Setting up a Seismic analysis model

[Anonymous]

Hello!

I am setting up a seismic modal response spectrum model following your youtube tutorial. I have some questions...!

Following the modal analysis I was struggling to get 90% mass participation, even using 200 modes, is this normal? Seems a large number compared to the example you were showing in the tutorial. I wondered if it was a symptom of a mistake? Additionally my minimum frequency is less than 5Hz, and this does not change with increased mode numbers – see table extracts below.

I am struggling to understand the difference between consistent, lumped with rotations and lumped without rotations – how much does matter which I choose and how will it affect my results?

I am not sure whether I should be choosing Modal or Seismic analysis? What difference will this make to my results. It does not seem to affect the mass participation, yet I read somewhere that if I choose the seismic option it should increase the number of modes automatically to reach 90% participation. This does not seem to happen? 

Which iteration should I choose? Subspace iteration is the default, I saw on the tutotial that Block Lanczos is faster, yet it was not used in the example – why is this? And what is the difference between Block Lanczos and Lanczos?

If I have missing eigenvalues but I choose ‘Yes’ to continue does this resolve the issue? Is this anything to worry about?

I have chosen Newmark Groups 1 and 2, I assume the groups refer to X and Y? I do not need to include Z direction for this building according to Cl 4.3.3.5.2 (1).

Does it matter if the main mode is the same for X and Y?

I have set up my slabs as shell elements, would I gain any benefit from setting them up as rigid diaphragms? The youtube tutorial has unmeshed slabs in at each level - are these rigid links? Does this not mean the masses are not taken into account? Extract from tutorial:

Thanks,

Rosie

[Anonymous]

Hello!

I am setting up a seismic modal response spectrum model following your youtube tutorial. I have some questions...!

Following the modal analysis I was struggling to get 90% mass participation, even using 200 modes, is this normal? Seems a large number compared to the example you were showing in the tutorial. I wondered if it was a symptom of a mistake? Additionally my minimum frequency is less than 5Hz, and this does not change with increased mode numbers – see table extracts below.

I am struggling to understand the difference between consistent, lumped with rotations and lumped without rotations – how much does matter which I choose and how will it affect my results?

I am not sure whether I should be choosing Modal or Seismic analysis? What difference will this make to my results. It does not seem to affect the mass participation, yet I read somewhere that if I choose the seismic option it should increase the number of modes automatically to reach 90% participation. This does not seem to happen? 

Which iteration should I choose? Subspace iteration is the default, I saw on the tutotial that Block Lanczos is faster, yet it was not used in the example – why is this? And what is the difference between Block Lanczos and Lanczos?

If I have missing eigenvalues but I choose ‘Yes’ to continue does this resolve the issue? Is this anything to worry about?

I have chosen Newmark Groups 1 and 2, I assume the groups refer to X and Y? I do not need to include Z direction for this building according to Cl 4.3.3.5.2 (1).

Does it matter if the main mode is the same for X and Y?

I have set up my slabs as shell elements, would I gain any benefit from setting them up as rigid diaphragms? The youtube tutorial has unmeshed slabs in at each level - are these rigid links? Does this not mean the masses are not taken into account? Extract from tutorial:

Thanks,

Rosie

[Artur.Kosakowski]

Please watch the recording of the 7th Robot webinar on this topic available from:

http://forums.autodesk.com/t5/robot-structural-analysis-forum/robot-structural-analysis-webinars/td-...

If you find your post answered press the Accept as Solution button please. This will help other users to find solutions much faster. Thank you.

[Anonymous]

Hi Artur,

Thank you for responding. Yes I have watched this webinar several times! It was extremely helpful in terms of setting up my modal but it does not quite answer my questions on the above post. Additionally I am running into issues running the model - I cannot achieve 90% mass participation, even selecting the Seismic option which I believe mean that Robot should do it automatically. I am not sure if there is something wrong with my model?

Please could you also look at my other questions on the above post. 

Many Thanks,

Rosie

[Artur.Kosakowski · Accepted solution]

Following the modal analysis I was struggling to get 90% mass participation, even using 200 modes, is this normal?

