Risa Footings in Risa Foundation

[Stazz]

I’m trying to do a hand check on the RISA footing designs and I can’t figure it out.

Specifically I’m checking the Overturning moments and RISA help simply says that it divides the Resisting Moment (Mr) by the Overturning Moment (Ma).

The output has a good table of the Mr and Ma for each load case but what I can’t figure out is which forces are applied to these moments.

When I draw a FBD of the footing I see these forces…

1) Column Axial Load
2) Column Shear Load
3) Column Moment
4) Soil Overburden Pressure
5) Extra surcharge
6) Soil Active Pressure
7) Soil Passive Pressure
8) Soil Bearing Pressure


Does Mr and Ma consider the moments from all of these forces? When I take the moment about the corner of the footing I actually get a factor of safety of 1.0 regardless of the column loads (using excel).

I’ve always thought the concept of overturning moments and sliding is BOGUS. Has anyone ever seen a footing overturn or slide? The whole building would have to flip over or slide away.

 

[JoshPlumSE]

I've seen sliding and overturning failures before.... But, only for retaining walls and such. I heard that the Korean (or was it the japanese one) earthquake a few years back had some overturning failures. But, I never looked into the published reports much.

The key with the OTM stabilty ratio calclation is that you don't consider the soil bearing. But, you include all the other load applied to the structure. There is a good write-up in the help file for RISAFoot 3.0 about exactly how this calculation is done. You might want to check this out.

 

[Stazz]

I can buy that. If the footing overturns then I would imagine that either A) the soil fails miserably and crushes as the footing overturns or B) if the footing overturns then it won't even be in contact with the bearing soil. Either way this bearing force wouldn't be present to help overturn the footing. Why can't this be explained in text books.

Thanks for your help.