Moment eccentricies for footings due to wind/seismic 1

[Leftwow]

Need some direction regarding the distribution of bearing pressures and how to use this in shallow foundations. Is there a resource that you guys turn to when designing a foundation with large overturning moments?

 

[KootK]

I usually refer back to PPI's Structural Engineering Reference Manual for this. Most foundation engineering texts will have something. My text by Bowles and my text by Teng both have examples. At the risk of sounding condescending, it's really just statics. You establish reacting soil stresses that don't exceed the allowables and put the system in equilibrium. Then you design the parts and pieces for that set of forces. The only real choice is whether you assume a triangular stress distribution or a rectangular one. And that's best handled as a conversation with your geotech.

If you're looking for more specific guidance, let me know and I'll do my best to help.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[BAretired]

It's usual to assume that soil pressure varies linearly and cannot be tensile. The soil pressure, together with all other forces acting on the foundation must result in equilibrium. The foundation does a rigid body rotation in the soil to satisfy all of the applied forces and moments.

So far as I am aware, those are about the only rules you require to solve the problem. As KootK has stated, it is statics.

BA

 

[KootK]

Rigid body assumption! Yes, can't believe I missed that. Where issues arise, they usually seem to pertain to the appropriate load cases, how to treat overburden, and rocking in seismic country.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[Leftwow]

ok thanks guys.

 

[steellion]

I'll make same recommendation I recently made in a similar thread: The only equations you need are P/A + M/S < q(max) and P/A - M/S > 0. The equations provided in textbooks work if your situation matches the example exactly, but if you start with these two basic formulas, you can forget the rest.

 

[BAretired]

If P/A - M/S < 0, you need to consider a reduced area such that P/A - M/S = 0. For a rectangular footing and single axis bending, this is not too difficult but for biaxial bending, finding the location of the line of zero pressure is a bit trickier.

BA

 

[Joel900]

When P/A - M/S > 0 the eccentricity due to the moment is less than L/6 or the kern where L is the length of footing in the strong direction. A footing can be stable as long as the eccentricity does not exceed L/2 which will put the axial load outside the physical limits of the footing. Depending on the applied forces keeping the eccentricity within the kern can result in a conservative design.