One of the things you need to do when installing the Atlantic Arch is to be sure you "preload" the structure. They do mention this in their instructions, but if you talk to them they REALLY emphasize the importance of this step. If you skip this step, or under do it, the arch will be quite unstable and sway back and forth a LOT.
What they mean by this is the cut width of the arch should be several inches wider than the location of the mounting feet, and then pulling them together to complete the installation. I think mine was about 5 inches wide, and I used a spanish windlass to pull the legs together. My recollection of the instructions was that they didn't really talk about this much. If you are installing one, it is worth a call to their support line to get a detailed and specific recommendation
With the parts stressed in this way the side to side motion is dramatically reduced. I have pretty heavy solar panels on top of mine, and I added a diagonal brace because "it can't hurt". but I did not notice much change in motion with and without the brace.
Fort Lauderdale, FL