Motor torque transmited to ground

[ralcober]

Hi everybody

I am designing a foundation for a machinery which transmit an important torsion in addition to other moments, horizontal and vertical foces.
The foundation consists of a rectangular concrete slab.

My question is how to calculate the shear force produced by the torsion, in order to calculate global the slip safety factor whith the other horizontal forces.

I have seen similar calculations which transform the torsion into a shear distribution on the slab-ground interface and then integrate it into an effective shear force which is possible to combine with the horizontal forces.

Unfortunately I don´t know such formulation.
Maybe someone knows it.

Thanks
Ruben

 

[Ron]

Let's set the planar coordinates first, so I can keep what I'm telling you straight!

Consider your computer screen to be the X-Y plane. The Z axis would then go in to the screen.

I assume you are considering a torsion in the X-Z plane, or one that is trying to spin your foundation against the soil. You can compute the friction interface as a result of a couple to replace the torsion (moment). For instance if your torsion is 1000 ft-lbs and your preliminary foundation size is estimated to be 8'x6', with the short side oriented along the X axis (conservative approach) , then you would replace the torsion with a couple at 1/3 in each direction on the X axis or having a separation of 2 feet. Thus your force would be 500 lb in the Z direction at X=1' and 500 lb in the -Z direction at X=-1'.

Nothing magic about the 1/3, other than I consider that foundations are generally more heavily loaded near their centers and less so on the edges, thus a triangular load distribution. You could use 1/2 instead of 1/3 and be justified as well (average force on the area).

I would suggest that you get a geotechnical engineer and a structural engineer involved, since this is not a minor foundation consideration. You have soil dynamics and structural dynamics to consider. I offered the simplistic approach in direct response to your question for your edification, not for you to base your design upon.