Page 46 - Structural Steel Designers Handbook AISC, AASHTO, AISI, ASTM, and ASCE-07 Design Standards

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FABRICATION AND ERECTION*

2.8 CHAPTER TWO

2.4 FABRICATION PRACTICES AND PROCESSES: ASSEMBLY,
FITTING, AND FASTENING

The work in a well-run shop should flow from one activity or work station to the next. The main mate-
rial is prepared substantially in groups that are either efficient to build or that form erectable units, divi-
sions, or sequences. When the main material is ready, it is sent to fabrication stations where the detail
material is ready and in place for assembly to the shipping piece. Columns, beams, and braces in which
a rolled shape is the main piece are individually fit and fastened. Repetitive similar trusses are assem-
bled in jigs or fixtures that speed assembly and ensure uniformity. Built-up sections such as boxes and
three-plate girders are assembled using spreaders, braces, and stiffeners that are sometimes left as part
of the piece. Shipping pieces are assembled by a skilled worker called a fitter or fabricator. Bolting is
usually performed by the fabricator. Welding may be done by the fabricator or by a separate welder.

2.4.1 Bolting
Most field connections are made by bolting, either with high-strength bolts (ASTM A325 or A490)
or with ordinary machine bolts (A307 bolts), depending on strength requirements. Shop connections
frequently are welded but may use these same types of bolts. When high-strength bolts are used, the
connections should satisfy the requirements of the “Specification for Structural Joints Using ASTM
A325 or A490 Bolts,” approved by the Research Council on Structural Connections (RCSC) of the
Engineering Foundation. Joints with high-strength bolts are generally designed either as bearing-
type or slip-critical connections (see Chap. 3 and Art. 5.9). Some joints may be specially designed
to slip, such as by using oversized holes or slots.
Bearing-type connections have a higher allowable load or design strength than slip-critical con-
nections. Slip-critical connections always must be fully tightened to specified minimum values.
Bearing-type connections may be either “snug tight” or fully tightened, depending on the type of con-
nection and service conditions. Snug tight is defined as the tightness attained with a few impacts of
an impact wrench or the full effort of an ironworker using an ordinary spud wrench to bring the con-
nected plies into firm contact. AISC specifications for structural steel buildings require fully ten-
sioned high-strength bolts (or welds) for certain connections (see Art. 5.9.1). The AASHTO
specifications require slip-critical joints in bridges where slippage would be detrimental to the ser-
viceability of the structure, including joints subjected to fatigue loading or significant stress rever-
sal. In all other cases, connections may be made with “snug-tight” high-strength bolts or A307 bolts,
as may be required to develop the necessary strength.
Pretensioned bolts are tightened to 70% of the tensile strength of the bolt (see Art. 5.9.5). The
RCSC recognizes four methods of tightening bolts: turn-of-the-nut tightening, use of tension-control
bolts, use of direct tension indicators, and calibrated wrench tightening. All of the methods depend
on the installer first bringing the bolts to the snug-tight condition and then tightening them in a pat-
tern from the most rigid to the least rigid part of the connection. This procedure is intended to pre-
vent initially tightened bolts from coming loose when subsequent bolts are tightened.
Turn-of-the-nut tightening requires the installer to install the bolts to the snug condition and then
rotate the nut relative to the bolt by an amount specified by the RCSC. In critical connections, the
nut and end of the bolts are marked in the snug condition and the installer and inspector can confirm
visually that the nut has been rotated the proper amount after snug tightening.
Tension-control (TC) bolts are manufactured with a spline on the end. The installation tool
applies a torque to the bolt using the spline and the nut. A groove between the spline and the threaded
part of the bolt is calibrated to shear off when the bolt achieves the proper tension. Since the per-
formance of the bolt depends on the correlation between the torque on the bolt and the tension in the
bolt, the condition of the bolt and nut are important. TC bolts, as with the other types, must be stored
to prevent degradation of the threads or lubricant.
Direct tension indicators (DTIs), sometimes referred to as load-indicating washers or LIWs, are
manufactured with profiles that deform under bolt tension. The geometry of the deformed LIW can
be measured to assure that the bolt has reached the required tension.

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