The purpose of these plate stiffeners is to increase the shear capacity of the member (like when post-buckling tension field action controls the shear strength of plate girders). Is that correct?
If so, then those stiffeners do not affect the ANALYSIS results at all. You've still got the same A, I and J values used during the analysis. The only thing those stiffeners would affect is the code check on the member.
It also reinforces the web to prevent web crippling due to slenderness. It is related to the code check but because the web is slender RISA does not perform calculations on the member which I think means the loads are being transfered through that memeber.
