Civil Engineering Reference
In-Depth Information
FIGURE 1.14
The Forth Railway Bridge built over the Firth of Forth in 1890, Scotland.
(Courtesy of GFDL Andrew Bell, January 2005.)
proposed a cantilever truss for the Firth of Forth railway bridge crossing in Scotland.
∗
It was a monumental undertaking completed in 1890 (Figure 1.14). It is an example
of steel truss cantilever-type railway bridge construction on a grand scale with can-
tilever arms of 680 ft supporting a 350 ft suspended span. Baker used the relatively
new Bessemer steel in the bridge even though it was an untested material for such
large structures and some engineers thought it susceptible to cracking. The bridge is
very stiff and the 3
2
in deflection, measured by designer Baker under the heaviest
locomotives available on the North British Railway, compared well with his estimate
of 4 in. The bridge was further tested under extreme wind conditions with two long
heavy coal trains and the cantilever tip deflection was
<
7 in.
The Forth Railway Bridge used a large quantity of steel and was costly. This
prompted engineers such as Theodore Cooper (who had worked with Eads on the
St. Louis Bridge) to consider cantilever construction with different span types using
relatively smaller members. Two such statically determinate railway bridges were the
671 ft main span bridge crossing the Mississippi at Thebes, Illinois, and the 1800 ft
main span Quebec Bridge. The Thebes bridge, constructed in 1905, consists of five
pin-connected through truss spans, of which two spans are 521 ft fixed double anchor
spans (anchoring four 152.5 ft cantilever arms) and three contain 366 ft suspended
spans. The Quebec Bridge, an example of economical long-span steel cantilevered
truss construction for railroad loads, was completed in 1917 after two construction
failures
(Figures 1.15a
and b). The initial 1907 failure was likely due to calculation
error in determining dead load compressive stresses in the bottom chord members
∗
Before this, Baker may not have known of the work of engineers C. Shaler Smith or C.C. Schneider
who had already constructed cantilever railway bridges in the United States.
