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Ahsan Kazmi

FOUNDATION DESIGN IN SAFE

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1. To check soil pressure in SAFE v8,

- After running the analysis, go to Diplay menu,

- Select 'Show Reaction Forces...',

- Select 'Soil Pressures',

- Select the Load case or combination,

- Click on OK.

2. Minimum & maximum pressures are the Actual range of calculated pressures anywhere on the given foundation.

3. The limit of pressure is the Allowable Soil Bearing Pressure provided by a geotechnical engineer, after carrying out necessary field & laboratory tests on the at the project site or the soil samples taken from the site.

Regards.

 

Edited by EngrUzair
2-spelling corrections

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Why are you still using SAFE 8. The latest is SAFE 2016.

In ver 8, negative means (tension) and positive means (compression). After ver 8, it is opposite.

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@Rana i am using SAFE 14. Actually there is R C C Wall in boundary of structure and all columns are in periphery as well. So negative pressure in periphery is exceeding the allowable limit but i cannot increase area. So please suggest Solution.

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1. Depending on how much the value is exceeding you could increase thickness of the footing.

2. Soil improvement.

3. Reduce loads/stories.

 

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Rana,

What is the solution if I cannot

6 hours ago, Rana said:

1. Depending on how much the value is exceeding you could increase thickness of the footing.

2. Soil improvement.

3. Reduce loads/stories.

 

If it is not possible to do all these solution, what shall he do, because this is the local pressure at one side only.

 the average total pressure may be very less than the allowable pressure

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It depends on the ratio of local area vs total area. It also depends on the location and importance of localized stress area among other factors. This is engineering judgement beyond the number crunching drudgery. I cant provide this as I am not aware of the project details. You could refer to your seniors for this.

And remember do not stop at this. There is always a solution no matter how hard the problem is.

For example may be your geotech engineer might suggest a lesser soil stiffness value at this location which will help reduce the stresses. You might increase the local thickness to better distribute the pressure. You could optimize the load combinations or even the load path of superstructure not to have the concentration here.

An important point here is if this is localized compression at corner then is it due to lateral loads? If yes then watchout for the reverse that is tension (blue color contours). This will be another whole new problem.

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Dear Ahsan,

1- consider the relieving pressure of soil as your model shows basement.

2- provide stiff element at the edges of raft as there is basement wall all around.

3-increase in thickness also decreases the pressure, check increasing raft thickness.

4-make sure that actual raft area is provided in SAFE, sometimes it is mistakenly considered less when raft is drawn on center points of the columns, actually you should provide the offset of half column width or depth as per condition on edges. 

5- check loads applied on the floors, finishes, walls, decrease where-ever you can, make deduction of windows and doors, check if lighter elements are available, decrease thickness of slabs if possible.

6-Add one column in between all grids if possible, 

7-offset your columns some distance 2' or 2.5' inwards, ask the architect, share your problem.  

4-if none of the above options and solution provided by seniors work, have review of geotech and go for piled raft.

 

Regards

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20 hours ago, waqar saleem said:

1- consider the relieving pressure of soil as your model shows basement.

2- provide stiff element at the edges of raft as there is basement wall all around.

Dear Waqar Saleem, 

Thanks for such comprehensive reply can you please elaborate how can i apply these two points in SAFE.

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Dear Ahsan,

1-If you are excavating 10'(say) from NSL,soil strata is under load of this much soil 10x120=1200psf, you are provided bearing capacity at -10 by the geotech, 1tsf=2204psf, you can add 2204+1200=3404psf for bearing pressude check, but discuss this condition to your geotech, also there is safety factor of 3-5 in geotech calculations, decreasing this FOS to 2 or at some lower value as geotech allow, earing pressure issue can be addressed.

2-define a stiff element in safe where you define your foundation section, and assign the area below basement wall as stiff.

 

Regards

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22 hours ago, waqar saleem said:

Dear Ahsan,

1-If you are excavating 10'(say) from NSL,soil strata is under load of this much soil 10x120=1200psf, you are provided bearing capacity at -10 by the geotech, 1tsf=2204psf, you can add 2204+1200=3404psf for bearing pressude check, but discuss this condition to your geotech, also there is safety factor of 3-5 in geotech calculations, decreasing this FOS to 2 or at some lower value as geotech allow, earing pressure issue can be addressed.

2-define a stiff element in safe where you define your foundation section, and assign the area below basement wall as stiff.

 

Regards

anaylsis of raft should be run with option 3D OR 2D?

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