×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Are you an
Engineering professional?
Join Eng-Tips Forums!
• Talk With Other Members
• Be Notified Of Responses
• Keyword Search
Favorite Forums
• Automated Signatures
• Best Of All, It's Free!

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

#### Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

# Problems with mixed mesh: plates/beams

## Problems with mixed mesh: plates/beams

(OP)
Hi all
I'm having a problem with a mixed mesh composed by beams and plates for a static analysis. The system is well constrained but I receive errors typical of meshes with mechanisms, i.e. high pivot ratios.

The structure has a (qualitative) shape:

------------------------------------------------
|  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
------------------------------------------------

where vertical signs are beams and horizontal signs are plates (obviously the structure is 3D). I have tried to turn the parameter K6ROT on and off, and I have varied it 1.E-5, 1, 1.E+5 but nothing changes. AUTOSPC is turned on.
The plaets are parabolic CTRIA and using CTRIAR do not change the result.

I have tried modeling a small portion and have tried a modal analyis and I can see beams rotating "freely", although the K6Rot should avoid that.

When I tried to give
PARAM,BAILOUT,-1
the analysis run without further problems and the result was reasonable. I don't want to use rigid elements around the beams beacuse the beam pattern is very complex and too many meshing efforts would be needed. Unless there is no way to adjust this analysis.

Hint: There is a part of the model were other plates of larger dimensions are present; may the high pivot ratio be due to big stiffness differences between these two regions?

### RE: Problems with mixed mesh: plates/beams

the beams are 2d and the plates are 3d? or the other way around?

in both cases, you need an overlap of shell-on-solid elements to avoind a mechanism effect.

### RE: Problems with mixed mesh: plates/beams

(OP)
Hi

the structure is 3D, I only sketched an idea here. Imagine two disks meshed with plates which are kept together by those beams. The only dof which is free is RZ for plates, that is RX for beams; for this reason I expected that K6ROT would be enough. Beam torsion is NOT a load path, so I'm not scared of using K6ROT, but I would like to know what else I can't see here is causing the errors.

### RE: Problems with mixed mesh: plates/beams

Just as a check, use a low-stiffness beam in the plane of the plates connecting the end of each beam to the plate at a node adjacent to where the main beam intersects the plate.  Generally, this is called a "spider" and is used to handle the DOF issues.

I understand k6rot was perhaps designed to handle this, but this would at least tell you if it is working or not.

### RE: Problems with mixed mesh: plates/beams

Hi

Just as a simple check, run a eigenfrequency solution for the structure. If you get results with modes near 0.0 Hz you have free-free conditions but you will get a plot of what is it si that is free.

The limitation to this is that if you have many modes that are zero the solver probably won't give any results at all.

But it might be worth a try.

Good Luck

Thomas

### RE: Problems with mixed mesh: plates/beams

(OP)

@ GBor: Your solution is the best for sure; but I am operating on a complex unstructured meh, where the beams (more than 1000) are positioned on a complex patter: to add a spider I should remodel the whole structure to position support nodes in a proper way; or I should add a thousand of spyders by hand. I wanted to reach a solution which did not require heavy manipulation of the model.

@ThomasH: I've already run a modale test on a small portion of the model and the problem seems to be exactly what I suspected: I get free (or almost free = 10^-8 and 10^-5 Hz) rotations on the two beams I've kept.

#### Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

#### Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

#### Resources

Research Report - How Engineers are Using Remote Access
Remote access enables engineers to work from anywhere provided they have an internet connection. We surveyed our audience of engineers, designers and product managers to learn how they use remote access within their organizations. We wanted to know which industries have adopted remote access, which software they are using, and what features matter most. Download Now
eBook - Managing the Context of Product Complexity Using the Digital Twin
Keeping track of changes to complex products is difficultâ€”think Aerospace & Defense equipment, new generations of commercial aircraft, and software-based automobiles. A new way to managing the digital context of the physical product is required and the answer is the Digital Twin. This ebook explores the opportunity available for Operations and Maintenance for the Digital Twin. Download Now
White Paper - Trends in Industrial Filtration
Substantial progress has been made in filtration technologies in recent years. New filter media materials, designs and processes have led to filters that are more efficient, reliable, compact and longer lasting. This white paper will discuss the various trends that are impacting operational responsibilities of MROs today and the resources that are available for staying up-to-date on the latest filtration solutions. Download Now

Close Box

# Join Eng-Tips® Today!

Join your peers on the Internet's largest technical engineering professional community.
It's easy to join and it's free.

Here's Why Members Love Eng-Tips Forums:

• Talk To Other Members
• Notification Of Responses To Questions
• Favorite Forums One Click Access
• Keyword Search Of All Posts, And More...

Register now while it's still free!