AISC Design Guide 1 - Base Plate Thickness

[JoelTXCive]

I am attempting to design a base plate that is subject to a large moment in relation to the axial load applied (see attached)

My moment is 12.7 kip ft and the axial load is only 2.7 kip.

I am following the 'large moment' (load eccentricity is greater than bearing eccentricity) procedure in AISC's design guide #1. Using this procedure, I am come up with a really thick base-plate thickness.

It appears that the loading condition does not factor into AISC's equation for baseplate thickness. AISC just assumes you are going to need the full bearing capacity of whatever concrete you are bearing on. As a result, you will always get a really thick baseplate.

Am I understanding this correctly? Why would we always assume that we need the full 100% concrete bearing capacity under our plates? I would think that the plate thickness would be driven more by the loading condition applied to the plate versus the capacity of the concrete.

AISC's design guide #1 has an alternate procedure in the appendix of the design guide that yields much more reasonable base plate thicknesses.

I guess my question is....... Under what conditions would we ever want to use the primary procedure?

 

[dik]

I usually size my anchor rod based on shear and the tension developed. The moment arm I use is the centre of the anchor rod to the centroid of compression at the far edge of the base plate. I use a rectangular stress block and guess the 'd' value is 0.9 times the out dimension. I then use the force times the distance to the outside part of the column flange to determine the plate moment. I then use the plastic section bd^2/4 for design.

Dik

 

[bones206]

I looked at example B.5.2 (also a "large moment" case), which uses the alternate triangular distribution approach you mentioned. It looks like that approach also assumes full concrete bearing capacity to determine the plate thickness, just with a triangular distribution. The "small moment" case, however, sizes the plate thickness based on the applied loads.