27 May 2011

Helsinki Kruunusillat bridge design competition announced

An international competition has been announced to design new pedestrian, cyclist and tram bridges in Helsinki, connecting a new residential area to the city centre. The bridges, termed Kruunusillat ("crown bridges") fall within a designated heritage area, and are being partly promoted under the auspices of Helsinki's role as World Design Capital for 2012.

The budget for construction is €86m, and these are not small structures, but crossing major waterways. The longest bridge will be over 1km long, rising to 20m above water level.

Prequalification submissions are requested from teams of bridge engineers, architects and supporting specialists by 3rd August. Full details of the programme, jury and how to apply are available on the competition website. More information can be found at the Official Journal of the European Union.

Between 5 and 10 design teams will be shortlisted, and each one is to be paid €50,000 for their efforts (plus expenses towards producing a scale model, and some limited travel expenses). If the contest moves into a second phase, a further €25,000 becomes payable, and there is the prospect of a design contract for the winner. This is pretty generous, certainly when compared to UK standards, although the competition submission requirements are very detailed, including production of numerous drawings, photo visualisations, reports and animations as well as the scale model.

I expect they will attract some very high-powered entrants, although the nature of the site is that designs will tend towards the elegant rather than the spectacular.

26 May 2011

Landsberg footbridge competition winner

Knippers Helbig have kindly sent me details of their recent competition-winning design for a footbridge in Landsberg, Germany, won jointly with Birk und Heilmeyer Architects. I can't offer any information on the context of the competition, but the bridge spans a small river, and is a very unusual sculpted timber structure.

It builds on some of the ideas in their Margaretengürtel design for Vienna, which I discussed here last March, although with a much simpler geometry. The Vienna bridge was formed of a series of horizontal timber layers, glued together like wind-eroded stone strata. In Landsberg, the timber is turned vertical but the contour-like sculpting is the same, with a varying depth timber spine reflecting the bending moment distribution, and a raised bank of timber above the main pier, where the bridge kinks in plan. This provides enhanced structural stiffness but also establishes a seating and viewing area.

The timber layers will be glue-laminated together, and presumably also stressed together with tie bolts, as was the case both on the Margaretengürtel proposal and on their even more sculptural entry to the I-70 Wildlife Crossing competition (again, covered here previously).

It's an attractive design, and it would be nice to see one of these actually get built!

Click on any image for a full-size version. 

25 May 2011

Winner declared in North Sheen footbridge contest

This contest was run by Richmond Conservatives to try and find a better design than an off-the-shelf Network Rail structure. It has been won by a local Richmond resident, Stephen Speak, who beat seven other shortlisted designs. I discussed the contest in a previous post, so follow that link for more details on the bridge, which provides access across a railway at times when the adjacent level crossing is closed. Click on the image below for a full-size version of the winning entry.

It would be interesting to know what else was entered - this was a contest aimed at engaging local amateurs, rather than professional bridge designers. Reportedly other designs included a curved bridge deck, solar powered lights, and laser-cut metal panels.

The winning design essentially adopts the geometry of the previous solution, probably unavoidable given the highly constrained site, and strips out Network Rail's traditional flat steel plates in favour of something more transparent. Essentially, it's based directly on the treetop walkway at Kew, with a lightweight steel truss structure supporting timber handrails and with mesh infill panels.

This offers a number of advantages over the standard solution, chiefly in removing all the space for vandals to spray graffiti, but also in the improved appearance and surveillance that the more transparent approach facilitates. The major disadvantage is that doesn't comply with Network Rail's parapet standards, which require all pedestrian parapets to be solid panels, without any holes at all, let alone a mesh. It would be nice to think that Network Rail would be willing to depart from their normal practice, but they are notoriously rule-bound and averse to setting precedents in this way.

The design simply dispenses the overhead cage which featured in the original solution, which was probably the most unpleasant thing about it. Privacy for an overlooked garden is provided by a series of louvred slats, allowing light and air through but eliminating direct views, and again this is clearly an advance over the flat plates which served the same purpose on the previous design.

The design won't win any awards on visual grounds, but seems to have focussed on the key issues of security, vandalism, and practicality which must be of greatest concern to the body actually paying for the bridge, Network Rail. Assuming the designer is a non-engineer, I think he's done pretty well.

24 May 2011

The space of all possible bridge shapes: Part 3

In the last two posts, I introduced Stephen Wolfram's idea that it may be possible to use modern computational algorithms to develop entirely new structural forms for bridge, and discussed how difficult it might be to work out whether any of them are actually better than what we design already.

A more coherent idea of the problem can be seen in the example truss forms that Wolfram generated to illustrate his article.

Would any of these offer any improvement on a more conventional truss design? Perhaps there is a greater robustness, but it is unlikely the benefit-to-cost ratio is anywhere close to what can be achieved by simply using a conventional truss form with stronger individual members.

