Some thoughts from a DIYer, not an engineer:

First, a six by six footprint for a 16-20' tall structure sounds a little "tippy" to me. But I'll defer that point to someone else.

If 16' 6x6's are available, I'd rather see those than a deck on a deck.

I wouldn't bother with the 6x6's for the railing. Either buy 16' posts or go up 20'. Attaching notched 4x4 railing posts to the deck is easier than raising the extra 4' of 6x6. And, you won't lose as much floor space.

Way easier to build the ladder up the side and put a gate in the railing than a trap door in the middle. And it would give more flexibility for benches or seats. If you do the diagonal braces as pictured, getting into the middle won't be much fun, either.

2x6 joists will work for a 6' span. If you use joist hangers and set them first, on the ground, then hoisting a 6' 2x6 joist to the top shouldn't be too tough. I'd set up a rope and pulley.

More as I ponder it.

 

The "tippy" part is what bothers me. We build stands with the slanted posts mainly to keep that sway out of the picture. The "look" will trump practicality, especially going up 20'. We climb scaffolding all the time and there's no way I would install a "ladder" either in the middle or on the outside without some sort of cage for protection. You get up about 12' and your brain and hands begin to fight each other. Your hands don't want to let go of the rungs and your brain tells you that you should have stayed on the ground.

Glad the design is in its infancy, as it needs refinement and safety built in.

 

on the ladder, I was thinking a 8' ladder to the first floor and then a separate 8' ladder from there to the second. built into the middle and only 8' feet should be comfortable.

What if I made the base wider with the use of some built beams and diagonal bracing? ...through bolted at the base of the columns? Maybe a 6x10 horizontal beam at the base tied into the four uprights and anchored to the concrete? along with some diagonal bracing. See image below.

 

 

and the weight this water tower is holding is 10 times what I need and this water tower is more susceptible to wind sheer. If I went with columns that were wider at the base, what angle would be sufficient. Seems any amount off of plumb would be a dramatic improvement.

 

. Since this will be an open structure, wind forces will be be minimal.

You really believe that?

 

yes and it will be bolted to the footers.

 

The Tale of the Tower: Building a Timber Frame Observation Tower in Greenwich, CT - YouTube

 

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and build 6x8 beams out of 2x8 lumber. heck, beams prob don't need to be 6" wide. 4" wide is probably fine. I'm gonna look and see if those caps come in a variety to fit a 4" wide beam on a 6" post.

 

For the top you just notch in 3" for a double 2x6 and bolt ti thru , just on the side that will support the deck