Page 270 - Handbook of Structural Steel Connection Design and Details

Welded Joint Design and Production

Welded Joint Design and Production 255

region. FCAW-ss may employ longer electrode extensions which some-
what duplicate the access flexibility of SMAW. Since FCAW-ss does
not utilize a shielding gas, this process is unencumbered with gas
nozzles that further restrict visibility. If FCAW-gs is used, however,
allowance must be made for the operator to be able to place the weld-
ing gun, complete with a gas delivery nozzle, into the joint. For these
reasons, weld access holes may have to be larger than the minimum
dimensions prescribed in the applicable codes. It is usually best to let
the fabricator’s detailer select appropriately sized access holes for the
particular welding processes to be used.
The quality of weld access holes is an important variable that affects
both resistance to fabrication-related cracking as well as resistance to
cracking that may result from seismic events. Access holes usually are
cut into the steel by the use of a thermal cutting process, either oxy-
fuel cutting (frequently called burning) or plasma arc cutting. Both
processes rely on heating the steel to a high temperature and remov-
ing the heated material by pressurized gases. In the case of oxy-fuel
cutting, oxidation of the steel is a key ingredient in this process. In
either process, the steel on either side of the cut (called the kerf) has
been heated to an elevated temperature and rapidly cooled. In the case
of oxy-fuel cutting, the surface may be enriched with carbon. For plas-
ma cut surfaces, metallic compounds of oxygen and nitrogen may be
present on this surface. The resultant surface may be hard and crack-
sensitive, depending on the combinations of the cutting procedure,
base metal chemistry, and thickness of the materials involved. Under
some conditions, the surface may contain small cracks. These cracks
can be the points of stress amplification that cause further cracking
during fabrication or during seismic events.
Nicks or gouges may be introduced during the cutting process, par-
ticularly when the cutting torch is manually propelled during the for-
mation of the access hole. These nicks may act as stress-amplification
points, increasing the possibility of cracking.
To decrease the likelihood of notches and/or microcracks on thermally
cut surfaces, AISC has specific provisions that are required for making
access holes in heavy group 4 and 5 rolled shapes. These provisions
include the need for a preheat before cutting (to minimize the possibility
of the formation of hard, crack-sensitive microstructures), require-
ments for grinding of these surfaces (to provide for a smooth contour,
and to eliminate cracks and gouges as well as hard material that may
be present), and inspection of these surfaces with magnetic-particle
(MT) or dye-penetrant (PT) inspection (to verify a crack-free surface).
Whether these requirements are necessary for all connections that may
be subject to seismic energies is unknown at this time. However, for
connection details that impose high levels of stress on the connection,

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