It's a classic case where a specialist with little or no familiarity with another field (in this case, bridge engineering) thinks they can bring some special insight which others are blind to. It's something seen frequently in evolutionary biology, where criticism of neo-Darwinian theory generally comes from biochemists or (sadly) engineers with an essentially shallow understanding of the topic.

However, I wouldn't dismiss Wolfram entirely. There's little doubt that bridge engineers are highly constrained by habit - their own design experiences, and the traditional forms which have calcified into the standard processes of bridge fabricators. Often, new bridge concepts are dead-ended by the inability of steel and precast suppliers to invest in new equipment and technology. Even where new technology is brought in, as with the robotic welding more commonly seen in Japan than in North America or Europe, it is applied only to a very specific problem (e.g. welding of orthotropically stiffened steel plate), rather than to more radically expanding the range of what can be built economically.

As ever, footbridges offer an area where designers can experiment with smaller economic consequence, and are often encouraged to do so by promoters' ambitions. I'm quite confident there are ideas out there in academia, of which Wolfram's is just one, which are underused (or never used) by designers even in this most adventurous of bridge-building fields.

One field which is just about making it into the "real world" is that of topology optimisation. This has been used to show that the theoretically ideal catenary cable is not a single cable but a multi-stranded Hencky net,
and there are various other examples online relating directly to bridge engineering. One paper documents its application to the design of the Knokke Footbridge (pictured, right), which has been featured here previously. This shows the use of computer processing to progressively optimise steel plate thickness (or determine where it can be omitted), something that could have wider applications not just in optimising existing forms but also in generating new ones.

Was Wolfram right that we will see entirely new bridge forms which surprise us in their novelty and apparent randomness? I doubt it, but I do think there's plenty of scope to take use some of the methods discussed here to take a fresh look at bridge designs now and in the future.

23 May 2011

The space of all possible bridge shapes: Part 2

In the previous post I discussed Stephen Wolfram's proposition that there exists a space of all possible bridge designs, and that if we could use modern computer techniques to generate this using a set of simple rules, we could find new and unpredictable bridge forms within that space which may improve on traditional ideas.

A key challenge is how to find the better designs, a process which involves testing each option against whichever criteria are required. This can be computationally intensive, but in the age of cloud computing, becomes a little more feasible. Sensible engineers might reasonably object that to analyse a non-trivial array of structural models would defeat even the greatest computing resources currently available, and I would sympathise.

I wonder, however, whether this isn't primarily a flaw of traditional analytical technologies such as the finite-element stiffness matrix method.

I recall a project from some years ago (VISABO) which used Newtonian mechanics in a manner more closely related to Wolfram's cellular automata, exploiting "intelligent" structural elements each of which contained their own rules of physics, global behaviour emerging naturally from their relationships. This has the potential to allow the change of structural response resulting from a change in structural form to be analysed much more quickly: individual members react dynamically when another member is moved, added, or eliminated.

Another, perhaps more accessible example, is the series of Bridge Builder games (pictured above right), which appear to use the same principle (they certainly don't use finite element analysis!)

Closer to the professional arena, there is Daniel Piker's Kangaroo (pictured left), an add-on for Grasshopper / Rhino which carries out a similar physics-based simulation, and is being explicitly promoted for structural modelling purposes e.g. form-finding of catenary structures.

Nonetheless, I think that non-trivial analysis may still remain computationally too expensive, particularly for structures governed by continuum rather than discrete element behaviour (such as beams and frames), or where non-linear, dynamic or global buckling behaviour determine performance.

Analysis of the individual designs is only half the problem: it's also necessary to test them against pre-defined criteria to decide which are optimal (or, at least, superior to neighbouring designs). Researchers like Wolfram seem to believe that "economy" is readily measurable e.g. by least material. However, real-life economy in bridges is intimately linked to simplicity of construction, and structures which are regular and repetitious are generally cheaper to manufacture and assemble than those which are highly variable. A classic example is the simple rolled steel beam, which contains considerable quantities of material resisting very low stress, yet is almost always cheaper to supply than a latticework or variable section plate girder beam where the stresses have been made more uniform.

For trusses of the sort that Wolfram takes as his example, it is likely that his process will find an optimum 2-dimensional truss with irregular bay sizes or truss angles, reflecting the variation of shear; and with curved top and bottom chords, reflecting the variation of bending moment. But in 3-dimensions, truss members do more than carry shear and bending, they also resist out-of-plane buckling, and regular bays can make the deck design more economic. Curved chord members can similarly increase fabrication costs to a greater degree than the more uniform stress saves material. How then can economy be easily assessed?

If economy is difficult, what of robustness? How can that be readily measured in a manner which is quickly repeatable across a large array of possible designs?

To be continued ...

22 May 2011

The space of all possible bridge shapes: Part 1

I guess this is an old one now, dating back to 2007, but I hadn't seen it before.

