Bridge geometry - Computational Analysis and Design of Bridge Structures - page 564

Final design curve

eoretical casting curve obtained
by backward analyses
Figure 18.17 Difference between the final curve and the curve to control casting.

Pier

be achieved during the precasting of segments. Figure 18.17 illustrates the
difference of these two curves. Note that the theoretical casting curve may
be above the final curve depending on the quantity of prestressing tendons
and other loads.

18.6.1.3 Casting segment and match cast segment
In short-line casting system, only one segment is casted at one time, which
is called casting or wet segment. Comparing the short length of each precast
segment and the length of the entire span, it can be easily understood that
the reproduction of theoretical casting curve is solely controlled by the con-
nections between segments. The accuracy of the shape of each individual
segment does not control the geometry as a whole. Therefore, the geometry
control of precasting segments is really the control of the connection face for
any two consecutive segments. This is achieved by casting a segment against
the segment it connects that is already casted. The segment is called match
cast segment, that is, the segment used to be matched for a new casting seg-
ment. Figure 18.18 illustrates these two segments. Figure 18.16 shows an
actual short-line casting system.

18.6.2 casting and matching
As shown in Figures 18.16 and 18.18, the formworks for the segment to
be casted are laid on the fixed casting bed. One end form is the bulkhead,
which is fixed as well. On the opposite of the bulkhead, the connection face
of the matching segment is used as another end form directly. Before cast-
ing, debonder is applied on the connection surface to prevent bonding of
the concrete. This match casting against the hardened surface of its connec-
tion segment leaves an almost invisible joint when segments are assembled.
As the match cast segment sits on top of the supporting soffit, both vertical
and horizontal alignments can be reached by adjusting screw jacks beneath
or the horizontal locations of the soffit. In case the superelevation exists, the
casting segment may be twisted relatively as shown in Figure 18.16c. This
can also be achieved by adjusting the support of match cast segment.
Figure 18.19 shows a 3D rendering of a segmental bridge being assem-
bled. Coordinate system Long-Alt-Lat as shown in Figure 18.19 is the
global coordinate system in which both final and theoretical casting curves