Foundation Design Consultations - Need Guidance

[DamianP]

Hello everyone,

I am a civil engineer with some years of experience, but I find myself facing some challenges in developing a spreadsheet for foundations. I would like to ask the opinion of more experienced colleagues, as I know these topics are not always straightforward and any help would be invaluable.

I have two specific questions that have me a bit stuck:

1. *Why is it important that the max ratio between the maximum stress and minimum stress in a foundation be less than 1.25?* I have read that this is crucial, but I don't quite understand the logic behind this criterion.

2. *Regarding shear in a foundation, is it possible that it is absorbed by the beams or slabs rather than the foundations?* I have been told that shear in foundations (and therefore the moment it generates) is often neglected when there are beams or slabs that absorb the shear. If so, wouldn't it be more sensible to take only a fraction of the shear with the foundations?
I hope you can help me with this point, as I am not sure if it is the most adequate from a structural point of view.

I understand that these questions may seem basic to some, but I would really appreciate any explanations or references you can offer me. I am trying to learn and improve in these aspects, and your experience would be of great help.

Thank you very much for taking the time to read and respond.

Best regards

 

[BAretired]

1. That is a new one on me. Where have you read it? Sounds like baloney to me.

2. I would need to see an example of that. Again, it is a new one to me.

 

[CarlB]

Regarding #2:
I'm guessing by "shear", this is lateral load from the structure that will transfer to the foundation. But depending on how the load is transferred, some may be resisted by grade beams (setting on the earth) and grade slabs. It some cases it may be reasonable to assume that those elements may share in transferring lateral loads to the subsurface, such that not all lateral loads need to be resisted by the foundations. The design engineer would need to understand the load distribution and design accordingly. It may be simpler, and would be more conservative, to ignore contributions from these other elements. The possibility that those elements might not always be available in the future should be considered, such as floor slab is removed, ground settles under grade beam, etc.

 

[BulbTheBuilder]

For #1, are you talking about the pressures like the image shown? I'm guessing they want to maintain a uniform pressure (stress) at the base of the foundation rather than have uplift (tension)

 

[BAretired]

I assume that is what he is talking about, but why would the ratio never be less than 1.25? That is just plain ludicrous. Most engineers would be happy to have a ratio of 1.0. If he means "would never be more than 1.25", that is much too conservative imo. Maybe he means "would never be less than 0.80", but again that is too conservative. I asked the OP where he read it, but he has not responded.

1. *Why is it important that the max ratio between the maximum stress and minimum stress in a foundation be less than 1.25?* I have read that this is crucial, but I don't quite understand the logic behind this criterion.

 

[BulbTheBuilder]

I think he is trying to "play it safe". As a newbie, better to be conservative than trying to get it accurate. But I'd happy to know his reference and the soil conditions for that particular criterion.