Uplift load on footings

Hello Tom,

Thanks for getting in touch.  This behaviour could be related to a number of different things.  One of the first steps I usually take in this situation is to try and run the analysis as a Linear Static Analysis, rather than a Nonlinear Static Analysis, which is the default.

This serves two purposes:

1. We use nonlinear compression only springs by default, but in a linear analysis these will be converted to linear springs with tensile stiffness.  This may sound counter intuitive, but we can often use these to get the analysis to solve, and output some results.  We can then review the deflected shape/contour diagrams and ensure that the behaviour is along the lines of our expectations.  This can be set from the Design Input window, under the 'Finite Element Analysis' field.
2. Nonlinear ground springs have a maximum compressive capacity dictated by their ultimate bearing capacity.  When using a linear analysis, the capacity is removed, so it can be a good way of checking if your soil springs were over their capacity due to the applied loads.  More information on this can be found here: https://www.youtube.com/watch?v=0vTi2q3a9rw     When your loads exceed the maximum capacity of your nonlinear ground spring, that spring provides no further stiffness, and can cause instabilities. 

I would suggest investigating these two items, and if that doesn't solve your problem, please feel free to open up a support ticket (see link below) so you can share your model with us, and we'd be happy to investigate further.

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

Regards,
Matthew Sauer

Altair Support