Risa load combinations

[Arjun Mudkondwar]

Hi all,
I have few concerns and doubts regarding Risa Load combinations in Risa 3D and Risa Foundations.
I build the whole structure in Risa floor then import it in Risa 3D for lateral and finally Import in Risa foundation.
We consider the Load combination X+Z for wind and seismic in risa 3D
Then i import the model in risa foundation and assign the X+Z load combination for wind and seismic.
We have 3 types of wall foundation options as retaining wall , strip footing (gravity only) and strip footing (lateral )
so i model the strip footing but the model is showing instability." There could be net uplift or no boundary condition in the model.
If i increase the foundation size it is showing shape more than 12 ft width and depth of 40 inches for load of 4 story building.
So my question is:-
1. Please suggest what is the solution of this problem?
2. Do we need to consider the 2D only in load combination generator tab in place of X+Z ?

 

[jayrod12]

Think about what it is telling you. You have an end bearing type footing and you have net uplift or no boundary condition. That means you either have more overturning moment than can be resisted by the initial size of foundation you have entered, or you're missing a support somewhere.

Regarding the load combinations, in my experience the generator will give you exactly the same outcome as if you entered the appropriate code load combinations. So that shouldn't have anything to do with it.

Review your model in RISA3D and see if any of your supports for that same load combination have a tensile reaction. If so, then you have net uplift and need more dead load/soil load to resist.