Mat Foundation Design Parameters

[rlewistx]

I am going to design a mat foundation using IES's mat foundation from their Analysis group package. It is a dynamic machine load. My question is how to come up with soil design parameters. I think I need to get them from a geotechnical engineer. There is one for this project. I need to come up with spring constants. Is this determined from the modulus of subgrade reaction 'k' which is in pci units? Do I multiple the area of the mat with the 'k' value to get the spring value at the node? Are there other types of information I should get from the geotech for this design?

Do I need a soil dampening value as well, or do I use the dampening value from the standard equations for the foundation displacement checks? Do I add the dynamic displacement values to the static values from the computer analysis to get my final design displacements?

Thanks for any help.

 

[rlewistx]

I have another question to add to the above. How do you determine the resonant frequency of a mat foundation? How does this differ from the natural frequency?

 

[MToft]

Hi,
The spring constants should come from the element area and spring length ie stiffness = modulus * area / length. If this initial model treats the soil as linear, it's worth checking the soil strain (from displacement) to see if there's a risk of non-linear behaviour (depending on whether the stuff's compressive or granular etc).

I THINK you should probably look at static and dynamic results separately - the mat has to pass both, I guess!

Resonant frequency = most important natural frequency (highest amplitude or lowest frequency, which will tend to dominate in practice). This is an eigenvalue of the mat's stiffness matrix, and each code has its own syntax for working this out, eg in Abaqus you add a 1line command to the input file asking for however many frequencies you want - they usually come out lowest first (which you want), possibly in radians/time not Hertz.

Certainly speak to the geotech engineer - I hope he's done more soil dynamics than me!!

Good luck!

 

[rlewistx]

Thank you for your repsonse. I appreciate it very much.

Spring:
How do you know the spring length?
What do I ask my geotech for regarding the soil strain? Is there a maximum strain value that can be determined before nonlinear displacement occurs?

Resonance:
The resonant frequency is obtained from a computer run? Do I input plate elements in the computer on spring supports and ask it to give it to me? I will need to do this by hand.

Results:
I understand that the two are done separately, my question is then are they additive to one another to get the total deflection? Do I run the static analysis and get deflections, then run a dynamic analysis, get deflectiosn and then add them together? I don't think my mat program will do a dynamic analysis of the foundation.

Thanks.