Further Recommendations for Welding Jumbo Shapes

The new AISC guidelines apply only to applications where the members are subject to primary tensile stresses and spliced with full penetration groove welds. These requirements do not apply to situations where members are not subject to primary tensile stresses. However, as noted, these failures were not associated with service loading. If the types of details that are typically used for tension applications are applied to compression components, e.g., CJP groove welds, the same types of cracking problems may occur.

Additional techniques that minimize the accumulation of residual stresses should be employed when welding on jumbo shapes, even though they are not enumerated in the AISC specification. Selection of the specific welding joint detail is important. Double-sided joints reduce the amount of weld metal required by a factor of two. Reducing the amount of weld metal proportionately reduces the residual stresses. Double-sided joints require access from both sides, which in the case of field welding would dictate overhead welding. While out-of-position welding is generally discouraged, it may be advisable in this case to minimize the amount of residual stresses.

Furthermore, welding sequence can be important. On jumbo sections that involved cracking, when the flanges were welded first, the crack would form in the web; when the web was welded first, the cracking typically occurred in the flanges. Finally, cracking was more likely when the final weld passes were applied on the inside surfaces of the flanges (closest to the weld access hole). The following should be observed with regard to welding sequence:

1. As much as is practical, do not weld any specific joint to completion. Weld no more than 1/3 of the depth of any joint before moving on to a separate joint.
2. Utilize joint details that permit the application of the final weld passes on the outer surfaces of the weld flanges where possible. For shop fabrication where single vee groove welds may be used, as shown in Figure 5a, the last welds will automatically be made on the outer surfaces. If double vee groove welds are preferred (because of the reduced weld
   volume and reduced shrinkage stresses), the last passes should be on the outer flanges, as shown in Figure 5b. For field work where flanges are horizontal, a combination of these joints may be desirable. The top flange can be prepared with a single vee groove, while the bottom flange is prepared as a double vee, as illustrated in 5c. This necessitates some overhead welding, but the final passes occur on the other flanges, reducing cracking tendencies.

The AISC specifications do not impose any new requirements on welding processes or the consumables used to join these jumbo shapes. The failures that had been experienced were not weld metal failures, but rather were located in the base materials. True, the driving force for cracking was due to the residual stresses from welding, but the primary problem was one of inadequate base metal toughness.

Some confusion has resulted regarding the requirements for filler metals on jumbo shapes. The new specifications do not impose notch toughness requirements on the welding materials. This was confusing because the supplements were printed with an additional comment regarding the use of “mixed weld metal.” This provision is applicable under circumstances where notch toughness has been specified for the weld metal, and the composite weld metal consisting of
different compositions must have a composite notch-tough weld metal. However, the requirement applies only to situations where notch toughness has been specified for the weld metal – typically, in dynamic structures, since static structures rarely require the use of weld metal with increased notch toughness properties.

Conclusion

The New AISC specification requirements successfully addressed the variables that have been associated with the fracture of welded jumbo sections in the past. In addition, proper selection of the welding joint detail and careful consideration of welding sequence will contribute to the successful use of welded jumbo sections in tension applications.

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