zero stiffness and ill-conditioned model 2

[aquacata]

After I ran my ETABS building model, I got a warning message of zero stiffness at a joint. Also it specified my model is unstable and ill-conditioned. In your experience, could you tell what is the most possible reason for that?

I suspect that my ramp area is not connected to its edge beams. I did select the ramp object and one of its sloped edge beam and meshed the ramp area. But I couldn't tell if it is meshed or not. Do you know a way?

Ramp cannot be assigned to a diaphragm. How can I trick it into a ridig diaphragm?

Thanks a lot for your help!

 

[slickdeals]

Any sloped member in ETABS can never be assigned to a diaphragm. For a diaphragm to be assigned, the member has to be in a horizontal plane. What kind of elements have you assigned to the ramp? (shell, membrane?).

The way to see your final mesh is to go to set building view options and select "show mesh". This will show you the mesh and associated nodal connectivity. Do you have a curved ramp by any chance? More info would help.

 

[aquacata]

Thank you, slickdeals! In your way, I saw my ramp is not meshed. I will make it meshed.

My ramp is straight. I assigned it as a shell since it needs to take out-of-plane gravity load. Could you please let me know to make it behave like a rigid diaphragm, what stiffness modifier need to change and change to how much?

I am excited that you replied my post!

 

[slickdeals]

There is no way you can make it behave like a rigid diaphragm using diaphragm constraints in ETABS. How thick is your concrete slab for out-of-plane loads? If it is 8" thick or more, it is pretty rigid itself and would not need any additional modifiers.

However, if it is not very thick, then you can increase the in-plane stiffness (f11,f22 and f12) to make it behave like a rigid body. Remember that these modifier values are a percentage, meaning if you make it 2, then it will be 2 times more stiff in resisting in plane loads.

I would suggest that you model the ramp explicitly and not assign any diaphragm constraints. For seismic loads, the masses are calculated automatically and for wind loads, it should not matter because the diaphragm width is calculated by the building width extents and hopefully, the ramp extents are within the building extents. Hope that makes sense.

 

[aquacata]

Yes that makes a lot sense. Thanks for your valuable help!