Civil Engineering Reference
In-Depth Information
it needs to involve at least 250 segments, which implies a total length of some 850 m
for a single box cross section. A ratio of side to main span of 0.43 defi nes a main span
of 450 m. If there are two or more boxes in the cross section, the viable length is
clearly less.
Decks may also be built by partially precasting the deck, and assembling the precast
elements by casting concrete in situ. For instance, for a deck consisting of longitudinal
concrete beams and transverse steel beams carrying a concrete slab, the steel beams
could be erected fi rst, supported on the shutters for the longitudinal beams. The rear
end of the shutters would be attached to the completed deck, while the forward end
would be supported by a cable stay. This could be a temporary stay, or the permanent
stay, subject to the ingenuity of the designer in fi nding a way to transfer the stay
from the falsework to the permanent works. The slab would then be erected either
as precast sections, or as pre-slabs. The deck would then be completed by casting the
remainder of the concrete slab and the main beams.
18.4.8 Tower design
Towers for cable-stayed bridges are normally made of concrete. There seems so little
justifi cation for making a member that is principally in compression in steel that
the author has no hesitation in dismissing the idea out of hand, except for small,
architecturally driven bridges. However, the towers for the Tatara Bridge in Japan,
which had, when it was built, the world's longest cable-stayed span of 890 m, are
in steel, and several major suspension bridges also have steel towers. Consequently,
there is clearly an aspect of tower design and construction that is outside the author's
experience.
The transverse forces on a tower consist of wind on the tower itself, on the stays, on
the deck and on the traffi c. They also may include seismic forces, principally related to
the self weight of the tower and deck. Longitudinally the forces include length changes
of the deck, wind and seismic action and braking and acceleration of traffi c.
The design of the towers depends critically on the articulation of the bridge deck.
To illustrate one extreme, the Pont sur l'Elorn has one plane of stays, and the towers
are an integral part of the deck, which is carried on bearings on traditional piers that
provide the longitudinal fi xity and transverse stability, Figure 18.7. This is unusual,
and for most bridges the towers carry the loads of the deck to the foundations, and
stabilise the deck both longitudinally and transversally.
Transversally the tower will tend to buckle as a free cantilever, while longitudinally
its head will be held in place by the stays. Consequently, transverse stability is the most
critical. If the stays are precisely aligned with the column axis, any lateral defl ection of
the column head will be resisted by an incremental horizontal component of the stay
force, providing a system with considerable inherent stability. If the stays are pulling
off the column axis, clearly they constitute a disturbing force. If a tower consists of
two columns connected by a prop as at Ah Kai Sha (
18.4.11
), the geometric stability
provided by the stays is re-established, as long as the two planes of stays are equally
loaded. Clearly a curved cable-stayed deck will require careful consideration in this
respect. An 'A' frame tower which is inherently stable transversally is well designed to
resist transverse forces with minimum bending moments and materials.
In the author's experience, the towers of cable-stayed bridges are almost universally
under-designed initially, and gradually increase in size as the designer meets and
