Compressive Bending of a Base Plate.. How? 6

[MegaStructures]

Question not for a current design, just interested.

I'm going through AISC DG1 - Base Plate and Anchor Design and noticed that the guide recommends designing the base plate for bending when a compression load is applied to the column. How in the world would a base plate bend from an axial compressive force if it is continuously supported by the concrete underneath it? Is this to account for voids in the concrete? I get that the DG treats the bearing force as uniform across the bottom of the base plate and this force profile would cause "upwards" bending of the base plate if it were able to "travel", but the concrete cannot act on the base plate through a distance.

“Any idiot can build a bridge that stands, but it takes an engineer to build a bridge that barely stands.”

 

[Agent666]

Jlnj, I think in your examples you're incorrectly making the overarching assumption that going to a bigger base plate size actually utilises all of the available plate outstand to result in a uniform bearing pressure. This isn't realistic (or required), there is a limit to the effective outstand based on plate thickness (which is stiffness in other words).

Look into the stuff HTURKAK is posting regarding EC3 approaches as it covers/incorporates this aspect. Any plate outside of this "effective cantilever length" is ignored for deriving the moment acting on the plate due to bearing. Therefore going thicker just increases the available outstand length you can utilise. It's a little like bearing through steel, there is a spread angle of 2.5:1 or similar through the steel. Similar occurs through the baseplate in a way.

 

[r13]

...especially when we established at the beginning that the pier would be just fine with virtually no plate at all.

Without baseplate, the load will be concentrated on the flanges, and can result in very high contact stress that might exceed the allowable concrete bearing strength. If the concrete does have adequate strength, the baseplate is simply provided for positioning and fastening the column.