Civil Engineering Reference
In-Depth Information
to leave the prop loaded by an unbalanced cantilever for weekends, or even in extreme
cases, overnight!
The dynamic load case of a segment being dropped due to sudden failure of the
lifting mechanism is a realistic scenario when the deck is erected by shear legs. In the
absence of a rigorous calculation, the dynamic component of the overturning moment
is generally considered to be equal to the weight of the segment × its distance from
the axis of the pier or prop. The connection of the shear legs to the deck must be
designed with adequate redundancy, to make it inconceivable that the shear legs could
fall together with the segment.
If segments are erected by crane, and if the temporary prestress is designed with
adequate redundancy so that a bar may fail without loss of the segment, this dynamic
load case is probably not relevant.
When a deck is erected by gantry, the sudden failure of the segment-lifting
mechanism would apply a shock load to the gantry. The effect on the stability of the
deck will depend on the manner in which the gantry is supported. For instance, if it
spanned from pier to pier, this accident would have little effect on the deck. However,
if the deck carried one of the legs of the gantry, the dynamic effect of the sudden loss
of a segment could be transmitted to the deck.
15.5.6 Building the end spans
The discussion on this subject in the section on cast-in-situ construction (
15.4.5
) is
in general valid for precast segmental construction. The principal difference between
the two technologies is the speed of erection of the latter. It is easier to adopt the
progressive cantilevering solution, as the temporary props need only be loaded for
a matter of a few days, giving little scope for consolidation settlements. If there are
several end span segments beyond the point of symmetry of the balanced cantilever,
they may be assembled on falsework using temporary stressing to make them self-
supporting. This run of segments may then span freely from the abutment bearings
onto one temporary prop, which may be jacked to line up the segments with the
balanced cantilever, and then joined with a short stitch, Figure 15.27 (a).
When a long continuous viaduct is divided into expansion lengths, there are
inevitably pairs of end spans that meet on a pier. The use of falsework in such a
repetitive situation is not economical, and if the viaduct is in town, may not be possible.
On the STAR viaduct the expansion joint over the pier was temporarily blocked, and
three segments either side of the pier were cantilevered using temporary prestress.
These temporary cantilevers were then joined to the balanced cantilevers by a cast-in-
situ stitch and the permanent prestress installed. The temporary prestress could then
be removed and the expansion joint freed, Figure 15.27 (b).
15.5.7 The mid-span stitch
Once the cantilever erection is complete, it remains to cast the mid-span stitch and
connect the new double cantilever to the completed portion of the viaduct. The
stitch may be as long as a typical segment giving aesthetic advantages as the segment
joints become evenly spaced. This solution was adopted on the East Moors Viaduct.
However, it was found to be slow to build and not very economical, as it requires a
substantial formwork to be mobilised and a large reinforcing cage to be prepared.
