Civil Engineering Reference
In-Depth Information
• Bronze, copper-alloy, or polytetrafluoroethylene (PTFE) flat, cylindrical,
and spherical sliding plates—these bearing components enable translation
on low-friction surfaces. Bronze and copper-alloy sliding elements are self-
lubricating by providing graphite or other solid lubricants in multiple closely
spaced trepanned recesses. PTFE sliding plates should mate with stain-
less steel or other corrosion-resistant surfaces and contain self-lubricating
dimples containing a silicone grease lubricant.
• Roller bearings—these bearing elements allow translation through rotation
of single or multiple cylindrical rollers.
• Linked bearings—this type of bearing uses a double pin and link arrange-
ment between pedestals to allow for horizontal translation.
• Expansion hinged bearings—this type of hinged bearing uses a pin and
rocker (segmental roller) arrangement with the pin allowing rotation and the
rocker permitting translation.
• Elastomeric bearings—these plain or steel reinforced rubber, neoprene, or
polyurethane bearing pads allow translation through shear deformation of
the elastomer.
In addition to steel span expansion bearings, the bearings at the bases of columns
in steel bents and viaduct towers should be designed to allow for expansion and
contraction of the tower or bent bracing system.
There are many proprietary types of fixed and expansion bearings available to
the steel railway bridge engineer. Most are similar, or combinations of, the basic
elements described above (Stanton et al., 1999; Ramberger, 2002). Bearings of mixed
element types are not recommended (e.g., elastomeric fixed bearings with PTFE
slidingbearings).Detailedrecommendationsontypes,design,andfabricationoffixed
and expansion bearings for steel freight railway bridge spans are found in AREMA
(2008, Chapter 15). Due to the large vertical cyclical loads and exposed environment
of most railway bridges, bearing designs should generally produce simple, robust,
and functional bearings that are readily replaced by jacking of the superstructure.
3.3.3 B
RIDGE
S
TABILITY
Girders and trusses should be spaced to prevent overturning instability created by
wind and equipment-based lateral loads (centrifugal, wheel/rail interface, and train
rocking). AREMA (2008) recommends that the spacing should be greater than 1/20
of the span length for through spans and greater than 1/15 of the span length for deck
spans of freight railway bridges. The spacing between the center of pairs of beams,
stringers, or girders should not be less than 6.5 ft.
3.3.4 P
EDESTRIAN
W
ALKWAYS
Mostrailroadcompanieshavepolicies,basedonFederalandStateregulations,regard-
ing walkway and guardrail requirements for bridges. Width of walkways is often
prescribed by the railroad company, but should not be less than 2 ft. Guardrail height
is generally prescribed as a minimum of 3.5 ft by regulatory authorities but greater
