Subgrade Modulus (Mat Foundation) 3

[Raymund93]

Hi all,

I just want to clarify my design using FEM analysis for mat foundation. Please correct me if I'm wrong.

For the subgrade modulus, I used qa(FS)/allowable settlement.
This comes from the link below.

https://www.structuremag.org/?p=1239

Now for my concern, does the base pressure results from FEM analysis (Service Loads) need to be multiplied or divided by FS?

The author from the link above mentions that it will need to be multiplied by FS but in the comments sections they are confused because this will result to large bearing pressure.

Please do confirm on what will be the correct procedure.

Thanks

 

[MotorCity]

The subgrade modulus is a property of the soil. It depends on the actual "settlement" of the test plate under load, not the allowable settlement of your structure.

The allowable bearing pressure should already have a FS built into it, that's why it is the "allowable" bearing pressure.

You should verify the information with your Geotech engineer.

 

[BAretired]

If your analysis is using service loads, settlement should be compared with allowable settlement (unfactored).

If you want to reinforce the mat for moments, you must multiply by FS (factored).

BA

 

[Celt83]

The subgrade modulus is a property of the soil. It depends on the actual "settlement" of the test plate under load, not the allowable settlement of your structure.

To add to this it's been my understanding that there is no real direct relationship between the subgrade modulus, plate test result, and the spring stiffness, K, used in the structural analysis software.

You need to open a dialog with the goetechnical engineer and go thru an iterative process of comparing the deflection results from your analysis with a settlement analysis on the geotechnical end. This may result in your model having various zones of different K values. Another approach I've seen is to run the model with a couple different K values to see if the reinforcement and bearing pressures are sensitive to the soil stiffness.

Open Source Structural Applications:

https://github.com/buddyd16/Structural-Engineering

 

[Raymund93]

Thanks for all your inputs.

Unfortunately, our basis for soil properties are only the structural design data from our mother company which is from another country.
We will have a hard time getting in touch with the geotech engineer in this matter, so for simplicity and faster design, we opt for educated guess and estimation of some properties.

 

[Settingsun]

As StructureMag says, allowable pressure is the lower from shear strength and settlement. Safety factor is probably different for these two, say ~2.5 vs ~1.0. Make sure you're using the right safety factor when calculating K.

If using unfactored loads you would compare to allowable pressure directly. When doing ultimate concrete design, factor loads and ground reaction naturally increases in response but isn't an issue if this exceeds allowable (apples vs oranges comparison).

This method is really rough. Do a sensitivity check as recommended in an earlier response.