I think Design Guide 1 is still a good resource in terms of the behavior of the plate under a large moment. The difference will, of course, be the reaction from the supporting surface. You'll have to figure that out based on the stiffness of the flange in bending. Limit to a variable stress based on that stiffness, or a uniform bearing stress 1.8Fy, whichever gives you a lower allowable moment.

Despite the large eccentricity, these are still pretty small forces. A single pair of beam stiffeners below the post and a four bolt pattern connecting the plate to the beam flange ought to get the job done. Will the moments be locked into a single plane of rotation perpendicular to the beam or can it swing around and impose the moment at various locations?

It can rotate, so probably checking against a couple scenarios or a single worst case.

I'd start with the 45 degree case, assume the compression reaction is shared by the crossing points of the pipe with the beam web and stiffener, and then design the tension resistance using only the single bolt in the quadrant opposite the compression reaction(s). Some kind of simplified flexure check or yield line analysis over there. Maybe something for prying if the geometry warrants it.

I agree with KootK on the worst case design scenario. I've only done a few steel column-to-beam connections with wide flange shapes, so things are a little different with a pipe column. But take a look at AISC Design Guide 4 and Design Guide 16 as references. They have some good information on similar types of connections.

How would I go about checking the 45 degree case? The only examples that I have seen treat the moment as going normal to the plate sides.

For the 45 degree case I would start exploring:

1. Separate the 45deg moment into orthogonal components. Do your checks in X. Do your checks in Y. Use some sort of biaxial interaction formula to see where you sit.

2. If #1 works out. Go to #3. If you are tight or overloading, revise the design.

3. Check the non-orthogonal case by determining where the yield line will be in the plate. Maybe corner to corner? Probably a take the perpendicular at the face of the pipe which means you would also rely on one bolt. Yeah. Just design it for one bolt to handle the load.

check this PDF prepared by Bentley for a procedure on biaxial moment: https://communities.bentley.com/cfs-file/__key/tel...

Open Source Structural Applications: https://github.com/buddyd16/Structural-Engineering