I didn't know it was triangular... I've used a rectangular stress block for over 40 years...
It's probably in keeping with the rectangular compression block for reinforced concrete... no sense in using a triangular block because as the outer edge of the base plate deflects, the load is reduced...
It seems to me that the resultant force wants to be directly under the column flange with very high grout bearing stresses in the vicinity. I don't know that you could get a base plate thick enough to truly distribute the force in a uniform manner as depicted.
AISC's Engineering Journal, first quarter 1999 had a paper written by Rick Drake and Sharon Elkin which described this method. The idea in the paper was to move away from elastic / ASD methods to an ultimate load / LRFD method for Base Plate design. Therefore, they replaced the triangular distribution with a rectangular distribution more similar to that used in concrete design.
I was not aware that this had made it's way into the Design Guides at all. It is certainly possible that the method in the design guide is an improved upon version of the methods first introduced in the Drake / Elkin paper.
This idea has bothered me for a long time. If the concrete is to control, the baseplate must be rigid. To make the pictured diagram to work, you have to take a guess at something. Given the reaction load and moment, you would have to decide how far apart are the center of bearing and the bolt.
Back when I was young, I worried about the arbitrary use of triangular blocks when putting a baseplate against the concrete, considering that the plate could not be absolutely rigid and the column shape stiffened it differently. I satisfied myself by considering it as a cracked concrete section with the anchor bolt as the reinforcing. I then took the resulting force distribution and applied it to the baseplate. This allowed me to see that the triangular diagrams were reasonably conservative.
You might be able to do something similar with LRFD.
Michael.
Timing has a lot to do with the outcome of a rain dance.
