Monolithic Footings and Eccentricity

[theonlynamenottaken]

When designing the foundation for a building (such as a PEMB) with monolithic slab & footings (thickened slab footings) do you and how do you consider the eccentricity of column axial load on exterior & exterior corner footings?

I know this is in the "grey area" between this forum and the Foundation Eng forum so just say so if I should post this there....

 

[dbuzz]

If the load eccentricity is located outside the footing kern, then yes.

 

[theonlynamenottaken]

Ok, so if the e > L/6 (i.e. the interior side of the footing is trying to lift) do you incorporate the resistance of the attached slab? I have in the past determined at what distance the slab would flexurally fail from the interior footing edge lifting, and applied that portion of the slab's dead load to the interior footing edge. I thought that if the attached slab is substantial enough to completely negate the uplift I may be able to consider the footing hinged about that interior edge? (making for a triangular soil bearing pressure that goes to zero at the interior edge/edges) Thats my core question... ultimately all this leading to reducing the calculated max soil bearing pressure under the exterior edge. Thoughts?

 

[DaveAtkins]

I assume you are talking about a slab-on-grade type foundation, without foundation walls and footings. For this situation, I model the slab, with a thickened edge, on RISA-3D, using spring supports under the slab. The controlling situation is always where the slab reduces in thickness.
If you do it by hand, figure out how much thickened slab edge you need to resist the vertical load without overstressing the soil (assume a uniform soil pressure). The centroid of this uniform soil pressure will not line up with the imposed load above, thus causing a moment that must be resisted by the non-thickened slab. Again, the moment is critical where the slab reduces in thickness.

DaveAtkins

 

[kslee2006]

Uplift and over-stressed soil which causing uneven settlement usually are not problems of this type foundation - monolithic slab with thickened areas for columns, since if properly reinforced, the slab acts, just as the name indicated, monolithically to resists the forces delivered by the building as a whole (compared to typical subgrade and buildings, the reinforced slab is quite rigid). Locally, the punching shear usually controls the thickness and the soil bearing governs the area that need to be thickened. As implied, the thickened area can be conservatively analyzed as separate footing to provide required reinforcing.

 

[223HR]

Don't forget, if you are designing the footing with the slab incorporated for structural purposes and not mearly as a slab on grade special inspection requirements may come into effect. You may want to try moving the edge of the slab ouside the building just far enough that the load is not as eccentric.

 

[scottiesei]

For large loads where eccentricity is an issue, I have used bars/WWR from the slab turned into the footing to resist the overturning.