Steel Designers Manual - TheBestFriend.org

This material is copyright - all rights reserved. Reproduced under licence from The Steel Construction Institute on 12/2/2007
To buy a hardcopy version of this document call 01344 872775 or go to http://shop.steelbiz.org/
Steel Designers' Manual - 6th Edition (2003)
Bearings 845
able. In these instances the engineer has the option of designing the units using
available literature (see references to Chapter 28) and perhaps incorporating
standard bearings from a manufacturer as components of a completed assembly
or alternatively engaging a recognized manufacturer to design and produce
the item as a special bearing. For straightforward applications such as may be
required on a short single-span bridge, it may be worthwhile investigating the
relative costs of a simple fabricated bearing compared with the equivalent proprietary
unit. Bearings (particularly ‘special’ bearings) can prove to be a large
item of expenditure in a structure and an estimate of the costs involved should
be made early in the design stage.
(10) Summary of design requirements. Before selecting a particular bearing it is
suggested that a summary of all relevant parameters is prepared. This can then
be used if necessary for submission to the bearing manufacturers for examination
and recommendations as to particular bearing types. A typical format
for such a sheet is given in Table 9 of BS 5400: Section 9.1. 2
28.2.1.2 Materials
Generally materials fall into three groups:
(1) those able to withstand high localized contact pressures e.g. steel.
(2) those able to withstand lower contact pressures but having a low coefficient of
friction; these slide easily in a direction perpendicular to the direction of the
pressure and thus accommodate translational movement, e.g. polytetrafluoroethylene
(PTFE).
(3) those able to withstand contact pressure and also to accommodate translational
or rotational movements by deformation of the material (e.g. elastomers).
Certain of these materials may be confined within a steel cylinder in order to
increase their compressive resistance.
(a) Mild or high-yield steel
The coefficient of friction of steel on steel is of the order of 0.3 to 0.5, unless continuously
lubricated; in order to provide for movement alternative arrangements
are usually necessary. Traditionally this has been through the use of single or
multiple rollers or knuckles. Rollers will permit translation in one direction and,
if a single roller is used, rotation about an axis perpendicular to that direction.
Knuckles permit rotation about one axis only. Rotation in two directions may be
achieved using spherical-shaped bearing surfaces.
The allowable pressures between surfaces for steel on steel contact depend upon
the radii of the two surfaces and the hardness and ultimate tensile strength of the
material used. BS 5400: Section 9.1 2 gives expressions for design load effects in such