assuming subgrade modulus for finite element foundation desi

huangyhg

assuming subgrade modulus for finite element foundation desi
ok this is the procedure my officemate told me how to assume subgrade modulus (ks) and just want to verify if you agree..
inially divide soil pressure by 0.5 in.. then use the ks in the program.. if actual deflection is less than 0.5 in then your assumption is ok..
if actual deflection is higher then increase ks.. redo the procedure..
any comments?
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here is my procedure: get the value from the geotechnical engineer.
ditto kbvt
thank you.
greenman
i have been following a similar procedure to you where i have been modelling the changing the value of the spring stiffness based on what i feel will be appropriate displacements of the foundation. our geotechnical engineers don't give to much information on subgrade modulus.
if your geotech report does not give the k value, what you are proposing is reasonable and conservative.  this is because most of the time the allowable bearing pressure is not based on failure of the soil, but on settlement of about 1/2".  and so if you take the allowable bearing pressure in psi, and divide by 0.5, you obtain the pressure in psi which will cause a 1" settlement (which is by definition the k value).
daveatkins
it's always useful to analyse a range of values to determine the sensitivity of the model.
get the soil bearing capacity, multiply it by 40 and a factor of safety which is usually 1.5. bt the way values are in metric units.
no disagreements with the bearing pressure method here, but ensure the pressure is an allowable design soil pressure and not an ultimate limit state soil design pressure. because the allowable design pressure should have deflection controls inherently built-in, however the ultimate bearing pressure has no such requirement.
nomally checking the k value arrived at against published values just to ensure it is in the ball park.
note: if bearing pressure is below 100kpa and the building is tall, get the geotech to return to site and test for this stiffness value.  
p34 of the concrete societies technical report 34 gives typical subgrade moduli fro differnet soil types and even plots cbr's against exepected modulus values. detailed design should be based on geotech tests.
as everyone said, let the geotechs give you soil values.  but in my experience i've seen a range of about 100 pci to 300 pci as the values given for the subgrade modulus which i then use to model the soil springs.  i've found it doesn't make much of a difference in soil pressures or internal stresses in the concrete unless you are talking huge loads or thin reinforced concrete...or both.  good luck.