The Design of Modern Steel Bridges - TEDI

10 The Design of Modern Steel Bridges
destroyed by a storm in 1864 when the stays were left loose. At the site of the
present Rainbow Bridge, Keefer built a bridge of 1268 ft (387 m) span in 1869,
which was destroyed by a storm in 1889. John Roebling and his son Washington
went on to build several more suspension bridges, the most notable being the
ones at Pittsburgh and Cincinnati, and the Great Brooklyn Bridge in New York.
1.2.2 Steel bridges
In the second half of the nineteenth century steel was developed and started
replacing cast-iron as a structural material. The technique of using compressed
air to sink caissons for foundations below water was also developed. In 1855–
59 Brunel built the Chepstow Bridge over the River Wye and the Saltash
Bridge over the Tamar to carry railways. These were a combination of arch and
suspension structures. A large wrought-iron tube formed the upper chord
shaped like an arch; the lower chord was a pair of suspension chains in catenary
profile. The tube and the chains were braced together by diagonal ties and
vertical struts. The first glimpse of lattice girder bridges can be seen in these
designs. To carry railways over the Rhine in Germany, several bridges were
built in the second half of the century, the most remarkable among them being:
(1) Two bridges in Köln built in 1859, each with four spans of 338 ft (103 m)
with multiple criss-cross lattice main girders 27.9 ft (8.5 m) deep.
(2) A bridge at Mainz built in 1882 with four spans of 344 ft (105 m), with
a combined structural system of an arched top chord, a catenary bottom
chord and a lattice in-filling between them, as in Brunel’s Wye and
Saltash bridges.
In America, the end of the Civil War and the spread of railway construction
resulted in growing demands for building bridges. To connect the Illinois and
the Union Pacific railways a bridge was needed over the 1500 ft (457 m) wide
mighty Mississippi river at St Louis, for which James Eads was commissioned
in 1867. The sandy river bed was subject to considerable shift and scour, and
rock lay at varying depths between 50 and 150 ft (15–45 m). Swirling water
rose 40 ft (12 m) in summer, and in winter 20 ft (6 m) thick chunks of ice
hurtled down. Eads proposed to sink caissons down to rock level by compressed
air – a technique already being used in Europe (by Brunel in Saltash,
for example), but often at the cost of illness and fatality of the workmen. Eads
also decided that a suspension bridge would not be stiff enough to carry
railway loading; he proposed one 520 ft (159 m) and two 502 ft (153 m) spans
of lattice arch construction with steel – the first use of the recently discovered
material in a bridge. Bessemer had already converted iron to steel by adding
carbon in 1856 and Siemens developed the open hearth process in 1867. But
the problem was to produce the enormous quantity of this new material to
a guaranteed and uniform quality rightly demanded by Eads, for example