A test comparing plastic capacity in welded profiles when analysis with or without welds+ analyzing as simplified shell element.
Profile with welds

Profile without welds

Shell element

Geometry

Profile with welds

Profile without welds

Shell element

Conclusion
Analyzing steel structures from plates/sheet metal with simplified contacts and shell elements does not significantly impact the plastic capacity of a profile. The analysis did fail to converge within a 1.5% load difference compared to each other.