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Steel Designers' Manual - 6th Edition (2003)
Bearings 859
racy of setting out of radially-guided structures, and the magnitude of the forces
developed in tangentially-guided structures. 7
28.2.4.4 Structures with fixed bearings and flexible supports
An alternative to the use of systems of guided bearings is to provide fixed bearings
at more than one (possibly all) supports. In this case the supporting structures (e.g.
bridge piers) have to be designed to flex and accommodate the necessary movements.They
also have to cater for the forces developed by these movements in addition
to any other design loading effects. This arrangement may be appropriate when
it is required to share horizontal load effects over several supports, but it should be
noted that replacement of the bearings may be more difficult owing to horizontal
loads which may be locked into the bearing/support arrangement.
28.2.4.5 Other considerations
(1) Wedging action. It is possible to utilize a form of ‘wedging action’ to resist
horizontal loadings by setting two (usually elastomeric) bearings on planes
inclined to one another as shown in Fig. 28.11. Equally, it is also possible to
develop the action inadvertently by errors in bearing setting out, and thus
attract more loading than that for which the unit is designed. 4,5,7
(2) Shock transmission units (STUs). Although not strictly bearings, these units can
be utilized in conjunction with bearings to distribute certain components of
loading to other parts of the structure. The units typically consist of a cylinder
filled with putty-like material which is acted on by a piston with a hole in it
through which the putty can flow. Slow, steadily applied forces such as thermal
expansion forces will cause the putty to flow from one side of the piston to the
other, and allow dissipation of the force through movement. Rapidly applied
forces such as seismic loads, braking loads, or wind gusts are too fast to allow
the flow to occur, and the unit therefore effectively transmits this ‘shock’ loading
without significant movement. A description of the use of these devices is given
in Reference 9.
Fig. 28.11 Wedging action