Hi Zz...
OK, so it sounds like you are putting the tub in an area that will be surrounded by deck and the "platform" you want the tub to sit on will be lower.
First up, planning. You are what I would call "on the edge" of needing an engineered plan for building what you want to build. Height is your problem because a suspended static load of 6,500 LBS can exert SERIOUS torsion force (dynamic load) on a structure even with small wind or snow load, moving people, etc. Re-enforcement with cross ties, bolting into an existing structure, etc., is REALLY needed here.
Given that, I'll outline the basics. For footings, I'd recommend using cardboard concrete tubes with steel re-enforcing rods. The tubes should be 12" in diameter and should be placed deep enough to go below your frost line by at least 18" (check building codes for this requirement). Drop the rebar rods as you are pouring the concrete and make sure you "tamp" down the concrete during the pour to get a tight footing.
To connect your posts to the footings, use a post base connector made to be set in the wet concrete. Anchor the post to the connector with lag BOLTS and make certain you use 6X6 lumber for your post. Simpson Strong-Tie products are best. I'd suggest getting the HIGHEST grade 6X6 available from a lumber yard to avoid twisting. Also, don't cut the posts until you are ready to immediately install and bolt them. They have a tendency to twist if you cut and leave them lying for several days. Also, don't use a mounting plate with a screw-in bottom. The post will eventually twist and you will have a problem.
I'm thinking at least 4, preferably 6 post footings for your supports. Based on your 10' length requirement, a 4 footing plan would end up giving a joist span of 8' (assuming a 1' cantilever on the ends) which for 2"x8" dimensional joist lumber is probably close to load limit for your application. Why save maybe $150 for 2 extra footings and one beam when you can be "bulletproof" with 6 footings and 3 beams? The 6 footing plan should be 3 footings in line on the 10' length, with footings spaced 1' from each end and then in the center. It would look like this:
O O
O O
O OAfter installing your posts, BOLT your beams (2"x10", EACH side of the post) to the posts to span the SHORT span (total of 3 beams). Leave about a 1' cantilever on the ends. Over the beams, install 2"x8" joists on 12" centers. Use Simpson "Hurricane" connectors to tie joists into the beams, where each joist crosses a beam. I prefer this over "toe nailing" because the connectors provide MUCH more rigidity.
Given your height, you may want to consider 2"x6" lumber for decking. This adds stability to your platform.
Try and give me some idea about how you are planning to connect to the surrounding deck, or if the tub deck will be completely free standing. When you do, I'll post something on connections to other structures.
REMEMBER, my advice is cheap and not necessarily correct. Use a contractor if you have worries...
Good Luck!
Drewski
