Difference between rigid and flexible footings in S-Foundation.

Tom_Structural

I am trying to understand what S-Foundation is doing when you say a footing is rigid versus flexible. To this end, I have a simple 5m x 5m x 1m (thick) footing as shown in my attachment subjected to 1072 kN force. I run a linear static analysis with both flexible and rigid options. For the flexible case, I have set the modulus of subgrade reaction to 3300 kPa/m. As shown in my attachment (Capture1), the flexure and one way shear demand on the footing in the flexible case is about twice that for the rigid case.  I also find that the one way shear capacity for the flexible case is about two times lower than the rigid case.  

How is S-Foundation calculating the one-way shear demand and capacity on a flexible footing? It seems to me that it is using some reduced width or depth. Why is the flexure demand also doubled? 

Capture.PNG

Capture1.PNG

Matthew_Sauer1

Hello Tom,

Thanks for reaching out.

S-FOUNDATION uses two different methods to calculate the demands on your foundation, as you pointed out, Flexible and Rigid pad methods.

The rigid pad method uses the assumption that the pad itself is entirely rigid, and you have a linear soil pressure distribution under your pad.  This method applies to rectangular pads only, and utilizes the same methods we would use for hand calcs, making it handy for double checking.

The Flexible method, however uses the underlying Finite Element Model produced by S-FOUNDATION, and the stiffness of the shell elements produced will be relative to the pad thickness, material properties, etc.  This will not necessarily lead to a linear soil pressure distribution.  The demands on the model are also calculated by strip integration lines which will integrate the contour results at key locations to compare against design code capacities.  These results can start to diverge in some scenarios (ie, relatively thin pads).  With that said, it is difficult to say for sure why your results are much different without taking a closer look at your model.  This may be best accomplished if you can share the model with us in our support system.  You can create a support ticket here for us to take a closer look:

https://community.altair.com/community?id=csm_get_help&sys_id=de45c412c312310015519f2974d3ae1b

 

Regards,
Matthew Sauer
Altair Support