Modulus of sub grade reaction input value in CSI-SAFE 1

[AYMAN5000]

I need to design an isolated footing which shall be constructed above an existing raft foundation
What is the modulus of sub grade reaction used for the isolated footing in this case.
Mainly the modulus of subgrade reaction = 40*3*Bearing capacity as per Joseph Bowles
KINDLY REFER TO ATTACHED PHOTO.

 

[AYMAN5000]

please not that i am using CSI-SAFE not Midas, moreover I am just modeling the isolated footing

 

[Tstruct]

I think modulus of subgrade isn't applicable to a foundation resting on concrete, as concrete in not a compressible material like soil. Conservative approach would be to use the modulus of soil beneath the raft foundation and just place your isolated foundation on raft (I am assuming you have an existing raft foundation).

 

[AYMAN5000]

I thick i would use the modulus of subgrade reaction of the raft and compare the actual stress under the isolated footing by the bearing strength of the concrete 0.85fc'
Again i did not model the raft, aim modeling the isolated footing only (standalone)
Does anyone has a better approach ???

 

[Tstruct]

Your approach seems reasonable. Still keep some safety margin.

 

[milkshakelake]

Is it actually conservative to use a lower bearing pressure, like the one below the raft? I'd think that it's unconservative, since we typically calculate isolated footings as having rigid subgrade. A lower bearing pressure will concentrate more of the force distribution towards the concentrated loads, which is unconservative for flexure and one-way shear. Or maybe I'm missing something, but you can test it out with a low and high subgrade modulus and see how it affects the moment.

The actual subgrade modulus would be something between the soil per Bowles formula and the stiffness of the raft, which is nearly impossible to calculate.

 

[AYMAN5000]

Kindly see attached.

 

[AYMAN5000]

Kindly see the attachment
using a lower value of modulus of subgrade reaction will lead to conservative design

 

[Tstruct]

Modulus of subgrade is like springs beneath the foundation. The more stiff it gets, less the foundation settles. Concrete raft would offer more stiffness than soil so it would be conservative to use soil's modulus. This is what my understanding is behind my answer. Please correct if I am wrong.

 

[AYMAN5000]

yes applying lower modulus of subgrade reaction can be considered conservative approach

 

[milkshakelake]

Got it, makes sense.