Stress averaging in SAFE

[israr_sari]

Dear Safeians
I ma working on a 22 Storey Building. we have assume a Raft foundation of 5' for it.

Net bearing capacity after overburden relief is 8kip/ft2. And i am getting 10Kip/ft2 soil pressure under service loads at some locations.

one of my friend said use stress averaging to minimise the values as these soil pressures are nodal results of finite elements.

do somebody have any clue how to do that using safe .. or Excel. 

stress distribution.bmp

Edited July 4, 2018 by israr_sari
spelling mistake

[Ayesha]

As far as high-rise design goes, there is always areas of high concentration in a raft. General practice is to ignore them and focus on more general values and see if they are within the allowable limit or not. There is no guideline for stress average. People do it by using their engineering judgement.

[Areeb]

Hello Israr,

As madam Ayesha said there are areas of high stress concentration in raft. Could you specify the location where you are getting 10 Kip/ft2. As specified if this pressure is at corners in small concentrations then you can ignore, however if the area is large then need to apply changes to raft design.

Thanks.

[Hafsa Azmat]

from the pic, your area of allowable bearing capacity violation is large , better not to do average.

Though the method of averaging is by importing nodal soil pressure table  in excel and take joints avg.

[Ahsan Kazmi]

@Ayesha what do you mean by more general values? Please elaborate? actually i need help in this context.

 

[Ayesha]

Here is an example to explain what I mean by general values:

If you have a shear wall located on a raft foundation and the only area where you have high stresses is under the shear wall, and the rest of raft has values within the acceptable range, then ignore the high values right under shear wall and focus on more general values or focus on the rest of the foundation.

For your case, @Hafsa Azmat has correctly pointed out that averaging wouldn't work or it would be  very limited. For example you can average 11 and 10 E3 zones (The purple colored stresses) as that is more of a concentrated zone. Not all of the raft. Hope that helps.

 

[israr_sari]

@Ayesha @Hafsa Azmat thanks for both of you.
@Areeb you can check in the picture that i have attached above. basically it is spreaded  all in the middle of raft.

 

[WR1]

Yes, you need to do stress averaging which is not very straight forward. I do not rely on software stress averaging as it will average the stresses based on the given length. Imagine if stress at localized area is too high (peak), even after the stress averaging you will get higher stress and the extent will also be too much. You need to develop your own rational approach for stress averaging, or need to adjust meshing some times.

[Ahmed Waqar]

Have you checked the settlements. What is modulus of sub grade that you have taken. If you computed modulus of subgrade then multiply by FOS then you are using ultimate capacity of soil. For geotechnical design that's not workable. Check settlements if they are in range then ok. These are my view only...

[israr_sari]

@Ahmed Waqar Settlements is within limit which is about 0.8". And Allowed is 2"
Subgrade of Modulus is 186 kip/ft3 with bearing capacity of 8.01 Kip/ft2.

@Rana well Rana i tried to play with meshing .. no-matter what size i adopted  stresses were over and above the limit (there was very less effect of meshing). only stresses came down when i didn't use any kind of meshing. Which is definitely wrong approach... so finally i concluded that i can not bring down these stresses with str ess averaging so need some other solution.

MODEL-C1&C2 28-6-2018.bmp

[Waqar Saleem]

Salam, try by increasing the thickness of the raft and check the effect , if it works increase the raft thickness locally. If it does not work go for piled raft.

 

Reagrds

[WR1]

On 7/19/2018 at 1:16 PM, israr_sari said:

@Ahmed Waqar Settlements is within limit which is about 0.8". And Allowed is 2"
Subgrade of Modulus is 186 kip/ft3 with bearing capacity of 8.01 Kip/ft2.

@Rana well Rana i tried to play with meshing .. no-matter what size i adopted  stresses were over and above the limit (there was very less effect of meshing). only stresses came down when i didn't use any kind of meshing. Which is definitely wrong approach... so finally i concluded that i can not bring down these stresses with str ess averaging so need some other solution.

MODEL-C1&C2 28-6-2018.bmp

If the stress is over the allowed limit and is distributed on a large area as shown in your image, then it is not called 'stress concentration' locally. You need to increase the soil capacity.

[Ahmed Waqar]

How you computed the modulus of subgrade for settlements and bearing capacity checks. Not for structural design