Bridge geometry - Computational Analysis and Design of Bridge Structures - page 561

in plane. In Figure 18.14, cross symbols show such locations in 3D. With
modern computer graphics techniques adopted, detailed information about
each point such as plane locations and elevations can also be shown when
hovering over a symbol, as rectangle boxes shown in both Figures 18.13
and 18.14.
18.6 Precast segmental BrIdge
geometry control
Precasting concrete girder segments in yard while substructure is being
built and assembling in place later is a popular construction method for
concrete box girder bridges. Many advantages such as eliminating time to
curing concrete and reducing concrete creep and shrinkage at earlier ages
made segmentally precasting method widely adopted in concrete box girder
bridges. As girder segments are casted in yard and assembled later in place,
how to ensure the finished bridge curve in close agreement with theoretical
bridge curves in both horizontal and vertical directions becomes a critical
issue in this type of construction method. Geometry control of girder seg-
ments during casting in yard so that errors in the finished bridge curve are
under control is a common and very important topic of precast segmental
bridge. In this section, key concepts and principles in precast segmental
bridge geometry control will be introduced.
18.6.1 Basics
18.6.1.1 Long - line casting and short - line casting
When girder segments are casted in yard, there are two different types of
casting: (1) long-line casting and (2) short-line casting (Baker 1980). In long-
line casting system, all segments of a cantilever or a span are casted in their
correct relative position on a continuous soffit of sufficient length. When
one segment is cast, the forms will be moved to the next segment position
along the soffit. Figure 18.15 shows the schematic of a long-line casting sys-
tem. Geometry control in long-line system is established by adjusting forms
and soffit before pouring concrete. In the perspective of geometry control,
the long-line casting system is easy to set up. The disadvantage of long-line
system is obvious that substantial space is required.
In short-line casting system, only one girder segment is casted at one
time on the casting bed, and cured segments are moved to the storage yard.
Figure 18.16 shows the schematic of a short-line casting system. The pros
and cons of this type of precasting are obvious too. As there is only one
segment to be casted in one time, the length of casting bed is limited. Forms
can be used repeatedly for other segments. The most important advantage