The dock frame is welded aluminum. The top chord is 2x4 all around. Perhaps 16-18" below that is a 2x2 chord. The two are connected vertically with pairs of 2x2 pieces on each side, where the legs go. There are four pairs of 4x6 PT wood legs out over the 30 length, or a bit short of eight feet apart. The pairs of vertical 2x2 pieces have a small gap between them, and the bolts holding the legs strapped to the frame pass between the pair. A piece of peel & stick rubber roofing membrane keeps the PT wood separated from the aluminum.
Across the 6' width are 2x2 cross members, except 2x4 at the two ends. There also is 2x2 cross-bracing. The lifting ladder is rectangular, made of 2x4 sides and 2x2 "rungs." This is 24 ft long. It sets down over the lateral members and cross-bracing and, with the two sides of the dock, helps support the deck sections.
In the first pic, you can see the two whip mounting plates, one near the outer corner, the other 10' in. The second pic shows the frame mounted on the header bar assembly, which was welded up in place for the set of boulders at the shore. In that second pic, that's not the lake bottom under the header bar. It's a lot of rock debris from drilling into the boulders for the expansion bolts and from chiseling out a place at the top of one boulder to make room for the header bar. The ice was well over a foot below that dirt, right down to the lake bottom.
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