Civil Engineering Reference
In-Depth Information
movements due to temperature or other longitudinal effects.
∗
In addition to these
translations and rotations, the bearings must also transmit vertical, lateral horizon-
tal, and in the case of fixed bearings, longitudinal horizontal forces. Vertical forces
are transferred through bearing plates directly to the substructure. Uplift forces may
exist that require anchor bolts and many designers consider a nominal uplift force for
the design of bearings, in any case. Horizontal forces are usually resisted by guide
or key arrangements in bearing elements that transmit the horizontal forces to the
substructure through anchor bolts.
Elastomeric bearings may be used at the ends of short spans of usual form
†
to
accommodate expansion and rotation. However, for longer spans end rotation is per-
mittedusingsphericaldiscs,curvedbearingplates,orhinges,andexpansionisenabled
by low-friction sliding plates, rockers, or roller devices. Multirotational bearings may
be required for long, skewed, curved, complex framed, and/or multiple track bridges
orforthosebridgeswheresubstructuresettlementmayoccur.Constrainedelastomeric
(pot) bearings have been used with success in many applications. However, they are
not recommended for steel railway superstructure support due to experience with
bearing component damage from the high-magnitude cyclical railway live loads.
Typical fixed bearing components used on North American freight railroad steel
bridgesthattransmitverticalandhorizontalforceswhileallowingforrotationbetween
superstructure and substructure are
• Flat steel plates—this type of bearing component has limited application
due to inability for rotation and should not be used in spans
>
50 ft and in
any span without careful deliberation concerning long-term performance.
• Disc bearings—this spherical segment bearing component allows rotation in
anydirection(e.g.,longitudinalrotationscombinedwithhorizontalrotations
due to skew and/or radial rotations due to curvature).
• Fixed hinged bearings—this type of hinged bearing uses a pin and pedestal
arrangement to resist vertical and horizontal forces and enable rotation at
the pin.
• Elastomeric bearings—these plain or steel reinforced rubber, neoprene, or
polyurethane bearing pads allow rotation through elastic compression of the
elastomer. The design of elastomeric bearing pads is a balance between the
required stiffness of the pad to carry vertical loads and that needed to allow
rotation by elastic compression.
Typical expansion bearing components used on North American freight railroad
steel bridges that transmit vertical forces while allowing for rotation and translation
between superstructure and substructure are
• Flat steel plates—this type of bearing component has limited application
due to a deficient ability for translation unless maintained with lubrication.
∗
For example, translations due to braking and traction forces, construction misalignments and loads,
support settlements, and thermal effects (particularly concerning CWR as outlined in Chapter 4).
†
For example, elastomeric bearings might not be appropriate for spans greater than about 50 ft or for
heavily skewed bridges.
