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Steel Designers' Manual - 6th Edition (2003)
116 Industrial steelwork
based examination of the expected frequency and duration of such unusual conditions
is made, leading to the adoption of reduced load factors without reducing the
overall factor of safety. Comparisons with, for example, wind loading can be used
to establish on a reasonably logical basis the appropriate load factors.
By definition, items of fixed plant can be treated as dead load in accordance
with BS 6399: Part 1 2 when specific location and loads are known. At the early
stages of design it is usual to adopt a relatively large imposed loading which will
have to cater for fixed items of plant or equipment, the existence of which may not
even be known at this stage and certainly not the location and loading data. The
choice of what imposed loading to use at this stage can be assisted by the following
guidance:
very light industrial processes 7.5 kN/m 2
medium/average industrial processes 10–15 kN/m 2
very heavy industrial processes 20–30 kN/m 2
One factor which will influence the choice within these values is the timing of the
release of final plant and equipment design data in relation to the steelwork design
and fabrication detailing process. If a second-stage design is possible then a lower
imposed load can be allowed since local variations needed to account for specific
items of fixed plant which exceed the imposed plan loading allowance can be
accommodated.
Under these conditions it can also be worthwhile, particularly for designers with
previous experience of the industrial process, to design columns and foundations
for a lower imposed loading than beams. In certain layouts with long span main
beams or girders at wide spacings this preliminary reduction can also be used
for these members. It should be stressed that these proposals are not intended to
contradict or override the particular reduced loading clauses in BS 6399, 2 but are a
practical suggestion for the preliminary design stage.
When detailed plant layouts with location and loading data become available,
fixed plant and equipment can be considered as dead load and subject therefore to
the appropriate load factor, 1.4 instead of 1.6. Remaining zones of floor space
without major items of plant should be allocated an imposed load which should
reflect only the access and potential use of the floor and may therefore very well be
reduced from the preliminary imposed loading, often in the range of 5–10 kN/m 2 .
Specific laydown areas for removal, replacement or maintenance of heavy plant
are the only likely exceptions to this range of loading.
An alternative scheme for dealing with the second stage of loading information
is to institute a checking procedure where the equivalent loading intensity of plant
and equipment is calculated for each item as follows:
14 .
16 . ¥
weight ( kN)
plan area m 2
( )
the factor 1.4/1.6 being introduced to cater for the reclassification from imposed
load to dead load.