Civil Engineering Reference
In-Depth Information
of a typical highway bridge will not be very highly stressed in compression (except for
very long span bridges or through girders), the effect of shear lag generally does not
result in overstressing of the concrete in compression due to longitudinal bending.
Side cantilever slabs more than 2 m long usually have a variable thickness. This
reduces the intensity of the longitudinal shear stress and logically should thus increase
the length of cantilever that participates in the longitudinal bending. Conversely, where
the side cantilever is carried by transverse ribs, it can be thinner than a cantilevering
slab, reducing the ratio of the root thickness to the area of the slab, leading it to be
more highly stressed in shear and increasing the effect of shear lag. These factors are
not recognised by the simplifi ed rules of thumb, or by many codes of practice.
9.2.5 Prestressed cantilever slabs
In the author's experience post-tensioned cantilever slabs designed to Class 1 or
Class 2 (
4.2.3
and
4.2.4
) are rarely cost effective for the following reasons. Prestressing
tendons usually need to be anchored at or near the ends of the cantilevers and so
cannot be effectively curtailed, while reinforcing bars may be tailored to the local
bending moment. Also, due to the relatively large size of prestressing ducts as compared
with reinforcing bars, to the fact that tendons must be inside the surface grillage of
distribution bars and the fact that prestress is designed at the SLS while reinforced
concrete is generally designed at the ULS, the prestress lever arm is signifi cantly less
than that for reinforced concrete, Figure 9.3 and Figure 6.31.
As the deck becomes wider and the slabs become thicker, the defi cit in the lever
arm reduces and prestressing gains in competitiveness. Also, in the thicker slabs, it
may be possible to achieve a degree of curtailment of the prestress and save some steel
by providing intermediate buried dead anchors. If the bridge deck is made of precast
segments, it may well be possible to use pre-tensioning for the transverse reinforcement,
which is likely to be more economical than post-tensioning.
The former French code for transverse prestressing of decks allowed tensile stresses
to penetrate to the surface of the duct for normal live loading, and to the centre of the
duct for exceptional deck loading, with no tensions under permanent loads. This is a
sensible method which increases the prestress lever arm. However, the most rational
and competitive way of sizing a prestressed concrete slab is to design the section at the
ULS, and to check at the SLS that no tensions occur under dead load, and that the crack
widths under live loading are acceptable. A slab designed in this way will be uncracked
over the greater part of its life, and will be both economical and more durable than
reinforced concrete. Unfortunately, some codes do not accept this approach.
If the slab is subjected to loading that is highly repetitive and could constitute a
fatigue risk, such as mass transit trains, or a proportion of standard highway loading,
it would be sensible to ensure that the tendons are in uncracked concrete under these
loads.
9.2.6 Precautions at expansion joints
Particular care must be taken in the dimensioning of side cantilevers at the ends of
bridges. Here, the total defl ection of the cantilever under combined dead and live
loading relative to the fi xed abutment wall may adversely affect the roadway expansion
joint. It is usually necessary to carry the cantilever by a transverse rib that provides the
