Civil Engineering Reference
In-Depth Information
Q
5
'
FIGURE 4.28
Derailment load.
4.4.5
L
OADS
R
ELATING TO
O
VERALL
S
TABILITY OF THE
S
UPERSTRUCTURE
4.4.5.1
Derailment Load
Events such as train derailments on bridges are relatively infrequent. However, par-
ticularly on long bridges, train derailments can occur and create overall instability of
individual spans. AREMA (2008) recommends an eccentric derailment load be used
to ensure stability of spans. This derailment load,
Q
, is a single line of wheel loads,
including impact, at a 5 ft eccentricity from the track centerline (Figure 4.28). It is
used as a load case for the design of cross frames and diaphragms in beam and girder
spans requiring lateral bracing.
∗
AREMA (2008) recognizes that damage to some
span elements may occur in these relatively extreme, but infrequent, events. A 50%
increase in allowable stress is permissible when determining stresses in cross frames,
diaphragms, anchor rods, or other members resisting overall instability of the span.
Example 4.22
Determine the Cooper's E80 derailment load forces in the brace frame
modeled in
Figure E4.12.
The calculated impact factor for the span is 40%.
The derailment force applied to the cross frame at (a) is assumed to be
transferred to the opposite girder through the cross frame members
ab
and
ac
. The force in the members is
[
80
(
1.40
)/
2
(
5
)
]
(
8
)
[
1
+
((
5
−
4
)/
8
)
]
P
ab
=
=
142.6 kips compression.
cos 45
◦
142.6 sin 45
◦
=
P
ac
=
100.8 kips tension.
∗
The tendency for the span to “roll over” is prevented by lateral bracing between beams or girders.
