subgrade modulus to spring stiffness
subgrade modulus to spring stiffness
(OP)
I have a question about how the subgrade modulus relates to the assumed spring stiffness of a soil.
I have a rather large mat foundation to design (using a beam on elastic foundation) and the geotech report gives a subgrade modulus in kcf. I can input this directly in PCA Mat, but I would like to run a (not so) quick hand calc for comparison since I've never used PCA Mat before. That being said, my notes for beams on elastic foundations only incorporate the spring stiffness of the soil, not the subgrade modulus. Is this a quick and easy conversion?
I have a rather large mat foundation to design (using a beam on elastic foundation) and the geotech report gives a subgrade modulus in kcf. I can input this directly in PCA Mat, but I would like to run a (not so) quick hand calc for comparison since I've never used PCA Mat before. That being said, my notes for beams on elastic foundations only incorporate the spring stiffness of the soil, not the subgrade modulus. Is this a quick and easy conversion?






RE: subgrade modulus to spring stiffness
So if you have 200 lbs/cu. inch soil subgrade modulus, and your spring governs an area of 1 sf, then your spring would be
200 lbs/cu. inch x (1 sf)(144 cu. inch/cu. foot) = 200 lb/ft = 28,800 lbs/inch or 28.8 kips/inch spring.
But keep in mind that soil isn't always linear like this. The spring stiffness is a way to model it in an analysis but can be more complex than this.
RE: subgrade modulus to spring stiffness
RE: subgrade modulus to spring stiffness
RE: subgrade modulus to spring stiffness
RE: subgrade modulus to spring stiffness
You cannot model continuous springs--it is FINITE element analysis, not INFINITE element analysis. I don't use PCA MATS--I use RISA 3D, and I space the springs 1' oc most of the time.
If you are given k in terms of pcf (which would be unusual--usually it is given in pci), you would multiply by 1728 to convert to pci.
DaveAtkins
RE: subgrade modulus to spring stiffness
PCA Mat allows the input of the subgrade modulus directly, you don't need to use a spring stiffness.
RE: subgrade modulus to spring stiffness
I don't know about PCA Mat. It may take your modulus and then convert to discrete springs in a FEA.
RE: subgrade modulus to spring stiffness
RE: subgrade modulus to spring stiffness
The spring constant has the basic unit - k/in, which implies uniformly continuous from inch to inch (similar to load and pressure terms - klf, psf...). I hope this will help you in thinking if I didn't misunderstand your question.
RE: subgrade modulus to spring stiffness
I understand what you are saying. JAE addressed my initial question. So, it seems as though you will get a different spring stiffness for a given subgrade modulus depending on the area of soil that you assume the spring to cover. As the area gets smaller, the spring constant gets smaller (i.e. you have more springs covering the same area, therefore each one can be less stiff to get the same stiffness per area).
The example given had real numbers applied to it.
The question now is how to account for the spacing between springs approaching zero (as you would assume if you are doing this calc by hand).
The whole point of this exercise is that I want to do this by hand before I do anything in PCA Mat so that I don't have their answer in the back of my mind as I'm doing my own calc. I would prefer to do mine first (by hand, hence the springs will be continuous with zero spacing, not spaced at some finite value), then run PCA Mat and compare.
RE: subgrade modulus to spring stiffness
htt