The dimensions are ~45’ deep by 38’ wide … these unusual dimensions are due to a tight building envelope we’re trying to work within

TJIs or similar can span 19'. You probably would go with 2 columns to break up the 45' dimension.

I get this feeling that you would benefit greatly by consulting a local architect. Steal beams are usually not elaborate an often cost effective. The architect will most likely have layout suggestions that will be a good solution to efficiently meet your goals with this project.

Not enough information.

Fair enough … I completely understand that point.

I’ll phrase the question another way:

Can someone suggest dimensions that would approximate this garage size that would make for the MOST cost effective approach to construction?

Find a local structural engineer to review your plans and make recommendations.

At least in my town the structural engineer approval will be required to get a building permit.

"Can someone suggest dimensions that would approximate this garage size that would make for the MOST cost effective approach to construction?"

My other comment tells you who that 'someone' should be.

We have a similar arrangement and a somewhat smaller garage in new construction that is underway. We have a couple steel beams in there. The house was designed by an architect and we had a structural engineer work out where more reinforcement was needed. Sounds like you will need something similar.

It's doable. I would split the 38' L-R span in half by by running a suitable sized glue-lam beam up the middle with one or two equally spaced steel support posts. Everything is a tradeoff, so you can decide on the number of posts when working with your lumber/joist/beam supplier. Then you can span the resulting 19' from the outside walls to the center glue-lam with suitably sized I-joists. From the attached I-joist span table (only use as an example) you have several joist depths and joist series to choose from to support your living area above. Again, the tradeoffs will be cost and availability/delivery. What you will need is a finished set of construction drawings for your local lumber/I-joist/glue-lam supplier/manufacturer to do their engineering takeoffs from.

https://www.fp-supply.com/st-louis-trusjoist-tji-i-joists.html

Our house has a flexicore garage ceiling--concrete hollowcore

To all of the engineers and architects who have responded on this thread:

Thank you, Captains of Obvious.

Was trying to approach said steps with a plan that was amenable to standardized materials and building techniques. Trying to avoid being ripped off by ANOTHER architect who fails to consider all of the things I asked them to consider.

No snarky comments permitted? This can't be Houzz. Lol

I have a house with a lower level garage and living above. I live in a hilly area on sand. I didn't build the house (over 30 years old). What the original owners did with the garage is install Flexicore hollow core concrete beams across the ceiling of the garage. These beams eliminate having support posts in the garage that block where/how you park your cars and give you a large span. I want to remodel the room that is over this garage and turn that into my kitchen, and it will be a bit interesting in turns of drilling into that flexicore to install kitchen water pipes and electric.

Since I live in a hilly community, I have a number of neighbors with living upstairs and garages downstairs. I don't really like that most of my house is upstairs. I have to drag all the groceries upstairs and my washer/dryer are downstairs.

One of my neighbors has a configuration where they can park by the front door (same level) but the garage is lower level. The main part of their house is then on the level of the front door.

https://www.coldwellchicago.com/blog/hollowcore--flexicore-construction/