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Steel Designers' Manual - 6th Edition (2003)
924 Tolerances
Fortunately in most cases it is possible to rely on the inherent improbability of
all unfavourable extreme deviations occurring together. Also the usually accepted
values for fabrication tolerances do make some limited allowances for the need to
avoid cumulative effects developing on site. They are tolerances that have been
shown by experience to be workable, provided that simple means of adjustment are
incorporated where the effects of a number of deviations could otherwise become
cumulative. For example, beams with bolted end cleats usually have sufficient
adjustment available due to hole clearances, but where a line of beams all have end
plate connections, provision for packing at intervals may be advisable, unless other
measures are taken to ensure that the beams are not all systematically over-length
or under-length by the normal permitted deviation. Other possible means for adjustment
include threaded rods and slotted holes.
Where it can be seen from the drawings that the fabrication tolerances could
easily accumulate in such a way as to create a serious problem in erection, either
closer tolerances or means of adjustment should be considered; however, the coincident
occurrence of all extreme deviations is highly improbable, and judgement
should be exercised both on the need for providing means of adjustment and on the
range of adjustment to be incorporated.
31.4.3 Full contact bearing
31.4.3.1 Application
The requirements for contact surfaces in joints which are required to transmit compression
by ‘full contact bearing’ probably cause more trouble than any other item
in a fabrication specification, largely due to misapprehension of what is actually
intended to be achieved.
First it is necessary to be clear about the kind of joint to which the requirements
for full contact bearing should be applied. Figure 31.1(a) shows the normal case,
where the profile of a member is required to be in full contact bearing on a baseplate
or cap plate or division plate. The stress on the contact area equals the stress
in the member: thus full contact is needed to transmit this stress from the member
into the plate. Only that part of the plate in contact with the member need satisfy
the full contact bearing criteria, though it may be easier to prepare the whole plate.
Figure 31.1(b) shows two end plates in simple bearing. The potential contact area
is substantially larger than the cross-sectional area of the member: thus full contact
bearing is not necessary. All that is needed is for the end plates to be square to the
axis of the member.Another common case of simple bearing is shown in Fig. 31.1(c).
By contrast, the case shown in Fig. 31.1(d) is one where, if full contact bearing is
needed, it is also necessary to take special measures to ensure that the profiles of
the two members align accurately, otherwise the area in contact may be significantly
less than the area required to transmit the load. Particular tolerances should be
specified in such cases, based on the maximum local reduction of area that can be