Understanding Modal Analysis and Harmonics in Autodesk Robot
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- เผยแพร่เมื่อ 10 ก.ค. 2024
- In this video, we'll explore the basics of modal analysis and harmonics in Autodesk Robot. We'll cover the fundamentals of creating a structural model in Autodesk Robot, including how to define the properties of the structure and how to apply loads and boundary conditions.
We'll start by discussing the importance of modal analysis and harmonics in the design process and how they can help engineers understand the behavior of structures under dynamic loads. We'll then dive into the specifics of creating a structural model for a simplified structure, including how to define the properties of the structure, how to create the different sections of the structure, and how to connect them together.
Next, we'll provide an overview of modal analysis and harmonics, including what they are, how they work, and why they are important. We'll cover the basics of defining the modes of vibration, including how to specify the natural frequencies, how to define the mode shapes, and how to apply the loads to the different sections of the structure.
Finally, we'll discuss the importance of interpreting the results of the analysis and how they can be used to improve the design of the structure. We'll provide practical tips and guidelines for designing structures with modal analysis and harmonics for a given application.
Whether you're a student, a practicing engineer, or simply interested in the behavior of structures, this video will provide you with valuable insights into understanding modal analysis and harmonics in Autodesk Robot using a beginner's guide.
Great Idea from Johan Salehi
... Regarding the weird modal shapes, I actually compare them using the mass participation ratio that is shown in the modal analysis table. The higher the number, the more important it is, ...
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Timestamps:
0:00 Introduction
0:34 Simple Column Structure
2:23 Setting Up the Analysis
5:13 Calculations
6:00 What are Modes (Eigenvalues)
8:16 Results Discussions
11:53 Harmonic Analysis
14:00 Effect of Freq.
17:00 A look on a full structure
23:00 Outro
Hello CEE, I am very excited to hear your old self energic voice again 😊, it is good to have you back.
I have done some browsing (quick shallow dives) after watching your video and I have found that this is one hell of a topic (in a good way of course). Lucky for me (and maybe for others), there is the CEE to help navigate through and there is RSAP to munch the mathematics of dynamic equations and systems. Thank you for the reference (A first course in the FE method).
Your introduction is both intuitive and enticing. Your comparative demonstration of the structure response due to static load case and due to a zero hertz (almost zero) frequency input loading and due to the same input loading at frequencies close, same (almost), and above the natural frequency of the mode shape of the structure quickly gives a glimpse of some practical applications of modal analysis [thank you for this].
I am looking forward to the CEE practical and illustrative models and examples (structural engineering structures) on the uses of modal analysis (in RSAP) and the interpretation of the results. During my shallow dives, I have come across the following possible uses of modal analysis which I am hoping that CEE may include on some of the planned future videos:
• Deciding which rotational speeds are dangerous for the structure - I am hoping for an example or illustrations of modelling and design of foundations for reciprocating machines (in RSAP).
• Where (locations in the structure) and how to increase or to decrease the frequency of a mode (of a structure). What are the practical examples and uses in structural engineering (in RSAP)?
The above are ideas only; I have no doubt that the CEE already have even more ideas and have a plan [after all it is the CEE that have introduced this topic].
I love energy and the humour that is all over this tutorial [there no doubt that you enjoy the topic]. Cool swing by the way 😊, the swing is a perfect choice for intuitive visualization of this concept. I could not stop laughing at the end when you said “….the higher the modes get….mode 10….I don’t know like now after that I have no words to describe this …😂🤣😂”.
Thanks a lot! yeah, being ill is not a funny business
Sure, I am unable to compress all dynamics theory in one video, would be too much. But I keep giving hints and intriguing thoughts for my viewers.
As for machinery rotation speed, I tried, but a bug in this video has stopped me, I still addressed your request in the video (is going to be available today) and will check that out in detail later.
Oooooh question 2 (locations to control modes and natural frequency) is an amaaaaaaaazing question. It is part of my PhD!!! I will keep it in mind and write it in my list as "masses and stiffnesses to control modes". It is a long story, but it is indeed an amazing one.
As for mode 10 and beyond, like: what are we even doing here?
It's kinda like DBZ and DBS where the only idea to powerup a character is to give him a transformation ^_^
Stay tuned, Regards,
CEE
It’s great to have you back CEE
This tutorial is more than beneficial to me since structural dynamics is also close to my heart just as it is to your heart ❤️. I hope to learn more from you sir.
