Steel Designers Manual - TheBestFriend.org

This material is copyright - all rights reserved. Reproduced under licence from The Steel Construction Institute on 12/2/2007
To buy a hardcopy version of this document call 01344 872775 or go to http://shop.steelbiz.org/
Steel Designers' Manual - 6th Edition (2003)
108 Industrial steelwork
3.2.4 Main and secondary beams
The plan arrangement of main and secondary beams in industrial steelwork structures
normally follows both from the layout of the main items of plant and from
the column locations. Thus a sequence of design decisions often occurs in which
main beam locations dictate the column locations and not vice versa. If major plant
occurs at more than one level then some compromise on column position and hence
beam layout may be needed.
Since large plant items normally impose a line or point loading there are clear
advantages in placing main or secondary beams directly below plant support positions.
Brackets, plinths or bearings may be fitted directly to steelwork, and for major
items of plant this is preferable to allowing the plant to sit on a concrete or steel
floor. Where plant or machinery requires a local floor zone around its perimeter for
access or servicing, it is common practice to leave out the flooring below the plant
for access or because the plant protrudes below the support level.
Deflection requirements between support points should be ascertained.They may
well control the beam design since stringent limits, for example relative deflections
of 1 in 1000 of support spans, may apply. In addition when piped services are connected
to the plant then total deflections of the support structure relative to the
beam-to-column intersections may also need to be limited. Relative deflections can
best be controlled by the use of deep beams in (lower-grade steel if necessary), and
total deflections by placing columns as close as possible to the support positions.
It is preferable to avoid the necessity for load-hearing stiffeners at support points
unless the plant dimensions are fixed before steelwork design and detailing take
place. Where this is not possible then stiffened zones to prevent secondary bending
of top (or bottom if supports are hung) flanges should be provided, even if the design
requirements do not require load-bearing stiffeners. This then allows a measure of
tolerance for aligning the support positions without causing local overstressing
problems.
The stiffness of major plant items should be considered, at least qualitatively, in
the steelwork design. Deep-walled tanks, bunkers or silos for example may well be
an order of magnitude stiffer than the steel supporting structure.
The loading distribution given by the plant design engineers will automatically
assume fully stiff (zero deflection) supports. When the stiffness of the supporting
structure is not uniform in relation to the support point locations and loadings, then
significant redistribution of loads can take place as the structure deflects. Where the
plant support positions and loads and the structural steel layout are symmetrical
then engineering judgement can be applied without quantitative evaluation. In
extreme cases, however, a plant–structure interaction analysis may be required to
establish the loadings accurately.
When hanger supports are required then pairs of beams or channels are a convenient
solution which allows for random hanger positioning in the longitudinal
direction (Fig. 3.7). Many hanger supports have springs bearings to minimize variations
in support conditions due to plant temperature changes or to avoid the plant
stiffness interactions described above.