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.
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.
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.
This is the below-stage machinery supporting the dramatic performance above, not really intended for public viewing. It all looks surprisingly well-maintained.
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.
- Google maps / Bing maps
- Engineering Timelines
- British Listed Buildings
- Where Thames Smooth Waters Glide (includes images of the previous bridge on the site)
- Civil Engineering Heritage: London (Denis Smith, 2001)
- Cross River Traffic: A History of London's Bridges (Roberts, 2005)
- An Encyclopaedia of Britain's Bridges (McFetrich, 2010)