Raft foundation stress check 2

[Eng. Tawfik Jdid]

Hello Engineers!,
I have an inquiry please,
If we were making an assessment for a raft foundation and The stresses exceeded the Soil bearing capacity. (Let say that soil bearing capacity was 150 kPa according to the soil report results)
So Since the geotechnical engineer Use safety factor of minimum 2, Which means that the real soil bearing capacity in our case is 300 kPa.
The maximum stress value was 170 kPa Which apparently is larger than solid building capacity, the value of 150 kPa.
The question is, can we compare the maximum Stress value With the real soil Bearing capacity without using the geotechnical safe factor Whether it was 2 or 3.

Sincerely,

 

[Luceid]

So you hired an expert to give you data and you want to disregard their safety factor without knowing what goes into that safety factor?

Sounds like a bad idea - especially if you think you can just do this without talking to Geotech first. The difference in numerical value isn't massive - the principle is suspect.

 

[JStephen]

There are some things you can look at.
In my area, most soils reports give the "net" allowable bearing capacity, which is the capacity in excess of the soil weight which was already there. Whether done this way or not, the design loads need to be coordinated with the assumptions used in the soils report.
And in my area, usually the allowable soil bearing is given assuming wind and seismic loads are also on an allowable stress basis rather than an ultimate strength basis.
And usually the allowable bearing assumes somewhat uniform bearing under the whole area. If you're coming up with high intensity in one spot only, that might be different.
In some cases, allowable bearing is based on factors other than strength (long-term settlement, for example). Allowing more settlement or doing various kind of soil improvement may increase the allowable.
In all of these cases, checking with the geotech would be the thing to do.