Footbridge 2011: papers roundup pt 1

Okay, I thought I would attempt a quick round-up of some of the papers and presentations from Footbridge 2011 which particularly caught my eye. Apologies to anyone whose presentation I saw and found interesting, but have missed out, I only have so much time so will have to be quite selective!

In addition to the three keynotes previously mentioned, I enjoyed Henryk Zobel's Contemporary structural solutions of timber pedestrian bridges, which offered a nice survey of the range of timber designs currently in use. One that struck me as especially attractive was the treetop walkwway in Tharandt, Germany, pictured right (click any image for a larger version). This 118m long curved, stress-laminated timber structure was explained further in its own paper, although I missed the presentation.

There were relatively few other presentations to suggest any acceleration of the use of timber footbridges, although the Margaretengürtel design in Vienna was presented (see my previous post for details), and another presentation addressed some of the reasons why timber bridges remain unpopular in the UK. Client inertia appeared to be the deciding factor, although I know from my own experience as a designer that client concerns over vandalism, fire damage and general durability are often hard to refute. Having said that, at one time, the UK was claimed as home to the longest timber arch footbridge in Europe (the Middlewood Way Bridge near Macclesfield, a 50m span built in 1992), although that claim has certainly now been overtaken by bridges over the River Lora, near Florence (72m span) and in Rimini, Italy (92m span).

One aspect of the conference which I thought was quite unfortunate was the separation of most of the footbridge dynamics papers into a separate strand, which meant that those of us interested in more general design case studies or topics probably saw very little about dynamics at all. The ghettoisation of the dynamics specialists did mean that the entirely non-technical could avoid the subject, but runs the risk of re-opening a gulf between academia and practising engineers which had been temporarily narrowed in the post-Millennium Bridge years.

Nonetheless, some of the dynamics presentations which I did see were quite staggeringly esoteric, and it was tempting to wonder to what extent they were researching areas which were likely to have real practical relevance.

One dynamics paper which clearly was highly relevant, was Aleksandar Pavic's keynote Vertical crowd dynamic action on footbridges: Review of design guidelines and their application. Pavic's paper reviews five current published methods of dealing with this problem (see table, left), provides worked example calculations for a simple structure (itself very useful, given the ambiguities present in some of these documents), and observes that they can lead to very different results, with serious implications both for economy of design and the potential for error.

Two of the guidelines, HIVOSS and SETRA, are available online. It's notable that when Eurocodes 0 and 1 were published, a methodology for dynamic analysis of footbridges was conspicuous by its absence, including the load models which had been expected to appear (and which are given in the 2005 fib document Guidelines for the design of footbridges). Some of this is because the state-of-the-art in this area has been a constantly moving target, but I think it's a shame that Eurocratic deadlines for publication of the Eurocodes were allowed to take precedence over the presentation of standards which would assist rather than confuse designers. For anyone who doesn't follow the footbridge dynamics literature, it must be a real struggle working out how to proceed.

I saw two presentations where weathering steel was treated in very different ways. Martin Knight and Simon Fryer presented Combining engineering and aesthetics: The Town Centre Link, London, which must be one of the largest scale footbridges shown, at some 135m long and 12m wide. It's enormous weathering steel Vierendeel trusses (pictured, right) are detailed to avoid water traps, and are protected against the risk of graffiti by their height above the rail station platforms below, and by the use of full-height glazing on their inner face.

In contrast, Xavier Font discussed the lovely Can Gili Footbridge (pictured left), which I have covered on this blog before. Here, the pedestrians are kept away from direct contact with the rusty weathering steel by an internal guardrail. However, the potential for graffiti clearly exists, and I understand graffiti is now present. The absorbent surface patina on weathering steel makes graffiti hard to clean off, and only a fresh blast cleaning can reliably remove it (in the UK, this is covered by Highways Agency standard BD 7/01). In addition to the cost, I guess that will affect the uniformity of colour of the weathering steel patina, which may be undesirable. I greatly admire the appearance of weathering steel bridges, but as with the timber designs already mentioned, clients may have to adopt a more flexible attitude to the issues of durability and vandalism.

I was also impressed by a couple of weathering steel footbridges presented by Mario Guisasola, but would like to prepare a full post on them some day, as they were easily amongst the best designs presented at the conference.

Okay, that's all I have time for now, I will say more on the conference presentations later.