25 January 2010

Bridges news roundup

Deal would let light-rail trains use I-90 express lanes
The significance for bridge engineers is the proposal to put a railway on a jointed floating bridge - has this been done anywhere else? Design of the rail joints will be challenging, I'd think!

Perryn Road footbridge over the A40, London by Grimshaw
There was an engineer too (Hyder), but let's not expect the Architects Journal to bother with trivia, even on a scheme were they report the key design challenge as being dealing with buried utilities.

Bednarski Origami bridge to move forward as part of Cardiff TV centre scheme
I had wondered what had happened to this bridge.

Softroom’s Bootle bridge hoisted into place
RIBA contest winning footbridge approaches completion.

19 January 2010

Open thread

There's not much happening here at Pontist Towers right now (well, on the blogging front, anyway, there's a positive overload of proper work!).

So for this, my 200th blog post since starting, I thought I'd just open the comments for ... anything you want.

If you're reading this, I take it you're interested in bridges, so what's exercising your mind in the bridges world? Are there topics, designs, news I've been missing out on? Click the comments link to let me (and other readers) know!

15 January 2010

Bridges news roundup

Design fantasies for obsolete Bay Bridge span
What to do with San Francisco's redundant Bay Bridge?

Covered bridge pops up in unlikely place
Unusual garage conversion in New Brunswick, Canada

Could Poole's Twin sails Bridge still be at risk?
Landmark opening bridge's 10-year long gestation not over yet?

Bridging the gap between the ages
How much would it cost to build the 1864 Clifton suspension bridge from scratch today? Construction Manager magazine has more details.

Design for Champlain Bridge announced
It's nice to see a network arch (or "Modified Network Tied Arch Bridge", as Vermont and New York states are calling it) used in this way, although I don't know if I'd agree with their contention [PDF] that it's much cheaper to build or maintain than a cable-stay design.

Hang Christiaan Huygens! Here's a Better Suspension Bridge
Researchers into structural optimisation software discover that a parabola isn't the most efficient shape for a cable to suspend or an arch to support a uniform load. But it's unlikely to change a single bridge, because of the complexity of the "Hencky net" truss required, and because for compression elements, they ignore the effects of buckling.

11 January 2010

I'll huff and I'll puff and I'll blow your bridge up

New Civil Engineer reports that Swiss architects have created an inflatable bridge. Philipp Dohmen and Oscar Zieta have built a 6m long steel-skin balloon that can hold a load of up to 1800kg, according to load test at the Swiss ETH institute.

It's all very exciting, but not really news as such - the ETH's department of Computer Aided Architectural Design (CAAD) has been working on this system for some time, with their so-called FIDU-Brücke having been load-tested in December 2007 (FIDU stands for FreieInnenDruckUmformung, or internal pressure forming). Essentially, it's a prestressed steel shell structure, where the internal pressure prestresses the skin (maintaining it in tension even when subject to applied compressive stress) and hence prevents it from buckling, even in the absence of stiffeners.

Although Zieta's website shows lots of clever applications in product design, including furniture, it's hard to see how any inflatable structure can be robust enough to act as a real bridge, with the danger that any escape of air (e.g. due to failure of a seam or accidental puncture) would immediately eliminate all the structural resistance. The advantages, of course, include the extremely light weight of the structure, which also allows it to be readily deployable in difficult locations.

Nor is ETH-CAAD the first to develop an inflatable bridge.

The US Army has a deployable air-inflated system which can carry military vehicles weighing as much as 80 tons. This however, is really a causeway system rather than a bridge as such: unlike the FIDU-Brücke, it isn't subject to bending.

The University of Maine has also used inflatable plastic arches in bridge construction, although these are filled with concrete after inflation.

More pertinently, the "tensairity" system promoted by Airlight has been proposed for a number of bridges, and applied to some. I wrote about this previously because an Airlight bridge was one of the entries to the Leamouth Bridge Design Competition (pictured left).

The tensairity system uses inflatable plastic beams shaped a little like cigars. These are inflated under relatively low pressure, and the system relies on the use of struts and cables fixed to the inflatable membrane to carry most of the load: the balloon only carries shear and compression between these, and stabilises the whole system against buckling.

An 8m span test bridge was built and shown to be capable of carrying a 3500kg test load [PDF], easily putting the Dohmen / Zieta span in the shade, and with a structure lighter in weight as well. Unlike the metal-skin solution, a further advantage of the Tensairity beams is their translucency, allowing them to be lit internally for architectural effect.

The Airlight website illustrates a number of projects for which tensairity beam bridges have been proposed, but not built. As well as Leamouth, these include footbridges in France and Switzerland, such as the one at Giubiasco shown on the right. They have also been proposed for temporary use to support construction vehicles.

