Lessons from history - the steel box girder story

Every year since 1989, the IStructE Study Group on the History of Structural Engineering has arranged an annual lecture, now titled the James Sutherland History Lecture in honour of the study group's founder.

Past lectures have been given by such notable figures as Ted Ruddock, Michel Virlogeux, Roland Paxton and Frank Newby, and many of the lectures have been published in the IStructE's magazine.

This year's lecture, in February, was by Ian Firth and covered the "steel box girder story".

All but the youngest bridge engineers will (or should) be familiar with the series of box girder bridge failures at the beginning of the 1970s, at Milford Haven, West Gate, Koblenz, etc. The subsequent investigations (by the Merrison Commitee and others) led to both a much greater understanding of the behaviour of plated steel structures, as well as new rules (such as the Interim Design and Workmanship Rules, see a very relevant paper by Bridle and Sims [PDF]) covering such issues as the need for checking of complex structures, involvement of designers in construction, and proper analysis of temporary conditions during construction.

My reason for posting is simply that the lecture is now available as an online webinar (with both video and slides), and although I've not had time to watch it all yet, what I have watched is excellent. This very important slice of bridge engineering history is covered in commendably thorough detail, but perhaps of more significance is the way it suggests that current procurement practices are forgetting the lessons learned from the 1970s (for example, on some schemes I have seen construction supervisors appointed who have had no involvement in the design, or understanding of it).

Trestles Beach footbridge competition finalists

Architecture for Humanity has been running a contest to design a safe footbridge spanning the railway at Trestles Beach in California. At this popular surf spot, there's not even so much as a level crossing at present, so the need for a bridge seems fairly obvious.

Out of 104 entries, they've recently announced the five finalists. As ever, I'm amazed at the outpouring of creative energy that can be engendered by contests like this, where there's no funding available to actually build a bridge, let alone to pay a designer, and there's not even a clear client who might actually promote the scheme (however, the finalists do each get a US$5,000 prize).

From that perspective, it's somehow irrelevant who actually wins, and clearly many entrants just see the contest as a good opportunity to get their name in the press, such as 24° Studio, whose wooden wave bridge can be found at inhabitat.com, or Dan Brill, whose weathering steel design graces Dezeen, Building Design, e-architect, trendhunter and others, mainly as a result of being the only UK semi-finalist. It might seem churlish to point out the folly of a weathering steel structure located within a few metres of the sea and its salty spray, but I imagine it would be as easy to pick holes in other entries, given that many entrants have no engineer on their team nor does there seem to be one on the jury. This is a bridge design competition in the vein of a fashion show, as is still so often the case.

The full 104 entries can be found on the Open Architecture website, but here are the five finalists. Links go to more detailed information on each entry. Click any image for a larger version.

kola / kle

CO-LAB design office

ERGO4

Murphy Burnham & Buttrick / Hage Engineering / Fieldwork

Ken Smith Landscape Architect Workshop West

Bridges news roundup

Spit threatens Indian bridge
Howrah Bridge under threat from corrosive power of saliva

Developments in FRP bridge design
Plastics industry remains keen to push its products into a new market (registration required to see full article)

Medvedev orders review of wobbly bridge
Did Volgograd bridge engineers never see that Tacoma Narrows film when they were students? Footage of the Russian bridge can be found on Youtube.

Octopus footbridge a fengshui buster?
I don't really know what the fuss is about, but it's a great headline.

Titanium bridge contest winner

The University of Akron's Blue Team has been announced as the winner of the US Defense Metal Technology Center's contest to design a titanium footbridge. This is proposed to be a real bridge in Akron, Ohio (as yet unfunded), although the main aim of the contest is to try and provoke interest in the structural use of titanium, ultimately to "demonstrate the versatility of titanium and make the metal more affordable in lifesaving military armament".

The winning design is perhaps the least elegant of the five finalists, although it is at least practical, unlike the runner-up, which demonstrates almost everything which is wrong about the present infatuation with digitally-developed geometry.

