23 June 2011

London Bridges: 9. Chelsea Bridge

Chelsea Bridge is the second in a trio of structures spanning the River Thames in London, all of which can be seen from each other.

Perhaps one of my more knowledgeable readers can tell me whether it is Britain's only self-anchored suspension bridge, or whether there are other examples? I'm not thinking of bridges like the Roxburgh Viaduct Footbridge, or the Royal Albert Bridge, although they are indeed self-anchored bridges with suspension systems, but structures which resemble the more conventional suspension bridge in form.

In a conventional suspension bridge, there are two (or rarely, more) towers, from which suspension cables are hung, with the bridge deck passing below and supported by (usually) vertical hanger cables. The main suspension cables are anchored into foundations which may either consists of ground anchorages, or massive blocks constructed with sufficient weight to restrain the pull of the cables. There are two significant advantages. The first is that the main structural elements are purely in tension, which allows a much lower weight of material than for elements subjected to compression and the attendant risk of buckling. The second is that the span can be constructed without the use of any temporary supports below the deck, minimising both cost and disruption to the obstacle spanned.

The self-anchored suspension bridge dispenses with the need for foundations to anchor the main cables by anchoring them instead to the bridge deck. The cost and scale of anchorage foundations can be considerable, so this seems to be a sensible approach, since the deck has to be present in any case to carry traffic, and might as well perform a second function. In practice, however, the self-anchored option is rarely, if ever, the best engineering solution. The main cable forces must be exactly balanced by a compression force in the deck, necessitating a much heavier deck than is required in the conventional option. More significantly, the cables cannot be erected until the deck is available to provide their anchorage, which in turn means that the deck must be built using extensive temporary support from below.

Megalomaniac megaprojects like the San Francisco Oakland Bay Bridge aside, this means that self-anchoring is rare, particularly for spans of any significant length. The disadvantages generally outweigh the fairly limited benefits.

Chelsea Bridge was built in 1937 to a design by Rendel, Palmer and Tritton (now High Point Rendel), with the architects George Topham Forrest and E. P. Wheeler. It replaced an earlier and much more ornate wrought iron suspension span designed by Thomas Page and opened in 1858.

The bridge is 213m long, with a 107m main span, matching the span arrangement of the nearby Grosvenor Bridge such that both bridges are easily navigated by river boats. The deck is 25m wide, with the footways cantilevering beyond the suspension cables and their towers. The entire bridge, which is now Grade II Listed, is built of riveted steel.

The main suspension cables consist of 37 locked-coil ropes tied together in a hexagonal arrangement, which is not a system I've seen previously. The deck hangers are clamped to these cables with bolted fittings. The main cables disappear into shrouds at the ends of the deck, so the way in which they are connected to the deck is not visible, which is a shame as I can't quite imagine how it works - the force from each of 37 individual cables has to be transmitted into the end of the main deck girders.

The main towers are in riveted steel box construction, tapering towards the top, where there are exposed saddles. They have something of the shape of Cleopatra's Needle. Most suspension bridges require cross-bracing between the tower legs to provide stability, but the span of Chelsea Bridge is short, and hence the towers aren't tall enough to require this. I think it looks good and wonder what size of suspension bridge renders it impractical.

The towers are hinged at their base, using heavily stiffened rocker bearings, which again is not something normally associated with suspension bridges. On a conventional suspension bridge, the towers have to be stable to support the main cables before the deck is in place, and hence are cantilevered rather than hinged.

I think the largest suspension bridge with hinged towers may be the Florianopolis Bridge, which has a 340m main span. There, the designer, David Steinman, saw the use of hinges as "the most economical and scientific design for suspension bridge towers", because of the reduced bending stresses in the permanent situation, but although Chelsea Bridge adopted the same choice ten years after Florianopolis, it's not a form that has prospered.

From below the deck, its structural form can be seen clearly. The main girders are formed of paired girders, with crossbeams and cantilevers at regular intervals, and extensive lattice-member bracing connecting the other members.

This is the below-stage machinery supporting the dramatic performance above, not really intended for public viewing. It all looks surprisingly well-maintained.

The good level of maintenance is evident throughout the bridge. The paintwork is all good, including the red on the cables and the elements of blue on the parapet infill (an improvement for local Chelsea residents over the previous red-and-white scheme, which reminded them uncomfortably of the colours of rival London football team Arsenal).

The cables and towers are studded with funfair-style lightbulbs, and the main streetlamps are in an unusual arrangement, with the posts carefully set around hanger cables, as can be seen in the photo above right if you look carefully (as always, click on any image for the full-size version).

In his book Cross River Traffic, Chris Roberts describes Chelsea Bridge as "a very striking, if odd, combination of Thunderbird One and seaside pier", which isn't far off the mark.

Further information:


Mark Yashinsky said...

Dear HP,

I really enjoyed your article on self-anchored suspension bridges. Please read my blog on the Konohana Bridge in Osaka Bay (www.bphod.com on June 11 and June 12, 2009) for information on another self-anchored suspension bridge.

However, I would take exception to the characterization of the self-anchored suspension span on the East Bay Crossing as a 'Megalomaniac Megaprojects'.

It certainly wasn't our intent, nor do I think any of the people involved in this project had delusions of grandeur. Brian Maroney who managed the project is one of the nicest people I know. Chris Arnold who helped chose the winning design was just trying to give California an attractive bridge that we could enjoy.

Caltrans is a public agency and our goal is to give the public the best bridge for their money. This project got involved in the complicated politics of the area which contributed to the steep rise in its cost, but when it's completed, hopefully everyone will appreciate all the hard work that was done to provide an attractive and seismically safe bridge across San Francisco Bay.

The Happy Pontist said...

I certainly didn't wish to cause undue offence!

I'm all in favour of providing attractive bridges, but I do find it hard to see how a structure on the scale of the East Bay Crossing SAS should not try to match that aspiration against the adoption of an economic solution. And I find it hard to see how a self-anchored suspension bridge on that scale can be an economic option when judged against feasible alternatives.

Will it look great when finished? I'm sure it will. Has public money been spent as effectively as possible? I believe there has already been plenty of ink spilled on that subject!

Bob Cortright said...

I read all of the Happy Pontist postings and learn something new from each. Keep them coming.

Bob Cortright, Bridge Ink