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Steel Designers' Manual - 6th Edition (2003)
956 Fabrication
At one time it was common practice to assemble components in the workshop
using bolts or rivets.With the increased implementation of welding, this practice has
declined due to the costs associated with bolted fabrications. Instead of a simple run
of fillet weld, holes need to be drilled and bolts introduced, increasing total labour
hours and cost. In many respects the ease with which welding can be undertaken
has diverted designers’ attention from the use of shop bolting.Today, steelwork contractors
are looking at increased automation to keep costs down, and machines have
been developed which considerably speed up hole drilling.
32.4.1 Shop bolting
There is still a demand for structural members to be bolted arising from a requirement
to avoid welding because of the service conditions of the member under
consideration. These may be low temperature criteria, the need to avoid welding
stresses or the requirement for the component to be taken apart during service (e.g.
bolted-on crane rails). For lattice structures, the designer should specify the bolting,
bearing in mind the effect of hole clearances around bolt shanks. HSFG bolts will
not give problems but other bolts in clearance holes will allow a ‘shake-out’ which
can cause significant additional displacement at joints. Typically, a truss with bolted
connections may deflect due to the take-up of lack of fit in clearance holes to such
an extent that it loses its theoretical camber. The use of HSFG assemblies avoids
this risk.
Large and complex assemblies which are to be bolted together on site may be
trial assembled in the fabrication shop. This increases fabrication costs but may pay
for itself many times over by ensuring that the steel delivered to site will fit. Restricting
trial assembly to highly repetitive items or items critical to the site programme
is to be recommended.
32.4.2 Types of bolt
The choice of which type of bolt to use may not necessarily be made on the basis
of strength alone but may be influenced by the actual situation in which the bolt is
used, e.g. in non-slip connections.
There are four basic types of bolts. They are structural bolts, friction-grip bolts
and close-tolerance bolts.
Structural bolts
Bolts with low material strength and wide manufacturing tolerance were until
recently known as ‘black bolts’ because of their appearance. Now they are called