Page 47 - Structural Steel Designers Handbook AISC, AASHTO, AISI, ASTM, and ASCE-07 Design Standards

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FABRICATION AND ERECTION*

FABRICATION AND ERECTION 2.9

Calibrated wrench tightening depends on establishment of a daily project torque and using impact
wrenches adjusted to that torque.
Inspection of structural bolting is done by observation during installation. Inspection of bolt
installation after the work is done is considered unreliable and tends to cause conflict.

2.4.2 Welding
Welding in the fabrication and erection of building structures is governed by the AISC “Specification
for Structural Steel Buildings” and the AWS “Structural Welding Code—Steel” (AWS D1.1).
Welding of bridges is governed by the Bridge Welding Code, a joint publication of AASHTO and AWS
(AASHTO/AWS D1.5). Other codes that may be used in steel structures are AWS D1.3 for sheet,
AWS D1.4 for reinforcing bars, and AWS D1.6 for stainless steel. Owners and engineers can sup-
plement the requirements of AWS to meet specific needs and experience.
The number of variables included, the skill levels necessary, and the quality demands on welded
joints require that welding be done as a controlled process. The fundamental scheme for that control
is to have qualified personnel use materials, joint designs, and procedure variables that have been
proven effective in combination, through tests. All welds must be inspected visually. Tests are expen-
sive and many of the material, joint, and procedure combinations have been used extensively and are
therefore considered prequalified. In summary, welders who have been qualified by test use weld
procedures that have been qualified by test or are prequalified. However, there are no prequalified
procedures in the “Bridge Welding Code,” or for reinforcing bar.
The use of prequalified procedures is desirable and prevalent in the fabrication of buildings.
Procedures are prequalified for four welding processes: shielding metal arc welding (SMAW), flux-
cored arc welding (FCAW), gas metal arc welding (GMAW), and submerged arc welding (SAW).
Other processes such as electroslag welding (ESW) can be used, but the procedures must be quali-
fied by test. Procedure prequalification demands the use of certain materials and a limited set of
joint geometries and electrical variables. The complete requirements are described in Chap. 3 of
AWS D1.1. The AISC “Manual of Steel Construction” also includes the joint geometries. Materials
approved for use by the AISC are included by the AWS in the list of prequalified materials. Filler
metals meeting the requirements of the appropriate AWS A5 series specifications and listed in Chap. 3
of AWS D1.1, and Chap. A of the AISC “Specification for Structural Steel Buildings” can be used.
Electrical variables must be within the filler-metal manufacturer’s recommendations. Weld proce-
dure specifications must be written, whether they are qualified by test or prequalified.
Shielded metal arc welding (SMAW) produces coalescence, or fusion, by the heat of an elec-
tric arc struck between a coated metal electrode and the material being joined, or base metal. The
electrode supplies filler metal for making the weld, gas for shielding the molten metal, and flux for
refining this metal. This process is commonly known also as manual, hand, or stick welding.
Pressure is not used on the parts to be joined.
When an arc is struck between the electrode and the base metal, the intense heat forms a small
molten pool on the surface of the base metal. The arc also decomposes the electrode coating and
melts the metal at the tip of the electrode. The electron stream carries this metal in the form of fine
globules across the gap and deposits and mixes it into the molten pool on the surface of the base
metal. (Since deposition of electrode material does not depend on gravity, arc welding is feasible in
various positions, including overhead.) The decomposed coating of the electrode forms a gas shield
around the molten metal that prevents contact with the air and absorption of impurities. In addition,
the electrode coating promotes electrical conduction across the arc, helps stabilize the arc, adds flux,
slag-forming materials, to the molten pool to refine the metal, and provides materials for controlling
the shape of the weld. In some cases, the coating also adds alloying elements. As the arc moves
along, the molten metal left behind solidifies in a homogeneous deposit, or weld.
The electric power used with shielded metal arc welding may be direct or alternating current.
With direct current, either straight or reverse polarity may be used. For straight polarity, the base
metal is the positive pole and the electrode is the negative pole of the welding arc. For reverse polarity,
the base metal is the negative pole and the electrode is the positive pole. Electrical equipment with
a welding-current rating of 400 to 500 A is usually used for structural steel fabrication. The power

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