Mating to a shell

Hello, I'm a new user and am in the middle of a short coursewhich has really been beneficial. I'm working on a tiny project andwant to make some headway before I go back to class tomorrow andask my question there.

I'm making a drawing of a simple steel building that's going to putup. In addition to showing the door placement and such, I'm tryingto place the I-beams and am experiencing frustration mating I-beamsinto place.

I have two parts, the building and an I-beam that I inserted intoan assembly. I't trying to put the beams into place by mating themto the walls, but am getting "over defined" errors. I made thebuilding shell, by extruding a boss of a sketch, then using theshell tool.

I note that when I place 2 of my mates, I can see the I-beam fromthe outside of the building.

I*think* that my problem stems from the fact that there is not aninterior and exterior wall. Is this correct? Should I have used the"offset entities" tool to creat an inner wall to mate to?

Additionally, this whole business of placing I-beams into theassembly seems to be a bit messy. Any suggestions on a betetr wayto do it?

Thank you for your time.

Michael KorneckiSolidworksGeneral