Well, its probably moot by now for this year, but I generally use 'B' when the long sides are being bolted together. This puts longer single beams facing together and should be a bit stronger if all of the 2x4 joints are equally strong. For those, I suggest two 5-inch long #14 flathead screws through the longerons and into the ends of the shorter bars.
A pair (or three if you're planning on dancing a lot on it) of 2x4 pieces parallel to the end pieces* will be plenty of bracing and even 1/2-inch plywoood should be ok if you secure it to the frame with c. 20 or so #10 or #12 wood screws at least 2-inches long. Drill and countersink everything, polyurethane the entire platform well to seal it and your platform(s) should last on the order of twenty years, even going out to Pensic every year. At least, all of the many lofts I've built this way have ... ;-)
* Again, two ~5-inch #14 flathead screws through the longeron and directly into the ends of the pieces. Also, perhaps its obvious, but the 2x4s should have the ~two-inch side against the plywood, such that their larger cross dimension is perpendicular to the plywood surface, as that's where the strength is needed. I've done it the other way in special curcumstances, but never without careful thought on how critical the need was. 'X's mark the location for holes for 3/8" carriage bolts to attach 2x4 leg pieces and you can add more holes along the length to bolt the platforms together.
PS: I had nice ascii diagrams of the design but lj inisisted on mangling them so I took them out. If they would be of use to you, let me know and I'll forward them via email or something.
(no subject)
A pair (or three if you're planning on dancing a lot on it) of 2x4 pieces parallel to the end pieces* will be plenty of bracing and even 1/2-inch plywoood should be ok if you secure it to the frame with c. 20 or so #10 or #12 wood screws at least 2-inches long. Drill and countersink everything, polyurethane the entire platform well to seal it and your platform(s) should last on the order of twenty years, even going out to Pensic every year. At least, all of the many lofts I've built this way have ... ;-)
* Again, two ~5-inch #14 flathead screws through the longeron and directly into the ends of the pieces. Also, perhaps its obvious, but the 2x4s should have the ~two-inch side against the plywood, such that their larger cross dimension is perpendicular to the plywood surface, as that's where the strength is needed. I've done it the other way in special curcumstances, but never without careful thought on how critical the need was. 'X's mark the location for holes for 3/8" carriage bolts to attach 2x4 leg pieces and you can add more holes along the length to bolt the platforms together.
PS: I had nice ascii diagrams of the design but lj inisisted on mangling them so I took them out. If they would be of use to you, let me know and I'll forward them via email or something.