Civil Engineering Reference
In-Depth Information
Variable factor of safety (FS) for compression members
1.96
1.94
1.92
1.9
1.88
1.86
FS (Variable)
FS AREMA
1.84
1.82
1.8
0
0.1
0.2
0.3
0.4
0.5
KL / rCcr
0.6
0.7
0.8
0.9
1
FIGURE 5.31 FS for compression members.
AREMA recommends an FS of 1.95 for axial compression members of all slen-
derness ratios. This may be appropriate unless the fabrication and erection can be
carefully controlled to avoid eccentricities or other unintended secondary effects in
axial compression members.
The allowable stress approach ensures all members behave elastically, which is
appropriate for steel with its well-defined elastic behavior and tensile yield stress.
Also, since stresses from various loads and load combinations are maintained within
the elastic region of behavior, load superposition is possible.
However, the ASD FS does not consider the real uncertainties associated with
loads or combinations of loads. AREMA (2008) recommends modification of the
FS (modification of allowable stresses) for design load combinations based on the
probability of the loads being applied concurrently to the member. Furthermore, the
use of a single safety factor against yielding for many different loads within a load
combination is a shortcoming ofASD. Methods that provide a probabilistic approach
to the estimation of loads and member strength (partial safety factors) have, therefore,
been adopted by many international building and bridge design guidelines, recom-
mendations, codes, and specifications. However, the use of a single safety factor is not
a significant shortcoming for ordinary steel railway superstructure design due to the
relatively high live load to dead load ratio and the importance of deflection and fatigue
criteria (both evaluated at service loads). In addition,ASD does not consider the local-
ized yielding and load redistribution of steel structures at failure. Railroad operating
requirements (see Chapter 3) make this a valid approach in regard to behavior at
failure.
Beams, girders, trusses, arches, and frames are subjected to internal normal and
shear stresses across cross sections caused by internal axial forces, shearing forces,
torsional moments, and bending moments. Steel beam and girder design are based
on shearing stresses and normal stresses caused by bending moments and shearing
forces. Steel arch design is based on shearing stresses and normal stresses caused
by bending moments, axial forces, and shearing forces. Steel truss design is con-
cerned primarily with axial forces causing normal stresses, although eccentricities
and secondary effects (e.g., due to deflections) might create additional normal (due
 
 
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