I could get 8' 2x10's and hang them from the girders with joist hangers. But instead I go with 16' joists and run them above. This way I don't have to buy all those hangers, and go through the extremely tedious process of hanging the joists just right, which I went through the year before on the barn.
I also actually want to raise the floor a little higher, so I can get underneath to insulate and bring in plumbing, and keep the joists well off the ground, and ultimately use the area underneath the building for storage. It's the strongest way to position the joists, having them sit up on top of the girders, with a full 16' board supported well at the middle and at either end.
Before I put in the floor joists, I put in the perimeter band (also 2x10) composed of both the rim joists and header joists (band joists). The 25' long header joists sit squarely on the outer boards of the outer girders - they are what each joist is connected to at each end, flush and at a right angle to the header joist. Since I don't have a 25' long board I will join a 16' with a 10' end to end and connect it with a scab.
The 2 rim joists are run just like the rest of the joists will go, perpindicular to the girders - the only difference is they go at each end, and with the header joists, form the outer band, or perimeter of the floor.
Here's a picture of the band in:Now the nightmare begins - getting the frame square, which means each corner must be 90 degrees, a right angle, and the length of each diagonal the same. Though a framing square can give you a ballpark estimate, and see if you're close, it's really the measuring of diagonals, and 3,4,5 triangles that gets it perfect. The problem is, measuring the diagonals isn't easy with something so large - you do the best you can.
I'd measure everything, conclude which corner needed to move, then start tapping it with a rubber mallet, then go back to measuring. I got very close to square, then realized that if I wanted it to be perfect I'd have to position a header joist slighty off the girder - and this I didn't want to do. So I just went with it. It was out of square by about 1/8", which over a 30' distance is not too bad. One outer girder wasn't absolutely parallel with another somehow - though they measured the same distance apart. After half a day of messing with it I picked what I thought was the best position and stuck with it.
Before I fastened it down to the girders I actually went ahead and put a few joists in, then went back again checking for square. I could still keep tapping the frame around as long as I didn't get too many joists in. After about a dozen joists the frame become more cumbersome and difficult to move. I could knock a corner around but that was it. Still I held off fastening the band joists down to the girders, until I was totally satisfied with the squareness of the frame:Here's a closer shot: