Civil Engineering Reference
In-Depth Information
Figure 18.16
Sundsvall Bridge project (Image: David Benaim)
18.4.9 Design of stays
The design of stays must address the issues of control of their operating stresses, their
method of erection, protection against corrosion, vibration and fatigue.
Stays for modern bridges may be composed of parallel strand, parallel wires or
locked coil cables. The fi rst two types are based on prestressing tendon technology,
while the latter is based on suspension bridge hanger technology. As the author has no
experience of using locked coil cables, the reader is referred to references [4] and [5].
This section offers only a brief overview of the design of stays. The brochures and
technical documents prepared by the suppliers should be consulted to obtain the most
up-to-date information in an evolving market.
a) Parallel wire stays
Parallel wire stays have anchorages which have evolved from the BBR prestressing
system, adapted to improve their fatigue resistance. Stays may use up to 421 wires of
7 mm diameter which are usually galvanised, giving an ultimate load of approximately
27 MN and a working load of some 12 MN. This system is prefabricated, with the
complete, sheathed stay being brought ready made to site. The prefabrication of
the stays allows very high quality protection. A tightly fi tting polyethylene sheath is
extruded onto the bundle of wires. The extrusion process may give rise to variations
in sheath thickness, and it is important to specify this thickness correctly and to check
the results. Sheaths of different colours may be specifi ed. The sheath is then pressure
grouted with a petroleum wax to fi ll all voids. As the stay is horizontal and in a factory
environment, several points of entry for the wax may be used, and the sheath may
be re-injected to fi ll voids caused by the cooling of the wax. The stays are generally
erected by a crane located at the top of the tower.
The anchors consist of a nut screwed onto the threaded outer casing and bearing
on an anchor plate. The threaded length of the live anchor has to take into account
the extension of the stay during stressing, including taking up its sag, as well as the
extension due to any re-stressing that may be required during construction, and any
tolerance on length. Clearly with this system, the cable lengths must be defi ned within
close tolerances.
