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Stretch of imagination

Bridges

Britain's innovative civil engineering over the last 100 years is perhaps best manifested in bridges. David Bennett reports.

Many of the world's landmark 20th century bridges were designed and built by British firms. Freeman Fox, Dorman Long, Cleveland Bridge, Maunsell and Arup have all produced great structures - across Sydney Harbour, the Severn, the Bosporus, the Humber and at Gladesville, and Kylesku.

But does each project mark a defining moment where British bridge engineering was so far ahead that it deserved inter- national acclaim?

Certainly there have been some record breaking spans. In 1904 the Victoria Falls Bridge was the longest steel arch in the world at 152m. The 300m span of Maunsell's Gladesville Bridge in Sydney held the record for the longest concrete arch for 15 years after it was completed in 1964.

And the 1.4km suspension span of Freeman Fox's Humber Bridge, built in 1981, held the record for longest span in the world for 17 years. But what was significant about these bridges?

The Victoria Falls Bridge was a rail crossing, built in a dramatic location, across a wide gorge on the fast flowing Zambezi River at the point of the falls. Its world beating span for a steel arch gives it a place in history, but as bridge it did not innovate nor push the boundaries of technology, no matter that it was well engineered for such a remote region in Africa.

Sydney Harbour, built by Dorman Long in 1932, is more than a bridge. It is an icon and symbol for Australia's emergence as one of fastest growing economies in the world. The bridge itself was not a world first, nor did it have the longest span. But the erection of the main span was one of the greatest engineering feats of the 1930s. The two halves of the steel arch were built out from each abutment held in position by 128 steel cables.

The Humber is a great bridge. Its aerodynamic deck, like that of the Bosporus Bridge, which was completed in 1988, was a refinement of the Severn, as were the hollow box tower structures. On the other hand, Arup's Kylesku Bridge has won plaudits for its skilful engineering and aesthetic beauty.

In recent times, claims have been made for a world first on the 1.37km span Tsing Ma Bridge designed by Mott MacDonald and built by Kvaerner and Costain in Hong Kong in 1997. It has the world's longest span for a combined road and rail bridge, an innovative rail track construction and a stiffened truss within an aerofoil bridge deck. The bridge is also the first all weather crossing that can remain open in typhoon winds gusting up to 290km/h.

Maunsell's small but innovative Aberfeldy Bridge across the Tay in 1992 was the first glass-reinforced plastic bridge in the world, but was only a footbridge. The A13 viaduct in Dagenham, designed by Robert Benaim & Associates and finished this year, claims to have the longest continuous concrete box section without an expansion joint.

The significance of these achievements has to be measured against what was known at the time, and how far an innovation has influenced bridge building and design thinking.

The most significant British engineered bridges of the century should not be judged on how big, long or slim they are, or on how quickly they were built. What matters is whether they introduced radically new ideas, departed from conventional practice or changed the way bridges are built.

In this respect the most significant bridge building achievement of the 20th century has to be the first Severn crossing. Designed by Freeman Fox and Mott Hay & Anderson, this was the most innovative suspension bridge of all time, with its streamlined deck and flexible deck stiffening truss.

Engineers tend to cite the same individuals as vital influences over 20th century bridge building. Leading the field are Guy Maunsell and Gilbert Roberts whose work in prestressed concrete and on the Severn Bridge established them as giants of the bridge world.

Among the others are the Freeman dynasty of Ralph senior, Ralph junior and Anthony; intuitive genius Oleg Kerensky, and Mike Parsons the brilliant mathematician who worked on the Severn and Humber. Parsons worked alongside Bill Brown, who resolved critical difficulties during the building of the Bosporus. All six worked for Freeman Fox.

Then there is the Arup contingent: Bill Smyth, now retired; Robert Benaim, who formed Robert Benaim & Associates; Srinivasan of Dar Consultants; plus Angus Low and Jorgen Nissen.

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