Civil Engineering Reference
In-Depth Information
in the design process. One option suitable for a single plane of cables would be to
use twin stays at each position.
If the site is suitable, the side spans may be provided with intermediate supports
that anchor the cables and virtually eliminate bending in the tower, Figure 18.9 (b).
Although allowing a slender tower, these supports add to the additional cost of the
harp arrangement and rather defeat the logic of a cable-stayed deck. When the side
spans do not require expensive long-span technology, an arch, tied or otherwise,
with short-span approach viaducts may be a more economical structural type.
The weight of stay steel and the compression in the deck are both greatest in
the harp arrangement. Despite its lack of economy, the harp is adopted for its
harmonious appearance.
(b) In the fan arrangement all the stays radiate from the top of the tower,
Figure 18.9 (c). Although the longest stay has the same inclination as in the harp,
the stays become progressively more vertical and the force in them necessary to
balance the load drops until the factor becomes unity for a vertical stay. When the
main span is loaded, the additional force in the stays is transferred directly through
the tower head to the backstays that are anchored to the end pier. Consequently,
the application of live loads on the deck or the removal of a stay does not impose
signifi cant bending moments on the tower.
The principal disadvantage of the fan arrangement is the diffi culty in housing
all the stay anchors in a very compact space. Such bridges often require a pier head
swelling that is not only odd in appearance, but is complex to design and build.
(c) Most modern bridges adopt the semi-harp arrangement, where the stay anchors
are concentrated on the upper part of the tower, Figure 18.9 (d). The stays are
spaced as closely as is possible, without resorting to expensive expedients such
as combined base plates. This generally means that the upper stays that are more
perpendicular to the tower will be spaced at about 1 m vertically, while the lower
more acutely inclined stays will be progressively further apart. The weight of stay
steel is marginally greater than in the fan arrangement. The bending of the tower
is also greater, but experience shows that the compression in the tower is such
that this bending moment does not cause diffi culty in design or require dense
reinforcement.
The spacing of the stay cables along the deck is a critical design decision. Early
cable-stayed bridges tended to use few very powerful stays, requiring a deck that had
the capacity to span 60 m or more between stays, Figure 18.9 (b). Such bridges are
relatively complex to build, as the deck needs temporary support as it cantilevers
towards the next stay, or it has to be erected in very long lengths that span from
stay to stay. The stays are very powerful, and generally adopt expensive locked coil
technology, while the transmission of their force to the deck also requires expensive
details.
The trend for modern bridges has been to use smaller stays spaced at between
5 m and 10 m. This arrangement of stays reduces the self-weight bending moments in
the deck to insignifi cant proportions, and opens the door to the very slender simple
decks described in
18.4.6
. The stays are based on prestressing technology (
18.4.9
),
and the smaller forces make the connection details simpler. Generally, the stay spacing
is related to the construction module, such that as each new section of deck is cast or
lifted into place, it is carried by a new set of stays.
