Suspension Bridges - Part Two
Just before the Severn bridge was built, the Firth of Forth road bridge was completed. It was made too soon to receive the aerodynamic treatment, and so it has the previously popular truss construction. It has a main span of 3300 feet/1006 m, and two side spans of 1340 feet/408 m. A model was tested in a wind tunnel at the National Physical Laboratory. Gaps were left between the two main carriageways, and between these and the cycle tracks. This allows air to flow between the upper and lower surfaces, helping to weaken any lift that might develop. Construction took from 1958 to 1964.
You can see several differences between the Forth bridge and the Severn bridge.
The bracing of the towers is triangulated in the Forth bridge.
The deck of the Forth bridge is a deep truss, contrasting strongly with the slim towers.
The hangers of the Forth bridge are arranged in vertical pairs. Those of the Severn bridge are triangulated.
The anchorages of the main cables of the Forth bridge are underground, and not visible anchorage blocks as in the Severn bridge.
Here are some pictures of the Forth road bridge.
The pictures above show the magnificent George Washington bridge across the Hudson river at the north end of Manhattan island. It was once the span record holder, with a span of about 3500 feet/1067 m. It raised the record span by the greatest linear step up to that time, 1650 feet/503 m, and also by the greatest ratio ever, 1.89, which will probably never be exceeded. For example, 1.89 times the span of Akashi Kaikyo would be 12340 feet/3761 m, or 2.34 miles, 3.74 km.
Irrespective of records, the George Washington bridge is a magnificent structure, with its unique towers, quadruple cables, double decks and fourfold hanger sets.
The truss was provided, not for stability, but as a later addition to support a second roadway below the original one. The bridge was completed in 1931, and remains a very impressive sight. The original structure was stable without a truss, and was designed by the great bridge engineer O H Ammann.
Early sketches of the bridge show the towers made of, or encased in, concrete or stone, as this web-page shows. Chapter one of this excellent book (Bridges, their art, science and evolution, by Charles S Whitney, ISBN 0-517-402440) is a discussion of the aesthetics of bridges, including the George Washington. These proposals look strange to modern tastes.
This bridge, carrying the A38 road over the river Tamar with a span of about 1100 feet/335 m, was completed in 1961.
It is a classic trussed design, built very close to I K Brunel's Saltash railway bridge. The towers, which are near the banks of the river, are made of reinforced concrete.
Like some other British bridges built in the 1960s, notably the M4 Severn suspension bridge, Tamar bridge was affected by huge increases in traffic volume before the century was out. Refurbishing in 2002 included the addition of cable stays, which not only helped in supporting the deck, but reduced torsion, induced for example by asymmetric traffic loads. These can occur during periods of dense traffic, if queues for tolls fill one side of the bridge while the other side has only a light load. The pictures show the extra stays on one tower, and one attachment to the deck. Note the provision for adjustment of the tension, so that the new cables can take the required fraction of the total load. The new cables change the forces in the deck, not only because they are localised much more than the array of old vertical hangers, but also because they put a large section of the deck into compression.
This picture shows the new suspension bridge with the old bridge which was built 102 years earlier. The picture was taken from the car park that the authorities have provided for people who want to look at the bridges. A short walk on to the new bridge provides superb views of the old bridge.
Click here for a series of superb photographs.
The Old Tamar bridge
The old bridge, designed by I K Brunel, is also a suspension bridge, albeit one of unusual, and probably unique, design. This bridge is described in its own page.
Budapest has two suspension bridges, the older of which, the chain bridge, is shown here. It was designed by William Thierney Clark and Adam Clark. Adam Clark also designed the tunnel near the Buda end of the bridge. Construction began in 1842, and the bridge was opened in 1849. Given the depth and speed of the river, and the tendency for slabs of ice to be carried in winter, this bridge was a great feat of engineering. The bridge was damaged in 1945, and only reopened for traffic in 1949. It was restored in 1987. Not far downstream is the elegant Elizabeth bridge, built in 1964 to replace the one of 1903, destroyed in 1945. Designs with few spans are ideal for fast flow and for avoiding obstruction of floating ice.
At Cruseille, in the Haute-Savioe, two bridges cross a spectacular limestone gorge in which flows the River Usse. The older one, a suspension bridge, dates from 1839. It was built to reduce the journey time between Annecy and Geneva. It is suspended from four cylindrical masonry towers by 24 cables.
See also Caille Bridges.
Small suspension bridges in towns are often relatively stiff in engineering terms; pedestrians do not generally enjoy a swaying bridge.
Suspension in Buildings
The suspension principle is not much used in buildings, most of which rely on beams, arches and cantilevers. But this one seems to be suspended on tubes, which are presumably formed into parabolas, as the building probably has a uniform distribution of weight.
This "catenary" carries only electrical power for electric trains. A complicated system of gantries, wires weights and springs ensures that the conductor is parallel to the rails. The locomotive carries a spring loaded pantograph to push the collector on the wire.
To continue reading about the problems in the design of suspension bridges please see Oscillation.
In recent years there has been a great upsurge in the building of large suspension bridges. Places with numerous islands, such as Japan and Denmark, are being linked by great bridges. Even different countries, such as Denmark and Sweden, are being joined together in the interests of communication. And the imagination and creativity of engineers shows no sign of flagging. To see how to estimate the length of the longest feasible span click here.
Some bridges are so long that the curvature of the earth has to be taken into account. If we take the radius of the earth as 6400 km, and we consider a 2000 m span, the ratio is 2/6400 = 1/3200. If the height of the towers is H, then the amount by which they are out of parallel from bottom to top is H/3200. For a 300 metre tower this gives 9 cm. The deck might also be curved upward by an extra amount, about 80 cm at the centre, to allow for the curvature of the earth. Measured from the anchorages, this extra height is even greater, as it goes like the square of the length.
The planet shown below might have amused St Exupéry's Little Prince. People are enabled to get from one side (A) to the other side (B) across the lake (C) by means of the bridge (D). Painting the bridge begins at A, and ends at B. From here the painters carry their gear around the bottom of the planet back to A, where they start again.
A fictitious living suspension bridge is described in "Dr Doolittle's Post Office", by Hugh Lofting. In this episode, the good doctor is privileged to see the "bridge of monkeys", which is seldom seen by people. See also Orbital bridges.
If you got this far, try a superb game about bridge building - http://firingsquad.gamers.com/games/pontifex/default.asp .
Links to other web-sites about suspension bridges
Interesting site about art - http://www.eskimo.com/~ofsound/temp.html
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Arch Beam Box Girder Cable Stayed Cantilever Pre-Stressed Truss Oscillations