Column base plate design (AISC 360) - I have a “Anchor location is invalid for anchors in tension” error. Why do I get this and how do I resolve it?

Question:

I have an “Anchor location is invalid for anchors in tension” error, or “The anchor arrangement is outside the scope of this calculation for anchors in tension” error. Why do I get this and how do I resolve it?

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Answer:

The Tedds Column base plate design (AISC 360) module requires that anchors in tension from uplift are limited to locations where a 45 degree load distribution to the column is clear. You can find this requirement listed in the Assumptions and limitations section of the Calc notes for the module, as shown in the screenshot below.

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This restriction on the anchor bolt placement comes from Section 3.2 of AISC Design Guide 1. As shown in the picture below, the Design Guide indicates one-way cantilever action to the closest column component using a 45 degree load distribution is a conservative model for base plate, weld, and local column failure for the tension only failure mode. As there is no other established way to determine the bending width to use for design in an uplift scenario, the anchor bolt placement must be restricted to locations where a clear 45 degree load path to the column can be achieved.

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In short, the restriction is defined by the bending behavior of the base plate specified by the Design Guide. In order to transfer the tension loads from the column to the anchor bolts, the baseplate will be under single curvature bending. The base plate must be checked for these bending forces, as well as the welds connecting the base plate and the column, and so the effective bending width of the base plate needs to be determined. The approach to perform this check given in Section 3.2 of AISC Design Guide 1 is:

"For tensile loads, a simple approach is to assume the anchor rod loads generate bending moments in the base plate consistent with cantilever action about the web or flanges of the column section (one-way bending) ... the effective bending width for the base plate can be conservatively approximated using a 45° distribution from the centerline of the anchor rod to the face of the column flange or web."

If the anchor bolts fall outside of this 45 degree distribution, the baseplate would potentially have different bending behavior than that assumed by the equations in the Design Guide and will therefore be beyond scope.

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To adjust the anchor layout in order to pass this validation, the spacing in either direction must be reduced to follow the 45 degree distribution rule. The 45 degree distribution is measured as a 45 degree angle from the center point of the bolt to either the column flange or column web. This distribution must intersect with either the flange or web face for at least half of the 45 degree distribution width to be within scope. You may observe that, per this rule, the max allowable spacing of the bolts would be equivalent to the extents of the column section, so this can be used as a guideline to determine acceptable anchor locations for anchors in tension moving forward.

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Note, you may be asking why the same or similar anchor layouts did not throw this error in previous versions of Tedds (Tedds 2021 and earlier). This is because there was previously an issue with the module causing the calculation to not properly apply this limitation to design the baseplate for uplift forces. This issue was fixed in Tedds 2022, and a product bulletin was issued for this as well: PBT-2201-1. This update is mentioned in the Revision History section of the Calc notes, and the product bulletin can also be viewed in the Bulletins folder of the Engineering Library.