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Steel Designers' Manual - 6th Edition (2003)
962 Fabrication
can cause excessive wear of the blast chamber and results in a surface which is more
pitted, giving a reduced paint thickness over high points.
The steel leaves the blast chamber and immediately enters the spray chamber.
Depending upon the requirements of the specification, the steel is then sprayed with
a primer paint which protects it from flash rusting.The use of prefabrication primers
should be discussed with the fabricator and paint system supplier as requirements
may vary. For most structural applications in building work, there is little need to
specify a prefabrication primer. Once the steel has passed through the spray
chamber, it is fed on to the outfeed conveyor and continues to its next process centre.
The other form of blast cleaning is called vacu blasting, a manual method where
the blasting medium is blown out of a hand-held nozzle under the pressure of compressed
air. This process is normally performed in a special sealed cabinet with the
operator wearing protective clothing and breathing equipment. It relies heavily on
the skill of the operator and does not give production rates approaching those of
mechanical blasting plants.
Local areas of damage caused by welding can be cleaned by vacu blasting or by
needle gunning. The needle gun has a set of hardened needles which are propelled
and withdrawn rapidly. The action of the needles on the steel effectively removes
weld slag and provides a good surface for painting. Needle gunning is not suitable
for large areas or for heavily contaminated surfaces.
Surface preparation is intended to provide a uniform substrate for even paint
application. It is important to consider the edges of plates, where paint thickness is
often low.
32.5.3 Cambering, straightening and bending
Each of these operations has a different purpose. Cambering is done to compensate
for anticipated deflections of beams or trusses under permanent loads, dead
loads or superimposed dead loads such as finishes. Straightening is part of the
fabrication process aimed at bringing sections back within straightness tolerances.
Bending is to form the section to a shape which is outside normal cambering limits.
Rolled sections are normally bought in by the fabricator and then sent to specialist
firms for cambering. The steel section is cold cambered by being passed
between rolls. The rolls are adjusted on each pass until the required camber is
achieved. The following cambers can be produced:
• circular profile (specify radius)
• parabolic profile (specify equation)
• specified offsets (tabulate co-ordinates)
• reverse cambers (circular, parabolic, offset or composite).
Bending is also carried out by specialists, the process being identical to that for
cambering. Curves can be formed on either the X–X or Y–Y axes.