Help for Industrial Theatre Glasgow

Hi everybody!

I'm a new member as my experience with Grasshopper.

I have to make the easiest version of the industrial theatre SECC conference center in glasgow by Foster for a project for the university.

i tryed to make an arc by graphmapper and then use mirror and extrude. the problem is that i can't move my curve as i want, to give it the arc shape. I used "series"with a panel and then "graphmapper", "construct point" and "interpolate" I'm probably on a wrong way. 

Can some of you give me any advice?

Thank you.