foundation partly on soil and partly on concrete

[avnei]

I am designing a foundation which rests partly on soil and partly on a existing concrete foundation. I am modelling it in Staad pro. I know the modulus of subgrade for soil. My question, how should I model the portion of foundation slab on the existing foundation? What are the constants that I may require at this stage?. Useful suggestions are welcome and appreciated. Thank you.

 

[JedClampett]

If you have a structure partly on soil and partly on concrete, the concrete will carry most of the weight of the structure. Unless you plan for it, there is likely to be cracking at or about the interface due to the difference in settlements.
Bottom line is that before anyone could help here, we'd need to know a lot more about the structure and the existing concrete.

 

[avnei]

JedClampett, Thanks for you reply.
Well, the foundation is for a pipe support. There are 4 points of loading on it. The main load is a 56 kips couple at 2 pairs of supports. These 4 points rest on the foundation. There is an existing foundation close by and part of the new foundation requires to be laid on it(old fdn). Preliminary size of fdn is 11'x 16'.

 

[jike]

If the existing foundation is bearing on the same type of soil as the new foundation, then I would think that the modulus of subgrade would be the same. The only difference is that the soil under the existing foundation has benn preconsolidated.

Make sure that you do not overstress the soil under the existing foundation. Do you plan to just pour over the existing foundation or dowel into it? You may also provide some sort of compressible material to isolate the new footing from the existing.

 

[avnei]

Jike, Thanks for your views.
Well, here's what I did. The portion of the foundation on existing foundation is anchored. The peripherial portion of the new fdn is connected to the existing fdn by driving anchor bars. So, I defined those support points as fixed in my model and all the other interior supports as pinned. I defined spring support for that portion of the foundation on soil. I could notice that the pressure experienced by soil was very low which meant that the foundation behaved as a cantilever. I adequately provided reinforcement for the maximum bending cantlever moment. Assuming that there may not be cent percent fixity, I reinforced the bottom portion of the slab too with the same reinforcement.
I believe this was the best approach I could utilize, but would certainly like to know your views for this case.
Appreciate further discussions.