Mat footing eccentricity within middle third, Overturning always OK? 1

[delagina]

If I model a mat foundation on soil using finite element and no tension (eccentricity within middle third), is it safe to assume it wont fail in moment overturning check?

The reason i ask is because overturning check is not checked using finite element software spmats. It does check soil bearing pressure.

And i was wondering if I could skip the separate rigid analysis for overturning.

 

[bgatto]

I think that this depends on the horizontal forces applied to the foundation.

 

[BAretired]

Overturning has to be checked with a lesser gravity load than you use to calculate bearing stresses, so you would have to consider the general case where the reaction is in the middle third under full dead plus live load, then write an expression for overturning. In this way, you will see what the effect of load reduction has on overturning.

BA

 

[msquared48]

The 45 degree load application could also be a factor to be considered here.

Mike McCann
MMC Engineering

 

[SteveGregory]

Am I missing something? Isn't the overturning check a simple hand calculation?

 

[msquared48]

Yes, but the chances are that if the resultaht force gets outside of the kern area that the soil matrix will be overstressed. A different equation would then apply for the maximum soil pressure.

Mike McCann
MMC Engineering

 

[delagina]

i checked at all load combination using finite element and in all cases qmin is positive. so all footing area is in compression at all load combination.

can i assume overturning is also ok.

anyway, i decided to check overturning manually also.

 

[graybeach]

Don't you have a certain required factor of safety against overturning? You need to calculate the stabilizing moment and make sure it is at least that factor times the overturning moment.

 

[bigmig]

not to beat a dead horse, but like others have mentioned, the overturning is based on the factor of safety, which has been discussed in detail in previous posts, especially for wind and dead combinations. You can run a finite model and start seeing your plates bending upwards (footing lifting off the ground), but this doesn't tell you if you meet the overturning requirements. Most finite programs let you do a summation of loads at various points on your model. This will let you get the loads so you can check global stability (i.e. overturning).

 

[jhenry]

I would consider a minimum area required for axial loading under full deal and live loads, then check the foundation mat reinforcement reqd for overturning with 1.5 SF. in x and y directions. Also check punching shear at the column. A hand calulation may be appropiate for these conditions to check againt the program and to check q-min under overturning.
This check is especially important for corner of edge columns.

 

[ishvaaag]

If for all the relevant hypotheses -including any of those required to check for overturning- the solicitations (axial, shearing forces plus their moment effects respect base of the footing, and moments) place the resultant within the central nucleus of inertia at the base of the footing, you are safe against overturning (assuming the ordinary response of the soil, without irregularities).

 

[JoshPlumSE]

The program I work with (RISAFoundation) used to have the same limitation. Eventually we added that calculation into the program. Yes, the hand calculation is easy, but folks want the calculation to be done automatically.

Back then(before the program automated that calculation), I drew up a quick image to help people get an intuitive feel for OTM safety factor based on the soil pressure diagrams. I have attached that image.

This image is perfectly accurate when the footing is considered rigid (and mats are not usually rigid). Even so, the goal was to give our users an approximate, intuitive and visual way to go from soil bearing to overturning safety factor.

FWIW: The most instructive aspect of this image is that if 50% of your mat is in compression then that approximately corresponds to a 1.5 safety factor for overturning. As long as you achieve that, you are usually in good shape.... I hope this helps.

 

[BAretired]

If, instead of a mat foundation, we consider a square footing A x A with a gravity load (D+L) and moment M applied at the center of the footing, then:

p = (D+L)/A^2 [±] M/S where p is the pressure at the edge of footing.

If the load is at the kern, then M = (D+L)S/A^2 = (D+L)A/6.

Overturning moment = M

Stabilizing Moment = 0.6D*A/2 = 0.3D*A

0.3D*A = (D+L)A/6 (equating stabilizing moment to overturning moment)

D/(D+L) = 0.556

If dead load is less than 0.556 (D+L), overturning will govern the design.

A similar argument could be made for the mat.

BA

 

[BAretired]

This can be re-stated as:

If D < 1.25L (or L > 0.8D) then overturning wil govern the design.

BA