That can turn out to be quite the arrangement...meaning that we fabricate a supporting structure to the building itself......usually its a supporting design made out of 2 x4 s and bolted together.......
When its finished you could probaly hang off of it that is how strong it is.
Make sure to check with your local building code. The NEC does not specify a minimum mounting height for ceiling fans in ordinary locations, most building codes do. I believe 7' AFF is the min. here in FL.
First of all you need to place 4" octagonal box that sits flush in a ceiling tile ,second the void between that box and the structure could be 6' difference in a commercial building ......
The goal is to support the box and keep it from wiggling around and falling down when the fan is turned on.
There are probably many ways to go about this but however you do it you better make sure it does not come down after you install it........like I said before it can be quite the task and a good solid mount is required.
We just had to do the very same thing. We had a room that showed 3 ceiling fans in a suspended ceiling.
This is what we did.
1) Cut hole in ceiling pad for ceiling fan dated box.
2) Mount box to unistrut above pad
3) hang threaded rod from framing members above to unistrut
4) Add cross braces to prevent wobble
Our application had bar joists about 2 ft above the suspended ceiling. We installed a cross brace (made of strut and angle brackets) for side to side movement and 1 for back and forth movement. When they were done, I could hang off of it and it didn't wobble at all.
It may be a bit expensive but I can sleep at night knowing that fan will never fall down and hit someone because of something I did.
B-Line and Caddy make a ceiling fan box/mount for grid ceiling. I have used the one from B-Line a few times and it is sturdy. The support of the actual grid is the key.