Bridge geometry - Computational Analysis and Design of Bridge Structures - page 566

Long′

Alt′

Match
segment

Lat′

Casting
segment

F i xe d b u l k h e a d
Figure 18.20 Local (casting yard) coordinate system and control points for alignment.

locating of the match segment to ensure a perfect (theoretical casting) vertical/
horizontal curve and superelevation, if applicable, can be achieved.

18.6.3 control points and transformation
As shown in Figure 18.20, adjustments of match segment are controlled by
measurements of certain control points on concrete segments. Mathematically,
three points on a rigid body are enough to determine its location in space. For
the purpose of easy practice and the need to control the shape of casting seg-
ment, six points on top of a segment are used as control points: two points
on each web centerline and two on the theoretical centerline. Longitudinally,
the control points are located at the segment edge as close as possible, 51 mm
(2″) offset from the edge, for example, so as to produce enough control of the
vertical alignment.
Measurements of control points are done in casting yard under a local
coordinate system (Long′-Alt′-Lat′), whereas theoretical curve values are
established in global coordinate system. Transforming of a 3D point from
the global system to the casting system is essential. As described in detail in
Section 18.6.7, given a defined local system, transforming between global
and local is simple.

18.6.4 Procedures of casting and control
The first segment to be casted is different from successive segments as both
ends are against bulkhead, rather than the normal casting as shown in
Figure 18.16. After the first segment is cured and before moving to match
cast position, all control points' coordinates in the casting system are sur-
veyed and are taken as as-cast values.