Elevated residential garage floors are quite common in my area and I have designed a lot of them. There are several good methods.
1. As previously noted, a 50 psf live load is usually adequate. Some times it is necessary to jack-up a vehicle in the garage so, although it rarely controls, be sure to check the 2000 lb wheel load requirement. Some of my clients have elaborate shops in the lower level and on occasion I've design the floor beam for hoisting loads.
2. Advise the client that condensation on the underside of the slab may become a problem if the floor is located in a cold climate. Condensation can be controlled when proper attention is given to ventilation, vapor barrier, and insulation.
3. Use a two-way slab or hollow-core planks if your client wants a column free space. A two-way slab of reasonable thickness can easily span the 23'x23' area, but either use a central drain or give it adequate slope because long term creep may cause a puddle to develop. 8" thick hollow core planks (my first choice and preferred method) are terrific for spans of about 27' or less. I usually use a 2 1/2" topping but never count on the topping structurally. When using hollow core planks most of the work is in detailing the edges to make sure the slab is properly tied to the walls. Use idiot proof details.
4. Use a single W8 central beam and design the slab to span one-way, if a column can be tolerated. I have used composite steel deck on several occasions when the owner insisted, but I prefer formed concrete slabs. I usually use studs on the beam to get composite action, but design the beam as unshored. The slabs are usually about 6" thick with #4 or #5 @ 12" BW supported on 1" slab bolsters. There are two ways to get slope to the garage doors. You can either keep the bottom flat and slope the top surface, or keep the slab uniform in thickness but slope the wall chase. I prefer the later.
5. Many carpenters shore the one-way slabs using telescopic aluminum beams that can be rented from a vendor who rents forms and equipment related to concrete construction. The plywood forming can be salvaged for use elsewhere on the house. Be sure to protect it from the concrete by covering it with poly before placing the concrete.
6. Open web joists are usually not a good choice because they reduce the headroom of the lower level, and thin slabs usually don't stand up well when exposed to deicers and long term moisture.
7. Watch your headroom in the lower level. It is very disappointing to the client to spend a lot of money on an elevated slab only to find that lower level is unusable.
8. An epoxy floor coating is the perfect finishing touch.
