Questions on Mass/Gravity Definition and Static Analysis for a Shaft Model in STKO

[liushun]

Hello STKO team,

I hope this message finds you well.

Recently, I have been investigating the reasons behind my model's unsuccessful runs. This time, I've primarily focused on correcting issues related to structural loads and mass definitions. Following your previous suggestions - and admittedly due to my own oversight in not thoroughly verifying these calculations earlier - I have now performed the following calculations for face mass, face force, volume mass, and volume force:

Face force = rho× g × S × t (where g represents gravitational acceleration, S denotes the shell area, and t is the shell thickness)

Volume force = rho × g × V (where V represents the total volume of the solid structure)

For mass definitions, I understand that gravitational acceleration (g) should not be included in the calculation.

However, for solid elements, I noticed that it's possible to directly apply body forces by inputting -rho × g in the brick element properties. In this case, is it still necessary to separately define the mass?

Similarly, if the soil is modeled using standard brick elements instead of brick u-p elements, which I believe cannot automatically account for self-weight, does this mean I need to apply additional loads (either through element properties or volume forces)? In such cases, is it also necessary to define volume mass for the soil?

I must sincerely apologize for the numerous inquiry posts I have submitted in this thread. It's impossible to overlook the fact that my questions have now accumulated to the point of spanning from the first page to the second. As a relative novice in this field, I have encountered many challenges throughout this process. I realize that my questions in this thread might have become somewhat disorganized by now, and I genuinely hope you can patiently review and understand the various problems I've been facing recently.

Any suggestions you might have would be of tremendous help to me. I look forward to receiving your reply.

[STKO Team]

Regarding the model you sent in the previous post:
The interaction for contact elements is node-to-node, and the ElementPropery thus requires orientation from Local Axes.
In this case, it's up to the user to make sure that every contact interaction has a proper local axes, where the local z is the contact direction (from slave node to master node).
Otherwise you can use the node-to-element interaction for an automatic calculation. In this regard I'd suggest you to have a look at the webinar about the zeroLengthContactASDimplex element.

Face force = rho× g × S × t (where g represents gravitational acceleration, S denotes the shell area, and t is the shell thickness)

Volume force = rho × g × V (where V represents the total volume of the solid structure)

This is wrong.
Face force is a Force per unit area, so: rho*g * t (which is force-per-unit-volume times length -> mass per unit area)
Volume force is a force per unit volume (self-weight) (so just rho*g)

[liushun]

Hello STKO team,

Thank you very much for your continued responses. I would like to express my sincere gratitude once again.

In this regard I'd suggest you to have a look at the webinar about the zeroLengthContactASDimplex element.

I have carefully reviewed the webinar on ZeroLengthContactASDimplex and have made corresponding modifications to my model. Specifically, I changed the connection method from node-to-node to node-to-element and adjusted the mass and load applications on the structure. However, during execution, I continue to encounter convergence issues. The model has not even successfully completed the gravity analysis. I have reattached the recently revised model file and would be extremely grateful if you could help analyze the problems.

Additionally, my model involves groundwater conditions, but I have not applied any specific treatment to the soil-structure interface surfaces. I am uncertain whether my current approach is correct and would appreciate your guidance on this matter.

The second model file is intended for verifying geostatic stress balance. Upon examining the post-analysis results, I noticed potential errors, and I would greatly appreciate your assistance in addressing them. Additionally, could you please provide some guidance on how to determine whether geostatic stress balance has been successfully achieved? During this process, I also observed that applying volume forces directly to elements yields better convergence compared to applying loads separately. The latter loading method fails to execute successfully, and I have been unable to identify the specific cause. I would be grateful if you could help analyze this issue as well.

Finally, I would like to ask one more question regarding result data: in a 3D model, when a single node is shared by eight elements, how should the stress value at this node be accurately determined?

I have reattached my recently revised model file to this email for your reference.

Thank you once again for your assistance. I look forward to your reply.

[liushun]

Hello STKO team,

I hope this message finds you well.

I am writing to follow up on the issues I previously mentioned, which I have been actively working to resolve. Unfortunately, despite my continued efforts, I have not yet been able to find a satisfactory solution to these challenges. These unresolved matters are still hindering my progress, and I would greatly appreciate any additional guidance or support you might be able to provide.

Thank you for your time and help. I look forward to your reply.

[STKO Team]

Dear user, please share your file (otherwise it's impossible to tell where the issue comes from).
Furthermore, try to address 1 issue at a time, otherwise it's difficult for us to follow the post.

[liushun]

Hello STKO team,

Thank you very much for your continued support and feedback. I truly appreciate your assistance.

Following your suggestion, I have selected one of the most relevant issues from our previous discussions for this thread, while moving other questions to separate posts to ensure each thread remains focused and targeted.

Currently, I am working on the analysis of a Soil-Structure Interaction (SSI) model. As per your earlier recommendation, I used ZeroLengthContactASDimplex (node-to-element) to connect the soil and the structure. I also adjusted the mass and load applications on the structure. However, I am still encountering convergence issues during the analysis—the model fails to complete even the gravity analysis successfully. I have reattached the model file and would greatly appreciate it if you could help identify the potential problems.

It is worth noting that the model includes groundwater conditions, but I have not applied any specific treatment to the soil-structure interface (e.g., defining whether the interface is drained or undrained). I am uncertain whether this omission could be contributing to the convergence issues.

I would be extremely grateful if you could provide guidance on how to resolve the convergence problems in this model.

Thank you once again for your time and support. I look forward to your reply.

[liushun]

Hello STKO team,

I hope this message finds you well.

I have recently studied the relevant content from your webinars and have prepared a revised version of my model that appears more reasonable. However, the convergence issues persist unresolved. I would greatly appreciate it if you could help analyze the potential errors in the model.

It is worth noting that the case study in the webinar does not seem to include groundwater conditions, which represents a difference from my model. Additionally, I should mention that I have not applied any specific treatment to the soil-structure interface (such as defining drainage conditions).

Thank you very much for your assistance. I look forward to your reply.