The structure in question is a diving (read: jumping) platform that my father and I built. Here are some pics: All measurements/approximations were made in feet or inches, so I'll present them in that form if it's not too much of a problem: The utility poles used are 35ft long, and buried approximately 5ft in the ground. Each pole has a diameter of 10-12 in. at its base, and each pole becomes thinner towards the top. The bases of the poles form an 8ft by 6ft rectangle. The two decks are situated at about 10ft and 20ft above the ground, and are 16ft long and 8ft wide. Both decks hang out almost 10ft beyond the rectangle of utility poles. The decks are fairly rigid, and are securely attached to the poles with 8 in. lag screws. Now for the problem: the structure is prone to unwanted vibrations, which are especially apparent when standing on the 20ft platform (on the 10ft one, it isn't bad at all). Before my dad and I resort to reinforcing the thing willy nilly, I thought I would seek out some more educated opinions :) So far, we have considered adding various wood or metal trusses and/or wire cables anchored to the bases of nearby trees, but we don't know what designs would be most appropriate. We have also considered cutting off a few feet from the tops of the poles to see if that helps, but I thought we might end up putting some trusses up there, so we've held off on that for now. Anyway, any suggestions would be greatly appreciated.