The Design of Modern Steel Bridges - TEDI

Types and History of Steel Bridges 13
heralded the era of great trussed cantilever spans, led by the mighty Forth
railway bridge. Designed in 1881 by John Fowler and Benjamin Baker, and
construction completed in 1890 by Messrs Tancred, Arrol & Co, this bridge
had two massive spans of 1710 ft (521 m), each consisting of two 680 ft (207 m)
cantilevers and a 350 ft (107 m) suspended section. The depth of the truss at the
piers was 350 ft (107 m). A German engineer called Gerber first developed the
cantilever and suspended technique of bridge construction and quite a few such
bridges were also built in America; it had the advantage of requiring no falsework
over the gap. Projecting out in both directions, a cantilever structure was
built on each pier and then a short suspended span was hung in between the tips
of the two cantilevers. In the Forth Bridge, Baker built a third main pier on an
island in the midstream, the bridge thus consisting of a triple cantilever with
two suspended spans. The bridge carried two railway tracks 150 ft (46 m) above
water. The specification for the steel required a minimum ultimate strength of
30 ton/in 2 (463 N/mm 2 ) for tensile members and 34 ton/in 2 (525 N/mm 2 ) for
struts, and working stresses were a quarter of the ultimate strength. Over 50 000
tons of steel and 6 million rivets were used. This was the first major bridge in
Europe built with steel. Unlike cast iron, steel suffers from rusting; paint was
the answer to this problem. The scale of the routine painting operation needed
for the maintenance of Forth Railway Bridge is another facet of its fame.
Steel truss bridges started going up all over the world. The Forth Railway
Bridge in Scotland was followed by the Queensboro Bridge over the East River
in New York which had two main spans of 1182 ft (360 m), a central span of
630 ft (192 m) and two anchor spans at the two shores – all made continuous in
triangulated truss form, without any suspended spans of the Forth sort. This
was followed by the start of construction in 1904 of the Quebec Bridge over
the St Lawrence River in Canada which had a central span of 1800 ft (549 m).
The bridge consisted of two giant truss cantilevers on two main piers, with
a suspended span in the middle. The two anchor spans were first built on
falsework; then the cantilever arms on the river were erected member by
member by cranes operating on the already erected structure. The two
cantilevers having been completed on the two piers in this way, the members
of the suspended spans were also being erected from both sides in this way
when there were signs of buckling on the web plates of the compression chord
members near the south pier and some rivets were found broken. Theodore
Cooper, the respected elderly consulting engineer, who was not present on site,
sent orders to stop erection, but work continued, and on 29 August 1907, the
whole structure collapsed into the river, killing 75 men. In the subsequent
inquiry and investigations it became clear that the lacing system and the splice
joints of the compression members were not able to resist the effects of the
buckling tendency of the compression members. In 1916 a new, slightly wider,
structure was being rebuilt on new foundations; the two cantilevers had been
completed and the entire 5000 ton suspended span, built on-shore and floated
out, was being lifted up by hydraulic jacks. Then a casting support block at one