Etabs area spring

[frameingineering2014]

Hello mates,
Please i want to perform a soil structure interaction in etabs 2013. I have assigned area spring of 60000KN/M2/M, in the 3 direction, is my definition correct. I was confused if the modulus of subgrade needs to be negative.
I also want to know how to place a restraints in the x and y direction of the raft.

Thank you.

 

[avecillasjavier]

Yes, your definition is correct. First of all, the modulus of the subgrade is positive if the area local axis 3 is in the same direction as the global Z axis. Also, to place restraints in the x and y direction you just have to assign values to the Local 1 and Local 2 'Springs Constants', respectively. I'm not an expert in soil interaction but if you want to get more conservative results, you need to choose the 'compression only' option for the nonlinear local 3 direction.

 

[JonnyFive]

Gents,

Indeed it is important when considering uplift on base slab to ensure that the "soil" is not going into tension. So why is it, that when I model my basement structure (held in place with tension piles) and tick the non linear "compression only" box for an area spring I seem to get the soil giving tension reactions?

Any ideas?

Regards,

J5

 

[JonnyFive]

doh! solved it - remember to make sure analysis load case/combo is static non-linear!

 

[frameingineering2014]

Can you share your non linear load cases? Did u model the soil structure interactions in etabs, how do you identify when there is an uplift?.
@avecillasjavier, please what value should be used for the restraint in the X and Y direction. What I did was to select the nodes of the raft and assign only UX AND UY. What about that?

Regards.

 

[ke27on]

I did a similar thing of assigning soil springs in my model. Talking with tech support I also assigned some springs in the 1 and 2 direction so the building would not slide all over during lateral loading.

One thing to watch for is if you have an uplift case (fluid pressure uplift) you may want to watch your load patterns/cases. What was happening in my building is in the Hydrostatic case i had no moments because it wanted to lift up the building and there was no resistance since it only looked at that load case.