Civil Engineering Reference
In-Depth Information
Cooper's equivalent E load
110.0
100.0
90.0
80.0
70.0
60.0
50.0
40.0
Span length(ft)
30.0
0 102030405060708090100110120130140150
80
'
-432 k - 6 axle locomotives bending
36
'
-4 axle - 315 k cars bending
36
'
-4 axle - 315 k cars-UB bending
100 k alt bending
80
'
-432 k - 6 axle locomotives shear
36
'
-4 axle - 315 k cars shear
36
'
-4 axle - 315 k cars-UB shear
100 k alt shear
FIGURE 4.2
Equivalent Cooper's E loads for some modern railway freight locomotives and
equipment on simply supported bridge spans up to 150 ft in length.
E80 design load for spans less than 80 ft in length.
∗
Figure 4.2 also indicates that
the alternate live load is appropriate for short-span design where the effects of short
heavy axle cars with unbalanced loads can be considerable. The alternate live load
governs superstructure design for simply supported spans up to about 55 ft in length.
On short simply supported spans less than 30 ft in length, the alternate live load is
equivalent to about Cooper's E94 in flexure and has even greater effects in shear.
Some railroad companies may vary from the Cooper's E80 bridge design load
based on their operating practice. It is usual that the magnitude of the axle loads is
changed (e.g., 72 or 90 kips), but the axle spacing is unaltered. Therefore, different
bridge designs can be readily compared.
However, the flexural cyclical stress ranges created by Cooper's E80 design load
do not necessarily accurately reflect the cyclical stress ranges created by modern
railway freight equipment.
Figures 4.3a
and b show the variation in mid-span bending
moment when 25 and 60 ft simply supported spans, respectively, are traversed by
various train configurations. The Cooper's E80 live loads appear to be conservative
representations of the design stress range magnitude for both the 25 and 60 ft spans.
†
∗
These cars are very short and heavy, and not typical of those routinely used on NorthAmerican railroads.
They are, however, representative of equipment currently used on some specific routes and, in terms of
weight, the potential direction for future freight equipment.
†
This is accounted for with adjustments to the number of equivalent constant amplitude cycles based on
the ratio of typical train loads to Cooper's E80 load (see Chapter 5).
