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Steel Designers' Manual - 6th Edition (2003)
28.2.2 Types of bearing
28.2.2.1 General
Bearings 849
Structural bearings can be broadly divided into two types: elastomeric and mechanical.
Elastomeric bearings comprise blocks of elastomeric material reinforced as
necessary with other materials. They can be made to accommodate movement by
shearing of the elastomer block. Mechanical bearings are generally formed from
metal and employ sliding surfaces of PTFE to achieve any necessary movement
capabilities. It should be noted that the above divisions are not exclusive as, for
example, some bearings are commercially available which employ elastomer blocks
with sliding surfaces which cater for larger movements than could be accommodated
by shearing of the block, but which do not merit the larger expense of a
mechanical bearing.
In the following sections the principal types of bearing are briefly described.
28.2.2.2 Elastomeric bearings
Elastomeric bearings rely for their operation on the interaction between vertical
load, rotation and translation. As a consequence, design of most elastomeric bearings
must be carefully checked. Large proprietary ranges are available, and although
load tables of the various capacities are published by manufacturers, it is prudent
to ask the supplier to confirm that the selected bearing is suitable for the load/movement
conditions under which it will be used. The basis of design of these bearings
is related to controlling strains and stresses in the elastomer and any reinforcing
material and ensuring that the bearing does not deform excessively, become unstable,
lift off, or slip under the anticipated design effects. Further guidance on
this subject is given in BS 5400: Part 9.1 2 and by Long. 5 The latter reference also
gives a detailed discussion of the properties of elastomers.
Bearing types are:
(1) Rubber pad or strip bearings. As the name implies, these bearings consist simply
of a block or strip of elastomer. They have the advantage of being inexpensive
and simple although their load-carrying and movement capability is limited.
(2) Fabric-reinforced bearings. In order to increase the capabilities of the simple
pad bearing, use is made of fabric (e.g. compressed cotton duck) to reinforce
the elastomer. Movement in these bearings is usually provided by use of a PTFE
surface bonded to the top of the block and sliding against a stainless steel plate
attached to the underside of the superstructure. In this manner, the elastomer
is used to provide rotational capability only, rather than rotation and movement
as in the case of other elastomeric bearings.
(3) Elastomeric-laminated bearings. This type of bearing consists of a block of elastomeric
material reinforced with steel plates to which the elastomer is also