Winner
University of Akron Blue Team

Runner-up
Ball State Dept of Architecture and Institute for Digital Fabrication

First honorable mention
University of Akron Gold Team

Second honorable mention
Kent State

Third honorable mention
Miami University

Bridges news roundup

Floating footbridge that can sail down the river unveiled
Bristol proposes to draw on West India Quay footbridge as its inspiration. It's not clear what this means for the competing scheme to build a footbridge at the same site using the old disused Brunel span from Cumberland Basin.

Pyramids and Taj Mahal to have a new arch rival – the Forth Bridge
Forth Rail Bridge to be put forward as World Heritage site (again). Note that despite the article's claim, it is clearly not the world's first major steel bridge, as residents of St Louis would undoubtedly attest!

Titanium bridge design competition for University of Akron narrowed to five finalists
Designs chosen from amongst the weird and wonderful. Well, mainly just weird.

Calgary's Peace bridge contract skirted policies
Audit report finds that the sole-sourcing of the Peace Bridge to Calatrava was symptomatic of a far wider lack of open tendering, and a failure to seek best value for the local council. Rick Bell offers a more pungent editorial at the Calgary Sun. See also previous post here.

Referendum to refurbish or replace Victoria’s Johnson Street Bridge likely for November
Mammoth exercise underway to rethink the decision to go with a Wilkinson Eyre designed replacement bridge (see previous post).

Scottish Bridges: 7. Myreside Road Footbridge

While I was up in Scotland in April, I spotted by chance another footbridge worth featuring here, although certainly it's not in the same league as the Forthside Footbridge - it cost only £169,000 for a start.

Myreside Road is a fairly quite street in Edinburgh, splitting two sets of playing fields belonging to George Watson's College. The footbridge was built across it in 2000 to provide a safer crossing for schoolchildren, although I wonder whether a pedestrian crossing may have been better value.

The bridge was designed by Patience and Highmore architects, with Peter Stephen & Partners as the engineer. The contractor was Edwards Engineering.

What's interesting about the bridge is simply its unusual configuration, with its tall arch founded at ground level closely followed in profile by the staircases. The arch looks like it has a circular or nearly circular profile, which is not efficient for uniform load let alone for six point loads as applied here, but the lack of efficiency is not a major issue for this small span and loading.

On most steel arch footbridges, the bridge deck is much more level, with staircases or ramps kept well outside the arch, and in many cases the arch is sprung from the same level as the bridge deck. Myreside Road is clearly a constricted site, and every effort has clearly been made to avoid encroaching onto the playing fields.

Ramps have been omitted, presumably for reasons of space and cost. I believe that the Disability Discrimination Act of 1995 exempted schools from its provisions.

The use of non-vertical parapet posts on the staircase is also an unusual detail, and I think partly a visually successful one. The ribbed parapet texture has the potential to look good, but seen from closer at hand, it's simply too industrial, like some kind of security barrier. This last photo also shows the bolted arch splices, which are very awkward.

It's not a great bridge, but hopefully was of enough interest to feature here.

Further information:

• Google maps
• Patience and Highmore

Scottish Bridges: 6. Forthside Footbridge

A recent trip to Scotland offered the chance to visit the Forthside Footbridge in Stirling, a landmark design by Gifford and Wilkinson Eyre (built by Nuttall), which was opened in May 2009. I discussed the design extensively at the time, so was pleased to be able to pay the bridge a visit in person.

The £6.5m bridge essentially consists of two 113m long "inverted Fink trusses", side by side. It's not strictly an inverted Fink truss - such a structure would have large diagonal compression struts holding it up - but never mind, that's just a term popularised by the similar Royal Victoria Dock Bridge. Another way to think of it is a series of cantilevering cable-stay spans, each cantilever acting as the root of a subsequent cantilever.

Each of the two trusses decreases in height from one end to the other, and each in the opposite direction, in a kind of "handshake" motif. At the same time, the vertical struts gradually tilt outwards from one end to the other, giving a cradling effect which is consciously designed to reflect the oblique pedestrian desire line.