I love the see-saw approach to understanding dynamics and the simple harmonics of a simple bar. That was insightful. Many Thanks.
These are a few observations.
*Model 1* The mode shapes and modal periods of the simple column makes lots of sense, the first 2 fundamental mode shapes are all purely translational with higher modes coupled with rotation along either the x or y direction of the columns.
The number of times the vibrating column crosses its original at rest position corresponds to the mode number and mode shape of the column, which makes soo much sense.
For instance the 3rd mode shape in the weaker Y direction of the column crosses the column along it height 3 times with 3 points along the column height.
I was happy to see this.
*Model 2* The confusion.
For a typical and regular building with uniform mass and stiffness for model 2 however, I found that only the first 3 mode shapes make sense. The rest of the mode shapes seemed off. I actually expected the modal behavior of a regular building in this case to behave in the same way as the regular column, most especially the higher modes shapes.
For instance, the 3rd mode shape in the weaker y direction of the building is expected to cross and have 3 points along the building height in the Y direction.
Which is not the case.
I wish to see in this tutorial series the dynamics report on this building.(most especially the modal participating mass ratios for each mode, the 2D mode shapes to understand the mode shapes of the higher modes etc)
Overall this tutorial was awesome one. I love ❤️ insight and awareness you always bring with your tutorials.
Thank you CEE
I look forward.
Hello there, thank you very much for your comment.
Model 2 indeed needs a deeper look. I added a small discussion to it yet again in the video that will be published today.
But a quick hint:
It has to do with the stiffness of the beams.
I am glad you like dynamics like I do. It is really amazing to see how structures behave dynamically.
As for dynamics reports, I will add it to today's video ideas under: more details about modal participation / report? point.
Stay tuned,
CEE
Thank you very much
You are most welcome
Stay tuned for more content.
Thank you for providing such a clear explanation of what modal analysis is. Regarding the weird modal shapes, I actually compare them using the mass participation ratio that is shown in the modal analysis table. The higher the number, the more important it is, as you said, just the few first rows are the most important one for our calculations.
Most welcome. I totally agree with mass participation. it slipped my mind to mention it in the video. Thus, a most welcomed comment. It helps the other viewers take note to that great idea.
I will even modify my video description with your hint.
Stay tuned for future videos, and consider helping the channel by suggesting it to your colleagues.
Regards,
CEE
it was complex before, you made it easy. Thank you.
You are most welcome,
possible, please share this channel with your friends. It helps a lot.
Regards,
CEE
@@CivilEngineeringEssentials sure, doing so.
It all make sense now to me, tks alot, it cost me a lot of time just to understand it.
Thank you for your comment .I am happy it helped ^_^
Please support the channel by suggesting it to your friends.
Regards,
CEE
Thanks for your sharing! this VDO is very helpful for young Engineer like me.
Thank you for your comment. I am happy you liked it and it was helpful.
Stay tuned for more. And recommend this channel to your friends.
Regards,
CEE
Incredible CEE tutorial!
I have a straightforward query regarding modal analysis: Do all modes of deformation occur simultaneously or independently?
In a real structure, the vibration of a structure is a mixture (or superposition) of all modes. Depending on the location of the force, you can "Excite" one mode more than the other, so that it becomes the "dominant" mode.
For example, imagine you see a swing and a kid inside it.
If you push the swing near its bottom end, the swing will move in its first mode "just a simply back and forth movement"
However, if you push the swing somewhere in the middle, you will see the second mode.
Note here, we actually intentionally pushed a swing at specific locations to get a purer form of vibration (for each mode)
In reality, your forces are arbitrary and stochastic in nature, which provides you with a mixture of modes.
Hope it helps,
Regards,
CEE
Great !
I am happy you liked it ^_^
Excellent class, I will check the standards in my country, so that I can take the same example!
I am happy you liked it. Stay tuned for more content like this.
Thanks for the explanation - it really helps to understand what's happening. As well as checking the deflection of the member, can you also assess the stresses in the member with RSA due to the vibration as part of a codified design check?
Hi there,
I am happy your found the video helpful.
Now I am assuming that by "codified design check" you mean the verification of members conforming to design codes such as the ACI code and the AISC code.
