is that pic from the DBG?

What you're fighting against with any outdoor structure are uplift and racking (lateral movement). There shouldn't be much uplift on an open-roof structure, but we're still required to consider it.

Lateral movement is the killer. In your inspiration photo I'm not seeing any angle bracing between the posts and the beams, which leads me to believe that the posts are buried deeper than 3' and probably in concrete. I'd bet money that those posts are at *least* 5 ft deep, or else the plans were certified by a structural engineer and signed off on at the end.

Awesome looking inspiration piece. You're now obligated to share pics when you're done - sorry, I don't make the rules!

Let me make it clear: I'm not a structural engineer so can't answer regarding the project pictured here. And this is more likely an engineering question specifically. Without a diagram I can't really grasp the design portion of this but here goes anyway: The sheer weight of the structure would likely hold it in place but I would still pin it to Terra firma via a hidden fastening system within the pipe itself. Why not create footings at each post, or even every other one (with sona tubes sunk 4' min. and filled with concrete) Then before the concrete has set you will set your hidden fastener within the wet concrete (making sure its corrosion resistant metal/steel). The fastener would then slide into its post and pinned/fastened on the exterior somewhere unobtrusive. Tying it to the ground should then eliminate any worry of the thing shifting. And by using an internal connection system you are NOT sinking your posts directly into concrete as well. In effect that makes it possible to pick up and move at the end of the day should you need to down the road.

PS: good point about the bracing! I would factor that into your design def. if you don't opt for the above. But do get some professional advice on this!

Then once you have done all that come over to my place and build me one just like it, kay!?