Soil Springs 1

[PEinVA]

Can't I just model a spring stiffness as related to my soil sub-grade modulus of reaction, and review the results to determine if I'm having any uplift forces, instead of "jumping through hoops" to create tension/compression only springs/links?
Is this not correct? I'm trying to get loads that are to be carried down to my 'mat' foundation under my shear wall frames.
Let me know if this is not correct.
Thanks.


RC
All that is necessary for the triumph of evil is that good men do nothing.
Edmund Burke

http://www.tbastructures.com

 

[stressed]

Yes, you can do this easily. Model the basemat foundation or footings as a slab type area element using section properties equivalent to the actual foundation or footing thickness. Then select the basemat or footings and Assign>Shell/area>Area springs. If you had a subgrade modulus of reaction of .25 Kip-in, for example, you would assign it as .25 Kip-in area spring in the local 3 direction (vertical). You would also need to add soil springs in the local 1 and 2 directions. Either use springs, or restrain it in those in-plane directions.

Even if your mesh changes, ETABS should automatically handle those area springs as you assigned them

 

[PEinVA]

Thanks stressed for the response. I know we've done this in RISA3D before, which you can model with compression only, but I figured as long as review of the soil springs showed no tension, then it would act as it should. We did just that with the sub-grade modulus of reaction.
However, when applying the reaction to different size areas per Terzaghi's research, we found conflicting formulas for how you apply this. Do you know what the formulas are? I have a Bowles foundation book, as well as a 5th edition Das Foundation book. One states to multiply the sub-grade reaction by the width and one states to divide by it. I feel like it should be somewhere in the middle. Which one is correct?
Thanks.



RC
All that is necessary for the triumph of evil is that good men do nothing.
Edmund Burke

http://www.tbastructures.com

 

[stressed]

I'm not sure that I understand your question..the subgrade modulus represents k, right? a pressure vs deformation relationship. Subgrade modulus is an area value, so it's handled on a per area basis, and engineering programs use these values to calculate nodal spring constants. On a large mat foundation, you may have variable soil conditions supporting the foundation, but you could mesh your basemat elements and make multiple assignments to account for that. Since you're modeling the basemat foundation using area shell finite elements, the basemat is not treated as rigid.

CSI has a special purpose program for design and analysis of foundations called SAFE.

 

[PEinVA]

Sorry for a confusing question.

Yes, I agree k is a pressure vs. deformation relationship ksi/in, or whatever units you want to use. This relationship of the k value with the shape and size of the foundation, as discussed in Das' Foundation Engineering, 5th Edition, doesn't have a linear relationship, as one would assume. Das claims that Terzaghi found a relationship with different areas (because k is based on a 1'x1' plate and this could change as the size of the footing changes) and in Das this relationship is stated as k*(1/B) where B is the width of the footing (ft). In Bowles' 2nd Edition book, it states the relationship is k*B, where B is the width.
In both texts, they claim the formulas are derived from Terzaghi's research in the 50s.

Does anyone know which formula is correct?
Thanks.

Also, if you want to design your shear walls correctly, the footings in etabs should have soil properties below to allow for shear to be transferred to the floor slab diapraghms and not all to the footing. This also helps reduce the possibility of having uplift on the your footings because most of the load on your walls is then transferred to the diaphragm and basement walls as a result of the allowed movement in the footing below your design in ETABS. We do utilize SAFE for footings once we have the controlling loads on them from the ETABS analysis.

Thanks for your help.



RC
All that is necessary for the triumph of evil is that good men do nothing.
Edmund Burke

http://www.tbastructures.com