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# Foundation beams design with Winkler springs - stiffness value???

## Foundation beams design with Winkler springs - stiffness value???

(OP)
I am designing a one-storey RC house, the foundation is a raft foundation 45cm thick, with foundation beams (45cm height) (fully embedded inside the raft) interconnecting the frame columns.

1. The superstructure was designed with fully fixed supports at column bases but the foundation beams will be designed with winkler beams at column supports.
Support definition at column bases is:

UX - fixed
UY - fixed
UZ - spring with stiffness - RELEASED
RX - spring with stiffness - RELEASED
RY - spring with stiffness - RELEASED
RX - fixed

So now how do i choose the stiffness value for the 3 releases?

### RE: Foundation beams design with Winkler springs - stiffness value???

(OP)
I used the ks calculator within Robot Structural analysis as shown in the picture, but i only get the one value of UZ KZ = 65876.60kN/m

UX - fixed
UY - fixed
UZ - spring stiffness, KZ = 65876.60kN/m
RX - spring with stiffness - RELEASED
RY - spring with stiffness - RELEASED
RX - fixed

My question now is: do i also need to calculate the HX and HY values for the rotation stiffness or Robot will use the value from KZ stiffness to automatically determine the HX and HY stiffnesses?

### RE: Foundation beams design with Winkler springs - stiffness value???

#### Quote (kellez)

do i also need to calculate the HX and HY values for the rotation stiffness or Robot will use the value from KZ stiffness to automatically determine the HX and HY stiffnesses?
https://forums.autodesk.com/t5/robot-structural-an...
The workaround in 2013 and 2014 release is replacing HX elastic ground along bars by nodal elastic supports in rotation distributed along bars.

### RE: Foundation beams design with Winkler springs - stiffness value???

Kellez -

I'm not an expert in Robot. Unfortunately, I don't know that you're going to find too many Robot users on Eng-Tips. It's just not that widely used of a program in North America.

That being said, I have some comments:
1) You need to understand what your program is doing internally. I suspect it's merely converting the subgrade modulus into a vertical springs based on the area of the footing. But, that is merely a guess.
2) If the program is adding vertical springs along the length of the grade beam, then YOU need to investigate this a little deeper to determine if it gives you the behavior you're looking for. Start with simple models and see if the results agree with what you think they should be.

3) Now if you want to convert the subgrade modulus and footing area into a rotational spring, then how would you do this? I've done it by assuming a unit rotation of the footing and then calculating the soil reactions induced by that unit rotation. Note: I assume the soil can take tension and compression for simplicity in calculating the rotational reaction.

### RE: Foundation beams design with Winkler springs - stiffness value???

(OP)

#### Quote (JoshPlum)

Kellez -

I'm not an expert in Robot. Unfortunately, I don't know that you're going to find too many Robot users on Eng-Tips. It's just not that widely used of a program in North America.

That being said, I have some comments:
1) You need to understand what your program is doing internally. I suspect it's merely converting the subgrade modulus into a vertical springs based on the area of the footing. But, that is merely a guess.

Well, yes this is exactly what the program is doing

2) If the program is adding vertical springs along the length of the grade beam, then YOU need to investigate this a little deeper to determine if it gives you the behavior you're looking for. Start with simple models and see if the results agree with what you think they should be.

Yes you can actually choose to add vertical springs along the whole length of the beams (elastic ground conditions), I actually did both, 1st i replaced the fully fixed supports at column bases with vertical springs and i also added elastic ground conditions along the whole length of the Foundation beams

3) Now if you want to convert the subgrade modulus and footing area into a rotational spring, then how would you do this? I've done it by assuming a unit rotation of the footing and then calculating the soil reactions induced by that unit rotation. Note: I assume the soil can take tension and compression for simplicity in calculating the rotational reaction.
Yes the soil will have both tension and compression to avoid any non linear analysis.

After reading some posts in the autodesk forum it seams that Robot doesnt take into account the rotational stiffness of the soil even if rotational springs are added but i am not sure yet.

I think i will proceed only with vertical springs for now.

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