That's the theory, and certainly it gave the elevations and plans of the bridge a certain geometrical elegance, which can also be seen in the view along the railway line, above right. Most views of the bridge however, are more confusing, if undoubtedly spectacular. Looking along the bridge, the rationale behind the inclined masts is hardly evident, and viewed from further away, the effects of perspective become positively unkind. The picture on the right is the view from Stirling Castle, showing the masts appearing to lean inwards rather than outwards (the same is true in the next photo below).

There's further potential for awkwardness in the bridge's end supports, one of which is pictured on the left. Due to the required clearance above the railway tracks, and the lack of space for ramps, a lift and staircase is provided at each end of the bridge. I'm not sure what I think about choosing to design the stairs as massive stone-clad structures, rather than a metal frame system that might be more obviously in keeping with the main bridge span.

The staircase is positioned behind the main bridge support, and provides part of the anchorage structure for the bridge's main cables. Seen on its own, rather than as part of the bridge, the staircase looks quite attractive, as on the photo on the right, where the striped shadows created by the stairs give an attractive effect. Open risers, as seen here, are forbidden by the UK footbridge standard BD 29/04 [PDF] (presumably because they can create a trip hazard?), but as is so often the case, ignoring the prescriptions of standards can lead to a better design overall.

From under the bridge, the main supports can also be seen, where the massive tubular masts come to ground via a concrete plinth. These strike me as visually awkward, because they make clear the lack of a simple structural rationale for the arrangement of the bridge's masts. In the absence of any significant lateral loading (as may be imposed, for example, by a bridge with a curve deck), you instinctively expect a bridge to deliver its load to ground vertically, in line with gravity, making the inclined support here look odd. The discrepancy in size of the two columns also looks peculiar, as engineering instinct would predict the vertical one to be the larger.

You can also see from below how the deck has been designed to resist the significant twisting forces which results from the cable-stay arrangement. The main deck members are two 550mm hollow steel sections, rotated 45° into a diamond configuration. These are connected by a grid of cross-members, onto which are fixed steel cover plates turning the whole thing into a very wide, not very deep steel box girder. The "cranked" appearance is, I think, purely for visual effect.

Back above deck, the pylons are consciously very simple in character, although they have odd little caps at their tip which I'm not sure I understand - perhaps they relate to hidden lighting in the masts? While the overall layout of stays and masts is visually confusing from many angles, individual sections of the bridge are simple and comprehensible, as shown on the right. This image, with white steelwork against blue sky ("azure a saltire argent", in heraldic terms), makes me wonder whether the choice of an inverted fink truss was a deliberate attempt to create a series of ascending Scottish saltires (although I doubt it).

The bridge's complex geometry is also apparent in the glazed parapets. These comprise a 1m high panel of semi-opaque glass inclined inwards at 12° to the vertical, topped by a 0.6m panel of clear glass inclined at the same angle as the adjacent mast. That means that the upper part warps as you go along the bridge, and this is intended to subconsciously reinforce the pedestrian desire line, with the bridge opening out in the direction you want to go. It's an interesting idea, but I don't think the effect achieved really justifies the fussiness introduced. In any case, the use of glazing is welcome over a railway line, as anyone who has walked across a conventional Network Rail solid steel parapet bridge can attest.

On the whole, I found Forthside Footbridge to have many aspects to admire, despite the many unnecessary complexities which arise from its geometric formalism. The choice of an inverted Fink truss is appropriate to the span, construction challenge, and the visual context. A taller conventional cable-stayed bridge would probably have been visually inappropriate. It looks very attractive in elevation, even while resembling a stack of cascading toothpicks from other angles. As a designer, I'd certainly have gone for something with a simpler geometry, as the many challenges the bridge resolves are all of its own making. It's undeniably an impressive achievement by the structural engineers, and one deserving of recognition through awards.

Further information:

• Structurae
• Google maps (the satellite view shown at the time of posting predates the bridge construction, but the bridge is visible in Google Street View)
• Gifford
• Wilkinson Eyre
• Forthside Bridge, Stirling, Scotland [PDF] (paper by Gifford / Wilkinson Eyre)
• G Hillan Masonry
• Craig & Buchanan (glazing & handrails)