The structural integrity of a backyard structure
As with any structure you need to define the loads on it (wind, snow, furniture, people etc) and from there define the structure or framing capable of supporting those loads.

There may well be local codes or regulations that need to be respected as well.

Since this does look like a neat design, why not check out a local structural engineer or architect to see what it would cost for them to set it up properly. (often, tasks like this are given to the office "junior" to improve their experience - under the "eye" of the engineer / architect to make sure it is correct).

That way, any contractor will have fewer problems, and if something is wrong you have a clear path of responsibility to the contractor &amp; architect / engineer.