Instabilities in Concrete Shear Walls - Mid Pier Model

[Anonymous]

Hi All

I'm in the process of modeling concrete shear walls in Robot using a Mid-Pier model procedure (See attached). I understand that Robot has wall panels and core wall functions for this type of application but I'm comparing results with another model that has been created using mid pier in another software. Please don't resond use Wall Panels instead.

I have constructed a mid pier model using column elements with section properties matching the wall and connected to each other using bars (very rigid) acting like rigid links. I'm getting a type 3 instability at the edge of the two intersection bars (see attached pdf)

My only loads are self weight of the columns and a lateral wind load applied at node 22 and 23. Notice the two ways I modeled the rigid link between the columns from level 1 and level 2. In level 1 the rigid link goes from column to column and level 2 the rigid link follows the profile of the wall (correct way of modeling). The instability is occuring only at locations where two bars intersect forming a cantilever shown in the attached pdf. I have checked my realeases and both i and j are fixed-fixed.

My original model is a 35 story concrete building but I was able to track the instability to be consistant at the canteliver ends of two intersection bars. The attached model is a very simplified version only showing two stories

What is this instability telling me? Is there a better way of modeling this?

[Artur.Kosakowski]

What is this instability telling me?

That you have large difference in stiffness of the elements of the model which is caused by modelling rigid links with RLinks (very rigid elements). If you switch off this option the instability will no longer be reported.

If you find your post answered press the Accept as Solution button please. This will help other users to find solutions much faster. Thank you.

[Anonymous]

Artur

Thank you for your swift response.  When I switch off this option what does it mean? What is the functionality of that option?

Eventhough that helped in the simple model I sent it did not solve my instablilities in the full size model... I have tried several things to track the instability but haven't had any luck.  Can you please take a look and let me know where the instabilites might be comming from?  Any help will be greatly appreciated. I'm new to ROBOT and just trying to get comfortable with the software.

[Artur.Kosakowski · Accepted solution]

Thank you for your swift response.  When I switch off this option what does it mean? What is the functionality of that option?

RLink is the automatically generated calculation element of stiffness which is significantly higher comparing to other elements that are defined in either of its nodes. When you switch this option off the the rigid links are modelled as the kinematic constrains (which behavior is similar to the situation when you use rigid elements instead).

Eventhough that helped in the simple model I sent it did not solve my instablilities in the full size model... I have tried several things to track the instability but haven't had any luck.  Can you please take a look and let me know where the instabilites might be comming from?  Any help will be greatly appreciated. I'm new to ROBOT and just trying to get comfortable with the software.

The reason is the setting of the solver type to Auto. For the small model Auto means Skyline whereas for the large one Sparse_M. The latter solver forces the use of RLinks regardless of your settings in Job Preferences. When you set the solver in the large model as Skyline (and switch off RLinks) there will be no instability reported.

If you find your post answered press the Accept as Solution button please. This will help other users to find solutions much faster. Thank you.

[khansayab]

Sir the RLINK element option was already turned off but still i had this instability warning 

i even tried deleting the wall and making it again but the error remained even meshing refinement didn't work either 

could you kindly help me 

if you need i can upload the file 

thank you

[Artur.Kosakowski]

Hi @khansayab 

You cannot rely on such defined rigid links (with vertical movement allowed) for a cantilever action of the floor. I'd suggest model it as a meshed panel instead.

 

If I managed to answer your question(s) press the Accept as Solution button please. This will help other users to find solution(s) much faster. Thank you.