Bridge geometry - Computational Analysis and Design of Bridge Structures - page 552

Figure 18.8 Plane view of a transverse curve transition example.
can be gained from vehicle gravity to help balancing centrifugal forces. The
change of transverse slope from a normal rate to accommodate turning is
called superelevation of a roadway. When designing superelevations, the
superelevation change rate, the final transverse slope, and the runout/run-
off length should meet certain requirements. However, the transition rate
of superelevation or the change of transverse slope within the runout/runoff
length is usually linear as shown in Figure 18.7.
When turning along a curve segment, vehicle wheels are easily off-tracking,
hence curve roadway widening is needed to ensure safety and to protect
shoulders from impacting. Similarly, transitions from normal-width segment
to widened segment, as shown in Figures 18.7 and 18.8, should be designed.
As superelevation transitions, superwidening transitions are usually linear,
that is, the change of the width is linear in terms of transition length.
18.2.5 Bridge curves
As part of a roadway, most bridge structures have to comply with geometries
set forth by a roadway globally. Therefore, the geometry of a bridge axis
or mainline is the same as a roadway curve in both horizontal and vertical
directions (Figures 18.1 through 18.3). Deck curves in transverse direction,
including superelevation and superwidening, must also meet requirements
from road transverse curves.
For most girder bridges, geometry of a girder axis follows the geometry
of a bridge axis, or mainline, so as to form the deck plan accordingly. For
example, a girder in a multiple-girder bridge is parallel to the bridge main-
line in horizontal curve and has a vertical curve as defined by the vertical
curve of the bridge mainline and transverse curve of the roadway. However,
cases where deck curves in both horizontal and transverse directions are
made up of deck components themselves are common too. For example,
straight girders are often used in small-curvature bridges, especially those
in simply supported multiple-span bridges.
In addition to bridge mainline geometries, girders in a continuous bridge
may be haunched longitudinally to incorporate changes of internal forces.
Haunches can happen in both concrete and steel bridges. Figure 18.1 shows