Page 50 - Structural Steel Designers Handbook AISC, AASHTO, AISI, ASTM, and ASCE-07 Design Standards

Brockenbrough_Ch02.qxd 9/29/05 5:01 PM Page 2.12

FABRICATION AND ERECTION*

2.12 CHAPTER TWO

Stud welding usually is done with a device, or gun, for establishing and controlling the arc. The
operator places the stud in the chuck of the gun with the flux end protruding. Then the operator
places the ceramic ferrule over this end of the stud. With timing and welding-current controls set, the
operator holds the gun in the welding position, with the stud pressed firmly against the welding sur-
face, and presses the trigger. This starts the welding cycle by closing the welding-current contactor.
A coil is activated to lift the stud enough to establish an arc between the stud and the welding sur-
face. The heat melts the end of the stud and the welding surface. After the desired arc time, a con-
trol releases a spring that plunges the stud into the molten pool.
Direct current is used for stud welding. A high current is required for a very short time. For example,
welding currents up to 2500 A are used with arc time of less than 1 sec for studs up to 1 in diameter.
(O. W. Blodgett, Design of Welded Structures, James F. Lincoln Arc Welding Foundation,
Cleveland, Ohio.) See also Arts. 5.9.1 to 5.9.4.

2.5 SHOP ASSEMBLY

When the principal operations on a main member, such as punching, drilling, and cutting, are com-
pleted, and when the detail pieces connecting to it are fabricated, all the components are brought
together to be fitted up, that is, temporarily assembled with fit-up bolts, clamps, or tack welds. At
this time, the member is inspected for dimensional accuracy, squareness, and, in general, confor-
mance with shop detail drawings. Misalignment of attachments and holes should be detected at this
time and corrections made before the piece is completed and shipped.
The foregoing type of shop assembly and fit-up is an ordinary shop practice, performed routinely
in virtually all work. There are other classes of fit-up, however, that may be performed on some
work. Sequential “lay downs,” discussed below, or vertical, three-dimensional shop assembly may
be required on some large complex pieces. This type of assembly usually includes drilling of holes
for the mating connections in the assembled position. These assembly methods may be performed at
the option of the fabricator but are not routine practices. If the designer or contractor deems that one
of these assembly methods is necessary, it must be included in the contract documents. These assem-
bly methods are commonly required on large bridge girders.
Assembly is helpful for pieces that are too large to “drift” during erection, where pieces are so
large that normal shop tolerances might accumulate to inhibit connection on the site, where pieces
are large enough to prevent the use of routine erection practices when minor fit-up problems occur,
and where elastic deflections during erection can make “pinning” of connections difficult. When
those conditions exist, as with large bridge girders, lay-down or vertical assembly in the shop may
be specified.
Shop assembly is expensive and requires the extended use of valuable space. Accuracy of hole
placement within a hole pattern, which at one time was a cause for assembly, is less of an issue with
the automated drilling equipment used in contemporary shops. However, the distortion in large
pieces or the complexity of pieces that mate at many points may still demand the extra precaution
provided by shop assembly.
Where assembly and drilling in position are required, the following guidelines apply.
Splices in bridge girders are commonly drilled assembled. Alternatively, the abutting ends and
splice material may be drilled to templates independently.
Lay-downs are the assembly of three or more pieces that will mate in the field into position rel-
ative to one another but are laid flat on supports on the shop floor. Splice materials are positioned
between the connecting pieces, and the holes are drilled through the splice material and the main
members while the pieces are in this position. Lay-downs are used where the main members will be
in or near a plane in their final position, such as girders in a line. Where the main pieces are not in
or near a plane, such as with a cross girder mating to stringers, a vertical assembly is used. In this
case, pieces are placed in position relative to each other and usually nearly in their correct final ori-
entation. Vertical assemblies can be reoriented to minimize blocking in the shop and keep the pieces
accessible for work.

Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com)
Copyright © 2004 The McGraw-Hill Companies. All rights reserved.
Any use is subject to the Terms of Use as given at the website.