I plan to extend my bedrooms above the garage. since I will be doing it, I want it done right. All the construction I see uses wood framing which I don't like due to fire susceptibility. I am considering metal studs and beams but would like to know if there are other materials that are resistant to fire but strong and durable. What are the advantages and disadvantages of each?
I agree with Jimmy Fix-it (in a comment on the question): residential sprinklers will protect best, if you have a good (dependable) water source. They are relatively inexpensive, because the sprinkler lines can be run from the nearest water line in the house provided the domestic system can supply the flows needed (13gpm to all design sprinklers and 18gpm to any single sprinkler operating). You don't need a separate waterline, riser, fire department connections, etc. However, be aware that when your insurance salesman says you'll save money in house insurance, you should ask him how much to add "water damage" if they accidentally go off.
But, to answer your question, the Code divides construction into 5 categories:
Type I and Type II: These types are "non combustible ", which primarily refers to concrete and steel construction. The main difference between I and II is whether the exterior and interior bearing walls need to be protected longer… it's 3 hours for Type I and 1 hour for Type II.
Type III is masonry construction using combustible inner structural members, or "ordinary construction".
Type IV is "Heavy Timber" construction. This is wood (fire treated and untreated) that is primarily thick (8" diameter!) timbers for structural members with masonry for load-bearing walls. Partitions (non-load bearing) walls can still be 2x framing.
Type V is anything or everything else… including wood framing, straw-bales, etc.
However, all these materials are divided into sub-categories of with and without fire sprinklers. There are many many requirements with each category… like everything, it's in the details. (I.e.: non-bearing wall construction, enclosed shafts, attic construction, etc.)
In addition to what you called "fire susceptibility" construction, the code identifies "flammability " of materials. All materials fall into 3 classes: Class A: 0-25 flame rating, Class B: 26-75 flame rating and Class C: 76-200 flame rating. Most often this is what kills occupants of burned buildings. Usually, victims are dead long before the building burns up and falls down on them. Fire sprinklers will protect against this too.
I'd also invest in the best fire alarm system (with battery backup)… and some come with additional horns and carbon monoxide detectors too.
You can use steel studs, but by the time the fire gets far enough to have ignited wood studs (in their usual gypsum cocoon), the steel studs (which are quite thin) will have failed from the heat.
Indeed, a heavy timber frame structure usually lasts longer than a steel frame structure in a fire, as the contents of the building burning cause the steel to sag and collapse long before enough of the wood frame (which is rather hard to ignite in a heavy timber frame) has burned away to cause it to collapse.
In any case, what you build the room above the garage from is almost irrelevant to the fire survivablity of such a room, in a building that's already been built with whatever materials it was built from - if "ignition from the garage" is your concern, adding two layers of type X with the seams offset to the garage walls and ceiling would do a lot more, as would removing any fuel other than that in the vehicle tank from the garage (i.e. build a shed in the yard to put the lawmower, mower gas, etc. in, and don't fill the garage up with boxes, recycling and other fuel.)