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Steel Designers' Manual - 6th Edition (2003)
the plane of the flanges, rather than the plane of the web, always controls the pure
axial load case, the comparatively wide flanges ensure that the strong-axis moment
capacity Mcx is not reduced very much by lateral–torsional buckling effects for most
practical arrangements. Indeed the condition Mb = Mcx will often be satisfied.
In building frames designed according to the principles of simple construction,
the columns are unlikely to be required to carry large moments. This arises from
the design process by which compressive loads are accumulated down the building
but the moments affecting the design of a particular column lift are only those from
the floors at the top and bottom of the storey height under consideration. In such
cases preliminary member selection may conveniently be made by adding a small
percentage to the actual axial load to allow for the presence of the relatively small
moments and then choosing an appropriate trial size from the tables of compressive
resistance given in Reference 1. For moments about both axes, as in corner
columns, a larger percentage to allow for biaxial bending is normally appropriate,
while for internal columns in a regular grid with no consideration of pattern loading,
the design condition may actually be one of pure axial load.
The natural and most economic way to resist moments in columns is to frame the
major beams into the column flanges since, even for UCs, Mcx will always be comfortably
larger than Mcy. For structures designed as a series of two-dimensional
frames in which the columns are required to carry quite high moments about one
axis but relatively low compressive loads, UBs may well be an appropriate choice
of member. The example of this arrangement usually quoted is the single-storey
portal building, although here the presence of cranes, producing much higher axial
loads, the height, leading to large column slenderness, or a combination of the two,
may result in UCs being a more suitable choice. UBs used as columns also suffer
from the fact that the d/t values for the webs of many sections are non-compact
when the applied loading leads to a set of web stresses that have a mean compressive
component of more than about 70–100 N/mm 2 .
18.5 Basic design procedure
Basic design procedure 523
When the distribution of moments and forces throughout the structure has been
determined, for example, from a frame analysis in the case of continuous construction
or by statics for simple construction, the design of a member subject to compression
and bending consists of checking that a trial member satisfies the design
conditions being used by ensuring that it falls within the design boundary defined
by the type of diagram shown as Fig. 18.3. BS 5950 and BS 5400 therefore contain
sets of interaction formulae which approximate such boundaries, use of which will
automatically involve the equivalent procedures for the component load cases of
strut design and beam design, to define the end points. Where these procedures
permit the use of equivalent uniform moments for the stability check, they also
require a separate strength check.