Page 279 - Handbook of Structural Steel Connection Design and Details
P. 279

Welded Joint Design and Production

                    264   Chapter Three

                    require that weld bead shapes meet prescribed requirements of
                    maximum height and widths. This has an indirect effect of limiting
                    heat input.
                      Current density is determined by dividing the welding amperage by
                    the cross-sectional area of the electrode. The current density is there-
                                          2
                    fore proportional to I/d . As the current density increases, there will
                    be an increase in deposition rates as well as penetration.
                      Preheat and interpass temperatures are used to control cracking
                    tendencies, typically in the base material. Excessively high preheat
                    and interpass temperatures will reduce the yield and tensile strength
                    of the weld metal as well as the toughness. When base metals receive
                    little or no preheat, the resultant rapid cooling can promote cracking
                    as well as excessively high yield and tensile properties in the weld
                    metal, and a corresponding reduction in toughness and elongation.
                      All of the preceding variables are defined and controlled by the
                    welding procedure specification. Conformance to these requirements
                    is particularly sensitive for critical fabrication such as seismically
                    loaded structures, because of the high demand placed upon welded
                    connections under these situations.

                    Fit-up. Fit-up is the term that defines the orientation of the various
                    pieces prior to welding. The AWS D1.1 code has specific tolerances
                    that are applied to the as-fit dimensions of a joint prior to welding. It
                    is critical that there is ample access to the root of the joint to ensure
                    good, uniform fusion between the members being joined. Excessively
                    small root openings or included angles in groove welds do not permit
                    uniform fusion. Excessively large root openings or included angles
                    result in the need for greater volumes of weld metal, with their corre-
                    sponding increases in shrinkage stresses. This in turn increases dis-
                    tortion and cracking tendencies. The D1.1 tolerances for fit-up are
                                          1
                    generally measured in  / -in increments. As compared to the overall
                                           16
                    project, this is a very tight dimension. Nevertheless, as it affects the
                    root opening condition, it is critical in order to avoid lack of fusion,
                    slag inclusions, and other unacceptable root conditions.
                    Field versus shop welding. Many individuals believe that the highest-
                    quality welding naturally is obtained under shop welding conditions.
                    While some aspects of field welding are more demanding than shop
                    welding situations, the greatest differences are not technical but
                    rather are related to control. For shop fabrication, the work force is
                    generally more stable. Supervision practices and approaches are well
                    understood. Communication with the various parties involved is gen-
                    erally more efficient. Under field welding conditions, maintaining and
                    controlling a project seems to be more difficult. There are environ-
                    mental challenges to field conditions, including temperature, wind,




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