Civil Engineering Reference
In-Depth Information
• The longitudinal force due to locomotive traction is LF
T
=
(
9
/
8
)
829
=
933 kips on the entire bridge. However, this is not likely (unless a string
of powered accelerating/decelerating locomotives traverses the bridge)
and other portions of the bridge should be investigated. For example, the
longitudinalforceduetolocomotivetraction,LF
T
=
(
9
/
8
)(
400
/
600
)(
612
)
=
459 kips on the deck truss span when the train is on spans 7-10 only and
(
9
/
8
)(
400
/
980
)(
783
)
=
359 kips when the train is on spans 1-7 only.
• The longitudinal force due to locomotive traction is LF
T
=
(
9
/
8
)
500
=
563 kips on the 400 ft deck truss span.
The longitudinal force due to train braking, LF
T
=
574 kips, is likely to be
used for design of the deck truss span.
AsnotedinExample4.8,thedistributionoflongitudinalforcesinthesuperstructure
maybeoflittleconcernforsomespantypes(e.g.,multiplelongitudinalbeamanddeck
plate girder spans). However, for other types of superstructures, the longitudinal force
path from rails to bearings is of considerable importance (e.g., floorbeams with direct
fixation of track and span floor systems). The horizontal axial force resistance of deck
plates from diaphragm behavior may preclude the need for bracing elements to carry
longitudinal forces to the main girders or trusses. Nevertheless, in some open deck
spans, specific consideration of the lateral bracing (traction bracing) requirements
is necessary to adequately transfer longitudinal forces to the main girders or trusses
for transfer to the substructures at the bearings. A typical instance where traction
bracing may be required is within the panel adjacent to the fixed bearings in an open
deck span with a stringer and floorbeam system supported each side of the track by
long-span main girders or trusses. In order to preclude the torsional and/or lateral
bending of floorbeams that might result from longitudinal forces transmitted by floor
systems without connection to the lateral bracing
(Figure 4.14a)
,
traction bracing is
used (Figure 4.14b). Traction bracing is provided through connection of the stringers
to the lateral bracing and addition of a new transverse member (shown dashed in
Figure 4.14b) between the stringers at the bracing connections. Provided the main
girder or truss fixed bearings are adequate to transfer the longitudinal forces to the
substructure, the traction bracing truss (Figures 4.14b and
4.15)
will avoid lateral
loading of floor beams (member 1-1 in Figure 4.15) since the stringers (members 2-3
in Figure 4.15) can carry no longitudinal force. Other traction bracing arrangements
may be used in a similar manner at the fixed end of long single and multiple track
spans to properly transmit longitudinal traction and braking forces to the bearings.
4.3.2.3
Centrifugal Forces
Centrifugal forces acting horizontally at the vehicle center of gravity (recommended
as 8 ft above the top of the rails in AREMA, 2008) act on the moving live load as it
traverses the curved track on a bridge, as shown in
Figure 4.16.
The centrifugal force
corresponding to each axle load is
m
A
V
2
R
CF
A
=
,
(4.29)
