Civil Engineering Reference
In-Depth Information
Plan of curved bridge with right spans
L
m 1
s
S
m 2
Floorbeam
Main girder or truss
Stringer
Plan of curved track on right span
FIGURE 3.7 Horizontally curved bridges using right spans.
3.2.4.1.2 Bridge Geometrics
Track curvature can be accommodated by laying out bridges using right or curved
spans. Right spans must be laid out on a chord to form the curved track alignment.The
individual right spans must be designed for the resulting eccentricities as the curved
track traverses a square span (Figure 3.7). The stringer spacing, s , may be adjusted
to equalize eccentricities in the center and end panels ( m 1 =
m 2 ); and in some cases,
such as sharp curves, it may be economical to offset the stringers equally in each
panel. However, fabrication effort and costs must be carefully considered prior to
designing offset floor systems. It is common practice on freight railroads to use m 1
between M /6 and M /2, where M
=
m 1 +
m 2 is given by Equation 3.22.
The superelevated and curved track creates horizontal eccentricities based on the
horizontal curve geometry (track curvature effect), e c , and vertical superelevation
(track shift effect), e s . These eccentricities must be considered when determining
the lateral distribution of live load forces (including dynamic effects) to members
(stringers, floorbeams, and main girders or trusses). The shift effect eccentricity, e s ,
is ( Figure 3.6)
h cg e
d
e s =
.
(3.28)
For example, through girder or truss spacing and design forces are increased due to eccentricity of the
track (curvature effect) and superelevation (shift effect).
An eccentricity, m 1 = M /3, is often used, which provides for equal shear at the ends of the longitudinal
members.
 
 
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