Why foundation models in safe (or SAP2000) isn't be matched with real world conditions ?!

[Sarkheil]

i have a simple question about SAP2000 or safe in foundation modeling and analysis.
below picture shows the settlement profile and soil pressure of various kinds of foundations in different soil conditions.

question : what is the presumption of safe (SAP2000) in foundation modeling?!
i think it should be elastic assumption. however, why in modeling of foundations in SAFE (or SAP2000) under uniform loading, both settlement and soil pressure of foundation is uniform in every place of foundation ???? it seems be false and not match with any kind of real conditions.

 

[JoshPlumSE]

What are the soil springs or modulus of subgrade reaction used in the model?

There is an ACI document titled "suggested analysis and design procedures for combined footings and mats" (ACI 336.2-R). This document gives recommendations on how to set the soil springs in different areas of the mat in order to account for the type of soil that is supporting it.

 

[Sarkheil]

it means that although the type of soil under a foundation can be similar in every place, but modulus of soil is different ?!
in our soil investigation report, it reported an exclusive Ks (subgrade modulus) for one foundation (according to its width). Is soil report incomplete ? or different subgrade should be calculated ?! if yes, how ?!

 

[JoshPlumSE]

You should read that ACI document yourself for best understanding. However, my interpretation would be the following:

1) Most analysis models use uncoupled / Winkler springs to model the soil.
2) The problem with this model is that is does not capture cross-coupling terms in the soil/slab interaction.
3) To correct for that, the ACI document suggests modifying your soil springs in certain regions of the slab in order to better capture the true behavior of the foundation.

Essentially, the modeling technique is meant to approximate the real behavior of the slab when using a simplified Winkler spring model that doesn't include cross coupling. If you had already accounted for cross coupling in some way, this would not be necessary. But, I don't think most engineers are really doing that. In fact, I'd argue that most engineers aren't even doing what we're talking about (modifying the springs in various areas of the mat per that ACI document).

 

[JoshPlumSE]

I should point out that I'm really not a SAP guy. Rather I'm a RISA guy. So, while I assume that the soil springs used in SAFE and / or SAP do not account for cross-coupling terms, that is just an assumption. Their documentation might address this issue for all I know. If not, then that ACI document is probably the best place to start.

 

[Sarkheil]

Thank you so much for your worthwhile help.