I just bought a new house that has a deck that sits about 20 feet off the ground. it is roughly 10ft by 20 ft. it has two rows of 4x4 posts as support spaced about 5 feet apart. one row in the back and one row in the front. there is no middle row. when walking on it you can really feel it sway. i don't see any other bracing like a 45degree brace or anything else. my question is what can i do to limit the sway based on these conditions.