there will not be anything stored above this ceiling. insulation will be used between the beams plus electrical fictures.
Talk to the local city structural engineer. They will be more than happy to answer your question. For free.
24 feet is a large span ...im a builder in uk ...you never gave the other dimension ..if its smaller ..that's the way to go ...however if we used a steel beam for that span it would need to be at least 12deep by 6wide ..a lot for a non loadbearing situation ...the joist would be 7x2 at 16centres....on any steel you will need padstones where they sit ..either concrete or blue brick
Contact your City/Local Building Codes
You should get a rail kit when you buy your tiles. Just ask them at the hardware store when you go and get them. tell them how long and wide the room is and they will figure out how many feet you will need. Measure along one wall and find the center, then do the same along the other wall, that will give you center. You will need some wire and U nails to suspend the rails from as you put them up. Make sure the line at the top of the wall where you plan to hang the ceiling is Level, may need to fudge this a little, if one corner is higher then the other.
I will tell you how contractors would do this. We would attach wires up to the existing roof or ceiling long enough to hang the grid-work off of for the suspended ceiling. The trouffer lights 2'x4' fit right into the grid pattern and then you lay insulation on top of the tiles. You are going to spend a lot of money to span 24'. You probably do not know what a suspended ceiling is - this is usually meant to be a grid/acoustical ceiling. If you want a framed ceiling using wood to span 24' ? You are going to be spending lots of money. No building inspector I know of is going to come and tell you how to do it. Building inspectors are not going to do anything other than inspections during/after construction to see if work meets codes. A little more info such as width/length of room. Height of existing ceiling roof above the floor level?
