Read the text about the most astonishing British bridges and tell about them

In 1779, Abraham Darby wrote a new page in the world’s bridge engineering history by building the first cast-iron bridge to span the Severn at Coalbrookdale in Great Britain. Designed by Thomas Pritchard, the bridge, constructed of cast-iron pieces, imitated stone construction with nearly semicircular 30-metre arch span (fig. 8.1a; 9.4a). Cast-iron plates covered each of its five thin cast-iron arch ribs. Despite the arch is slightly humped, the bridge is still used today but not for vehicles. It is now a British national monument.

Figure 9.4 Bridges of Great Britain

a – the earliest cast iron bridge across the Severn River; b – the Britannia Bridge;

c – the Tower Bridge

The upcoming of railways during the 19^th century resulted in new bridge forms strong enough to resist the dynamic loads of trains. The most significant of these early railway bridges was the Britannia Bridge by Robert Stephenson across the Menai Strait. He supported a pair of completely enclosed iron tubes of rectangular section by a pier in the centre of span. He was the first to employ the hollow box girder, which gave the deck the extra stiffness of a truss. The workers delievered the iron tubes afloat, and then a capstan and hydraulic power lifted them. The lengths of spans are 140 m and each abutment span is 71 m long (fig. 9.4b). The bridge builders completed the structure in 1850.

There were towers for chains on the bridge. The towers were originally to carry the wrought iron boxes through which the trains ran. Now they are useless because there are no cables on the bridge. In 1960’s the structure was severely damaged by fire, and during the repairs the tubes disappiered completely. Steel arches support modern concrete deck. At present, the bridge carries railway and motor traffic (fig. 9.6a).