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Steel Designers' Manual - 6th Edition (2003)
926 Tolerances
31.4.3.3 Squareness
If the ends of a length of column are not square to its axis, then after erection either
the column will not be vertical or else there may be tapered gaps at the joints,
depending on the extent to which surrounding parts of the structure prevent the
column from tilting. Under load any such gap will try to close, exerting extra forces
on the surrounding members. In addition, both a gap or a tilt will induce a local
eccentricity in the column.
A practical erection criterion is that the column should not lean more than 1 in
x (where x is 600 in NSSS and 500 in ENV 1090-1). This slope is measured relative
to a line joining the centres of each end of the column length, referred to as the
overall centreline. The column is also allowed a lack of straightness tolerance of
(length/1000), which corresponds to end slopes of about 1/300 (see Fig. 31.2(a)). It
is thus necessary to specify end squareness criteria relative to the overall centreline,
rather than to the local centreline adjacent to the end (see Fig. 31.2(b)).
There is generally a design assumption that the line of action of the force in the
column does not change direction at a braced joint by more than 1/250, requiring
an end squareness in a simple bearing connection (relative to the overall axis of the
member) of 1/500 (see Fig 31.2(c)). However, full contact bearing generally arises
at column splices which are not at braced points, so an end squareness tolerance of
1/1000 is usually specified, producing a maximum change of slope of 1/500 (see Fig.
31.2(d)).
Once a column has been erected, it is more practical to measure the remaining
gaps in a joint. These gaps are affected not only by the squareness of the ends but
also by the second criterion, flatness.
31.4.3.4 Flatness
Ends have to be reasonably flat (as distinct from curved or grossly uneven) to enable
the load to be transferred properly. Following a history of arguments over appropriate
specifications, the American Institute of Steel Construction (AISC) commissioned
some tests, which are the basis for their current specifications.
It was found that a surprisingly high tolerance was quite acceptable, and that
beyond its limit (or to compensate for end squareness deviations) the use of
localized packs or shims was acceptable. Basically similar rules are now beginning
to appear in other specifications including the CEN standard (see section 31.5.6 in
relation to erection tolerances). This is an essentially simple and effective method
of correcting excessive gaps on site (see also section 31.5.6). However, inserting
shims into column joints is not a matter to be undertaken lightly. It is normally more
economic to avoid the need for shimming by working to close fabrication tolerances
in joints where full contact bearing is required.