01 March 2015

Tower Bridge ... some musings

I recently posted about Tower Bridge in London, and observed that it is one of the world’s most unusual suspension bridges.

Most suspension bridges have a suspension cable or chain from which the deck is hung. Tower Bridge, unusually, is suspended from a suspension member made up of riveted steel plates (pictured).

Additionally, most suspension bridges have the deck stiffened with trusses or girders to prevent excessive deflections. On Tower Bridge, the suspension system is itself stiffened, comprising trussed elements in the form of an inverted three-hinged arch.

Both these features are unusual, and I wondered whether the latter one is in fact unique.

Since my post, suspension bridge expert Bridgemeister has introduced me to a number of other bridges which share either or both of these features, and I thought I'd cover some of them here.

I already knew of one other suspension bridge which has riveted steel plates as its suspension "cable", the Grunwaldzki bridge in Wrocław, which I visited in 2011. This bridge was built in 1947 and it's really difficult to understand why plates (pictured, left) were used instead of cables.

Bridgemeister also highlights the Trukhanov Island footbridge in Ukraine; the Lahn River Bridge at Nassau in Germany (pictured right); and a hybrid bridge over the Salzach between Laufen in Germany and Oberndorf in Austria.

Of those the 75m span Lahn River Bridge can be counted twice, both for its 2005 incarnation and its 1926 predecessor (this is the one in the photo). It is a thoroughly delightful structure, especially the detailing of its towers. It deserves to be much better known.

The plated suspension elements on these bridges are in some cases very different from the riveted plates of the Tower Bridge. The welded steel flats on the Lahn bridge are simple and elegant by comparison, yet the bridge still retains a great deal of character, mainly thanks to the marvellous towers.

Bridges with a trussed suspension "cable" are also hard to find, and often not well known. The 1895 Mill Creek Park, pictured left is a good example, but others are more questionable.

An 1889 suspension bridge in Rome has only minimal stiffening, while several examples were either never built (Gustav Lindenthal's spectacular Hudson River design in New York, pictured right) or have been dismantled (St Louis's Grand Avenue bridge, and Lindenthal's Seventh Street Bridge and Point Bridge in Pittsburgh).

The Kindee bridge at Ellenborough in Australia adopts broadly the same form as Tower Bridge, a three-hinged arch formed from two crescent trusses. It's a glorious monument to engineering idiosyncrasy.

The 1947 Lumberville-Raven Rock bridge (pictured left) has two-hinged stiffening trusses, and is another triumph of engineering. In this arrangement, there are actually two suspension cables one each side of the bridge, separated by an arrangement of lightweight trussed steel ties and struts. These are held together with a fascinating assemblage of custom-made clamps and connecting nodes. It's like a modern-day Schlaich Bergermann design transported back in time.

Another example of the genre can be found at the former John Roebling offices in Trenton, New Jersey, providing a high-level walkway connecting two buildings (pictured right). This shares the Lumberville bridge's two-hinged truss arrangement, although it has had to be further stiffened with tie-down stays below the deck.

It's not hard to see why both variants on the normal suspension bridge have found little favour. The plated "cable" solution is a step up from historic chain bridges, being less vulnerable to single-point failure than chains. However, it simply can't compete with any kind of wire cable, as wires are inherently stronger than plain structural steel, and don't need to be peppered with rivet or bolt holes, which further reduce the capacity. Wires are lighter and much easier to install. However, I do like the Lahn River Bridge and wonder whether there are not some nice bridges still to be designed along similar lines.

As for trussing, again, this is not an efficient means of construction – the trussed "cable" requires expensive temporary support while it is assembled. It's much easier to install a relatively lightweight cable first and use that to provide all the necessary support while the stiff bridge deck is assembled.

Picture sources: Bridgemeister (Lumberville-Raven Rock, Trenton); out of copyright (Hudson River, Lahn River); rg998 at Wikipedia (Mill Creek Park); the author (Grunwaldzki, Tower Bridge).

1 comment:

Imre Laufer said...

One guess why the Grunwaldzki bridge may have been built with plated "cables" in 1947:
proper cables were probably not available at the time of post-WW2 reconstruction of Europe.