William Arrol - Forth Bridge

Forth Bridge

The 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). At its peak, approximately 4,600 workers were employed in its construction. Initially, it was recorded that 57 lives were lost; however, after extensive research by local historians, the figure has been revised upwards to 98.

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.