Civil Engineering Reference
In-Depth Information
3.3.2 S
TEEL
R
AILWAY
B
RIDGE
S
UPERSTRUCTURES
Railway bridges transmit loads to substructures through decks, superstructures, and
bearings. The superstructure carries loads and forces with members that resist axial,
shear, and/or flexural forces.
The steel superstructure forms typically used in freight railway bridge construc-
tion are beams, trusses, and arches. These superstructures have the rigidity required to
safely and reliably carry modern heavy dynamic railroad live loads and the lightness
required for transportation to, and erection at, remote locations. Beam, truss, and arch
bridges can be constructed as deck or through structures depending on the geometry
of the crossing and clearance requirements (Figure 3.12). Steel frame and suspended
structures (i.e., suspension and cable-stayed bridges) are less common but some-
times used in lighter passenger rail bridge applications. Simple span construction is
prevalent on North American freight railroads for performance,
∗
rapid erection,
†
and
maintenance considerations. AREMA (2008) recommends simple span types, based
on length, for typical modern steel railway bridges as follows:
Base-of-rail
hrough plate
girder span
Deck plate
girder span
Base-of-rail
hrough truss
span
Base-of-rail
Half-through
arch span
FIGURE 3.12
Basic forms of steel railway bridges—beams, trusses, and arches.
∗
The high railway live load to steel superstructure dead load ratio often precludes the use of continuous
spans due to uplift considerations. AREMA (2008) recommends that dead load reactions exceed live
load reactions by 50% to avoid uplift.
†
Simple span construction is generally preferred by railroads due to relative ease of erection in comparison
to continuous spans or spans requiring field splicing.
