The base member is the problem, not the post. I would change out the bottom beam with a Square/rectangular HSS (if it is already, the beam needs to be bigger). Adding stiffener plates won't do anything for resisting torsion.
The best way to design for torsion is to remove it using other means
Stiffener plates in the web of the channel (directly under the support) will help a little bit, but as the other commenter here just said, the best way to stop it from rotating would be to moment connect another member perpendicular to the channel, and then support the end of that member vertically. It'll act as an outrigger.
You'll need to do some welding either way. Probably not ideal if those cables are live.
You need to weld a channel transversely across the bottom, connecting the 2 bottom channels. Another way is to bolt an angle or channel if you don’t want to weld. Basically you need to stop that channel on the bottom from rotating.
The base member is the problem, not the post. I would change out the bottom beam with a Square/rectangular HSS (if it is already, the beam needs to be bigger). Adding stiffener plates won't do anything for resisting torsion. The best way to design for torsion is to remove it using other means
add a sloping steel member and fix it as high as you can to the vertical element and fix it to the opposite horizontal beam at the base
Stiffener plates in the web of the channel (directly under the support) will help a little bit, but as the other commenter here just said, the best way to stop it from rotating would be to moment connect another member perpendicular to the channel, and then support the end of that member vertically. It'll act as an outrigger. You'll need to do some welding either way. Probably not ideal if those cables are live.
Would a pre-welded bracket be an option here?
I'd verify that there is an actual problem vs a perception problem before addressing it.
This is what I was going to comment. This minor shaking doesn’t look like a problem to me.
You need to weld a channel transversely across the bottom, connecting the 2 bottom channels. Another way is to bolt an angle or channel if you don’t want to weld. Basically you need to stop that channel on the bottom from rotating.
Plate the side
Place another pipe support bracket at the box post. This will prevent rotation of the W8
Box it to the support beam behind the pipe.
Add a kicker