How rigid is a 1" x 4" flat bar ?

[bdog]

Loading will never be truly lateral. Side loads on the dock will resolve into tension and compression in the arms that locate the dock. Since the arms are relatively close together in comparison to the standoff of the dock the alternate push/pull forces applied to your mooring will be pretty big.

The orig I beam attachment @ 4' up would allow some significant "spring" in the beams as they took their loads.​

You drastically limit that spring attaching lower - but not to worry. - The arms themselves are springs of a sort, compressing and extending elastically.

I think your idea is fine but you must allow for fatigue of the mooring tabs at the transition into concrete. -- Use 1 X 6". - Exit the concrete with the full 6" to a height of about 1" and taper the front and back sides equally from that point to achieve 4" width at the attach point. Before placing this shape in concrete radius the corners of the rectangular shape from about 1" below concrete grade all the way to the top. The corners are where a crack would occur, if ever.​

A flap disc wheel will allow you to accomplish this well iteratively in a controlled and forgiving fashion. Any rounded break of the abrupt edges is improvement. A sizable radius, ~1/4", is better. No magic there. Just eyeball it.

Good advice - but a little late now. They were concreted in yesterday and are finished now.

 

[TractorGuy]

I am getting a 15' x 30' aluminum two story fishing dock installed. The pond it is going on has a plastic liner thus no anchor poles in the water. The manufacturer use two stiff arms, one on each side, to anchor it to the shore.

Below are pics of how they recommend doing it. Basically an I beam is set in concrete and then they supply a T shaped aluminum bracket that bolts to the I beam and then the stiff arm bolts to the bracket using a 1" bolt.

My setup at my pond is different in that my water level does not fluctuate as it is well fed and the ground is flat leading up to the pond. Instead of my stiff arms being attached 3-4' above the ground they will only be 12" above the ground.

I was thinking of using 1" thick by 4" wide flat bar set 5' deep in concrete for this and then simply drill a 1" hole in it. It will provide a much cleaner look.

The flat bar would only be sticking up 12" out of the concrete. This bar should be rigid enough with only 12" of stick out shouldn't it? Alternatively I could set an I beam and weld a tab to it or use their bracket I am just trying to clean up the look of it on the shore.

I would go with the I Beam attachment illustrated in your first post.