STM - Wall on wall detail 1

[hardbutmild]

Hello everyone,

I have a question about STM and would like to hear your opinions.
I have two perpendicular walls, one sitting on top of another (so the top wall is some sort of deep beam supported on an orthogonal wall). This is a quick sketch of the problem:

Both of the walls are 20 cm thick so their contact is relatively small. I made an STM model of the whole top wall (it has 2 supporting walls, this is a detail of one of them) and this is just a part of that model -a detail of the connection:

Now, of course, I know that it is better to divide the support force into two, one for each diagonal, but that is not important here. I also know that the left diagonal will probably fail here, but these are all things that I can find a solution to.
My problem is the contact point of the two walls, so the transfer of 1476 kN. What is the allowable pressure in your opinion? If I divide the force by area I get 36,9 MPa (1476000 / 200*200) - this is more than the design strength of 20 MPa.
The question is does this need to be satisfied or can this stress be distributed to a larger area of the supporting member like in this picutre:

Eurocode has provisions for "partially loaded areas" where the strength increases by 2-3 times. This would help significantly in situations like these.

I know that I can always do something like the following picture to reduce the stress and it's certainly a better solution, but is it strictly necessary?

 

[hardbutmild]

As you said, that would be hard to sell to an architect, especially when a lot of other engineers in the area will agree to any architectural solution.

 

[isidor_]

This is interesting as I was looking into this recently!
I have some assorted thoughts about the subject so here we go:

The provisions that OP alludes to are the image below, for the given indata (loaded area 0,2m*0,2m and 0,2m perpendicular wall below supporting) the compressive stress can be increased by a factor of (A_c1/A_c0)^0,5=...=1,732, making the compressive stress capacity = 1,732*(30/1,5)=34,641MPa
The maximum increase in compressive stress for this section is a factor of 3.

(It is also specified that reinforcement should be provided for the tensile forces so the basic check for compressive stress is not enough)

If one were to look at the CCC node at the top the compressive stress capacity becomes (according to 6.60 in 1992-1-1) f_cd=1*(1-30/250)*(30/1,5)=17,6MPa.
Thus this simplified check makes the capacity twice as large 34,641/17,6=1,97..

If 6.7 is applicable here is another thing though.
The figure and text in 6.7 does not state any minimum size of the surrounding concrete member, it only specifies the supporting area A_1 to be less than b_2*d_2 (b_2<=3*b_1 and d_2<=3*d_1).
But when looking at the figure it sort of implies that the supporting concrete should extend outside of the supporting area (A_c1).
In the case for OP there is only distribution in one direction as drawn in the original post.

The EC chapter about STM (6.5 in 1992-1-1) has a text about increased compressive stress in a "Triaxially compressed node", there the maximum compressive stress can be increased by a factor of maximum 3 ( although multiploed by the factor v=(1-f_ck/250MPa).
The check is then done by equation 3.24 or 3.25.

What I am getting at is that maybe the simplified check assumes a large concrete plinth such that triaxial compression can be assumed to occur at the support / where the load is applied, and thus the increased concrete stress is OK.
The sketch below shows how I visualize it. The square at the top is loaded, divided into four to make a STM model, below one of the "subsquares" there are only compressive forces acting on the node thus replicating the triaxial stress.

If it can be applied for a wall where there is unclear though, hopefully someone can clear this up