Civil Engineering Reference
In-Depth Information
may be cranked inwards above the deck, so the zone of stay anchorages is co-axial with
the stays, Figure 1.10.
When a deck is carried by two planes of stays, it has a high degree of inherent
aerodynamic stability. This does not mean that it cannot suffer wind-induced torsional
oscillations. In particular, there is a possibility that the columns may oscillate out of phase
along the line of the bridge, permitting a torsional deck oscillation, Figure 18.8
.
The tower may also be an 'A' frame, with the anchors either concentrated at the
top in a fan arrangement or distributed down the two legs in semi-harp or harp
arrangements. This type of tower eliminates the possibility of the columns oscillating
with the deck, providing the best resistance to wind-induced torsional oscillations.
There must be a safe clearance, usually 1.5 m, between the stays and the highway
clearance diagram which makes 'A' frame towers unsuitable for wide decks on
short spans.
c) Three planes of stays
For very wide bridges, the transverse bending of the deck as it spans between two
planes of stays becomes the main source of expense for the deck. Some designers have
considered it viable to reduce the cost of this transverse bending by providing a third
plane of cable stays. It is diffi cult to be dogmatic about the merits of this choice. It
is certain that the more planes of stays adopted, the more times a concentrated live
load is carried. If the tower for a three-plane arrangement consists of three columns,
this will inevitably be more expensive than two columns, and in addition the deck
will have to be widened. However, savings will be made in the self weight and the
transverse bending of the deck; it is up to the designer to make his own assessment.
The author's experience with the Ah Kai Sha Bridge (
18.4.11
) is that despite its 42 m
width, it was more economical to maintain two cable planes and to fi nd an effi cient
way of spanning the deck transversally. However, this was a particular case, and one
should not generalise from it.
18.4.4 Arrangement and spacing of stays
Stays may be arranged in the harp, fan or semi-harp patterns.
(a) In the harp arrangement, all the stays are parallel, Figure 18.9 (a). The force in
each stay is such that its vertical component balances the dead load of the section
of deck it is supporting. Thus if the tower height is span/5, the longest stay has an
inclination of 1/2.5, and the force in the stay is 2.7 times the weight it is carrying.
As all the cables are parallel and equally spaced, they all have the same force.
The application of live loads on one span will increase the force in the stays
supporting that span and impose unbalanced loading on the tower which will
generate in it substantial bending moments. The tower will also be subjected
to bending moments if any stay is removed, either through damage or for
maintenance. These bending moments are likely to govern its dimensions, and will
certainly greatly increase the weight and congestion of reinforcing steel required.
The towers of some cable-stayed bridges have required more than 600 kg of
reinforcement per m
3
of concrete, which is close to the limit of the constructible.
It is necessary for the designer to consider the implications of stay removal early
