Risa Footings in Risa Foundation
Risa Footings in Risa Foundation
(OP)
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.
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.





RE: Risa Footings in Risa Foundation
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.
RE: Risa Footings in Risa Foundation
Thanks for your help.