Shortly after the collapse of the I-35W Mississippi River Bridge in August 2007 (pictured right, courtesy of pmarkham), Stephen Wolfram published a blog post titled "The space of all possible bridge shapes", wondering whether new developments in science could have anything to offer to bridge designers and hence help prevent future disasters. In order to come up with designs which maximise robustness while minimising economy, Wolfram speculates that designers will need to find entirely new structural forms, which may look nothing like those that have emerged from engineering history.

Wolfram is the developer of the popular Mathematica software, and a researcher into computational systems such as cellular automata. The best known of such systems is perhaps John Conway's Game of Life (pictured left, courtesy of kieff at Wikipedia), which demonstrates in a very graphic way how a wide spectrum of behaviour both random and structured can emerge from applying simple rules to the on/off state of image pixels. Genetic algorithms can be used to mutate the results, compare them against various tests, and "evolve" the system over many generations in search of some desired optimum. I have seen some experimental use of genetic algorithm techniques by architects in building design, but in structural engineering it seems to be largely confined to the academics (one of many examples here).

Wolfram's main interest is in the ability of very simple systems to be processed and combined by simple rules to create highly complex outcomes. The range of possible outcomes forms a kind of computational landscape, which can be investigated to determine whether there are useful results other than those that might have initially been predicted. Some of this is explored in Wolfram's book, A New Kind of Science.

Wolfram notes that before the 19th century, there were only a limited number of bridge forms in use (the beam, the arch etc), but with the advent of the railway age, a Cambrian explosion in bridge shapes occurred, all variations on the metal truss. As in the evolution of organisms, a certain feature had to arise before a wide array of new forms could build upon the opportunities it presented (the evolution of evolvability). This image of truss variations is taken from Wolfram's blog post:

Most of the famous truss types (Warren, Pratt, Howe, Fink etc) arose through a process not dissimilar to natural selection: inventors of truss forms were competing in terms of strength, ease of construction, and economy, and simple economics meant that only the fittest survived. It would be interesting to trace the history of the metal truss bridge through some kind of developmental tree, complete with extinctions, hybridisation etc.

As an aside, the generation of truss forms using simple rules was the subject of an interesting paper by Yoshiaki Kubota at IABSE's Venice symposium, which I discussed here before. It forms a subset of the wider systematisation of bridge types, as illustrated in one of Kubota's diagrams below:
Wolfram's proposition is that a wide range of otherwise unpredictable variations in form can be readily generated by combinations of simple rules e.g. add a brace, subtract a brace, subdivide a bay, shorten, lengthen, rotate. It is therefore straightforward to generate a multi-dimensional "design space" containing a myriad of options which a rational designer would never consider. The question is then whether any better designs exist within the space of possible bridge shapes, and Wolfram's experience in other areas makes him believe quite strongly that they would. His other work also suggests they may look like nothing we have seen before, possible quite "random" in appearance.

This post is getting quite long, so I'll continue this tomorrow.

19 May 2011

Bridges news roundup

UNESCO science committee approves construction of Haliç metro bridge
New bridge won't affect Istanbul's World Heritage Site status

Another bridge crossed in bid to mark US steel links
Will Sheffield get its replica of Brooklyn Bridge?

Burley Bridge Association carries on its century-old fight
They've been trying to get stepping stones replaced with a bridge for 113 years - see http://www.burleybridge.com/ for more.

Tony Meadows' 10-year wait ends as Borough Market viaduct installed
Only 10 years? You should try waiting 113 years, mate.

IQ Winnersh footbridge up for award
Ramboll's tree-lined structure is an unusual design, to say the least.

Fort York bike bridge project in limbo
The original headline to this story read something like "Fort York bike bridge dead", to which somebody presumably responded "it's not dead, it's just resting". Having already called a halt to this iconic bridge scheme, Toronto Council were challenged by one local councillor to reconsider, but have decided to stand their ground. Council officials are still being asked to identify a cheaper alternative. The desire to move it ahead was partly motivated by the fact that the railway it spans will be closed for other work in 2012, and any delay will miss this opportunity, potentially increasing construction costs considerably. Apparently, they were relying on the rail closure to allow the curved deck and arch to be temporarily propped. The structural form was never well suited to building across a railway to begin with, and is unlikely to survive the rethink.

16 May 2011


I haven't done a roundup of what you could read on other blogs (if you weren't too busy reading this one) for some time, so here goes ...

As always, my favourite of the bridge blogs is Tabikappa, who seems to post a seemingly inexhaustible supply of minor suspension bridges in his native Japan. My recent favourite was a marvel of bridge-building on the cheap, and I also very much liked this example of a bridge where you sit in a basket and pull yourself across (the only UK example I can think of, in Glen Etive, is called a "bucket bridge").

Bridge Photo of the Day is currently documenting an extensive trip around the spans of Brisbane, Sydney and elsewhere in Australia, including the bizarre Kurilpa Bridge and, of course, the Sydney Harbour Bridge.

