Civil Engineering Reference
In-Depth Information
For a member with a 25 ft long loaded length, the allowable fatigue stress
range is the CAFL stress range (greater than 2
10
6
cycles). Therefore, the
25 ft long stringers should not contain any details less than Category A (as
shown in
Table 5.5
and
Figure 5.36)
.
×
5.3.2.3
Other Design Criteria for Steel Railway Bridges
There are other specific design criteria relating to both the strength and serviceability
design of steel railway bridges that require consideration by the design engineer.
5.3.2.3.1 Secondary Stresses in Truss Members
Truss members may be designed as axial members (see Chapter 6) provided that
secondary forces from truss distortion,
∗
force eccentricity, end conditions (unsym-
metrical connections), or other effects do not create excessive bending stresses in
the members. AREMA (2008) recommends that truss members be designed as axial
members provided that secondary forces do not create stresses in excess of 4 ksi in
tension members and 3 ksi in compression members. For secondary stresses in excess
of 4 or 3 ksi for tension or compression members, respectively, the excess is super-
imposed on main primary stresses and the member designed as a combined axial and
flexural member (see Chapter 8). Combined axial and flexural stress
†
can occur in
many superstructure members. For steel railway superstructures, the members most
likely to be subjected to combined stresses are
• Truss members with secondary stresses in excess of 4 or 3 ksi for tension or
compression members, respectively.
• End posts in trusses, which are subject to bending and axial forces due to
portal bracing effects superimposed on the axial compression as a member
in the main truss (see
Section 5.2.2.1.2).
• Truss hangers
‡
are stressed primarily in axial tension but the effects of out-
of-plane bending must also be investigated in their design. Hanger allowable
fatigue stress range is of critical importance due to the cyclical tensile live
load regime and relatively short influence line.
• Girders and trusses with external steel prestressing cables (Dunker et al.,
1985; Troitsky, 1990).
5.3.2.3.2 Minimum Thickness of Material
Material thickness is related to strength and serviceability. AREMA (2008) recom-
mends that steel members should not have any components less than 0.335 in thick
(with the exception of fillers), but some design engineers specify a greater minimum
material thickness (often 3/8 or 1/2 in.). Gusset plates used to connect chord and web
∗
Truss distortion effects on a member are generally negligible for relatively slender members where the
width of the member parallel to the plane of distortion is less than 10% of the member length.
†
The classic example is the so-called “beam-column.”
‡
Truss vertical members without diagonals at the bottom chord panel point.
