Portal:Scotland/Selected article/Week 39, 2012

Sir William Arrol (1839–1913) was a Scottish civil engineer, bridge builder, and Liberal Party politician. The son of a spinner, he was born in Houston, Renfrewshire, and started work in a cotton mill at only 9 years of age. He started training as a blacksmith by age 13, and went on to learn mechanics and hydraulics at night school. In 1863 he joined a company of bridge manufacturers in Glasgow, but by 1872 he had established his own business, the Dalmarnock Iron Works, in the east end of the city. In the late 1870s he went on to found Sir William Arrol & Co., a leading international civil engineering business. In 1878, he secured the contract for the Caledonian Railway Bridge over the Clyde, and in 1882 he was awarded the reconstruction contract for the Tay Rail Bridge, which had collapsed in 1879.

His company went on to construct the Forth Bridge which was completed in 1890. At the time, the Tay and Forth bridges were the largest of their type in the world. They were notable not just for their size but also the use of steel in the Forth bridge, and the riveting method developed by Arrol to attach the girders to one another. The Forth Bridge bridge is, even today, regarded as an engineering marvel. It is 2.5 km (1.6 mi) in length, and the double track is elevated 46 m (approx. 150 ft) above high tide. It consists of three main spans of 1,710 ft (520 m), two side spans of 675 ft, 15 approach spans of 168 ft (51 m), and five of 25 ft (7.6 m). Each main span comprises two 680 ft (210 m) cantilever arms supporting a central 350 ft (110 m) span girder bridge. The three great four-tower cantilever structures are 340 ft (104 m) tall, each 70 ft (21 m) diameter foot resting on a separate foundation. The southern group of foundations had to be constructed as caissons under compressed air, to a depth of 90 ft (27 m). Both bridges are known for their high safety factors, a natural result of the under-design of the first Tay bridge by Thomas Bouch, and both bridges have recently (2008) been renovated.