Rolland the Thompson Gunner,
Sorry you have a great name that reminds me of Zevon.
My two cents:
It sounds like you are trying to span 30' at a gable and then have two bays at 12'. Without getting into the structural of 6 X 6 not being big enough for the task I would suggest just a couple of design ideas.
1. If you build a pavilion with a form of a gable that is 30 feet wide and 20' long it will be a small window when your shelter is actually shaded underneath regardless of orientation. It would not be fun to have the sun shining on you in a shelter most of the day.
2. Be aware that a slab of concrete can be overwhelming for some tastes. Please look into crush and run for your flooring as a possibility. It has much more character and is not nearly as "sterile and cold". You would just most likely need some banding to hold in the smaller aggregates once you get to finish level.
And don't forget, "That when you pay for quality you only cry once"
Cheers.
