Rectangular HSS Column/Round Base Plate Design

If you aren't struggling to justify the capacity in general, probably the easiest way would be to idealize the baseplate as a rectangle with corners located one edge distance away from your (4) "in use" anchors. The additional baseplate steel beyond the anchors can just be conservatively ignored for failure modes where it would improve the connection performance. (don't forget to account for the real size anywhere it hurts though, ie. the weight)

RenHen said:

If you aren't struggling to justify the capacity in general, probably the easiest way would be to idealize the baseplate as a rectangle with corners located one edge distance away from your (4) "in use" anchors. The additional baseplate steel beyond the anchors can just be conservatively ignored for failure modes where it would improve the connection performance. (don't forget to account for the real size anywhere it hurts though, ie. the weight)

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I had considered that but I'm not sure how to determine which configuration would be the controlling one to design to. There would be a round bolt pattern with a rectangular column and square baseplate. Assuming only one of the anchor rods was in tension at any given time would likely simplify it. I went down this path some and end up getting a required plate thickness of 2.75" which seems way over designed to me which lead me to think this isn't the way to do it. But may be the only option without FEA. I appreaciate your response.

The Simpson Anchor Design software does circular baseplates to the rigid plate assumption. Might be worth a look.

I don't see a problem using the Yield Line method by hand and assuming two different column rotations to be possible. If in doubt, make the plate a little thicker.

The aspect I've noted may not be relevant / significant, but would you need to take into account that bolts in the round base plate placed on a circular line will have different radial distances to the rectangular column, and, as such, wouldn't the bolts closer to the rectangular corners pick up more load than the bolts placed in the middle of the rectangular sides, being further away? This will also have an influence on any heel / toe (prying) bolt load effects?

Just some thoughts, you could do a BGA of the plate, introducing a weighting factor to account for bolt to rectangular column distance (stiff column / flexible plate). If you use a 'fixed-fixed' beam stiffness approach for the weighting method, your deflection results could be used to determine the prying moment. Just noting, 'fixed-fixed' for the tension side, cantilever for compression side? But then the compression load path would probably be through the plate and up the column? Food for thought!

Consider the case with the anchor furthest from the face of the HSS and determine the effective width of the circular plate based on 0.95 of the HSS per AISC DG 1. If you really want to get fancy you could consider prying, though this is typically not done for baseplates and would be less beneficial for a circular plate.

The plate doesn't need to be circular. Simply add a hole half way between the holes in the square base plate to permit 45 degree rotation.
Better still, ask the contractor to place his anchors as designed.