Erik Kostson
Ansys Employee

Hi
I would share the geometry in order not to have contacts there (stiffener and plate), and so to have shared nodes and a compatible mesh between them (stiffener and plate).

Finally if it is still there try to put a reasonable fillet on those 90 degree corners between stiffener and plate.
Also use a regular mesh as shown below.

https://www.engineering.com/story/fea-simulation-how-to-use-shell-element-results-to-improve-design

See our getting started course and if there is any meshing on that.

All the best
Erik