According to Eurocode 8 (earthquakes), the mass participation of the structure must be greater than 95%. Not considering the mass of the basement structure because the seismic action occurs immediately above the ground, I consider it correct, but the truth is that the code nowhere states that this should be the procedure. Thank you CEE, as always your videos are great.
There was a comment by @calypsotravel that is very good in explaining it. "I would turn on disregard densities, separate the selfweights, i.e superstructure selfweights and basement walls as two separate load cases. Then, I would only select the superstructure selfweights and the live load as participating masses. The basement will be excluded for the modal analysis."
what does Eurocode say about scaling of base shear reaction? other codes talks of scaling base shear between static and response spectrum. I don't seem to get any info from Eurocode.
Thank you for such valuable information, I have two questions to ask, I hope you can help me: 1) In ETABS you can show the percentage of rotational modal participation, that is, the percentage of mass that is rotating in the Z axis, my question is, how can we analyze what percentage of mass participates in the rotation of the Z axis in ROBOT 2) In the video it is concluded that the mass above the base level should be separated and the mass below the ground level should be neglected. That said, is it correct to model the foundation together with the superstructure? What would be the considerations when modeling the foundation as a whole, that is, foundation depth, is the design of the concrete columns affected by the soil confinement or not?
Hi there, you are most welcome. in RSA, you can show rotational mass participation by going to the modal mass table and: 1) right click on any column of that table 2) Go to the Table Columns 3) Mass Sums 4) Enable anything you need. Hope it helps. For modeling both foundation and superstructure, the reason why you may want to do this is to get the loads on the foundation itself and design it all using 1 model. You still can separate it into two models. One for the superstructure and one for the substructure. The only hassle might be in load transferring.
Hi CEE Great content as always. Any advice for converting vibration readings using the fast Fourier transform to find the operating frequency range to avoid. Thanks.
Hi there and thnx for your comment. You comment may be understood in two ways, and I will address both. The first way I understood it is: You have a vibrating machine (let's say a sieve or a crusher in an aggregate industrial plant). This machine will of course vibrate as follows (names may be different, but ideas are the same): Transient vibration (this happens when the machine starts up or shuts down) Sustained/ steady state vibration (this is what happens during normal operations) Kind of like your car, when you start the ignition, the car vibrates differently for 2 seconds before it goes to the steady state Now if that is the question, then you can find those information from the manufacturer of the equipment. He is required to provide you with: 1) steady state vibration frequency 2) force amplitude. The second way I understood it is: you have a vibration of unknown source, a fourier analysis is to be done. I am assuming you are not asking about "the math" of fourier, but rather about how to interpret the results of fourier. Interpretting the results is actually easy, if you have the transformation plotted in a power spectral diagram, you will notice spikes at certain frequencies which are the dominant frequencies of the vibration. You an use those frequencies to recreate the vibration you measured using cosines and sines. Note that you will not be able to create it with 100% accuracy because you are only taking "dominant" spikes when recreating and modeling the signal in RSA. I hope I understood your point correctly and addressed it, if there is anything I missed, or if you have further questions, please let me know, you are more than welcome Regards, CEE
Your suggestion not to include the walls is correct. Base shear will be at ground level and not basement level. I would turn on disregard densities, separate the selfweights, i.e superstructure selfweights and basement walls as two separate load cases. Then, I would only select the superstructure selfweights and the live load as participating masses. The basement will be excluded for the modal analysis.
Etabs seems to have great output of results when it comes to seismic analysis. Could you please talk about scaling of base shear reactions between static and response spectrum? What does Eurocode say about this?
Hi there and thnx for your input. I might not have understood what you mean fully (I blame myself for this). So, just for my own understanding, can I ask about what you exactly mean? is it basically that a dynamic / response spectrum analysis = factor x static analysis?
@@CivilEngineeringEssentials there is a requirement in other codes like ASCE to match base reactions due to equivalent static method and linear dynamic method like response spectrum method. I can't find it in Eurocode.
