Civil Engineering Reference
In-Depth Information
from the fi xed stop-end, defi ning the length of the new segment to be cast. This stop-
end segment projects a short distance (
100 mm) into the side shutters and over the
new segment trolley. This allows the side, soffi t and core shutters to clamp onto the
stop-end segment, providing a grout seal.
The stop-end segment is then put approximately (to the nearest millimetre) into the
correct relative position to the fi xed mould to defi ne the deck alignment. This is done
by acting on the adjustments of the trolley in all three dimensions.
The new segment is cast, generally in the late afternoon. Four studs are inserted into
the top surface of the segment, over the webs near each end, and a small steel bar is
cast into the top surface of the segment on its centre line at the end remote from the
fi xed stop-end. The concrete is allowed to harden overnight.
The critical segment survey is then carried out, generally before the site has fully
opened, so that vibrations due to moving plant do not disturb the measurements. The
survey is carried out by the theodolite aiming at the target which defi nes a centre line
and a horizontal plane. The levels of the four studs on the new segment are recorded
using a staff, and the axis of the line of sight scratched onto the steel bar. The elevation
of the studs on the stop-end segment and the offset of its centre-line scratch are also
measured.
The accuracy of these measurements is critical to the method and usually the
theodolite is read twice by different operators to avoid the risk of booking or reading
errors.
The achieved geometry of the stop-end segment may then be compared with the
theoretically correct geometry necessary to create the deck alignment. The error with
respect to the theoretical alignment is calculated, and the set-up position of the new
stop-end segment for the next cast is chosen such that this error is corrected.
After the survey, the side shutters are removed and the core shutter extracted. The
stop-end segment is then taken away to storage, and the new segment rolled forwards
on its trolley, becoming the next stop-end segment.
Mistakes or bad practice in the casting yard may create errors in the alignment.
Other than gross mistakes such as erroneously booking the results of the survey,
there are more subtle errors that may derive from poor procedure or defects in the
equipment. As a result, there are in effect two levels of this technology. Where the
deck includes cast-in-situ joints at no more than say 50 m intervals when built span-
by-span, or 100 m intervals when built in balanced cantilever, a faultless mathematical
analysis of the alignment allied to a good understanding of the theory and practice of
the method is suffi cient to ensure adequate dimensional tolerance of the completed
alignment. It is however possible to build decks many hundreds of metres long without
cast-in-situ joints, or to cast complicated sections of deck such as bifurcations. These
sophistications require a full in-depth understanding of all the possible sources of error
in the casting yard and their effects on the erected alignment, allied to very careful
procedure.
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14.3.6 Length of segments
As the basis of the method requires the precise survey of the relative position of two
segments in the casting cell, this becomes less accurate as the segments become shorter.
The minimum practical length of match cast segments is about 2.5 m, except for
