Very simple to find the stress on soil in ETABS, Assign the K spring factor Get the displacements values for all joints in Uz directionFrom the relation K=P/delta, find the maximum stress on soil at any point.

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EagleI've created a test model and applied DL, LL and WL on all floors. Assigned Kspring of 10000 kN/m3, Divided the raft into 1x1m mesh. Extracted the displacements of all points in the raft and multiplied their values by Kspring. Please refer to the attached Etabs and Excel files from the link below.

http://attchd.in/D4vp-

frameingineeringPlease, Can you create the test model of a regular 10 story with the 10x10m raft.

EagleK = P / Delta

Kjoint = Pjoint / joint displacement

Yes, this is correct. But it has to be done with several points in a spreadsheet. Let's have an example on Etabs of a very small raft 10m x 10m and discuss.

See, this is all about finding the stresses applied on soil from wind (service load combinations), seismic (ultimate load combinations) and is not necessary for the gravity case as it is easy to be done simple hand calculations.

frameingineeringOkay, so supposed I manually meshed my raft slab let say 1x1m, there would be four points around this meshed slab. So the deflection of a point served by this meshed area multiply by the kz should give me the bearing stress, is that correct please?

What if you use the joint reaction, so that you can use the formula, B. C=P(reaction FZ) /area (meshed area)

Another thing, what load combination do u use for the deflection? Do you have to run a nonlinear analysis?

Hoping to get a response from you soon.

Samuel

EagleHi @framengineering

The above is a rule of thumb is the concept of calculating the modulus of subgrade reaction. Well your figure fully depends on the area served by the point. I applied this method on one of the projects for all the points in the raft. Extract the displacements of these points in a table.

frameingineeringOk, so u mean if i have a joint and it's has a displacement of say 0.0121mm, that's 0.0000121m, and my subgrade of reaction, that is kz is 60000KN/m^3, then my bearing pressure at that point is 0.0000121*60000=0.726KN/m^2.

is that correct?

If my allowable bearing pressure is 500 KN/m^2, what conclusion can i draw from this.

Thank you.