Page 247 - Handbook of Structural Steel Connection Design and Details

Welded Joint Design and Production

232 Chapter Three

In the SMAW example, the electrode cost is approximately 6% of
the total cost. For the FCAW example, primarily due to a decrease in
the labor content, the electrode cost is 25% of the total. By using
FCAW, the total cost of welding was decreased approximately 65%.
While the FCAW electrode costs 85% more than the SMAW electrode,
the higher electrode efficiency reduces the increase in electrode cost
to only 39%. The first priority that must be maintained when select-
ing welding processes and procedures is the achievement of the
required weld quality. For different welding methods which deliver
the required quality, it is generally advantageous to utilize the
method that results in higher deposition rates and higher operating
factors. This will result in reduced welding time with a corresponding
decrease in the total building erection cycle, which will generally
translate to a direct savings for the final owner, not only lowering the
cost of direct labor, but also reducing construction loan costs.

3.8 Techniques to Limit Distortion
3.8.1 Why distortion occurs
Distortion occurs due to the nonuniform expansion and contraction of
weld metal and adjacent base material during the heating and cooling
cycles of the welding process. At elevated temperatures, hot, expanded
weld and base metal occupies more physical space than it will at room
temperatures. As the metal contracts, it induces strains that result in
stresses being applied to the surrounding base materials. When the
surrounding materials are free to move, distortion results. If they are
not free to move, as in the case of heavily restrained materials, these
strains can induce cracking stresses. In many ways, distortion and
cracking are related. It should be emphasized that not only the weld
metal, but also the surrounding base material, is involved in this con-
traction process. For this reason, welding processes and procedures
that introduce high amounts of energy into the surrounding base
material will cause more distortion. Stresses resulting from material
shrinkage are inevitable in welding. Distortion, however, can be mini-
mized, compensated for, and predicted. Through efficient planning,
design, and fabrication practices, distortion-related problems can be
effectively minimized.

3.8.2 Control of distortion
Design concepts to minimize distortion. The engineer who is aware of
the effects of distortion can design measures into the welded assem-
blies that will minimize the amount of distortion. These concepts
include the following:

Downloaded from Digital Engineering Library @ McGraw-Hill (www.accessengineeringlibrary.com)
Copyright © 2009 The McGraw-Hill Companies. All rights reserved.
Any use is subject to the Terms of Use as given at the website.