Ok so short answer: you need a time-history analysis.
Long answer:
Please bare with me.
The modal analysis does not provide you with stresses to be used in the design of elements. I know this might sound insane, but it is. The only thing a modal analysis does, is to predict how a structure would vibrate (resonance) and the frequency of this vibration.
Now how would this be helpful in the design? you might ask.
You would need to know both the frequency and amplitude of the force to be able to asses the stresses in the structure.
Now you would think: wait a minute, the harmonic analysis does define the frequency, right? so shouldn't I be able to use it in the design?
Unfortunately, the answer is still no. The reason behind this is because you still need to define a magnitude of the force to assess the stresses correctly.
This is where the time history comes into play. Time history allows you to define a certain force (a magnitude) that may fluctuate however you see fit. you can even let it fluctuate harmonically.
This is something you could use in the design, and there is a video introducing this on this channel.
Sorry for the lengthy reply, but I wanted to try give the best possible explanation for your question.
Regards,
CEE
@@CivilEngineeringEssentials Thanks for the reply. Yes, I'm in the UK, so I meant a design check to EC2. The modal analysis seems academic, for consultants, we want to understand how much additional rebar (or the size of steel section) we need to provide for a given load and frequency. I'll check out your time history video, it seems like this is what I need. Thanks again.
Very interesting video with the concepts explained well. I would be interested in a video relating to the design of a beam or floor supporting machinery! Eg. A crusher. We are currently busy with a similar project for one of the South African mines.
Hi there and thnx for your comment,
I think that is a really nice idea, I will try cover it once I finish my RC design series that I am currently doing on this channel.
But here is a heads up, to investigate the effect of machinery on floors, there are two types possible:
1) A machine producing harmonic forces: (this video would help): Like a motor or generator where rotational motion is expected, that translates into a vibration. Here, the frequency of vibration is the RPM of the rotation.
2) A machine producing transient/non harmonic forces: here a time history analysis would be more appropriate. I have a video on this channel talking about it.
I hope those videos help you for the time being.
Stay tuned for more content,
CEE
Very nice! Keep it coming.
Of course. Stay tuned for more.
Help the channel by subscribing and encouraging your friends to subscribe to the channel.
Very good!! Keep up the good work!
Most welcome,
Stay tuned for more content ^_^
Regards,
CEE
Thanks a lot
You are most welcome🌹👍
Excelent video!
thnx a lot ^_^ stay tuned for more content.
Hi Sir Many thanks for your videos. I started watching from retaining walls, and now i'm stacked with your channel. A question Sir please
I want to do Static analysis, and want to Access the rotation of nodes(Rx,Ry,Rz) for a continuous beam to see if the results IS same as my theoritical results. Does RSA permits us to see those results ?
Thanks in advance
Boris
You are most welcome.
Yes, RSA allows you to see all deflections and rotations for any node in the structure.
To see it, after analysis, go to "Results Menu" then "Displacement"
Regards,
CEE
Good mode work🎵
Thanks. Stay tuned for more.
Help the channel by subscribing and encouraging your friends to subscribe to the channel.
Hi Sir;
I've a structural building with both static and dynamic loads. One level of the story is subjected to vibration(dynamic loads). The rest of the building is static load. From what i see if as if we can't do Modal analysis for a part of a building??. So for a person that has static and dynamic loads in the building, does he has do remodel the dymaic structure serparately??
Hi there,
First of all, you have to apply the dynamic load using the full model, because the full structure will resist the dynamic loading.
You do not need two models to achieve this. You can have a load case where your apply the dynamic loads, and another load case where you apply static loadings.
Sometimes you can even make combinations our of those.
I hope it helped,
CEE
Hi! Your posted videos are very constructive. I wish you would discuss why a modal analysis of a structure will not give 90% and more participation of masses under what Circumstances
Hello there,
definitely a good idea. I will post me a video (maybe next one of after that) about it. Stay tuned for more content.
Hi CEE, I work at a large consulting firm based in South Africa. Is there a way to connect you directly to assist with Modal analysis and Harmonic in Autodesk Robot?
Hi there. Thnx a lot. Yes, I am available for that, and you can contact me via email:
civ.eng.essentials@gmail.com
Regards,
CEE
Thank you for this and other video! ☺️