The most significant bridge to be completed is at Val Cenis ski resort in France, a 52m footbridge for skiers (also carrying substantial snow load) - pictured below. This is essentially a timber frame bridge with steel bars providing the suspension cable below - the two elements are separated by the Airlight balloons which hold the main structural pieces in place. It was designed by Charpente Concept, who provide further details of the design and construction on their website.

See also:

08 January 2010

River Wear Bridge goes for planning consent

It's been a while since I've had any news to report on Sunderland's River Wear Bridge, the structurally ambitious competition-winning highway bridge design by Techniker and Spence Associates.

Just before Christmas, the project made its planning consent submission, depositing a mammoth package of drawings and reports that must total at least 400 pages. You can find it yourself by visiting Sunderland's planning portal and searching for the application reference 09/04661/LAP.

The reports include a number of interesting engineering documents as well as the normal planning statements, and I've extracted some of the more interesting points here.

It's clear that lead designer Techniker has devoted considerable effort to proving the feasibility of a design that seems by far their most challenging project. They've contracted the prestressed concrete bridge expert Nigel Hewson to provide engineering support, and their design has been peer reviewed both by Roughan O'Donovan and Aecom.

For the first time, we're given a cost estimate for the bridge itself, separated out from the highway scheme's overall cost of £133m. At £46.1m, it's pretty much in line with the budget of £43m specified in the original RIBA bridge design competition (contrary to my previous speculation). The bridge is 336m long, with a maximum span of 144m, and the deck is 25m wide. The cost therefore works out at about £5.5k per square metre in plan, which I would say is pretty cheap for such an unusual design. I've previously noted that it's comparable with Calatrava's Alamillo design, which, although controversial in its own right, is a considerably more structurally efficient bridge.

The planning submission contains a Statement of Community Involvement, which reports on a survey of 161 people, of whom 93% rate the Techniker design as "fairly good" or "very good". That's nice, but it ignores Sunderland Council's previous and larger survey (see council report [PDF]), where out of 1,641 responses, 49% wanted a "tried and tested design", and 58% wanted to minimise the impact on taxpayers (which this design hardly does).

The masts (one 140m high and the other 190m, making it the tallest bridge tower in the UK) are a very interesting construction. At the very bottom, they're of prestressed concrete with a stainless steel skin, but for most of their height they comprise prestressed concrete composite with a 12mm thick painted structural steel skin. The very top section is pure structural steel. Essentially, the prestress cables could be thought of as the "missing" back stays, hidden within the pylon.

The reports acknowledge the difficulties of fabricating the very complex steel plates involved, which are constantly changing in cross-section, rotating in plan and elevation, and hence much of the steel plate is curved in two directions (see drawing extract on the left, all drawings are taken from the planning submission).

Given the size of the towers, the steel skin will provide very little structural benefit. Indeed, its interaction with the tower prestress and with the creep behaviour of the internal concrete make it a significant complication in the design.

It seems to be there partly to provide an ultra-smooth finish sought by the architect (welds are to be ground flush up to 40m height, which strikes me as rather pointless), and partly to act as permanent formwork for the hollow concrete tower cores, which would otherwise be very awkward to form. The downside of course is the enhanced maintenance cost associated with painted steel. Compare the steel-clad concrete pylons on Stonecutters Bridge: these use a 20mm thick stainless steel skin providing a genuine composite benefit (indeed, the composite system replaced the original steel-only design), but I presume the budget at River Wear ruled out stainless steel.

The pylons sit on a caisson foundation 40m by 24m (or alternatively, on a grid of 78 1.2m diameter piles, they haven't made their mind up yet). Quite a bit of thought seems to have been devoted to carefully positioning the foundation relative to the pylons so that the forces, including massive torsions, are as well balanced as possible.

The deck is a steel and concrete composite ladder-beam system (see image below right), with short outriggers picking up the cables. That's a reasonably economic choice, although it struggles near the pylons, where the hogging bending moment is highest. At these locations, the designer suggests the deck concrete may need to be post-tensioned, which strikes me as an unusual and interesting approach.

The deck is likely to be launched, with the pylons built at the same time and the cables stressed only after both are complete. That seems at first to be very odd choice for a cable-stay bridge, particularly given the expense involved: a number of temporary launching props will need to be installed in the river, and subsequently cut off or pulled out through holes in the deck. The normal approach for a cable-stay bridge is to cantilever the deck in segments from the pylon, as was done on Alamillo (where the pylon and deck were both cantilevered segmentally in phase). The ability to use the stay cables both as the temporary and permanent support for the deck is normally the great economic advantage of this type of bridge.

At River Wear, however, the lack of back stays means that the cantilevers would be unbalanced, and the loads imposed on the pylon and foundation during construction could substantially exceed those in the final condition (where the deck acts as a continuous beam, or propped cantilever). The design is therefore entirely driven by the architectural rather than the structural logic - a cable-stay bridge is a means to an end (the production of an icon), rather than the logical response to the engineering challenge.

