SAP2000 Non Linear Time History (FNA or Direct Integration)

[MrMaritime]

Hi,

First time poster and fairly new SAP2000 user. I am also still learning the rope of seismic analysis.

I am currently modelling an existing jetty structure. Flat RC deck supported by vertical and transverse raker tubular piles. Deck is modelled using shell elements and piles modelled using frame elements.

I have ran a response spectrum analysis but finding that there are tension forces in the piles exceeding their pull out capabilities. Essentially my model has now become non linear as I have assigned the piles to be compression only members in SAP. This is the only non linearity in my model. Having run the model non linearly for static load cases I am looking to assess it for seismic load cases.

Would running FNA time history in SAP be acceptable for my model? Information on CSI is a bit vague on FNA applicability. Direct integration would likely take forever to run (more likely abort) given the size of my model and amount of mesh.

Thanks in advance for your help.

 

[JoshPlumSE]

I would think FNA would work in your type of model.

FNA is essentially the same as using modal superposition for time history in that it uses the mode shapes (or Ritz Vectors) to solve the response history. It's just that it's modified to work with non-linear elements.

The best usage is when your non-linear elements are fairly limited in number. Right? If everything is non-linear then you'd be better off doing direct integration.

Most importantly, documentation (in the CSI Analysis Reference Manual) states that FNA only works if your non-linearity is confined to link elements. So, you'd have to make sure you have a link element which creates the non-linearity in the piles.