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The Forth Bridge

Few structures are as immediately recognisable as the Forth Bridge. But the mighty rail crossing, the great achievement of Victorian engineering, is in this world-famous form only because of an earlier disastrous collapse.

The Tay Bridge fell during gales in 1879, killing 75 people and destroying with it confidence in designer Sir Thomas Bouch.

Construction had already started of a Bouch-designed suspension bridge over the Forth; this was shelved a year after the Tay fell.

But a crossing was still needed and the board's consulting engineers Sir John Fowler, WH Barlow and TE Harrison were asked to develop proposals. Fowler then worked with his colleague Benjamin Baker to come up with the design that was built.

The pair had the courage, skill and confidence to take forward techniques and materials that were relatively new, and contractor Sir William Arrol's ingenuity was also essential to achieving the result (NCE 8 March 1990).

Fowler and Baker's design has a series of cantilevers and suspended spans, rather than being a continuous girder. Baker had been advocating the use of cantilevers for long spans, but such a bridge had never been seen in the UK.

Mindful of the Tay disaster, many restrictions were imposed, and Parliament instructed that every stage of the work should be closely scrutinised so that public confidence would be restored.

And while the structural form in itself was not a world first, its scale surpassed anything seen before. It was for many years the largest girder type bridge in the world and its cantilever dimensions and material volumes eclipsed previous structures.

Furthermore, this was Britain's first major steel bridge. One of the worries about the material was its tendency to rust. Paint was the answer, and the scale of the bridge's painting operation remains another facet of its fame.

Regulations for steel construction were in the early stages of development and the engineers had to derive their own design rules. Modern analysis

shows the bridge is not as over-designed as is often supposed.

The bridge has three main towers, each with cantilevers. There are two suspended spans and approach viaducts in steel and masonry. The three towers stand each on four piers, and compressed air was used in construction of half of them, again a new technique to the UK.

The bridge was opened on 4 March 1890, sadly not to universal acclaim. It was, according to William Morris, 'the supremest specimen of ugliness'. Baker's answer was simple. The design expressed the lines of force through the structure, and 'critics must first study the work to be done by the structure before they are capable of settling whether it is beautiful or ugly'.

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