Cables - Structural and Stress Analysis - page 114

Chapter 5 / Cables
Flexible cables have been used to form structural systems for many centuries. Some of
the earliest man-made structures of any size were hanging bridges constructed from
jungle vines and creepers, and spanning ravines and rivers. In European literature
the earliest description of an iron suspension bridge was published by Verantius in
1607, while ropes have been used in military bridging from at least 1600. In modern
times, cables formed by binding a large number of steel wires together are employed
in bridge construction where the bridge deck is suspended on hangers from the cables
themselves. The cables in turn pass over the tops of towers and are fixed to anchor
blocks embedded in the ground; in this manner large, clear spans are achieved. Cables
are also used in cable-stayed bridges, as part of roof support systems, for prestressing
in concrete beams and for guyed structures such as pylons and television masts.
Structurally, cables are extremely efficient because they make the most effective use
of structural material in that their loads are carried solely through tension. Therefore,
there is no tendency for buckling to occur either from bending or from compressive
axial loads (see Chapter 21). However, many of the structures mentioned above are
statically indeterminate to a high degree. In other situations, particularly in guyed
towers and cable-stayed bridges, the extension of the cables affects the internal force
system and the analysis becomes non-linear. Such considerations are outside the scope
of this topic so that we shall concentrate on cables inwhich loads are suspended directly
from the cable.
Two categories of cable arise; the first is relatively lightweight and carries a limited
number of concentrated loads, while the second is heavier with a more uniform dis-
tribution of load. We shall also examine, in the case of suspension bridges, the effects
of different forms of cable support at the towers.
5.1 L IGHTWEIGHT C ABLES C ARRYING C ONCENTRATED L OADS
In the analysis of this type of cable we shall assume that the self-weight of the cable is
negligible, that it can only carry tensile forces and that the extension of the cable does
not affect the geometry of the system. We shall illustrate the method by examples.
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