Architecture - steel I beam calculation

I am building a mezzanine over my garage. The size of the garage is 27 feet long by 24 feet wide. I need a steel I beam supported on the ends by steel columns that spans 27 feet. The joists are going to rest on the side walls and on the I beam. Therefore the joists will be 12 to 14 feet long. I have done some research and found out that I should estimate a 40 psi as a combination of dead plus live load. Can anyone tell me what size beam and columns do I need?

Your best bet for an answer is to talk with the company supplying the steel. They should have the engineering background to provide you with an answer, including what may be required above minimum by local building codes.

Based on your description, I would say W12x22 would suffice, which is roughly 4" flange width and 12.25" deep. Any manufacturer can help you with the columns. But certainly do not take my word for it, get it engineered as this is a very important step in your construction. I would need a full set of plans to give you a good beam size, this is intended as a guideline.

Consider contacting a local residential architect. We will occasionally take on small jobs like this for $100-$200. That gets you a signed/sealed letter including the beam specification based on the conditions you describe. A local architect will be aware of state codes regarding allowable deflection and bearing lengths, and may also be able to specify the posts needed to support such a beam.

Good luck!