The requirements for welding of structural steel are specified in Specification Article 2408.03 and Materials I.M. 558 (558 Appendix A and 558 Appendix B). When possible, all welding of structural steel is to be performed in a qualified structural steel fabrication plant that has an established and approved Welding Procedure Specification (WPS).
The welding of structural steel involves a very controlled process that is more easily achieved in the control setting of a fabrication shop. Although quality welding of structural steel can be achieved in a field setting it is much more difficult and as such is not generally permitted. That is why designs involving structural steel assembly in the field have bolted rather than welded connections.
There have been instances in the past where contractors have requested the use of temporary welds on structural steel. Specification Article 2412.03 states that temporary welds will not be authorized….. and there are few exceptions. Any consideration on the use of welds in the field must be reviewed by the structure designer.
The reasons for restricting the welding of structural steel to qualified structural steel fabrication shops include the following:
- Prior to welding of any structural steel, the contractor must submit and have an approved WPS.
- After approval of a WPS, they will verify the qualifications of the contractor and their welding personnel. During the welding process, a materials inspector will observe and inspect the structural steel welding being performed including review of all required testing of completed welds.
- Critical aspects of the welding process include: correct current settings, joint preparation, welding position, preheat temperature, and interpass temperature. The last two if not well controlled can result in damage to both the structural steel base metal and the actual weld metal being applied.
If the preheat of the structural steel base metal is not increased to the required temperature prior to welding, the base metal composition may be altered and the hardness of the structural steel affected. This can result in the structural steel becoming brittle and susceptible to cracking.
In a similar fashion the interpass temperature on multiple weld passes must be controlled and maintained at the required temperature or the composition of the weld metal may be altered and cracking within the weld metal may occur.