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306 Barry’s Advanced Construction of Buildings
it strongly binds them together. The edges of the members to be joined are cleaned and
also shaped for certain types of weld. For a short period, the weld metal is molten as it runs
into the joint, and for this reason it is obvious that a weld can be formed more readily with
the operator working above the joint than in any other position. Welding can be carried
out more quickly and accurately in a workshop where the members can be manipulated
more conveniently for welding than they can be on site. Welding is most used in the prefabrication
of built-up beams, trusses and lattice frames. The use of shop welded connections
for angle cleats to conventional skeleton frames is less than it was due to the possibility
of damage to the protruding cleats during transport, lifting and handling of members.
In the design of welded structures, it is usual practice to prefabricate as far as practical
in the workshop and to make site connections either by bolting or by designing joints that
can readily be welded on site. The advantage of welding as applied to structural steel frames
is that members can be built up to give the required strength for minimum weight of steel,
whereas standard members do not always provide the most economical section. The labour
cost in fabricating welded sections is such that it can only be justified in the main for longspan
and non-traditional frames. The reduction in weight of steel in welded frames may
often justify higher labour costs in large, heavily loaded structures. In buildings where the
structural frame is partly or wholly exposed, the neat appearance of the welded joints and
connections is an advantage. It is difficult to tell from a visual examination whether a weld
has made a secure connection, and X-ray or sonic equipment is the only exact way of testing
a weld for adequate bond between weld and parent metal. This equipment is somewhat
bulky to use on site, and this is one of the reasons why site welding is not favoured.
Surfaces to be welded must be clean and dry if the weld metal is to bond to the parent
metal. These conditions are difficult to achieve in the UK’s wet climate out on site. The
process of welding used in structural steelwork is ‘fusion welding’, in which the surface of
the metal to be joined is raised to a plastic or liquid condition so that the molten weld
metal fuses with the plastic or molten parent metal to form a solid weld or join. For fusion
welding, the requirements are a heat source, usually electrical, to melt the metal, a consumable
electrode to provide the weld metal to fill the gap between the members to be joined,
and some form of protection against the entry of atmospheric gases which can adversely
affect the strength of the weld. The metal of the members to be joined is described as the
parent or base metal and the metal deposited from the consumable electrode, the weld
metal. The fusion zone is the area of fusion of weld metal to parent metal.
The method of welding most used for structural steelwork is the arc welding process,
where an electric current is passed from a consumable electrode to the parent metals and
back to the power source. The electric arc from the electrode to the parent metals generates
sufficient heat to melt the weld metal and the parent metal to form a fusion weld. The processes
of welding most used are:
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Manual metal-arc (MMA) welding
Metal inert-gas (MIG) and metal active-gas (MAG) welding
Submerged arc (SA) welding
MMA welding
This manually operated process is the oldest and the most widely used process of arc
welding. The equipment for MMA welding is simple and relatively inexpensive, and the