I've done a few timberframe shelters, in fact doing one right now with my building construction class. We are "far north" as well and I have buried precast concrete footings, but I have also used a post hole digger and used stove pipe for sono tubes and I have buried those "big foot" forms and used sono tube on top mof that. You definitely want to fasten the frame down in that field. It will uplift with only a roof in a strong wind.