Hi dear CEE outstanding as always, trying to implement your knwoledge into my own projects i may ask, how would you implement mononobe okabe method (for seismic forces) to design retaining basement walls in rsap? Thank you so much! (When you reach 100k subs remember that i was one of the first ones haha!)
You are most welcome. Sorry for being late with my reply "again". As for the M O method, I only marginally read about it. But I will give it a try. And yes, dear Engr. Jesus, you are an OG sub ^_^
Hi again Dear CEE have one more question how could you run a calculation with only compression springs (non linearity?) how do you define their kh modulus? Thank you so much!
Hi there. You can do this by going to: Geometry -> Supports Then go to linear support tabe There, click on new then, you can remove the Ux direction (for example) but enable uplift, and tell him which direction. (positive or negative) I think this is worth a video really. Thank you very much for giving me the idea ^_^
Therefore, according to general opinion, the steps to follow are: 1) Create a dead load for the weight of the structure of the slabs, beams and pillars above level 0. 2) Click Checkbox "Disregard Density" 3) Perform modal analysis 4) Check mass participation ensuring 95% OK. You know, it's easy to follow this reasoning but then I question whether this is the correct procedure and that's where insecurity comes in, even more so when this doesn't appear anywhere in the EUROCODE as being permissible. Thanks to the CEE channel. Greetings to all participants
Eurocode and even american codes do not talk a lot about mass participation. The only thing you hear is some heresay about xx% being "acceptable". Thus, I believe that you can do something like this only if you can explain it via sound engineering logic. In my humble opinion, since the mass underground is not expected to vibrate, you may get away with this. The other option is of course to go for hundreds of modes. Actually, I want to thank all the dear engineers for their lively discussion. I feel that a community is building here. I am really honored.
According to Eurocode 8 (earthquakes), the mass participation of the structure must be greater than 95%. Not considering the mass of the basement structure because the seismic action occurs immediately above the ground, I consider it correct, but the truth is that the code nowhere states that this should be the procedure. Thank you CEE, as always your videos are great.
Hi there Engr. Pedroso and thank you for the model. 🌹
I am happy that the vid was beneficial.
You can still increase the number of modes in that case.
@@CivilEngineeringEssentials one question how you separate the Wall mass and columns in the basement from the rest of the structure?
There was a comment by @calypsotravel that is very good in explaining it.
"I would turn on disregard densities, separate the selfweights, i.e superstructure selfweights and basement walls as two separate load cases. Then, I would only select the superstructure selfweights and the live load as participating masses. The basement will be excluded for the modal analysis."
what does Eurocode say about scaling of base shear reaction? other codes talks of scaling base shear between static and response spectrum. I don't seem to get any info from Eurocode.
@@calvoh9715 me too. This subject for me is very strange.
Thank you for such valuable information, I have two questions to ask, I hope you can help me:
1) In ETABS you can show the percentage of rotational modal participation, that is, the percentage of mass that is rotating in the Z axis, my question is, how can we analyze what percentage of mass participates in the rotation of the Z axis in ROBOT
2) In the video it is concluded that the mass above the base level should be separated and the mass below the ground level should be neglected. That said, is it correct to model the foundation together with the superstructure? What would be the considerations when modeling the foundation as a whole, that is, foundation depth, is the design of the concrete columns affected by the soil confinement or not?
Hi there, you are most welcome.
in RSA, you can show rotational mass participation by going to the modal mass table and:
1) right click on any column of that table
2) Go to the Table Columns
3) Mass Sums
4) Enable anything you need.
Hope it helps.
For modeling both foundation and superstructure, the reason why you may want to do this is to get the loads on the foundation itself and design it all using 1 model.
You still can separate it into two models. One for the superstructure and one for the substructure. The only hassle might be in load transferring.
Hi CEE
Great content as always.
Any advice for converting vibration readings using the fast Fourier transform to find the operating frequency range to avoid.
Thanks.
Hi there and thnx for your comment.
You comment may be understood in two ways, and I will address both.
