The Design of Modern Steel Bridges - TEDI

polyethylene or even steel tube. Ordinary plastic as a material is of doubtful
durability; fibre-reinforced plastic is better. Tubes must be hermetically sealed
at their connection with the end anchorages to prevent ingress of water and
oxygen. In some instances, instead of using tubes, the cables have been
covered by directly extruded thermoplastic polyethylene jackets in the factory.
In China, a pressed rubber protective cover has been tried in place of polyethylene,
in order to save time and cost associated with the latter.
Cable-stays made with prestressing wires are commonly protected from
corrosion by enclosing them in steel or polyethylene tubes. The cables are subjected
to the dead load tension of the bridge, and then cement grout is injected
into the tubes, a practice similar to that adopted for post-tensioning tendons in
prestressed concrete construction. Steel tubes require regular painting, unless
stainless type is used. Polyethylene has thermal coefficient several times that of
steel wires and thus there could be problems of distortion and cracking with
polyethylene tubes. There have been worries about separation of water during
grouting, voids, and cracking in the cement grout due to live load tension, vibration
and shrinkage; for these reasons other types of grout based on epoxy, wax,
polyurethane or polybutadiene, that are more ductile and shrink less, have been
used with some success. Use of specialised water-retaining admixture removes
the problem of water separation in cement grout. But the presence of some free
water after grouting is very difficult to avoid completely. Grouted tubes remove
the opportunity of future inspection of the condition of the cables. In some cases
polyethylene tubes have cracked, probably due to thermal effects and/or prolonged
or too-small-radius reeling. Polyethylene tubes are black in colour, due
to the inclusion of carbon to ensure durability against ultra-violet rays from sun;
however, the black colour and high-thermal coefficient can cause distortion or
even cracking of the tubes due to temperature. For this reason the tubes have
sometimes been wrapped in white or other colour plastic sheets or film.
In USA epoxy-coated wires, instead of galvanised wires, have been used to
make up parallel-wire strands.
It will be clear from the foregoing that fully reliable methods for protecting
cable-stays against corrosion for the expected lifespan of bridges have not yet
been established. For this reason it is desirable that provision is made for
regular inspection of the condition of the stays and for their possible replacement.
Painted spiral strands of galvanised wires offer these facilities at moderate
cost. To avoid the problem of corrosion altogether, attempts are being made to
develop cable-stays from materials like graphite and aramid (i.e. Kevlar) that
do not suffer from corrosion.
7.4 Cable properties
Cable-stayed Bridges 195
When a cable is suspended from two points, it sags due to its own weight. The
amount of the sag depends on the tension with which the cable is pulled at its