Civil Engineering Reference
In-Depth Information
On the other hand, the Redding Bridge, crossing the Sacramento River in California,
designed by Jiri Strasky, Figure 18.24, [11, 12], has a much shallower sag and the
stressed ribbon itself is used as the bridge deck. As the author only has experience of
the latter type, the description will be limited to such bridges.
A correctly designed concrete stressed ribbon bridge has the same characteristics
of durability as conventional reinforced or prestressed concrete decks. Although
the torsional and bending strength of the slab give it good inherent stability, the
aerodynamic behaviour and the risk of excitation due to pedestrian footfalls of such
fl exible structures should always be investigated.
18.5.2 Design of the deck
The deck consists of a thin concrete slab hanging in a catenary: a concrete cable.
Loads are carried in direct tension, not in shear and bending as in a beam. As concrete
cannot carry tension safely, the slab is prestressed with cables. The force in the cables
must equal the tension in the catenary, plus an additional force which compresses the
concrete. If more load is applied to the deck, the tension in the catenary increases. This
will decrease the compression in the concrete; more of the force of the prestressing
cables is used for resisting the tension, and less for compressing the concrete. If the
load is further increased, the compression in the concrete will become zero, and the
catenary tension will equal the force in the cables. At the ULS, when the loads are
increased still further, the concrete will crack, and all the additional tension will be
carried by the cables.
Theoretically, a stressed ribbon could be built in reinforced concrete. However,
the concrete would be cracked through by the tension, exposing the reinforcement to
corrosion and creating the risk of fatigue as every change in load or geometry would
change the stress in the reinforcement. For a prestressed deck, variations in tension in
the ribbon alter the compressive stress in the concrete, but do not change substantially
the stress in the cables, and at working loads the concrete is not cracked through. Thus
the prestressing cables are not exposed to the risk of fatigue or corrosion.
The tension in the catenary and its sag are related by the equation
T
=
pc
2
/8
s
(Equation 1)
where:
T
= tension in the band
p
= the load per metre on the band
c
= the chord length or span
s
= the sag.
Variations in tension in the ribbon change its length and hence its sag. Temperature
changes, and creep and shrinkage of the concrete also affect the length and the sag. For
instance, if the temperature falls, the ribbon shortens and its sag
s
will decrease. If the
applied load
p
remains constant, the tension
T
must increase.
From this brief description, it should be clear that the structure is completely non-
linear; the principal of superposition does not apply as each new load or change
