Bridge geometry - Computational Analysis and Design of Bridge Structures - page 562

Forms moving direction

Movable forms

Casting segment

Cast segments

(a) Plane view of fixed sot and casting sequence of a long-line system

Segments to be cast Casting segment

Cast segments

Vertical casting curve

(b) Elevation view of fixed soffit and casting sequence of a long-line system
Figure 18.15 (a) Plane and (b) elevation view of a long-line casting system.

is that the forms can be built as machinery so as to be easily unfolded and
folded, for both quantities and size permit to do so. Therefore, higher qual-
ity of precast can be achieved. The disadvantage of short-line system is the
geometry control during the casting of each segment. Imaging a curved
box being sliced into many short segments, the deliberated geometry con-
trol measurements have to ensure different segments are casted in their
right shapes so that the theoretical girder can be reproduced when all are
resembled in place. This section will mainly discuss on the geometry con-
trol during precasting in short-line system.

18.6.1.2 Final curve and theoretical casting curve
As casting segments are laid on casting bed or supported, conditions are
different from when they are assembled where structural displacements due
to dead loads and/or poststressing happened. There are two types of girder
curves involved during precasting and assembling of a precast segmen-
tal bridge. The first one is called the final curve , which is what engineers
designed and expected after a bridge is built. For a segmental constructed
bridge, there will be many permanent load applications after a segment is
assembled that cause the girder curve change from the initial condition.
Examples of these loads include structural weight of a girder segment, pre-
stressing, concrete creep or shrinkage, and superimposed dead loads. The
second curve is called the theoretical casting curve , which is what geometry
control is aiming at. It can be imagined that the theoretical casting curve is
what all segments should form after assembled without any load applica-
tion, as there is no load applied on segments while casting in yard. The the-
oretical casting curve can be obtained by backward analyses, in which each
applied permanent load is removed one by one from closure stage. From
the perspective of precasting geometry control, it can be simply taken that
a theoretical casting curve, which is different from the final curve, should