Silly question regarding instability in ETABS 2

[LinhLe]

Hi guys, I'm sure this is a very basic/silly question but I couldn't figure it out myself.

I have a simple model as attached. When I run it, it always says "MAXIMUM NUMBER OF NULL STEPS REACHED FOR CASE: ~P-DELTA SUBSEQUENT RESULTS WILL NOT BE AVAILABLE" and doesn't show any results.

Any help or suggestion to locate the problem is much appreciated.

Regards.

 

[Luceid]

I don't have access to an ETABS license right now - but does your model run fine without P-Delta effects active?

For regular instabilities - switching the analysis solver to the "Standard Solver" typically helps you locate the instable nodes. I haven't investigated a P-Delta failure, however if this is a simple model it may not take too long to check.

You may also for some reason or another just need to add more steps to your P-Delta analysis, or your model is simply too flexible and the P-Delta is breaking it.

 

[LinhLe]

Thank you,

My model is just a 2x4m concrete wall, 0.3m thick, fixed at base bears 2kN horizontal (as shown in the image).
I ran without P-delta but it says the model is unstable (as shown in the log). I don't quite understand how that happens.

 

[EZBuilding]

Under the Analyze tab - try setting your active degrees of freedom to just the plane in the direction of your horizontal load and the elevation.

 

[Luceid]

I was thinking that EZ - but it seems weird that it would be found unstable without P-Delta considering the base has fixed connections. Wouldn't this technically be 3-dimensionally stable? I could see P-Delta causing issues with the low out of plane bending stiffness, but a linear run should come out as stable if that was the issue.

 

[LinhLe]

Yeah, I haven't tried 2D analysis but I think 3D should work as it's fixed at base.

Could you please have a look into the model and might see what I've missed. Thank you.

 

[BulbTheBuilder]

Not an ETABS person, but what are constraints for the 4 elements? Are they acting as 1?

 

[Luceid]

I guess that's true Bulb - are your elements modelled as membranes or shells? That could be it

 

[EZBuilding]

I was thinking that EZ - but it seems weird that it would be found unstable without P-Delta considering the base has fixed connections. Wouldn't this technically be 3-dimensionally stable? I could see P-Delta causing issues with the low out of plane bending stiffness, but a linear run should come out as stable if that was the issue.

Agreed that it should - but I've found that removing layers of complexities is the best way to troubleshoot an issue

And also further agree - those elements being modeled as membranes could lead to an instability.