10
STEEL DETAILERS' MANUAL
number of manufacturers to dimensions particular to the
Since the early 1980s many workshops have installed
supplier and are usually galvanised. Ranges of standard
numerically controlled (NC) equipment for marking, sawing
fitments such as sag rods, fixing cleats, cleader angles,
members to length, for hole drilling and profile cutting of
gable posts and rafter stays are provided, such that for a
plates to shape. This has largely replaced the need to make
typical single storey building only the primary members
wooden (or other) templates to ensure fit-up between
might be hot rolled sections. Detailing of cold rolled sec-
adjacent connections when preparation (i.e. marking,
tions is not covered in this manual, but it is important that
cutting and drilling) was performed by manual methods.
the designer ensures that stability is provided by these
Use of NC equipment has significantly improved accuracy
elements or if necessary provides additional restraint.
such that better tolerances are achieved without need for adjustments by dressing or reaming of holes. However, the
Open braced structures such as trusses, lattice or Vierendeel
main factor causing dimensional variation is welding dis-
girders and towers or space frames are formed from indi-
tortion, which arises due to shrinkage of the molten weld
vidual members of either hot rolled, hollow, fabricated or
metal when cooling. The amount of distortion which occurs
compound shapes. They are appropriate for lightly loaded
is a function of the weld size, heat input of the process,
long span structures such as roofs or where wind resistance
number of runs, the degree of restraint present and the
must be minimised as in towers. In the past they were used
material thicknesses.
for heavy applications such as bridges, but the advent of automated fabrication together with availability of wide
To an extent welding distortion can be predicted and the
plates means that plate girders are more economic.
effects allowed for in advance, but some fabricators prefer to exclude the use of welding for beam/column structures
1.4 Tolerances 1.4.1 General
and to use all bolted connections. However, welding is necessary for fabricated sections such that the effects of distortion must be understood and catered for.
In all areas of engineering the designer, detailer and con-
Figure 1.7 illustrates various forms of welding distortion
structor need to allow for tolerances. This is because in
and how they should be allowed for either by presetting,
practice absolute precision cannot be guaranteed for each
using temporary restraints or initially preparing elements
and every dimension even when working to very high
with extra length. This is often done at workshop floor
manufacturing standards. Very close tolerances are
level, and ideally should be calculated in consultation with
demanded in mechanical engineering applications where
the welding engineer and detailer. Where site welding is
moving parts are involved and the high costs involved in
involved then the workshop drawings should include for
machining operations after manufacture of such compo-
weld shrinkage at site by detailing the components with
nents have to be justified. Even here tolerance allowances
extra length. A worked example is given in 1.4.2.
are necessary and it is common practice for values to be specified on drawings. In structural steelwork such close
When site welding plate girder splices the flanges should
tolerances could only be obtained at very high cost, taking
be welded first so that shrinkage of the joint occurs
into account the large size of many components and the
before the (normally thinner) web joint is made to avoid
variations normally obtained with rolled steel products.
buckling. Therefore the web should be detailed with
Therefore accepted practice in the interests of economy is
approximately 2 mm extra root gap as shown in figure
to fabricate steelwork to reasonable standards obtainable
1.13.
in average workshop conditions and to detail joints which can absorb small variations at site. Where justified,
Table 1.7 shows some of the main causes of dimensional
operations such as machining of member ends after fabri-
variations which can occur and how they should be over-
cation to precise length and/or angularity are carried out,
come in detailing. These practices are well accepted by
but this is exceptional and can only be carried out by
designers, detailers and fabricators. It is not usual to
specialist fabricators. Normally, machining operations
incorporate tolerance limits on detailed drawings although
should be restricted to small components (such as tapered
this will be justified in special circumstances where accu-
bearing plates) which can be carried out by a specialist
racy is vital to connected mechanical equipment. Figure
machine shop remote from the main workshop and
1.8 shows tolerances for rolled sections and fabricated
attached before delivery to site.
members.