How to apply masses in a N-mm-s unit system - STKO

[bruznunez]

Dear community,

I hope you are all doing well. I would like to ask about the correct way to apply masses to a structural plate system in STKO.
On one hand, I can input a RHO value within the material properties, but I have doubts regarding the units I should consider for this RHO. On the other hand, I know that I can also input masses using facemass. However, I have noticed a difference in the total mass of the system.
Why does this difference occur? What is the correct way to apply mass to plate elements?

Best regards.

Benjamin

[STKO Team]

If you define the mass density in the materials (rho) then it should be a mass density (mass per unit volume).
If you use the FaceMass, it's a mass per unit area (i.e. rho * thickness of the shell).

Whatever you choose, make sure to be consistent with the units you are using!

[bruznunez]

How it works the masstoLoad command in the analysis step?

Does exit any difference between use face masses and face loads in a TH analysis, considering the modal analysis step before the dynamic analysis?

[STKO Team]

How it works the masstoLoad command in the analysis step?

There is not masstoLoad command in STKO/OpenSees

Does exit any difference between use face masses and face loads in a TH analysis, considering the modal analysis step before the dynamic analysis?

A load is a load and a mass is a mass... you should apply both

[bruznunez]

What about the load pattern named "massToLoad"? Maybe it's not a command but it does exist. How it works?

[STKO Team]

It's an automation we make in STKO to simplify generation of mass and self weight for frame elements.

Problem:
In OpenSees, frame elements do not automatically get the mass density from the material. Therefore the user has to specify, for each element, the mass-per-unit-length (i.e. mass density * cross-section-area) either in the Element Property, or in the Mass->EdgeMass.
If you have a model with a lot of different cross sections, this can be tedious.

Solution:
We implemented a Condition->Mass->AutoEdgeMass, where you can specify the mass density (rho) and assign this condition to all frame elements with the same mass density. This automation will automatically process all elements and their cross-section to compute edge_mass = rho * section_area.

Optionally:
You can also check the "Convert to load" checkbox in the Condition->Mass->AutoEdgeMass, and add this in the load pattern under the option you mentioned

[bruznunez]

See i was right