I'm always slightly amazed that someone is letting the author of Always Civil post close-up photos of bridges he has recently inspected, complete with shoddy defects, but it's great that they are, as the blog is a rare public representation of how bridge engineering is for most civil engineers: the day-to-day maintenance issues that pay the bills and are a world away from the fancy footbridges niche market.

I've linked several times recently to Frame and Form, a Spanish blog (with English translation) about bridge and structural design. They've recently been providing links and images for the structures shortlisted in the Footbridge 2011 Awards, but often cover other interesting topics, like tensairity bridges or the work of Thomas Heatherwick.

Tallbridgeguy has a wide-ranging interest in bridge design, and writes probably the most personal and personable blog on the subject. Recent posts are a mix of observation on the architectural and engineering worlds, thoughts (and videos) on using the Sketchup software for bridge visualisation, and a few entirely original bridge concepts (daft yet endearing).

The last blog I'll mention this time is the Bridgehunter's Chronicles, part of the movement in the US to preserve historic bridges (often metal trusses), many of which have limited legal protection and are threatened with neglect or reconstruction. The Chronicles often include lengthy, detailed reports on bridge preservation news, as well as from the author's own bridge visits (currently covering the Schleswig-Holstein area in Germany).

12 May 2011

"The World of Footbridges: From the Utilitarian to the Spectacular"

I've recently received a copy of "The World of Footbridges" by Klaus Idelberger (Ernst and Sohn, ISBN 978-3-433-02943-5, 192 pp, 2011) [amazon.co.uk] (also available in a cheaper German edition).

The book presents a collection of modern pedestrian and cycle bridges, all of steel construction, sourced from Europe and Asia, although by far the largest number are in Germany.

The book is aimed very much at practicing bridge design engineers. Each bridge is given a one or two-page spread, with CAD or simplified dimensional drawings, photographs, and detailed descriptions of the structure. A typical page is shown below (click on it for a larger version).

These pages include detailed facts regarding structural members, protective treatment, cable materials etc - the sort of thing which makes the book of considerably less interest to non-engineering designers such as architects, or indeed to non-specialists. The text is clear and well translated throughout, and not without a sense of humour, as when the author notes how the decking on a suspension footbridge near Lavertezzo, Switzerland, was sized to suit the private owner's dog. The author is also not afraid to criticise when appropriate, drawing attention to occasional poor details such as climbable balustrades.

The photographs are mostly in colour, although they are sometimes a little too small to properly appreciate the more beautiful structures. However, this is not a coffee table tome devoted only to the spectacular and aesthetically sublime. Many of the bridges are functional rather than beautiful, and some are frankly ugly, often the result of an attempt to do something offbeat or unusual. This isn't a bad thing, indeed it's great to see a very different range of bridges from what is more commonly portrayed.

"The World of Footbridges" is organised by structural type: suspension bridges; cable and bar-stayed bridges; girder bridges; arch bridges; and enclosed skywalks. This throws some interesting bridges against each other, particularly in the final section, which is perhaps the one I found most interesting - enclosed bridges are always difficult to design well.

The book features a number of bridges which should be well known: SBP's Inner Harbour Bridge at Duisburg, and their Gahlensche Strasse bridge at Bochum; the Miho Museum Bridge; the Dreilanderbrucke; and the Corporation Street footbridge.

In one or two cases, the books strays beyond the factual. Although the author personally visited most of the bridges, he clearly didn't go to the Royal Victoria Dock footbridge, which he states has a suspended transporter pod (actually never installed), nor the South Quay Footbridge, which he describes as having two masts (one was removed some time ago). These errors didn't detract much from the rest of the book for me, but it may be worth treating some of the other "facts" with a pinch of salt.

"The World of Footbridges" introduced me to several remarkable structures that I hadn't previously encountered, such as the Regnitzsteg in Bamberg, with its intriguing cable net system (pictured); Stefan Polónyi's Doppelbogenbrücke in Gelsenkirchen (pictured on the book cover - despite appearances, the two arches are parallel and equal in span and height); and a number of excellent, ultra-economic suspension spans in Switzerland (see one example).

Even amongst the more straightforward structures, there are several examples of good detailing to be seen. The bridge may also help draw attention to practices that are simply local customs rather than based on real merit. For example, many of the bridges featured are galvanised but not painted, which is not what would normally be done in the UK, where normally bridges are painted without being galvanised, except for parapets, which are both. Whether examples from elsewhere can encourage procurement authorities to make exceptions to their traditional requirements must remain doubtful, however.

A similar point that I noted is a number of European footbridges over railways which take what, in the UK, would be regarded as an unacceptable approach to the parapet design. In Britain, tall (1.5m or 1.8m) parapets which are solid without gaps (normally in steel plate), are invariably required, supposedly to reduce vandalism (although what is to stop people lobbing bricks over a tall parapet is anyone's guess). The solidity is supposedly there to stop people trailing cables down onto overhead wires or onto the track. "The World of Footbridges" has several bridges with perforated or mesh parapet above a railway, and again, it's depressing to be reminded of the UK's rulebound, jobsworth tendency.

