Civil Engineering Reference
In-Depth Information
A considerable proportion of the shrinkage of the concrete will also have been
completed before erection. As a result of the reduced shortening of the deck, expansion
joints may be further apart, and longer lengths of deck may be pinned to, or built into
the columns. Also, early thermal stresses are virtually eliminated by the single-phase
precasting, giving rise to crack-free units.
Balanced cantilever bridges may be erected by crane, by shear legs (or beam and
winch), or by overhead gantry. The choice of method depends on the scale of the
bridge, on the weight of the segments, on the height of the deck above ground level
and on the nature of the terrain crossed.
The erection and attachment of the pier segment, the stability of the balanced
cantilever, the erection of the end spans and the construction of the mid-span stitch
are common to more than one method of erection, and they will be covered fi rst.
15.5.2 Erection of the pier segment
The erection cycle starts with the placing of the pier segment that must be cast with
precisely dimensioned downstand bearing plinths that accommodate the transverse
and longitudinal falls of the deck. Due to the presence of the diaphragm, this is usually
the heaviest segment and is frequently made shorter to limit its weight. For very deep
decks the pier segment may be subdivided, either vertically into two or more very
short segments, or horizontally, with match cast joints allowing its assembly without
casting concrete. The pier diaphragm needs to be very carefully designed to minimise
its weight, which involves thinking carefully about the purpose of each component.
When designing a major segmental bridge one cannot lazily adopt a diaphragm
consisting of a 2 m thick solid wall! It is also possible to cast the diaphragm in-situ
once the pier segment has been erected although this slows down the construction
programme and is rarely adopted.
As the crane or gantry cannot place the segment accurately enough, it is lowered onto
falsework which is equipped with jacks that correct its position before it is fi xed to its
bearings. The segment must be accurately positioned, in level and in crossfall as these
parameters cannot easily be corrected subsequently. In general, the fi ne adjustment of
the longitudinal gradient may be achieved by rotating the complete double cantilever
on its bearings, as long as this remains within their rotational capacity. If the deck rests
on sliding bearings, or if one is fi xed and one sliding, the orientation in plan of the
completed cantilever may be corrected by rotating it about a vertical axis. However, if
both the bearings are fi xed, either the pier segment has to be placed with near perfect
orientation, a diffi cult operation on a short base length, or one of the bearings must be
modifi ed to allow a small amount of adjustment before it is locked in position.
Mechanical bearings are conventionally attached both to the pier and to the deck
with cast-in anchors or dowels. The connection detail must allow the bearings to be
replaced. For precast concrete construction this causes problems of tolerance, as the
items that are cast into the pier segment must match exactly those cast into the pier
head. However, it is frequently unnecessary to connect the bearing to the segment.
Clearly, for all free-sliding bearings the friction between the steel top plate of the
bearing and the concrete segment will always be greater than that of the PTFE sliding
layer. Consequently no mechanical connection is needed.
For guided and fi xed bearings, it is necessary to investigate the critical combinations
of vertical and horizontal load, and compare them with the lower bound estimates
