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Steel Designers' Manual - 6th Edition (2003)
104 Industrial steelwork
Naturally, there are many different methods of achieving lateral stability. Where
floor construction is reasonably complete and regular through the height of the
structure, then the design can be based on horizontal diaphragms transferring load
to vertical stiff elements at intervals along the structure length or width.Where large
openings or penetrations exist in otherwise conventional concrete floors, then it is
important to design both the floor itself and the connections between the floor and
steelwork for the actual forces acting, rather than simply relying automatically on
the provision of an effective diaphragm as would often and justifiably occur in the
absence of such openings.
It can be worthwhile deliberately to influence the layout to allow for at least a
reasonable width of floor along each external face of building, say of the order of 1 – 10
of the horizontal spacing between braced bays or other vertical stiff elements.
When concrete floors as described above are wholly or sensibly absent, then other
types of horizontal girders or diaphragms can be developed. If solid plate or open
mesh steel flooring is used, then it is possible, in theory at least, to design such flooring
as a horizontal diaphragm, usually by incorporating steel beams as ‘flange’
members of an idealized girder, where the floor steel acts as the web. However, in
practice, this is usually inadvisable as the flooring plate fixings are rarely found to
be adequate for load transfer and indeed it may be a necessary criterion that some
or all of the plates can be removable for operational purposes. A further factor is
that any line of beams used as a ‘flange’ must be checked for additional direct
compression or tension loadings; the end connections also have to be designed to
transfer these axial forces.
It is therefore normal to provide plan bracing in steelwork in the absence of concrete
floor construction. The influence that this will have on plant penetrations,
pipework routeing and many other factors must be considered at an early stage in
liaison with the plant engineers. Naturally, the design considerations of steel beams
serving a dual function as plan bracing are exactly as set out above and must not
be neglected. Indeed, it may be preferable to separate totally the lateral load
restraint steelwork provided on plan from other steelwork in the horizontal plane.
This will avoid such clashes of purposes and clearly signal to the plant designers the
function of the steelwork, as a result preventing its misuse or abuse at a later stage.
Many examples exist where plan bracing members have been removed owing to
subsequent plant modifications. A practical suggestion to further minimize the possibility
of this happening is to paint such steelwork a completely different colour as
well as to separate it completely from any duality of function with respect to plant
support or restraint.
Where it proves impossible to provide any type of horizontal plan bracing, then
each and every frame can be vertically braced or rigid-framed down to foundations.
It is best not to mix these two systems if possible, since they have markedly differing
stiffnesses and will thus deflect differently under loading.
If diaphragms or horizontal girders are used, then the vertical braced bays that
receive lateral loading as reactions from them are usually braced in steelwork. In
conventional structures tension-only ‘X’ bracing is frequently used, and whereas this
may be satisfactory for some straightforward industrial structures such as tank