There's nothing wrong with pouring your piers/floor first, if what you're looking for is a firm, level place on which to park your vehicles and complete the rest of your construction.
I built such a building about twenty years ago. The "poles" (4x4's) sat on top of the concrete piers (8' on center, max).
It was
not an easy thing to put up. The first reason was that no matter how hard one tries, you'll never get the tops of your piers to be all at the same height, which makes determining the length of each pole time consuming.
Second, and of particular relavance to your idea, is that such a building frame doesn't adapt well to the "build the wall on the deck and erect" method of construction. The "poles" themselves are the main reason for this, since they interrupt the shear plane of the walls.
You mention "cutting the tops" off of square tube in which you'll set your poles.
There are commercially available products that serve the same purpose. True, they cost you some , but they'll save you time in the long run and you can put your tube to better use.
Here's what I would do:
1) Pour your piers and floor, leaving the piers about 2" proud of the floor.
Use the commercially available sill bolts in the piers. 3/4 allthread is
expensive overkill.
2) Determine the height of your highest pier. That will be the base plane of
rest of your building.
3) Lay down a pressure treated sill (2 layers of overlapped 2x6's is the
usual), shimming atop the piers as needed to achieve a flat and level
perimeter on which to build. You can loosely fasten the nuts on the sill
bolts.
4) Your walls will be constructed as follows: The bottom chord should be
2 2x6's with 1/2 plywood sandwiched between. Overlap the joints and be
sure to leave a gap in the plywood where the wall will sit over the piers.
This gap will form a mortise. The top chord should be built the same
way, but use 2 2x8's instead.
The vertical pieces (poles) should be 2 2x4's with 1/2" plywood
sandwich. 2 2x6's would be better, but you decide. The poles, you
remember, will have the sandwiched plywood protruding out each end
a sufficient amount to form a tenon that fits the mortises you have in
the top and bottom chords. You'll also need some intermediate studs to
which you'll attach your sheathing, but those can be toe-nailed to the
chords.
Use a generous amount of construction adhesive.
You can attach the sheathing before you erect the walls, but the walls
are going to be quite heavy that way. If you decide to sheath the walls
later, you'll need to add temporary diagonal bracing.
What I've described is basically "continous header" framing and it's
very strong.
5) Be sure that you make the walls tall enough to allow for deep headers
over your doors.
6) As others have mentioned, 24' trusses are not expensive. Save yourself
a bunch of time and buy them.
Have fun