I wouldn't say this is a book which is going to be a huge source of inspiration to designers, in the way that some of its coffee-table competitors may be. It's more of a reference source for bridge forms and details which are interesting rather than aspirational. I certainly enjoyed seeing designs I'd not otherwise be exposed to, and stored away several ideas to consider in my own work in the future.

11 May 2011

Footbridge Awards 2011 - technical up to 30m

Okay, I've previously provided some commentary on the 2011 Footbridge Awards shortlists for short, medium and long-span bridges under the "aesthetics" heading. Now it's time for those that fall under the "technical" heading, although there is some overlap.

Frame and Form have posted images of the short-span technical shortlist, and as before, please just visit their blog if you would like to see the pictures!

Three bridges on the technical shortlist are also on the aesthetics shortlist: Castleford Footbridge, the Glass Bridge in Lisbon, and the Buitengracht bridge in Cape Town. I don't have anything to add to what I said previously on the Castleford or Lisbon structures.

In the case of Buitengracht, it appears F&F may have pictured the wrong bridge. I think the right one is the one now posted at Future Cape Town. There are also several images from the contractors, Vusela Construction, and a technical description at Skyscrapercity.

The challenge with this footbridge was to minimise the length of the approach ramps, while allowing access to the bridge from the side. Reducing the ramps means minimising the construction depth, which is the depth between the footway surface and the underside of the bridge. This is normally achieved by raising the bridge girders above the deck (in the "half-through" form), or suspending the deck from above (with a bowstring arch or cable-stayed layout).

At Buitengracht, the designers opted for a "quarter-through" design, raising a box girder above one edge of the deck, and relying on its torsional stiffness for overall stability. That allowed exits from the opposite edge of the bridge deck. It partly explains why the parapets are very different, with a post-and-rail arrangement on one side, and a glazed screen above the edge girder, which also acts as a wind-break.

There are two other bridges on the technical shortlist.

The Marinic bridge in Slovenia, would appear to be a footbridge in the spectacular Škocjan Caves, possibly the one known as the Cerkvenik Bridge. This spans an amazing underground abyss. I can't find much information online, but would imagine the challenge was to design a bridge which could be built in a remarkably inaccessible and difficult location!

The Stalhille Footbridge is another design by Ney & Partners, their fourth on the awards shortlists. This is an opening bridge over a canal in Flanders, with an opening system which may be unique.

Generally, there are six basic systems available for an opening bridge:
  • rotation about bridge's longitudinal axis (tilt, or "blinking eye")
  • rotation about bridge's transverse axis (bascule)
  • rotation about bridge's vertical axis (swing)
  • translation along bridge's longitudinal axis (retracting)
  • translation along bridge's transverse axis (possibly used on some floating bridges?)
  • translation along bridge's vertical axis (lifting or submersible)
There are some feasible combinations of these, particularly for retracting bridges which move at an angle to the transverse and longitudinal axes, and for "driven" bridges such as passenger boarding bridges in airports. There are also a very small number of oddities which involve transformation of the bridge's geometry, as with Schlaich Bergermann's Duisburg and Kiel bridges, or Thomas Heatherwick's Rolling Bridge. And of course, there are transporter bridges which solve a related problem, but aren't really opening bridges as such, perhaps having more in common with the cable ferry.

Ney's Stalhille design is a variation on the second and sixth types, whereby there are two pivot points lying above the bridge deck, allowing the entire deck to remain level while it is swung up and away from the canal. Structurae puts it in a class of its own, a pendulum bridge.

Spanning a mere 26m, it's far from clear why such a system would be chosen in preference to the more conventional options. Ney note that the bridge deck's static system (a simply supported beam) is preserved in all situations, but the same is true of a lifting bridge. I imagine that coordinating the rotating mechanisms so that the bridge remains level requires a carefully designed control system. I also wonder quite how the joints at either end of the deck are detailed, to avoid one being "clipped" as the bridge deck falls into place.

I do particularly like the bridge's filigree parapets, which are a lovely combination of ancient and modern.

Updated 20 June 2011:
The bridge in Slovenia is actually this one, which remains an impressive piece of engineering at a difficult location - I'd recommend following the link to see the photos!

10 May 2011

Fort York Pedestrian and Cycle Bridge, Toronto

This scheme has been under development since the late 1990s. It's a project to build a footbridge in Toronto linking two areas severed by a huge set of railway tracks, and connecting the historic Fort York site.

The design was for a bridge following an S-shaped curve in plan, with the deck supported on alternating inner edges by inclined arches. The alignment follows that of a historic watercourse, the Garrison Creek. The bridge deck is 234m long and 5m wide. Preliminary designs were developed by Stantec with Montgomery Sisam Architects (see PDF for more images).

