Bridge geometry - Computational Analysis and Design of Bridge Structures - page 567

The first segment is moved to match cast position, and the casting bed
is ready for the second segment. A new local system aligned in face of
the second segment is established. Of the 12 points, six points for the
first segment (match cast segment), which is already casted, and six for
the second segment (casting segment) are transformed from the global
system to this new local system. Transformed values for the match cast
segment are used to guide the adjustment of supporting jacks so as to
ensure the end form of the casting segment is in correct position and ori-
entation. Values for the casting segment are used to guide the punching
of six marking bolts on top of casting segment (through the connection
to rebars beneath). This step is a main part of geometry assurance and
is called setup . Control values in this setup are also called setup values
(LoBuono 2005).
Again, after the casting segment is cured and before moving to match
cast position, as-cast values are surveyed again. Theoretically, as-cast
values are the same as setup values. The earlier process will be repeated till
all segments are casted.
18.6.5 error finding and correction
However, as-cast values are not exactly the same as setup values in real-
ity. Their difference from setup values indicates the existence of geometry
error. If these errors are simply ignored, they will be accumulated along the
casting, and thus the final curve after assembling will be out of control. To
make sure the as-cast curve is in close agreement with the theoretical curve,
the error must be detected and corrected during casting.
It should be noted first that the as-cast values of a segment are mea-
sured in the segment's own local system. For example, as-cast values of
the second segment are under the local system of the second segment
and as-cast values of the third segment are under the third's local system.
Transforming these local as-cast values from each individual local system,
as-cast curve in the global system can be obtained. Finding casting error
can be done by further comparing as-cast curve with the theoretical curve.
Figure 18.21 shows the elevation errors between as-cast curve and theo-
retical cast curve. Errors on plane and twist along the longitudinal axis are
in the same manner.
As shown in Figure 18.21, corrections can be done using adjusted setup
values for match cast segment when casting segment is set up. The adjusted
setup values are obtained by transforming the as-cast points of match cast
segment, instead to the local system of casting segment. Chances are that
accumulated as-cast errors are too big to be fully corrected in the imme-
diately followed casting segment, for too big a kink in vertical, planar, or
longitudinal twist not satisfying the smooth geometry requirements. In that
case, as shown in Figure 18.21, the error can be partially corrected in the next