Building Design and Construction Handbook - Merritt - Ventech!

7.120 SECTION SEVEN
There is an established sequence for completing the connections. The raising
gang connects members together with temporary fitting-up bolts. The number of
bolts is kept to a minimum, just enough to draw the joint up tight and take care of
the stresses caused by deadweight, wind, and erection forces. Permanent connections
are made as soon as alignment is within tolerance limits. Usually, permanent
bolting or welding follows on the heels of the raising gang. Sometimes, the latter
moves faster than the gang making the permanent connections, in which case it
may be prudent to skip every other floor, thus obtaining permanent connections as
close as possible to the derrick—a matter of safe practice.
Some erectors prefer to use permanent high-strength (A325 and A490) bolts for
temporary fitting up. Because bolts used for fit-up are not tightened to specified
minimum tension, they may be left in place and later tightened as required for
permanent installation.
7.41 FIELD-WELDING PROCEDURES
The main function of a welding sequence is to control distortion due primarily to
the effects of welding heat. In general, a large input of heat in a short time tends
to produce the greatest distortion. Therefore, it is always advisable, for large joints,
to weld in stages, with sufficient time between each stage to assure complete dispersal
of heat, except for heat needed to satisfy interpass-temperature requirements
(Art. 7.3.5). Equally important, and perhaps more efficient from the erector’s viewpoint,
are those methods that balance the heat input in such a manner that the
distortional effects tend to cancel out.
Welding on one flange of a column tends to leave the column curled toward the
welded side cooling, because of shrinkage stresses. A better practice for beams
connecting to both sides of a column is to weld the opposite connections simultaneously.
Thus the shrinkage of each flange is kept in balance and the column remain
plumb.
If simultaneous welding is not feasible, then the procedure is to weld in stages.
About 60% of the required weld might be applied on the first beam, then the joint
on the opposite flange might be completely welded, and finally, welded on the first
beam would be completed. Procedures such as this will go far to reduce distortion.
FIGURE 7.61 Indication of sequence in welding
a connection.
Experience has shown that it is good
practice to commence welding at or near
the center of a building and work outward.
Columns should be checked frequently
for vertical alignment, because
shrinkage in the welds tends to shorten
the distance between columns. Even
though the dimensional change at each
joint may be very small, it can accumulate
to an objectionable amount in a
long row of columns. One way to reduce
the distortion is to allow for
shrinkage at each joint, say, 1 ⁄16 in for a
20-ft bay, by tilting or spreading the col-
umns. Thus, a spread of 1 ⁄8 in for the two ends of a beam with flanges butt welded
to the columns may be built in at the fabricating shop; for example, by