The scheme had a budget of CAN$18m (£11.4m), which I would have thought was sufficient for a bridge of this type and scale, working out at nearly £10k per square metre (compare the somewhat similar Peace Bridge in Londonderry, with a reported budget of £8.7m).

However, tenders have come in at just over CAN$22m, and Toronto's council has decided it can't proceed. This is hardly the first time a local council has struggled with cost escalation on a landmark footbridge scheme, I've featured several similar cases here in the past. The council has sent this one back to their staff, asking them to come up with a cheaper scheme.

"No one ever dreamed that we would be looking at something that looked like a Golden Gate Bridge," said one councillor. Really? The bridge design has been around for some time, with the design team drawing clear comparisons to the York Millennium Bridge and Gateshead Millennium Bridge in their presentations [PDF, see page 64]. So, no Golden Gate, but surely nobody ever thought they were looking at a bargain-basement design either.

Looking at the design images, there are clearly options which will save money (including going back to simpler alignments, as previously considered and discarded). It would be possible to keep one of the two inclined-arch spans over the wider railway corridor while reverting to shorter spans over the smaller railway lines and the area of land in between.

Presumably, the cost of erecting the inclined arch over the railway is a major contributor to the over-budget tender prices, as it's a form of structure completely unsuited to this sort of space. It's not suitable for launching, nor is there obvious space for the temporary supports required while a 100m+ long arch is assembled piecemeal. From that perspective, you might think it was an odd choice to begin with, one which ignored rather than worked with the engineering constraints.

The local press are lining up to support or attack the council's decision, while a Facebook / Twitter group has been set up seeking to protest the cancellation via petition.

09 May 2011

Footbridge Awards 2011 - aesthetics above 75m span

Once again, those kinds folks at Frame and Form have posted pictures of the shortlisted entries to the Footbridge 2011 Awards, this time for the "long span / aesthetics" category. As on my two previous commentaries, I won't repeat the images here, just visit Frame and Form to see them.

I haven't covered any of the bridges on this shortlist previously, which is a shame, as several are very impressive.

The Center Street Bridge, in Iowa, is a variant on the now classic typology whereby a curved or straight bridge deck is hung from an inclined arch. In Iowa, the bridge has a vertical arch, and twin curved decks hung either side which to some extent counterbalance each other. The decks are in the form of closed steel box girders to provide the necessary torsional stiffness, given that they are supported by cables on only one edge.

The twin decks allow foot and cycle traffic to be separated, although there is also a short transverse "strut" deck between the two main spans which provides space to stop and admire the view. From the photos I've seen, it looks to be an attractive, well-detailed bridge, which has a touch of the iconic without appearing inappropriately brash.

Ney and Partners have done well to receive a number of shortlistings, and their College Bridge in Kortrijk, Belgium, is a very interesting design. It's an S-curved suspension bridge, 203m long, with a main span of 86m. The main cable follows the funicular line of forces, and supports the deck using a "warren truss" type arrangement of hangers, which provides greater stiffness than the normal arrangement of parallel hangers. The tightness of the S-bends is sufficient to allow the two masts to be inclined towards the deck. The masts are held against movement by tie-down cables connected via the deck to ground, which must contribute enormously to the overall stiffness of the system.

The deck is stiffened further by lateral edge trusses, although I'm not sure I see how these work, as I would have though the deck itself was wide enough to provide sufficient lateral stiffness. Vibration dampers have been avoided, which is a respectable achievement for a lightweight cable-supported bridge like this. In some photos the bridge looks inappropriately long and massive, but in others it looks just right.

The bridge at Esch-sur-Alzette is also by Ney, but is a very different beast. It's remarkable in many ways, including its bold response to a very challenging site, where it must span a railway, avoid overhead service cables, and carry people across a considerable difference in levels (achieved via a lift and stair tower at one end). Unlike a number of structures, the lift tower is completely integrated into the structural form, which lies somewhere between stressed-skin construction and truss design. The truss frame can be seen either as a membrane structure with stiffening ribs, or as a simple truss where the gusset plates have been allowed to grow unhindered. Overall, it takes the form of one half of a three-pinned portal frame.

The frame is painted grey on the outside, red on the inside, and is remarkable in its appearance, particularly the illuminated interior at night. Ney & Partners are easily amongst the most consistently interesting European bridge designers, and it's great to see them getting recognition on this shortlist.

One firm who have also long held such acclaim is Schlaich Bergermann & Partners, who have two bridges on this particular shortlist. The first is the Passerelle La Defense, in Paris, which was designed in collaboration with Feichtinger Architectes. It's an 88m long bridge in the almost-popular "inverted Fink truss" form (which really needs a new name, since it simply doesn't match the topology of an actual Fink truss, inverted). What makes it remarkable is that it curves round the outside of an existing building, seemingly defying gravity. Most of the truss masts appear to hang mysteriously in mid-air, balancing on a set of horizontal cables which are actually there to restrain twisting in the deck, not to hold it up.

