Modelling as per the actual structure.

Suppose you have a foundation at two levels. The foundation of the structure is at two different heights/elevation, A and B. And the two are connected by a wall. 

The scenario.

The wall will start from the lower foundation B and so it will be monolithically connected with it. Thus, you will have to model it accordingly. 

Whereas A can be modelled and designed in two different ways:


The two approaches.

One where you place the foundation over the wall. But then the foundation would be able to move sideways in case of horizontal forces, like the first image below. To avoid the horizontal movement, it is designed to be monolithic by providing the required reinforcements. And it becomes a normal wall slab connection design, the third image below.

                                  

Another way to go through it is by placing the foundation on the top level of the wall. Case II among the two approaches. By doing so, you restrict the horizontal movement, like the second image above. As you can see, it now acts as a pinned support. All the reactions other than the moment will be transferred below, there won't be any moment generated in this kind of connection. 

(Actually, there will be moments, but it will be quite insignificant that it is acceptable in the Engineering world. I copied this line from Claudio Gianini).

So now that we have finalized on the type of connection, we have to model it accordingly.

As foundation are modelled as shell elements, we select the edge which will connect with the wall and provide moment releases to it. What this does is exactly what we desire of a pinned support, it will transfer all the reactions other than the moment. 

Shell release.

Likewise, there are releases for each reaction and it ultimately depends on the kind of support/connection you provide.

One might ask why not model the whole wall and the foundations together. There are two reasons for it. Firstly, one cannot design a shell element on ETABS Plus, you need Ultimate for it. And secondly, SAFE doesn't really have a Z coordinate to it, it's just a 2D-plane based software and one cannot define levels without it.

This whole approach is just breaking one big problem into tiny bits. 

Always open for discussion.

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