Despite the bridge's utter lack of any basic structural rationale, I found myself strangely impressed reading though the documentation. If you accept the architectural vision as sacrosanct (Ove Arup style), it's clear that the designers have risen to the challenge posed by this deeply unconventional, flagrantly contrary structure. I understand that detailed design is now underway, and no doubt the technical challenges which remain are tremendous but perfectly capable of being resolved. Once complete, the bridge will be a major engineering achievement.

I think I can admire that, without in any way liking the bridge.

07 January 2010

Bridges news roundup

Architect plans Elliott Bay Bridge to replace viaduct
Here's a story that leaves me slightly confused. Seattle architect Roger Patten (pictured, right) has proposed a big bridge in place of a tunnel-and-viaduct proposal, and has been arguing for some years against the local state Department of Transportation. Criticising his scheme, the Washington DOT has stated [PDF] that "the bridge has a center span more than a half mile in length, longer than all but one completed suspension bridge in the world." Really? Half a mile is a piddling 805m, which is shorter than the top four cable-stay bridges and beaten by thirty suspension bridges.

While the DOT is therefore talking nonsense, it's unclear whether Patten's bridge makes much sense. Defending his concept, he noted: "They told Eiffel to take down his tower after the fair because it was an eyesore. They said the same thing about the Golden Gate Bridge. Maybe we should take that down, too." Hubris aside, his design relies on "buoyancy stabilised piers" (i.e. caisson foundations filled with air to offset the weight of the bridge), a daft idea that entirely ignores the fact that heavy foundations are normally required to stabilise the bridge pylons against overturning, which governs design more than the purely vertical load.

What will we name the new Forth bridge?
"Forth Replacement Crossing" is too dull; "Fourth Forth Bridge" likely to cause confusion because it's actually the Fifth Forth Bridge; so just what will they call it? Incidentally, according to the type of people who like to generate ridiculous newspaper headlines, the bridge is being designed by the same people who design Ferrari F1 cars.

Bridge petitioners declare victory
Reprieve expected for Victoria's Blue Bridge (see previous posts): campaigners against the Johnson Street Bridge replacement raise enough votes to put at least a temporary brake on their council's attempt to borrow CAN$42m towards the cost.

Bridges to prosperity
The always excellent Bridge Photo of the Day blog has several interesting posts on low-cost suspension bridges in Ecuador, which make a nice follow-on to my own posts on the same theme.

03 January 2010

The curious case of the copycat coils

When Santiago Calatrava's Peace Bridge design was revealed last summer, it was hailed as a radical departure for the maestro of bone-white structural flamboyance: a truss, no less, and a red one at that.

Of course, its helical truss design was ambitious but by no means unique, and I've covered a couple of similar designs here before. But I had perhaps underestimated quite how many there were, so I'm taking this opportunity to put that right.

I'm not seriously suggesting that any of these designs are direct copies of any other bridge (with the obvious exception of the twin Happold bridges), but it is interesting to see just how this typology, has appeared, spread, and extended its ambition with ever greater spans.

I've put them in reverse chronological order. Click on any image for a larger version.

Peace Bridge, Calgary, Canada

Due to open late 2010. Designed by Santiago Calatrava (architect) / Stantec (structural engineer). Spans 130m. See my previous post Calatrava springs a surprise for more.

Double Helix Bridge, Singapore

Due to open 2010. Designed by Arup (structural engineer) / Cox Group (architect). Spans 65m. For more details, see Wikipedia, Bentley Generative Design article, and Singapore's URA.

Roche-sur-Yon Bridge, France

Opening February 2010. Designed by HDA Paris (architect & engineer) / Bernard Tschumi (architect). 70m long but I don't know the span. Lots of great pictures on flickr; also see Morphocode, Archdaily (especially the discussion), a photo of the previous bridge on this site, and HDA's Complexitys blog. This one deserves its own post, really, it's by far the best design of any shown here!

Harthill Footbridge, near Glasgow, UK

Opened October 2008. Designed by Buro Happold. Spans 70m. More details from when I visited it.

T. Evans Wyckoff Memorial Bridge, Seattle, USA

Opened October 2008. Designed by SRG Partnership (architect) / Magnusson Klemencic Associates (structural engineer). Spans 42m. More details at the Museum of Flight website, and at modernsteel.com [PDF].

Randstad Rail Station at Beatrixlaan, The Hague, Netherlands

Opened 2006. Designed by Zwarts and Jansma (architects). Spans 40m to 50m. More details at Archdaily.

Greenside Place Link Bridge, Edinburgh, UK

Opened 2003. Designed by Buro Happold (structural engineer) / Broadway Malyan (architect). Spans 46m. More details from my visit.