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Soane's souvenirs

John Soane and the Wooden Bridges of Switzerland At Sir John Soane's Museum, Lincoln's Inn Fields, London WC2, until 14 April

Sir John Soane knew as much about the practicalities of construction as any architect of his generation: not just how to build in brick and stone, but with other materials as well, especially timber.

One aspect of timber construction, which he studied during his return journey from Italy in 1780, continued to fascinate him and resurfaced in his Royal Academy lectures more than 30 years later. It forms the subject of the opening exhibition for the Soane Museum's year of celebration, marking the 250th anniversary of his birth.

Soane cut short his stay in Rome because of an irresistible offer from Frederick Hervey, Bishop of Derry, to work in Ireland.

Hervey seemed to be an ideal client because he was obsessed by building. Soane knew that during a tour of Europe in 1770 he had visited Switzerland in order to see the engineering marvels of the time, the timber bridges of the Grubenmann brothers: he had even commissioned drawings of them. The same enthusiasm, and the knowledge that it would count well with Hervey, led Soane to direct his route home via Switzerland so as to study those very bridges.

Soane had time to see the Grubenmann bridges in three places - at Reichenau, Wettingen and Schaffhausen, all of them built in 1756-66. These were not tourist hit-and-run visits. He made careful annotated drawings of each bridge as part of his travel diary, and these notes (which mercifully remained with him, despite the loss of the rest of his luggage) are the heart of the exhibition. True, they are not as spectacular as the wonderful models on display, or the large drawings he had made to illustrate his Royal Academy lectures, but they show us exactly how he understood the bridges as structures.

The Grubenmann brothers had been successful in devising an economic form of bridge-building using short timbers; a system that was part arch and part truss, characterised by struts running diagonally from the masonry piers to either the top chord or a central king post. Beams and bracing between the trusses carried the roadway, which was protected from the weather by a continuous gabled roof. The sides of the bridges were boarded, but it is unclear how far the boarding contributed to their stiffness.

The most interesting of the bridges was at Schaffhausen, north of Zurich, with two unequal spans of 52m and 59m. This was originally designed as a single span of 119m, but that was too adventurous for the civic authorities who insisted on the use of an intermediate pier.

Soane did not wholly approve of everything he saw. When he reached the larger of the two bridges at Reichenau, he commented: 'The whole of the framing of this bridge is exceedingly rude, and very great want of iron.' It seems that the Grubenmanns hadn't fully understood how to distribute iron straps and ties. And even though the bridge was only a few years old when he visited it, Soane found it 'exceedingly decay'd, several of the (principals) split. It is sunk and twisted exceedingly in many different directions.'

However, the durability of the bridges was never fully put to the test. The examples Soane had seen were destroyed during the French invasion of Switzerland in 1799.

Because they did not survive, his vivid record of them was crucial in perpetuating their memory.

The irony is that Soane really ought not to have returned early from Italy, since the Bishop of Derry proved a thoroughly unreliable patron. But if he had not sped back via Switzerland, this crucial episode in structural history would be less well-known, and we would not have this intriguing exhibition.

Robert Thorne is a historian with Alan Baxter & Associates

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