Civil Engineering Reference
In-Depth Information
in reinforced concrete action if the code of practice allows. The parasitic moments
increase the length and magnitude of the sagging moment envelope, creating a vicious
circle as yet more tendons are required to carry the increased moments, increasing
the parasitic moments in their turn. Consequently, the designer should make every
effort to reduce the number and length of these Stage 2 tendons. The sagging parasitic
moments may be slightly reduced by stressing some top fi bre Stage 1 tendons after
completing the stitch. Sagging parasitic moments diminish the total hogging moments
at the supports, reducing the number of Stage 1 cables. However, the saving in weight
of prestress at the supports is always less than the increase at mid-span, particularly
when the deck is of variable depth.
Stage 2 tendons may either anchor in bottom fl ange blisters, or rise up through
the web, providing shear relief around the quarter points of the span, a section that is
often the most critical for shear. This may allow any web thickening to be moved back
several segments. They anchor either in top fl ange cut-outs or in top fl ange blisters
either side of the pier diaphragm. When they are anchored beyond the pier, they
supplement the Stage 1 tendons for service loads.
In some schemes there are anti-symmetric Stage 2 tendons which anchor in a bottom
blister, cross the mid-span, and then rise up through the web and anchor in blisters
beneath the top fl ange beyond the pier, Figure 15.19 (c). In heavily prestressed bridge
decks, such tendons help overcome a shortage of anchorage points for Stage 2 tendons,
as they provide two mid-span continuity tendons for each symmetrical pair of bottom
blisters. However, such tendons are diffi cult to design as their ducts have inevitably to
be rotated about each other in the limited space of the bottom slab haunches.
15.5 Precast segmental balanced cantilever construction
15.5.1 General
The most widely used method of erection of precast segmental bridges is balanced
cantilever. It is adaptable to spans from 25 m up to about 150 m, and can cope with
virtually any succession of span lengths and deck alignments. The upper limit on span
is generally imposed by the weight of the deeper segments and the cost of the casting
cells, although if there is enough repetition, longer spans are viable. A typical deck
consists of pier segments, and a number of span segments that are usually symmetrically
placed about each pier, in balanced cantilever. The span is closed by a mid-span stitch,
cast in-situ. The joints are usually glued, although this is only essential when internal
tendons are adopted.
Cast-in-situ bridges built in balanced cantilever have been discussed in
15.4.
Only the characteristics that are specifi c to precast segmental construction will be
described here.
One signifi cant difference from cast-in-situ construction is that the segments are
several weeks old when they are erected, reducing the changes in bending moment
due to creep. Whereas typically the creep coeffi cient for the concrete of a cast-in-
situ deck is between 2 and 3, for a precast deck it is of the order of unity. (Clearly
these are broad generalisations; the creep coeffi cient needs to be calculated for each
specifi c case.) Except for very long spans, the effects of creep on the erection geometry
are reduced to virtual insignifi cance by the speed of erection for precast decks; six
segments in a day is not unusual.
