The Design of Modern Steel Bridges - TEDI

186 The Design of Modern Steel Bridges
successively attaching the closely spaced stays. Thus the structural system
consisted of vertical towers, inclined stays and the deck as the compression
chord, the three forming a triangulated frame. As all applied loading was resisted
by axial forces in these structural members rather than bending of a stiffening
girder, this system possessed substantial rigidity with acceptable deflections for
highway or railway traffic. Another advantage of the multi-cable system is the
ease with which an individual stay can be replaced if needed. The economical
span range for cable-stayed bridges was thus increased substantially.
The first multi-cable bridge was the Friedrich Ebert across the Rhine at
Bonn, designed by Homberg and completed in 1967. It had three 120–280–
120 m spans; it was supported by 80 stays of single locked-coil strands, 20 on
either side of the two towers, in a single plane. The stiffening girder had to
resist torsion for the full length of the bridge and hence had to be a large box
girder. This was followed closely by the Rhine Bridge at Rees, with closely
spaced cable stays arranged in a harp fashion in two planes supporting two
plate girders as stiffening girders, with an orthotropic steel deck in between.
This bridge achieved the full benefit of a slender deck structure with adequate
stiffness and aerodynamic stability provided by the multi-cable system. Both
these bridges had substantial lengths of the stiffening girder in mid-span and
near the towers unsupported by stays. In the Knie Bridge across the Rhine in
Düsseldorf (Figure 1.25), opened in 1969, 16 cable-stays were arranged in two
planes in a harp fashion, eight on either side of one two-legged tower near one
bank. The spans on the bank side were supported on intermediate piers and the
back stays were also anchored to them. This increased the longitudinal rigidity
of the stiffening system and enabled the construction of the 320 m long span
over the river supported by cable-stays from only one tower. The same
technique was used to build the symmetrical 350 m span Duisburg–
Neuenkamp bridge over the Rhine in 1970.
The 325 m span Köhlbrand Bridge in Hamburg (Figure 1.28), completed in
1974, was the first bridge with multiple cables in two inclined planes anchored
from the upper part of two A-shaped towers in a modified harp fashion.
Cable-stayed form of bridge construction has now virtually superseded all
other forms of bridges for spans between 200 and 500 m, and is competing with
suspension bridges for up to 1000 m spans. The advantages that a cable-stayed
bridge has over a suspension bridge of the same span are that the former does
not require substantial anchorages and its erection is simpler. A cable-stayed
bridge is also stiffer than a suspension bridge for live and wind loading. Multistay
form of cable-stayed bridges may not have the simplicity of bridges
supported by single or twin stays or the classical elegance of suspension
bridges, but their profile of a slender deck held by an array of thin cables in a
linear pattern from one or two tall towers has a striking attraction.
Aerodynamic stability, both of the completed bridge and of the incomplete
bridge during construction, is a major concern for cable-stayed bridges, and
this aspect can only be investigated by wind-tunnel tests.