Civil Engineering Reference
In-Depth Information
V B =
150 lb/ft vibration load at the bottom lateral bracing.
Top lateral bracing design will be based on W T =
705 lb/ft in addition to
other lateral loads such as those due to live load (see Sections 4.3.2.3 and
4.3.2.4). Bottom lateral bracing design is based on W B =
450 lb/ft.
4.4.3 F ORCES F ROM THE CWR ON S TEEL R AILWAY B RIDGES
Continuously welded rail is used in modern track construction because it diminishes
dynamic effects (no impact forces due to joints in the rail), provides a smoother ride,
and results in reduced rail maintenance and increased tie life.The rail may be fastened
to the deck to provide lateral and longitudinal restraints. The deck is also fastened
to the superstructure to provide lateral and longitudinal restraints.
The longitudinal forces generated due to restraint of thermal expansion and
contraction of the rail may need to be considered in the design of some steel
railway superstructures. The distribution of longitudinal forces through the super-
structure may be of little concern for some superstructure types (e.g., multiple
longitudinal beam spans and deck plate girder spans). However, for other types
of superstructures and long spans, the longitudinal force path from rails to bear-
ings may be of importance (e.g., floorbeams with direct fixation of track and
some span floor systems). In addition, the CWR may experience internal stresses
due to bridge span movements from thermal actions or live load bending. The
magnitude of the CWR-bridge thermal interaction is governed by the following
conditions:
• Movementofthebridgespans,inparticularthemaximumspanlength,which
may freely expand in the bridge.
• The rail laying temperature (neutral temperature) and ambient temperature
extremes at the bridge (the temperature ranges experienced by the rail and
superstructure depend on neutral temperature, and maximum and minimum
ambient temperatures).
• The type of bridge (open deck, ballasted ) and bridge materials.
• The connection of rails-to-deck and deck-to-span interfaces.
• The cross-sectional area of the rail and the coefficient of thermal expansion
of the bridge.
• The location of fixed and expansion bearings (spans with adjacent expansion
bearings on the same pier create a long expansion length and generally
provide the governing condition for design).
Longitudinal restraint by elastic hold-down fasteners, friction, and/or rail anchors applied at the base-
of-rail against the ties.
Ballasteddecksaregenerallyrigidlyconnectedtothesuperstructure.However,opendeckspansmayhave
various degrees of longitudinal restraint depending on deck-to-superstructure connection (see Chapter
3). Open decks are often fastened to the superstructure with bolts or “hook bolts” installed at regular
intervals (e.g., every third tie).
For ordinary ballast deck bridges, the differential thermal movements are generally accommodated by
the ballast section.
 
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