Civil Engineering Reference
In-Depth Information
The next simplest method of transferring the force is to cross the stays and anchor
them on the far faces of the tower, Figure 18.15 (a). This allows the use of a solid tower
which may be very compact. Clearly the forward and back stays have to be offset to
avoid each other. In order to avoid introducing a substantial torque into the towers,
the planes of the stays should be alternated, such that the resultant passes through the
tower axis. This alternation tends to be visually untidy. In a deck with two planes of
cables and an inverted 'Y' frame tower with the stays anchored on the vertical leg, this
problem does not arise, Figure 18.15 (b).
Placing the anchors on the outside faces of the tower gives rise to problems with
respect to their weather protection, and complicates access for their initial installation,
later inspection and eventual replacement. These problems are not insurmountable,
and their solution depends on the expectations of the client, as well as on the size of
the bridge. For a small footbridge, for instance, temporarily closing the bridge to allow
the erection of a staging may be acceptable, while it may not on a higher tower or on
a heavily used highway or railway bridge.
Alternatively, the stays may be anchored on the near faces of a hollow tower. This
gives the best protection for the stay anchorages, and also offers the best facilities
for inspection and maintenance. Clearly, the tower will not be as compact as a solid
tower carrying the same loads. However, if the bridge is reasonably large, say with
a main span of 250 m or more, a box section tower appears in proportion to the
overall scale.
If the stays are to be stressed from this anchorage, there has to be enough room
within the void to install and operate the jack. Even if the tower houses only dead
anchorages, it must be possible to pull the stay through its ducting and to handle the
heavy anchorage components. It is also necessary to accommodate either access stairs
or, on large bridges, both stairs and a lift.
In order to transfer the stay force from the front to the back face of the tower,
both the Pont de Normandie and the Pont sur l'Elorn used steel fabrications, where a
pair of forward and back stays was connected by a steel box. These fabrications were
bolted onto each other as the tower was erected. Concrete was then cast around them
to complete the vertical strength of the tower.
The Ah Kai Sha Bridge and many others have been designed with all four faces of
the box section tower prestressed to carry the stay forces to the side walls and then
from front to back of the tower, Figure 18.15 (c).
It is necessary to position the stay anchorages very accurately so that they are
truly co-axial with the stays. Whereas this is easily done with a steel fabrication, in a
concrete solution it is more diffi cult. The solution adopted for the Ah Kai Sha Bridge
is described in
18.4.11
.
A development of the conventional concept is the use of 'V' shaped towers. These
lower the height of the tower which may offer environmental benefi ts in some cases
and save a signifi cant weight of stay cable. This option was adopted by Benaim as
a competition entry for the Sundsvall Bridge in Sweden, Figure 18.16. Whether the
savings in stays outweigh the extra cost of the towers needs to be proven by a fully-
fl edged preliminary design.