The first way I understood it is:
You have a vibrating machine (let's say a sieve or a crusher in an aggregate industrial plant). This machine will of course vibrate as follows (names may be different, but ideas are the same):
Transient vibration (this happens when the machine starts up or shuts down)
Sustained/ steady state vibration (this is what happens during normal operations)
Kind of like your car, when you start the ignition, the car vibrates differently for 2 seconds before it goes to the steady state
Now if that is the question, then you can find those information from the manufacturer of the equipment. He is required to provide you with:
1) steady state vibration frequency
2) force amplitude.
The second way I understood it is:
you have a vibration of unknown source, a fourier analysis is to be done. I am assuming you are not asking about "the math" of fourier, but rather about how to interpret the results of fourier.
Interpretting the results is actually easy, if you have the transformation plotted in a power spectral diagram, you will notice spikes at certain frequencies which are the dominant frequencies of the vibration.
You an use those frequencies to recreate the vibration you measured using cosines and sines. Note that you will not be able to create it with 100% accuracy because you are only taking "dominant" spikes when recreating and modeling the signal in RSA.
I hope I understood your point correctly and addressed it,
if there is anything I missed, or if you have further questions, please let me know, you are more than welcome
Regards,
CEE
Your suggestion not to include the walls is correct. Base shear will be at ground level and not basement level. I would turn on disregard densities, separate the selfweights, i.e superstructure selfweights and basement walls as two separate load cases. Then, I would only select the superstructure selfweights and the live load as participating masses. The basement will be excluded for the modal analysis.
Thnx a million🌹
That's a very important point. I slipped my mind talking about seismic action.
Etabs seems to have great output of results when it comes to seismic analysis. Could you please talk about scaling of base shear reactions between static and response spectrum? What does Eurocode say about this?
Hi there and thnx for your input.
I might not have understood what you mean fully (I blame myself for this). So, just for my own understanding, can I ask about what you exactly mean?
is it basically that a dynamic / response spectrum analysis = factor x static analysis?
@@CivilEngineeringEssentials there is a requirement in other codes like ASCE to match base reactions due to equivalent static method and linear dynamic method like response spectrum method. I can't find it in Eurocode.
Hi dear CEE outstanding as always, trying to implement your knwoledge into my own projects i may ask, how would you implement mononobe okabe method (for seismic forces) to design retaining basement walls in rsap? Thank you so much! (When you reach 100k subs remember that i was one of the first ones haha!)
You are most welcome. Sorry for being late with my reply "again". As for the M O method, I only marginally read about it. But I will give it a try.
And yes, dear Engr. Jesus, you are an OG sub ^_^
Hi again Dear CEE have one more question how could you run a calculation with only compression springs (non linearity?) how do you define their kh modulus? Thank you so much!
Hi there.
You can do this by going to:
Geometry -> Supports
Then go to linear support tabe
There, click on new
then, you can remove the Ux direction (for example) but enable uplift, and tell him which direction. (positive or negative)
I think this is worth a video really. Thank you very much for giving me the idea ^_^
If selfweight is already included in modal analysis.
I think you should activiate the (disregard density) option.
You are 100% right. Thnx for pointing it out.
Therefore, according to general opinion, the steps to follow are:
1) Create a dead load for the weight of the structure of the slabs, beams and pillars above level 0.
2) Click Checkbox "Disregard Density"
3) Perform modal analysis
4) Check mass participation ensuring 95%
OK.
You know, it's easy to follow this reasoning but then I question whether this is the correct procedure and that's where insecurity comes in, even more so when this doesn't appear anywhere in the EUROCODE as being permissible.
Thanks to the CEE channel. Greetings to all participants
Eurocode and even american codes do not talk a lot about mass participation. The only thing you hear is some heresay about xx% being "acceptable". Thus, I believe that you can do something like this only if you can explain it via sound engineering logic.
In my humble opinion, since the mass underground is not expected to vibrate, you may get away with this.
The other option is of course to go for hundreds of modes.
Actually, I want to thank all the dear engineers for their lively discussion. I feel that a community is building here. I am really honored.
@@CivilEngineeringEssentials waiting for the next subject video. Greetings