Using the sum or average of static von Mises stresses in an element set as a response ?

Hello everyone,

For the purposes of a shape optimization I need to conduct for my Master's thesis, I wanted to optimize a response that is partially based on a sum of the Von Mises stress values of several elements (100+) in a certain element set of my model mesh. My initial idea was to do it using a DRESP2 card, set the function as sum, and give as a response input my static stress > Von Mises response; I then realised that the sum function actually sums the response values of multiple separate responses, and not the individual values of multi-output Optistruct responses such as stress or displacement.

Is there any feasible workaround for this, other than maybe writing a DRESP3 external function that would take all Von Mises stress values of elements within a certain element set, and churn out the sum of them all or any other scalar value I would need? Defining a stress response for every single element in my set would not be a feasible solution because of the very high number of elements I have in the set of interest.

Thank you and appreciate any insight from you on the matter