Civil Engineering Reference
In-Depth Information
Unpopular sections are rolled once a quarter. It is worth
avoiding these to use heavier popular less costly sections in
the same depth range. A 'minimum weight' design could be a
high cost design if infrequently roll sections are specified, so
it is suggested that:
Consider using a minimum number of maximum column lengths
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consistent with transport, erection and crane capacity.
The lowest storey column design will normally be the critical load
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based on buckling about the weak axis dictating the section size
of, a UC. A heavy load, say from localised roof plant, can affects
only a few columns consider welding plates across the toes of the
ground to the first floor section, say 500 mm from the base plate
up to within 500 mm of the first floor connection. This avoids any
change of the column section size at the beam connection points
maximising connection repetition.
Use higher grade steel to minimise column sizes. Consider using
When specifying UBs avoid the narrow flange serial sizes such as:
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457UB152; 356UB127; 305UB127; 305UB102; 203UB102.
Avoid use of UB <203. These sections can be costly to connect
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due to small proportions. Shallow, <150 mm deep sections can
be costly. A heavier common section may be more available, eco-
nomic and require a thinner intumescent fire coating.
Channels deeper than 200 mm are costly - use UBs if
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countersunk splices to maintain a minimum standard size column
casing throughout the building (see
Figures 18.24(a,b)
. This stand-
ardises column to ceiling finish junctions and maximises net let-
table floor areas.
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possible.
Avoid specifying an excessive variety of serial sizes.
Maximum optimisation might be the use of three section sizes
in a project: a column, a primary beam and a secondary beam
(see
Figure 18.23
). Hollow sections and smaller sections are
perhaps rolled once in a quarter and minimum order tonnages
should be checked with the mill. Remember stiffening of open
sections subject to torsion is likely to be more costly than using
hollow sections.
Sometimes if loads are heavy or there are long spans, rolled
sections are replaced with special fabricated sections. Before
this decision, a cost effective solution for resisting a heavy load
is to use twin rolled sections particularly for a transfer beam
or if there is a limit on the maximum depth allowed. Where
down stand floor beams are used if the services zone shares the
structural zone holes may need to be cut or a Cellform section
selected. For preliminary cost purposes the uplift on the rate of
a basic unfabricated piece of steelwork is:
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Perimeter columns often require corrosion protection, perhaps
galvanising if set within the walling thickness to minimise/avoid
loss of lettable internal space from column bulkheads. Identify the
Hollow sections
+£200 ($300,
€240)/tonne
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Girders fabricated from plate
+£250 ($400,
€300)/tonne
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Cellform beams
+ £200 ($300,
€240)/tonne
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It is worth noting that Cellform beams have limited suppli-
ers with the potential for a cost premium. Connection posi-
tions normally need the cells filled in with welded plate and if
connections are frequent use of a UB or plate girder cut with
circular holes can be advantageous on cost and/or programme
criteria.
18.6.3 Fabricated steelwork cost - columns
Once order of magnitude calculations determine approximate
column section sizes practical framing decisions are needed
such as:
Multi-storey columns can have a section size change to suit storey
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loads but compare the cost justification of a splice, perhaps £150-
200 ($300, €240) with the cost saved by section weight reduction.
A safe initial assumption might be to change section size every
three storeys.
Figure 18.23 Sometimes the main framing members need only three
serial sizes to be ordered in bulk by the steelwork contractor, assuming
a 'minimum weight/high complexity' frame has been avoided
