Make the part a single plate (build a equivilent plate and check the results). Try the following web page <A HREF="http://femci.gsfc.nasa.gov/hcplate/" TARGET="_new">http://femci.gsfc.nasa.gov/hcplate/</A> .
A good book for Stress Concetrations is a book by Peterson. You can always build a FEA model. Depending on the application you may be over designing the part. Stress concentrations are typically looked at during LCF, I am not sure how you margins of safety are being calculated.
I am looking for a handbook for Designing Bolted Frame Structures, like a bridge made from I-beams. The book would be similar to "Roark's Formulas" and have formulas for standard bolted joints (not bickford). It should calculated bolt distribution of load. max stress location, etc.
I am not familiar with foundations but have seen where thin shells are considered anything with a 10:1 ratio (span versus thickness). Some have considered 3:1 as well. This comes into play with Plane Stress versus Plane Strain when considering the material properties such as fracture mechanics.
If I am interpreting your question correctly, all of the extermal load does not go through the bolt/fastener once a preload is applied. Once the joint is preloaded, and the external load applied the load in the bolt should follow a different slope since the stiffness of the joint now comes into...
