Anchor Bolt Chair

Some your questions appear to be answered if you do a Google search. Several items came up

The issue is that forces can be fairly large, but the tank bottom is thin, shell may be relatively thin. So it's impractical to just put holes in the bottom plate, and attaching to the shell, the attachment needs to be big enough and stiff enough to distribute the load. They may also be used for pressure vessels and stacks.

Many tanks can be built unanchored (or "self-anchored" in some of the standards). It depends on the tank dimensions and the wind, seismic, and internal pressure loads.

Design procedure is included here: This may also be in various other vessel handbooks, and is based on some older technical papers. Note that allowable shell stresses, etc., may be specified in the tank code in question.

We used them sometimes for pressure vessels. Some thoughts:
1) The increased length allows for some increased elongation / ductile lengthening of the anchor rods for seismic design.
2) We also used sleeves (grouted after placement) to help with alignment. Since you had so many large diameter anchors, it could be difficult to place them perfectly without the sleeves. I believe the anchor chairs can help with that a bit as well.

Also a bolt chair arrangement can give some additional free length on the bolts to allow bolt to be tensioned, by giving a longer unbonded length for the elongation due to tensioning to occur over. You might tension the hold down bolts to address fatigue design issues for example.

Good answers all, but as to whether this arrangement is only for tanks, no. Many industrial structures have columns with rigid bases, and "chairs" are used instead of very thick base plates, and to more effectively develop the flange forces.

SKJ25POL:
In any case, a large stl. column or a stl. tank shell, the idea is to get a large and concentrated loading up into the structure with a weld which is lined up with the line of the force. This, as opposed to a weld which runs across the direction of the load, which is somewhat less effective. In the case of the tank shell you have a fairly thin shell plate, and thus a fairly small weld btwn. the shell and the base plate, trying to transmit that large A.B. load over a small length of weld immediately around the A.B. And, the chair takes that load up into the tank shell.

The last time that I used these, I wasn't able to get enough weld between the bottom of my column and the base plate to get the tensile portion of a moment couple out of the column.

Once,
The flange tension goes from the vertical stiffeners into the bolts, not through the base plate.

Hokie, I think we're on the same page, maybe my original msg wasn't clear.
Originally my load path would have been from the column, through the weld, to the base plate, to the anchor bolts, but I couldn't get enough weld.
The revised load path was from the column, through the new vertical welds, to the new vertical stiffeners, through the cap new plate, to the anchor bolts. There's a lot more weld in this load path.

Thanks for the clarification.