How to decide if uplift is okay on raft foundation

[Grendizer]

Dear all,

How to decide if uplift is okay on raft foundation

I searched many rules, like IBC for this question, and i didn't find anything.
Shall we look at bearing capacity of the soil being exceeded?
Shall we look at upwards settlement?
Shall we look at percentage of area being lifted up?

Please enlighten me.

Regards

 

[oldestguy]

By not clearly saying what your project is all about and not giving pertinent details, there is no way you can expect useful comments here. Please provide some details, such as soil type area, typical soil profile, elevations of ground surfce and raft, ground water elevation, approx size of foundation and building type with loads.
Why the uplift and from what source?

 

[EireChch]

its all relative...is it a road subgrade over expansive soil? Or it a high structure that has high wi paso g that's causing overturning and uplift? Etc

 

[DaveAtkins]

Unless you have some overburden to hold the raft foundation down, no amount of uplift is OK. We don't assume there is cohesion between the bottom of the raft foundation and the soil it is supported on.

DaveAtkins

 

[Grendizer]

It is purely a theoretical question,
but my mind goes into a mid rise building under wind load, on marl (clay with limestone dust), typical in my country, with a deep water table (no effect)

10 story building, narrow base, say 15 m x 15 m

Thanks for your replies

If study is made with SAFE, can i accept some locally pressures above alloable pressure?
If study is made with Plaxis, can i accept some local plastification zones?

I remember reading clearly that some plastification is unavoidable...
If you want I can search that reading in my collection of documents.

 

[KootK]

But my mind goes into a mid rise building under wind load, on marl (clay with limestone dust), typical in my country

I do this all the time for transient loads like wind and earthquake. As far as I can tell, there's no other economical way to make some of these things work in a shallow foundation scheme. If I had uplift under permanent loading, I'd be concerned and would, at minimum, want to perform a very detailed study of settlement.

If study is made with SAFE, can i accept some locally pressures above alloable pressure?

I do this regularly as well. I ask the geotechical consultant for permission to do this and invariably get it. Usually a true soil shear failure does not govern and you're only interested in settlement. And the settlement that you're interested in is at the column/shaft center for the most part. As such, an average soil stress not exceeding maximum is often appropriate.


I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.