With all respect to the OP, Shelter Tech has TWO grades of shelter available.(ONE is the Standard grade, the SECOND is the CANADIAN grade). The Canadian grade has larger dia tubes with thicker walls and a thicker type of plastic cover. I have the Canadian grade, plus I have 20 extra duck-foot stakes ( one for every vertical; and extra wind straps 4 vs 2. I also have the ratchet type of lower attachment tighteners all along the bottom edge.)
When I built it, I used fine grade crusher dust two feet thick for the base and patio blocks for each vertical post, (everything dead level in two directions and well compacted) before installation. The cover was tightened to the max with the ratchets. These extra steps are why the shelter has withstood six winters, two hurricanes and amazing snow loads. I do not have to push the snow off, ever and wind does not make it sway, because I added braces (see Earlier posts) to prevent the structure from any longitudinal flexing. The whole job cost me 3,500 dollars taxes in. It was quite pricey, yes, but it is still standing. Cheaper jobs, even from Shelter Tech will collapse under heavy snow loads.
I cannot emphasize enough: If the structure gets off plumb or level, or if is allowed to flex in the wind, the weight will eventually break the tubes at stress points. It Must be dead plumb and square...and well secured and braced..just like a house is. The frame of the shelter will only give max strength if everything is lined up, is plumb, and stays that way. Overkill is way better than hoping.
Shelter Logic does not have the heavy grade tubing nor the heavy grade cover material and it cannot withstand heavy weather. It is a waste of time and money..and it endangers you and everything inside it, if it collapses, which it will..take a look around and check it out for yourself.. This is one thing you cannot do on the cheap!
Filling the tubes with concrete will make them stiffer, but would be very hard to do and then how would you get the screws into it. Also, it will not strengthen the cover, So I would not even consider doing that. Putting up a big frame is already hard enough to do.
Buy the best stuff, do it right and You WILL be satisfied, just as I am....after six horrid winters here in the Maritimes.
.. (no I do not have any financial interest in the company Shelter Tech), but they are a New Brunswick company and they have a six-month backlog of orders currently. Because they do it right. That is all good. It gives we NB guys pride.
JIX