I'm looking at the feasibility of constructing a hangar and I want to
estimate the cost of the steel needed. The building would measure
42.4x22.93 meters with reinforced concrete base and perimeter walls up
to 2m height. The "roof" would be a semi-circle arch constructed of
I-beams and clad with semi-transparent polycarbonite plates. What I
need is the size of the arched I-beams and determine the required
cross-bracing to comply with Eurocode3.

For a better idea of what the structure would look like see:

http://www.dallur.com/fileadmin/user_upload/perspective.jpg
http://www.dallur.com/fileadmin/user_upload/front.jpg
http://www.dallur.com/fileadmin/user_upload/side.jpg

Just so you don't think that your question has been ignored, I decided
to comment. Even though $100 is a reasonable price for a question, I
think that you had better consider spending quite a bit more and
consulting with an engineer familiar with Eurocode3. I looked at your
drawings and was very impressed.

Judging by your pictures, the hangar is very high relative to it's
width. Nearly 13.5m high and around 24m wide. I'm not into planes or
hangars at all, but I can't imagine any planes with a wingspan of
around 20m being nearly 13m tall. Make it a lower arc instead of a
circular shape. Will cut costs too.