Effective Area Method for rings + FEA question

[nicoga3000]

I have been looking into the Effective Area Method outlined in NAVFAC DM7-02 for rectangular and circular footings. However, for instances of designing foundations for silos or similar structures, ring type foundations are typically used (circular foundation with an outside and inside radius). Does anyone have any experience with or documentation on how to utilize the Effective Area Method for shapes like that? I know it CAN be done as I've seen results for it, but I've never been able to determine the method applied.

Also, for use of FEA for large foundations, I've found a few issues with designs. Typically, I use a simple P/A + My/I for determining service bearing. When using FEA, the same foundations that work under my method tend to have local bearing failure due to concentrated pressure directing under larger loads. Is this typical? Are there provisions to deal with this out there?

 

[aeoliantexan]

I used to write geotech reports for grain elevators. The elevators were typically built on continuous mat foundations 2 to 3 feet thick that extended about 2 feet beyond the walls of the slip-formed grain cells. Large settlements were common, but the structures were very rigid and tolerant of settlement so long as they didn't tilt too much. For an especially compressible site, I suggested overexcavating and replacing about 10 feet of the existing soil. The designer said he would rather make the mat wider and extended it about 8 feet beyond the walls. The extension broke off along one side. I believe this occurred because the contact pressure under a rigid foundation is higher near the edges than under the middle. Elastic analysis shows this; see Figure 7.3 in "Elastic Solutions for Soil and Rock Mechanics" by Poulos and Davis.(It's available on line for free) The designed had assumed a uniform bearing pressure.

Perhaps your FEA model is showing something that really happens. There is local shear failure of the soil under the edges of the foundation.

 

[LRJ]

As a very crude (and possibly conservative) approach, I'd calculate the effective area as if it were a solid foundation and then deduct the area of the hole. If you were to take this approach then you'd need to be careful to ensure the deducted hole area is only that area which would have been in contact with the soil if it was solid (since the hole area not in contact with the soil would've already been deducted when considering the effective area of a solid foundation).

Below is a picture to explain what I mean (based on Sargazi and Hosseininia - Bearing capacity of ring footings on cohesionless soil under eccentric load):

Of course, the above approach implicitly assumes the loading on your ring foundation is comparable to the loading on a solid foundation (i.e. typically in the middle of the foundation). This may not be the case for your ring foundation, so more detailed analysis may be warranted.

On your second point about the FEA: could you share an image to explain what you mean? The response by aeoliantexan seems reasonable - I just want to see what you are describing.