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Steel Designers' Manual - 6th Edition (2003)
which are likely to incur greater costs. These affect the fit-up, the boundary dimensions,
the finishes and the interference problems. Data are given on the normal tolerances
(to be expected and catered for in detailed design) under two headings:
(1) Site construction (table 1 of BS 5606).
(2) Manufacture (table 2 of BS 5606).
Unfortunately many of the values for site construction of steelwork are only estimated.
No specific consideration is given in BS 5606 to dimensional tolerances necessary
to comply with the assumptions inherent in structural design procedures,
which may in fact be more stringent. It does however recognize that special accuracy
may be necessary for particular details, joints and interfaces.
Another important point mentioned in BS 5606 is the need to specify methods
of monitoring compliance, including methods of measurement. It has to be recognized
that methods of measurement are also subject to deviations; for the methods
necessary for monitoring site dimensions, these measurement deviations may in fact
be quite significant compared to the permitted deviations of the structure itself.
31.3 Implications of tolerances
31.3.1 Member sizes
31.3.1.1 Encasement
The tolerances on cross-sectional dimensions have to be allowed for when encasing
steel columns or other members, whether for appearance, fire resistance or structural
reasons. It should not be forgotten that the permitted deviations represent a
further variation over and above the difference between the serial size and the
nominal size.
For example, a 356 ¥ 406 ¥ 235 UC has a nominal size of 381 mm deep by 395 mm
wide, but with tolerances to BS 4 may actually measure 401 mm wide by 387 mm
deep one side, and have a depth of 381 mm the other side. The same is true of continental
sections. A 400 ¥ 400 ¥ 237 HD also has a nominal size of 381 mm deep by
395 mm wide, but with tolerances to Euronorm 34 may actually measure 398 mm
wide by 389 mm deep one side, and have a depth of 380 mm the other side.
31.3.1.2 Fabrication
Implications of tolerances 921
Variations of cross-sectional dimensions (with permitted deviations) may also need
to be allowed for, either in detailing the workmanship drawings or in the fabrication
process itself, if problems are to be avoided during erection on site.