Civil Engineering Reference
In-Depth Information
and thus may be made thinner, recovering fl exibility. The essence of the design of such
split piers is to fi nd the correct compromise between the fl exibility and the elastic
stability of the leaves. In considering stability when the deck is in service, it is relevant
that usually only one leaf of a pair is highly compressed under any load case, with the
less compressed leaf providing stability by resisting translation of the deck. Also the
columns are usually only highly stressed in compression under transient loading, when
the higher, short-term Young's modulus is used to check buckling.
As well as considering the buckling of the individual leaves of the split piers, it is also
necessary to check the buckling mode where the whole deck translates longitudinally,
Figure 7.20. This will be controlled by the shortest piers, and may also be inhibited if
the deck is curved in plan.
A good example of a deck supported on split piers is the valley section of the Byker
Viaduct, Figure 1.5, described in
7.15.4
.
If the piers are very short, it may be necessary to further subdivide them in order to
reduce the shear stiffness enough, while maintaining adequate moment capacity. This
is demonstrated on the River Nene Bridge, described in
11.
6 and in Figures 11.16
and 11.17. It was necessary to make this bridge deck as slender as possible so that it
would fi t between an imposed highway alignment and river clearance requirements.
Thus a 700 mm thick prestressed concrete voided slab was allowed to span 39.9 m
by providing local haunches that were built into the piers. The piers, which were
divided into fi ve 200 mm thick precast concrete planks, provided a very high degree
of moment fi xity to the deck combined with very low shear stiffness.
7.13 Integral bridges
One of the aims of a designer of concrete bridges should be to reduce maintenance
by minimising the number and sophistication of mechanical engineering devices
required for its operation. Thus, wherever possible, piers should be built into the
deck. Where this is not possible, the next best option is to adopt concrete hinges
(codes allowing), then rubber bearings, then fi xed mechanical bearings, and last of
all, sliding bearings.
However, the greatest single item of maintenance expenditure on highway bridges is
the expansion joint. Not only do such joints need regular repair and renewal, but they
also allow salt-laden water to attack and corrode the substructure. Most attempts at
creating waterproof joints fail after a number of years of service. Expansion joints also
need to be inspected from beneath, which greatly increases the cost and complexity
of abutments.
Integral bridges take the philosophy of mechanical simplicity to its logical conclusion
by either pinning the deck to the abutment or building it in. This eliminates the expansion
joint and greatly simplifi es the abutment structure that becomes more like a pier.
The consequence of this type of design is that the abutment either rocks or slides
back and forth as the bridge expands and contracts, causing settlement of the backfi ll
behind the abutment, and disrupting the road surface. This is overcome either by
regular maintenance of the road surface, or by bridging the disrupted area with a short
transition slab. The slab is attached to the abutment, and so follows its movement,
sliding on the substrate. Consequently, a fl exible mastic type joint is required in the
blacktop at the end of the transition slab. Some authorities adopt transition slabs,
while others prefer to maintain the road immediately behind the abutment [6, 7, 8].
