High period values in modal analysis

[bruznunez]

Hello STKO team,

I need help with a 3D model. The case study is a two-story structure made of reinforced concrete block masonry. This is just a test to learn about modal analysis. The problem is that I have not been able to obtain periods shorter than 1 second, considering that this type of structure is highly rigid. The unit system used is N-mm. I already checked the measurement units, and as far as I know, they are correct. I would appreciate it if you could help me find the error or provide suggestions to solve this problem and obtain periods shorter than 1 second.

[STKO Team]

Your mass density is wrong for your unit system:
A concrete for example
Self weight = 25 kN/m^3 = 2.5e-5 N/mm^3
g = 9810 mm/s^2
Rho = 2.548e-9 N*s^2/mm^4

[bruznunez]

I fixed the issue, thank you for your help. Unfortunately, I am still encountering modeling errors that I cannot find a way to solve, and I have the following questions:

1. Should merge be applied to the entire structure, or can the analysis be run with only some merged planes of the structure?

2. When running the analysis, it first provides the analysis information but then displays a message saying, "need to specify more data."

3. In the post-processing visualization, when loading the modal response to observe the vibration modes, only the edge nodes of the slabs show displacements.

4. Does any special interaction need to be applied for wall-to-wall connections?

I would greatly appreciate your help in solving these and any other issues you may find in my 3D model.

[STKO Team]

Sorry, but there are several critical mistakes in the model.
I recommend starting with our webinars to better understand the correct way to approach this type of modeling.

Here are some key issues to address:

• All faces are separated from each other. If a wall is detached from the rest of the building, the model will be singular.
• In the recorder, under custom element results, you entered "section.fiber". This approach hasn't been covered in any of our webinars, and it's incorrect. This is the reason for the error in the console.

Before proceeding, I suggest reviewing the fundamentals to ensure a solid base for your modeling.

[bruznunez]

Could you provide me with any suggestions or a more specific approach to address this problem? I have already reviewed the documentation and webinars beforehand. In none of the videos have I seen this type of modeling based on 3D walls and slabs with shells, only frame-type structures and 3D slabs with shells.

Best regards.

[STKO Team]

Could you provide me with any suggestions or a more specific approach to address this problem? I have already reviewed the documentation and webinars beforehand. In none of the videos have I seen this type of modeling based on 3D walls and slabs with shells, only frame-type structures and 3D slabs with shells.

The concept is the same for lines/surfaces/volumes (and it's all over our webinars).
If 2 geometries are separated (even if their boundaries are coincident) they will not share the same nodes on the boundaries.
So you have 2 options:

• Merge the geoemtries so that they will share the same nodes on their boundaries (continuity)
• Or define and interaction between them, for example when you want to model internal releases or contact conditions