The Design of Modern Steel Bridges - TEDI

Types and History of Steel Bridges 9
compression members and wrought-iron tension members. In 1842 Charles
Ellet built a suspension bridge over Schuylkill river at Fairmont, Pennsylvania,
to replace Lewis Wernwag’s 340 ft (104 m) span Colossus Bridge destroyed by
fire. The latter was a timber bridge formed in the shape of a gently curved arch
reinforced by trusses the diagonals of which were iron rods – the first use of iron
in a long-span bridge in America. Ellet’s suspended span was supported by ten
wire cables. In 1848 Ellet started to build the first ever bridge across the 800 ft
(244 m) wide chasm below the Niagara Falls to carry a railway. To carry the
first wire, he offered a prize of five dollars to fly a kite across. After the first wire
cable was stretched in this way, the showman that he was, he hauled himself
across the gorge in a wire basket at a height of 250 ft (76 m) above the swirling
water! He then built a 7.5 ft (2.3 m) wide service bridge without railings,
rode across on a horse and started collecting fares. Then he fell out with
the promoters and withdrew, leading to the appointment of John Roebling, a
Prussian-born engineer, to erect a new bridge. In 1841 Roebling had already
patented his idea of forming cables from parallel wires bound into a compact
bunch by binding wire.
In 1848 John Ellet had built another suspension bridge of 308 m (1010 ft)
span over the Ohio river at Wheeling, West Virginia. In January 1854 it
collapsed in a storm, due to aerodynamic vibration. Roebling realised that ‘the
destruction’ of the Wheeling bridge was clearly ‘owing to a want of stability,
and not to a want of strength’ – his own words. He also studied the collapse of
a suspension bridge in 1850 in Angers, France, under a marching regiment, and
another in Licking, Kentucky, in 1854 under a drove of trotting cattle. His
Grand Trunk bridge at Niagara had a 250 m (820 ft) span and had two decks,
the upper one to carry a railway and the lower one a road; stiffening trusses
18 ft (5.5 m) deep of timber construction were provided between the two
decks – the first stiffening girder used for a suspension bridge. The deck was
supported by four main cables 10 inch (254 mm) in diameter consisting of
parallel wrought iron wires, uniformly tensioned and compacted into a bunch
with binding wire.
This was the birth of the modern suspension bridge, which must be ranked as
one of history’s greatest inventions. The deck was also supported from the
tower directly by 64 diagonal stays, and more stays were later added below the
deck and anchored to the gorge sides. The bridge was completed in 1855.
Roebling proved, contrary to Stephenson’s prediction, that suspension bridges
could carry railways and were more economical than the tubular girder
construction used by the latter at Menai. In reality, however, not many railway
suspension bridges were later built, but Roebling’s Niagara bridge was the
forebear of a great number of suspension bridges carrying roads. Its wooden
deck was replaced by iron and the masonry towers by steel in 1881 and 1885,
respectively, and finally the whole bridge was replaced in 1897. Two other
bridges were built across the Niagara gorge. Serrell’s road bridge of 1043 ft
(318 m) span was built in 1851, stiffened by Roebling by stays in 1855 and