Bridge geometry - Computational Analysis and Design of Bridge Structures - page 548

For the geometry modeling purpose, a term of mainline is introduced.
Roadway mainline is just the centerline of the roadway, the geometry at the
center of the roadway in terms of roadway design, not necessarily the middle
of the roadway. Not only many roadway geometric characteristics but also
their bridge components depend on the mainline geometry. Figures 18.1
through 18.3 show the relationship between the mainline of a roadway and
bridge girders. In general, the girder geometry should follow the mainline
in both horizontal and vertical curves as shown in Figures 18.1 and 18.2,
respectively (Wang and Fu 2013).
The location of the mainline or its alignment in transverse and verti-
cal curves is critical and should be unique. For the purpose of roadway
geometry design, mainline is always aligned with the control point in the
transverse direction on top of the roadway surface, which is not necessarily
the center of a roadway. The thick line in the middle of Figure 18.3 shows
the mainline and its location.
18.2.1 types of horizontal curves
When a vehicle runs on a curve, the horizontal centrifugal force is propor-
tional to the reciprocal of curve radius or curvature of the roadway. To pro-
vide an acceptable riding smoothness and to meet physical requirements,
(a)
(b)
Figure 18.1 Roadway and girder horizontal curves. (a) Horizontal curves of a roadway
centerline and span layout. (b) Girder axes follow roadway horizontal curves.
Figure 18.2 Girder axes follow roadway vertical curves, and curves in girder profiles.