SBP have designed some excellent bridges over the years, but I'd say this is probably one of their best efforts.

Their other shortlisted design is the Grimburg Harbour footbridge at Gelsenkirchen. This owes more of a debt to classic SBP designs of previous decades, being another hi-tech reinterpretation of ring-girder and suspension bridge forms. It's not at all a bad bridge, but it seems a lesser achievement when set next to the Paris design.

From a technical standpoint, the cable layout leaves me slightly queasy. The curved deck is supported on its outer edge by a suspension bridge type arrangement, with parallel hangers extending to a suspension cable. The main cable isn't supported directly on towers, as would be normal, but from two subsidiary cable which are held up by a guyed mast. What makes me queasy is simply the thought of how you might maintain the structure should any of its main cables ever require replacement. The arrangements required to maintain temporary stability would be awkward, at the very least.

07 May 2011

Worcestershire Bridges: 8. Upton-upon-Severn Marina Footbridge

This is the last of the current trio of bridges from Worcestershire, and it's the most modest of the set.

When a new marina was built at Upton-upon-Severn, it resulted in the lengthy of an existing footpath running alongside the River Severn. The inlet entrance to the marina passed directly across the line of the footpath. Following protests, the footpath was reinstated, with the aid of a short timber footbridge across the marina entrance.

It's essentially a simple structure, although with peculiar echoes of the main highway bridge in the same town.

It's a three-span bridge with an arched profile, as is the case with the Upton-upon-Severn bridge. It has two timber beams along each edge, which behave as continuous girders rather than via any arching action. Indeed, there appear to be half-joints in the timber beams, which should be visible in the photos if you look closely. For a bridge of this short a span, these are essentially unnecessary.

The main beams support simple plank decking, and the timber balustrade is simply bolted to the outside face of the main beams. The bridge deck sits on two timber trestle piers.

Is it pretty? Not especially. Ingenious? No, I wouldn't say so.

Timber bridges remain rare throughout the UK, largely due to a lack of familiarity with their design amongst engineers reared on steel and concrete, and indeed, I haven't been able to identify who designed or built this bridge. It would be nice to see more of them, albeit without the unnecessarily deep beams used here.

Further information:

04 May 2011

Worcestershire Bridges: 7. Upton-upon-Severn Bridge

The bridge over the River Severn at Upton-upon-Severn is the most recent of several structures at this site. It's the only crossing of the river for some distance upstream and downstream, and although Upton is now a somewhat quiet town, it must once have been an important centre for trade.

It isn't known when a bridge first replaced a ferry at Upton, but a wooden bridge was present when John Leland visited in 1539. By 1576, work had started on a stone bridge, but this was not fully completed until 1609. One span of the bridge was destroyed during the English Civil War, but later repaired. In 1852, the entire structure was washed away by floods.

Two years later, a new bridge was completed. This had four wrought iron spans, the furthest west of which could be retracted onto its abutment to allow taller vessels to pass. The opening procedure was not quick, and in 1882 the retractable span was replaced by a swing span, pivoting on the abutment.

Once into the motor age, it was apparent for some time that the swing bridge was unsuitable for the new loads it had to bear. In 1935, tenders were invited for a new, electrically-operated swing bridge, but the prices received were unacceptably high. In 1940, the present bridge was designed by Worcestershire County Council (county surveyor and bridgemaster B.C. Hammond), approximately 100m upstream of its predecessor. The contractor was Thomas Vale & Sons Ltd, with the steelwork fabricated by Horseley Bridge & Thomas Piggott Ltd.

The abutments from the old bridge remain, and the line of the former bridge is clearly visible in the Google and Bing aerial photographs linked below. There is an associated viaduct to the east over the Severn flood plain, which was built in 1940 of reinforced concrete, but recently rebuilt.

The new river bridge has three spans, with a main span of 200 feet (61 metres), keeping the piers out of the river and reducing impediment to flow. It was built at a higher level than the original bridge, a vital decision given the Severn's propensity to flood (see the images at SABRE, linked below). It was reported to be a larger version of the 1935 Jubilee Bridge built at Fladbury, but the latter bridge has girders only below deck level.

Upton-upon-Severn Bridge is unusual amongst highway bridges in having girders which extend both above and below the deck. The "half-through girder" design is common on rail bridges, where it minimises the depth from the rails to the underside of the bridge, in turn minimising approach gradients and disruption to existing highways. At Upton, this solution was presumably chosen to minimise the level of the roadway, reducing the cost and extent of the approaches.

Although unconventional, it's clearly not unattractive, and the bridge gives the impression in elevation of being more slender than it really is. The shallow arched profile disguises what is a three-span semi-continuous girder - continuous over the piers, with cantilever arms supporting a suspended middle span via half-joints.

