Civil Engineering Reference
In-Depth Information
TABLE E5.7
S
)
3
i
Stress Range,
Δ
S
i
Cycles,
n
i
g
i
=
n
i
/
N
i
(
Δ
1
0.70
1
0.25
0.086
2
0.90
1
0.25
0.182
3
9.7
1
0.25
228.2
4
10.4
1
0.25
281.2
S
—
4
1.00
509.7
trains typical of grain, coal, and other bulk commodity traffic on NorthAmerican and
other heavy haul railways.
It is based on locomotives and equipment with maximum axle loads of 80,000 lb.
In addition, for loaded lengths or spans greater than 100 ft, it is based on a maximum
equivalent uniform load of 6000 lb/ft (see
Section 5.2.1.3)
.
These loads are charac-
teristic of modern train traffic that typically create shear forces and bending moments
equivalent to between Cooper's E50 and E80 load on railway spans (see Chapter 4).
∗
Therefore, since cycles corresponding to typical characteristic load geometry and
loaded length are considered, the maximum Cooper's E80 load stress may be used
for the fatigue design stress range with design cycles adjusted for the characteristic
load magnitude.
The recommended number of effective constant stress range cycles,
N
, is based
on an analysis of loaded lengths for various member types and lengths subjected to
a 110 car unit train (AREMA, 2008). The AREMA (2008) recommendations assume
variable amplitude stress range cycles estimated from 1.75
10
6
trains (60 trains per
day over a design life of 80 years), in order to provide infinite life for loaded lengths
or spans less than 100 ft long (
Table 5.3).
It may be required to increase the number
cycles shown in Table 5.3 for spans greater than 75 ft long to account for specific
load patterns used in accordance with a particular operating practice.
†
The analyses
also considered the cyclical loading based on orientation and number of tracks for
transverse members (generally floorbeams) and the effects on longitudinal members
bytransverseloadsapplieddirectlyat,orwithin,panelpoints(generallytrusshangers,
subdiagonals, and web members). For spans or loaded lengths greater than 300 ft, a
more detailed analysis by influence lines (see
Section 5.2.1.2)
or using structural
analysis computer software may be required. The adjusted equivalent number of
constant amplitude design stress range cycles,
N
, considering an E60 characteristic
load magnitude, is
×
N
v
Δ
m
N
v
60
80
3
S
E60
N
=
=
=
0.42
N
v
,
(5.56)
Δ
S
E80
∗
For longer spans (greater than about 50 or 75 ft depending on car lengths), modern unit freight train
traffic typically creates forces equivalent to about Cooper's E50-E60. For shorter spans, modern unit
freight train traffic can generate forces equivalent to about Cooper's E60-E80.
†
For example, it is theoretically possible to generate 55 cycles on spans almost 100 ft long with a repeating
load pattern of two loaded and two unloaded rail cars (AREMA, 2008).
