Springs at the base of a Structure

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Hi, I have some questions:
While modeling a soil-structure interaction; I would like to know why there is a diference using static and dynamic simulation (coef. subgrade reaction vs. shear modulus), Am I doing something wrong?.

I've read that for lateral (horizontal) subgrade reaction of the soil I should multiply by 0.5 the vertical subgrade reaction. Is that correct ??.

To find the "k" constant of the spring while modelling, for an horizontal spring, which area of the footing do I have to use??

Thank you very much

 

I would not recommend use the subgrade reaction from tables, you have to make in-situ job.
Sorry