The footways are supported on stiffened steel plates, while the road deck, with its heavier loads to carry, relies on a steel troughing deck.

The main girders have very large vertical stiffeners at frequent intervals, particularly visible on their traffic face. Sizeable stiffeners are required in a half-through bridge to provide "u-frame" action, preventing the top flange from buckling sideways in the parts of the bridge where it is in compression. However, with the half-jointed bridge construction, there will be very little significant compression in the top flange, so it's less clear what purpose these stiffeners serve.

The girders have a hat-shaped top, which is non-structural and presumably there to discourage people from walking along the top of the girders.

A plaque on the bridge incorrectly attributes the building of the bridge to Worcestershire County Council, and also states that the bridge was "one of the last of riveted construction to be built in England". In fact, riveting only died out some two decades later, still being used on bridges such as the Barton High Level Bridge (1960), Runcorn Widnes Bridge (1961) and Thelwall Viaduct (1963).

Further information:

03 May 2011

Worcestershire Bridges: 6. Lowesmoor Railway Bridge No 10

I did a short series of posts on bridges encountered in Worcestershire last year. I have three stragglers, further bridges in the area which may be of interest, of which this is the first.

A canal was opened between Birmingham and Worcester in 1815, shortening the route for freight barges between the manufacturing centre of Birmingham and the River Severn. Within a few decades, railways were taking over as the principal means of long-distance transport, and a new railway from Worcester to Hereford was built in about 1860. Its line crosses the Worcester and Birmingham canal at Lowesmoor, in Worcester.

The railway had to cross both the canal and the narrow roadway of Westbury Street, two very different spans. It's not clearly recorded who designed the railway bridge, but Pevsner's guide attributes it to Charles Liddell, who was the Engineer with overall responsibility for the railway line.

Liddell was clearly familiar with metal bridges, having previously accepted Thomas Kennard's design for the trussed Crumlin Viaduct on the Newport, Abergavenny and Hereford line. However, a brick bridge was chosen at Lowesmoor.

I'm guessing the unusual arch form resulted because a single span over both canal and roadway would have required a fairly shallow arch rise to avoid encroaching into the roadway headroom. This would have led to greater arch thrusts and more expensive foundations. The twin span arrangement which was built is charmingly eccentric, entirely because of the presence of a large oculus above the small roadway arch.

The Pevsner guide already linked suggests that the round hole was to reduce the weight of the bridge, as was famously done on the Pontypridd Bridge, although the saving at Lowesmoor must have been small. A greater saving in weight could have been made by extending the roadway arch up to the full height of the bridge. However, the thrust from the canal span arch at a higher level would have unbalanced the central pier, requiring a strut at the pier's mid-height. The circular hole may have resulted from a combination of these considerations - saving weight while ensuring the arch forces had load paths in the right places.

The result is not a spectacular bridge, or even one which is especially ingenious. However, I've never seen another bridge quite like it, and it definitely has charm.

Further information:

02 May 2011

Selfridges Footbridge, Birmingham

The now-defunct Future Systems were a firm with a very distinctive architectural style, a cross between sixties day-glo and 21st century blobitecture. They were never noted for their bridges, indeed other than the subject of this post, the structure at West India Quay may have been their only other completed span.

The bridge at the Selfridges store in Birmingham is little more than a punctuation mark bookending the utterly dotty enterprise that is the building itself. It connects Selfridges to a multi-storey car park across the road, and so is effectively one of the shop's main entrances, but it still seems very much like an afterthought.

The Selfridges building, which has a fascinating interior with a lovely atrium, is somewhat intransigent on the outside, with its silver-speckled mass offering little idea of its layout or purpose, with no windows and a minimum of other openings. How do you use a bridge to create a gateway into such a place?

The pedestrian bridge which was built studiously avoids that question, proferring an appendage that seems barely integrated with the building, and which, once crossed, presents a blank wall behind which the department store's 3rd level is hidden.

Its 37m span comprises a steel box girder, fabricated from a series of warped flat plates and segments of bent steel tubes, and supported roughly in the middle from a Y-shaped arrangement of cable stays. It is curved in plan, giving the visitor an expectation that the initially obscured view will gradually open up, only to dash it with that unhelpful blank wall.

Structurally, it is neither innovative nor hugely interesting, although clearly it must have been a complete bastard to assemble.

Steel arches spring from the box girder at varying angles, carrying a polycarbonate canopy, which is dirty but not yet discoloured (a fate which appears to await most similar materials). The canopy is combined with a stainless steel mesh parapet system, which must have been a brave choice. This kind of mesh is generally chosen for its extreme immateriality, and this high above the roadway it would have been easy for it to appear simply too insubstantial for comfort. In combinatio with the see-through canopy, however, it works well.

The bridge was designed by Future Systems and Arup, and built by O'Rourke.

Further information: