel differently.

Many of the structures depicted are prosaic but interesting, but many are quite spectacular, especially the tall timber viaducts of the Devon, Cornwall and Vale of Neath lines. It's a tremendous shame that none still survive, even if quite understandable (there are still timber viaducts surviving elsewhere on the railways, of course).

I think, on the whole, that Brunel's Timber Bridges and Viaducts will be too fastidiously detailed for most readers. It's sufficiently packed with detail to be a book to nibble rather than to try and digest whole. For my part, I found I eventually achieved a second wind, particularly in exploring the copious illustrations included, so I'm glad to add it to my bookshelf.

13 November 2016

Bridges news

I'm keeping it short and to the point: a few items of bridges news I spotted recently ....

London bridge lighting contest shortlist unveiled
This is being promoted to make London more of a "24-hour city", as if we didn't have enough light pollution already.

Mathematical bridge model kit
Do you know anyone who'd like one of these for Christmas?

Vision of swivelling bridge is unveiled for Renfrew Riverside project
File under "what the hell were they thinking about?!"

Richard Meier’s Cittadella Bridge inaugurated in Alessandria
Italian bridge 20 years in the making.

St Saviour’s Dock footbridge to be replaced
21 year old bridge is officially knackered.

New £7 million footbridge in Telford gets council backing
30 year old bridge also knackered, to be replaced with covered bridge design

Santiago Calatrava wanted pretty lights for the Peace Bridge but they couldn’t stand the cold
$700,000 bill to replace 300 light fittings.

New South Side pedestrian bridge spanning Lake Shore Drive a striking design
"Sometimes, a bridge is more than just a way to get from Point A to Point B."

New Dedicated Tram Stop Pedestrian Bridge for QMC Approved
Check out the visualisations, which show the proposed bridge floating above its support pillars rather than connected to them with anything suitable!

06 November 2016

1961 film of Hammersmith Flyover construction

Three years ago I visited London's Hammersmith Flyover, one of the most innovative bridges to be built in Britain in the last century. It was one of the world's first precast, segmental, prestressed concrete viaducts, constructed using a complex kit of different concrete elements.

One of my readers recently alerted me to a 1961 film documenting the bridge's construction, produced by the Cement & Concrete Association and available on the BFI website.

This is an excellent film, well worth watching, for many reasons. It remains a hugely informative documentary, full of technical detail on all phases of the bridge's construction. There's plenty to learn not only about how bridges were built over 50 years ago, but about how they are still built today.

One further attraction is the somewhat Cholmondley-Warner style of narration, but probably the most interesting aspect is the difference in approach to worker and public safety from 1961 to today. You could watch this film while marking off a bingo scorecard of what would now be considered quite unacceptable safety practices. In some sections, you'd struggle to keep up, there are so many.

I definitely recommend taking a look, it's an excellent film of considerable historical value.

05 November 2016

Swedish Bridges: 9. Norrbro, Stockholm

This is the last bridge I visited in Stockholm, and I think it's also the oldest. Designed by Erik Palmstedt and Carl Fredrik Adelcrantz, it was completed in 1797, replacing a previous timber bridge which had been extensively damaged by flooding in 1780. The part of the bridge that I saw comprises three 16m span stone arches, although the structure continues south on a series of vaults on the island, and a further arch span over another waterway.

Klas Lundkvist's report Norrbro och Strömparterren, available online, has a great deal of information on the bridge and its construction. Lundkvist suggests the design was inspired by French architecture, particularly Perronet's 1774 Pont de Neuilly.

A 1781 drawing shows that the masonry piers are supported on timber piles. A later drawing shows the piers constructed on the pilings by layering stone within an open-topped timber caisson, the base of which was left in place to form a timber layer above the piles. This method was reportedly also used for the Pont de Neuilly. Below the bridge, it can be seen that there are actually two parallel sets of arches, and holes are visible which would have formed the support positions for temporary timber centering during construction.

The bridge is in very good condition, with the ornamental balustrades in particular looking like they have been recently renewed.

I think the arches and piers are too squat for the bridge to be considered truly elegant, but it certainly gives the impression of robustness, and I'd expect that having lasted for 219 years so far, it could quite easily last the same again.

Further information:

03 November 2016

Swedish Bridges: 8. Riksbron, Stockholm

Heading east along the Norrström, the next bridge is the Riksbron (roughly, the "State bridge").

This links Norrmalm to Helgeandsholmen, a small island which is home to the Swedish parliament building.

A bridge was proposed at this location in 1712, but nothing was built until 1907, when a steel truss structure was erected. This was replaced in 1931 by the bridge which stands there today, designed by engineers Axel Björkman and K A Wickert  with architect Ragnar Östberg (who was also involved in the Stadhusbron).

The bridge is a 44m long reinforced concrete arch, with a hinge at the crown. It's a simple, elegant design, helped by the shallowness of the arch and the inclusion of only limited detail to break up the flatness of the concrete. The metal parapets are attractive and lightweight, and it's a significantly better looking bridge than the next one I'll be sharing here.

Further information:

01 November 2016

Swedish Bridges: 7. Vasabron, Stockholm

I've got three more Stockholm bridges to feature, all of them spanning the Norrström waterway and connecting Stockholm's Old City, Gamla stan to Norrmalm.

Vasabron is named after King Gustav Vasa.

There was an unsuccessful attempt to build a bridge here by British-Swedish engineer Samuel Owen in 1843. A design competition was held in 1868, with the present bridge being built from 1875 to 1878 by AB Atlas, to the design of Emil Edvard von Rothstein.

The bridge has seven metal arches with a maximum span of 32m. A plaque on the bridge states that the bridge is cast iron, which is clearly incorrect as the bridge can be seen to be riveted from below. Wikipedia states that the design contest was for a cast iron bridge, and that the bridge is actually built of cast steel. Given the date, this sounds pretty unlikely, and there's nothing about its appearance to support this. The ornate parapets and spandrel infills are presumably cast iron, but the main structure and associated bracing give the appearance of wrought iron.

It's a simple and elegant bridge, although I can't help feeling that the ornate elevations would benefit from being painted in a more interesting combination of colours.

Further information: