Civil Engineering Reference
In-Depth Information
4
Loads and Forces on
Steel Railway Bridges
4.1 INTRODUCTION
The loads and forces on steel railway bridge superstructures are gravity, longitudinal,
or lateral in nature.
Gravity loads, comprising dead, live, and impact loads, are the principal loads
to be considered for steel railway bridge design. Live load impact (dynamic effect)
is included due to the relatively rapid application of railway live loads. However,
longitudinalforces(fromliveloadorthermalforces)andlateralforces(fromliveload,
wind forces, centrifugal forces, or seismic activity) also warrant careful consideration
in steel railway bridge design. An excellent resource for the review of load effects on
structures, in general, is ASCE (2005).
Railway bridges are subjected to specific forces related to railroad moving loads.
These are live load impact from vertical and rocking effects, longitudinal forces
from acceleration or deceleration of railroad equipment, lateral forces caused by
irregularities at the wheel-to-rail interface (commonly referred to as “truck hunting”
or “nosing”), and centrifugal forces due to track curvature.
4.2 DEAD LOADS
Superstructure dead load consists of the weight of the superstructure itself, track, deck
(open or ballasted), utilities (conduits, pipes, and cables), walkways (some engineers
also include walkway live load as a component of superstructure dead load), perma-
nent formwork, snow, ice, and anticipated future dead loads (e.g., larger deck ties,
increases in ballast depth, and additional utilities). However, snow and ice loads are
generally excluded from consideration due to their relatively low magnitude. For
ordinary steel railway bridges, dead load is often a small proportion of the total super-
structure load (steel railway bridges typically have a relatively high live load to dead
load ratio).
Curbs, parapets, and sidewalks may be poured after the deck slab in reinforced
concrete construction. This superimposed dead load may be distributed according to
superstructure geometry (e.g., by tributary widths). However, it is common practice to
equally distribute superimposed dead loads to all members supporting the hardened
deck slab. This is appropriate for most superstructure geometries, but may require
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