ETABS P-DELTA Setup Confusion

[qwop]

I'm fairly new to ETABS and P-Delta effects and I want to make sure I have things set up correctly. I have a building that's controlled by seismic loads and I want to make sure I'm capturing P-Delta effects. My controlling load cases are below

1.2D+1.0L+1.0Ex
1.2D+1.0L-1.0Ex
1.2D+1.0L+1.0Ey
1.2D+1.0L-1.0Ey

I watched the CSI tutorial for setting up P-Delta in SAP and in that video it says to set up a static non-linear load case for each load combination you want to consider P-Delta for. So if I were in SAP my understanding is that I create 4 new static non-linear load cases, apply the same load patterns and scale factors as above, and select "P-Delta" in the geometric non-linearity option.

Now my confusion with ETABS comes in with the Define > P-Delta Options menu. Since my controlling cases use 1.2D+1.0L can I simply select "Iterative - Based on Loads" and input Dead and Live loads with the corresponding load factors and run my model without creating 4 new nonlinear load cases? Or so I still need to create those 4 cases like in the SAP tutorial video? Will my seismic load cases need to be set to non-linear if I use the P-Delta menu? Will I need to select "P-Delta" for the geometric nonlinearity option in the seismic load cases, or is that not necessary once the Define > P-Delta Options menu is used?

 

[milkshakelake]

If it's a small building, like 8 stories or less, and it has no weird conditions like a cantilever, you can select "Iterative - Based on loads". The P-delta doesn't really matter and ETABS will make a good approximation. If it's a taller building, it becomes more complicated and I'm sure someone else will chime in.

 

[JoshPlumSE]

Since my controlling cases use 1.2D+1.0L can I simply select "Iterative - Based on Loads" and input Dead and Live loads with the corresponding load factors and run my model without creating 4 new nonlinear load cases? Or so I still need to create those 4 cases like in the SAP tutorial video?

Yes, that's my understanding. Keep in mind that this will base the P-Delta analysis on a single set of gravity loads. What you're essentially doing is a single Geometric Stiffness adjustment to the stiffness matrix based on this one set of loads. The benefit being that this then becomes a linear analysis for all load combinations. That's generally viewed as being accurate enough most of the time.

However, if you want to be a little more accurate then you could create the 4 non-linear static load cases instead. Heck, if it were me, I'd be interested in how different the results would be between the two methods.

 

[qwop]

Thank you for both of your replies. Fortunately the building is very regular and under 6 stories, so it sounds like I can use the Iterative based on loads and get a close approximation. I created a model with 4 non-linear static cases which takes a long time to run right now compared to using the P-Delta menu, but I'll compare and see what's different

 

[MSUK90]

I created a model with 4 non-linear static cases which takes a long time to run right now compared to using the P-Delta menu, but I'll compare and see what's different

Would like to know the difference you got in your models?(I believe you have already done the comparison)