Truss structure optimization

hi all,

I'm kinda a newbie and i'm trying to optimize a simple 2D truss structure using optimization solver.
the objective function is total strain energy of the structure (mod1.truss.Ws_tot) and the constraint is total weight (mod1.mass1.mass).
control variables are a set of three length parameters of position of truss members and cross section area.
I read everything available on comsol documentation about optimization, but i still couldnt figure out how to use derivative based optimization algorithms to this model.

I think the problem is that my objective and constraint is indirectly related to control variables but i dont know how to connect them in the right way.

And i have deadline soon, any kind of help would be really appreciated.

Regards, Kosar