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Steel Designers' Manual - 6th Edition (2003)
However, just as plant engineers can display a lack of understanding about the
necessity to design supporting steelwork afresh for each different structure even
though the ‘same’ plant is being built, structural steelwork engineers must be aware
that seemingly identical or repeat pieces of plant or equipment can in fact vary in
significant details. Most plant installations are purpose-designed, and the layout and
loadings are often provided initially to the structural engineer in terms of estimated
or approximate values. It is vital to be aware of the accuracy of the information
being used for design at any stage, and to avoid carrying out designs at an inappropriately
advanced level.A considerable margin should be allowed, provided that
it is established that the plant designers have not allowed a similar margin already
in estimating the plant loadings. Experience shows that loadings are often overestimated
at the preliminary design stage, but that this is compensated for by new
loadings at new locations that were not originally envisaged.
3.2 Anatomy of structure
3.2.1 Gravity load paths
Anatomy of structure 101
Vertical loadings on industrial steelwork can be extraordinarily heavy; some individual
pieces of plant have a mass of 10 000 t or more. Furthermore, by their very
nature these loadings generally act as discrete point loads or line loads rather than
as uniformly distributed loadings. Load values are often ill-defined (see section
3.3.1) at the steelwork design stage and frequently additional vertical loadings are
introduced at new locations late in the design process.
For these reasons, the gravity load paths must be established at an early stage to
provide a simple, logical and well-defined system. The facility should exist to cater
for a new load location within the general area of the equipment without the need
to alter all existing main structural element locations. Typically this means that it is
best to provide a layered system of beams or trusses with known primary span directions
and spacings, and then with secondary (and sometimes in complex layouts,
tertiary as well) beams, which actually provide vertical support to the plant.
While simplicity and a uniform layout of structure are always attractive to a structural
designer, the non-uniform loadings and layout of plant mean that supporting
columns may have to be positioned in other than a completely regular grid to
provide the most direct and effective load path to the foundations. It is certainly
preferable to compromise on a layout that gives short spans and a direct, simple
route of gravity load to columns, than to proceed with designing on a regular grid
of columns only to end up with a large range of member sizes and even types of
beams, girders or trusses (Fig. 3.4).
Similarly, although it is clearly preferable for columns to run consistently down
to foundation level, interference at low levels by further plant or equipment is quite
common, which may make it preferable to transfer vertical loading to an offset
column rather than use much larger spans at all higher levels in the structure. A