Bowles estimation formula of subgrade modulus reaction

[ramihabchi]

Hi,
as I understood from the following paragraph extracted from Bowles foundation analysis and design, that ks=12(SF)qa cannot be used to estimate differential settlements, but only can be used for design of foundation and soil pressure checking.am I right?
because an engineer I work with is saying to me that no need to check differential settlement in R.C buildings when the modulus of elasticity of the the previous formula is used, because the modeling software will take them into account.

." Since one does not often have values of Es, other approximations are useful and often quite satisfactory if the
computed deflection (directly dependent on ks) can be tolerated for any reasonable value. It
has been found that bending moments and the computed soil pressure are not very sensitive
to what is used for ks because the structural member stiffness is usually 10 or more times as great as the soil stiffness as defined by ks
. Recognizing this, the author has suggested the following for approximating ks from the allowable bearing capacity qa furnished by the geotechnical consultant:
SI: ks = 40(SF)^ kN/m3
Fps: ks = l2(S¥)qa k/ft3

 

[HTURKAK]

So, what is the question here ?

I will suggest you ;

- Have a look BEAMS ON ELASTIC FOUNDATION ( By Hetenyi)

- Analize a beam on elastic foundation with FEM , with proposed stiffness , then change length and soften the springs and see how the bending moments and the soil pressure are sensitive..







I cannot give you the formula for success, but I can give you the formula for failure..It is: Try to please everybody.

 

[ramihabchi]

My question is:
Because diffential settlements under the structure affect moments and shear in the superstructure, and the method of Bowles based on ks=12(SF)qa is not good in estimating settlement, we have to use other more accurate formulas or calculate the settlement in classical way to determine differential settlement and therefore the real effect they have on the superstructure.am I right?

 

[HTURKAK]

Yes..!! You are right..
Moreover, the modulus of subgrade reaction (ks ) obtained from other methods also SHALL NOT BE USED to predict the settlements..

Ks , is used for modelling the soil ( winkler springs ) for support model based on linear elastic springs at soil -structure interface. The springs deform independently , not coupled.







I cannot give you the formula for success, but I can give you the formula for failure..It is: Try to please everybody.