It may happen that have many local vibrations with similar frequencies which don't contribute to increase of the mass participation factors. The solution for such situation was presented during the webinar:

Additionally my minimum frequency is less than 5Hz,

The point is that you should reach the mode which eigenfrequency is above 5 Hz

I am struggling to understand the difference between consistent, lumped with rotations and lumped without rotations

See the slide from the webinar:

I am not sure whether I should be choosing Modal or Seismic analysis?

You can select any of them. The point is to reach the targeted values of participating masses. The results will be the same.

Which iteration should I choose? Subspace iteration is the default, I saw on the tutotial that Block Lanczos is faster, yet it was not used in the example – why is this?

Some methods are not available for certain selection of parameters of modal analysis. In such case the program will automatically switch to the 'right' method. 

And what is the difference between Block Lanczos and Lanczos?

Memory management.

If I have missing eigenvalues but I choose ‘Yes’ to continue does this resolve the issue? Is this anything to worry about?

Not for seismic analysis provided you reached the targeted mass participation factor.

I do not need to include Z direction for this building according to Cl 4.3.3.5.2 (1).

Then you need only 10 modes

Does it matter if the main mode is the same for X and Y?

If the current masses for it are the highest for both directions then no

I have set up my slabs as shell elements, would I gain any benefit from setting them up as rigid diaphragms?

Smaller size of a model and less modes required especially when you have local vibrations of floors. Mind that this can be used only when you can disregard Z direction and you don't need results for floors.

The youtube tutorial has unmeshed slabs in at each level - are these rigid links?

Correct 

Does this not mean the masses are not taken into account?

They are.

If you find your post answered press the Accept as Solution button please. This will help other users to find solutions much faster. Thank you.

[Anonymous]

Hi Artur,

Thank you for responding, this was very helpful. I have a few more clarifications to ask of you…!

R: Following the modal analysis I was struggling to get 90% mass participation, even using 200 modes, is this normal?

A: It may happen that have many local vibrations with similar frequencies which don't contribute to increase of the mass participation factors. The solution for such situation was presented during the webinar:

Yes following the webinar I have tried to use the Residual Mode function and I still do not get 90% participation. In fact it did not change my results at all. I have also been reading your website and it seems that this function is only permitted in French codes, is this not included in EC8? What is the implication of using this Residual Mode function in terms of my results? 

R: I am struggling to understand the difference between consistent, lumped with rotations and lumped without rotations

A: See the slide from the webinar:

How will the choice of mass matrix affect my results? i.e. how can I decide what is appropriate for my building?

R: I do not need to include Z direction for this building according to Cl 4.3.3.5.2 (1).

A: Then you need only 10 modes

Is this image you show here from my model? I am unable to reach 90% mass participation at 99 modes but yet you reach it at 10 modes?! What is the difference in how I am running this model?

R: I have set up my slabs as shell elements, would I gain any benefit from setting them up as rigid diaphragms?

A: Smaller size of a model and less modes required especially when you have local vibrations of floors. Mind that this can be used only when you can disregard Z direction and you don't need results for floors.

When you say less modes required – does this mean it would help me to get the 90% mass participation if I set up my slabs as rigid diaphragms instead?

R: The youtube tutorial has unmeshed slabs in at each level - are these rigid links?

A: Correct 

R: Does this not mean the masses are not taken into account?

A: They are.

What is the benefit of rigid links vs rigid diaphragm?

Many Thanks,

Rosie

[Anonymous]

Hi again Artur,

I have managed to get 90% mass participation now by modelling the slabs as rigid diaphragms instead of shell elements. I am now struggling to get the Total Uz Mass to equal the sum of reactions converted to kg. In addition to the questions above regarding model set up, do you have any tips on resolving this? 

Thanks,

Rosie

[Artur.Kosakowski]

R: Following the modal analysis I was struggling to get 90% mass participation, even using 200 modes, is this normal?

A: It may happen that have many local vibrations with similar frequencies which don't contribute to increase of the mass participation factors. The solution for such situation was presented during the webinar:

Yes following the webinar I have tried to use the Residual Mode function and I still do not get 90% participation. In fact it did not change my results at all. I have also been reading your website and it seems that this function is only permitted in French codes, is this not included in EC8? What is the implication of using this Residual Mode function in terms of my results? 

IMHO the principles of response spectra analysis are similar regardless of the code. Using the residual mode you will make some simplification / approximation but the whole analysis is like this. You do the same when you use diaphragm instead of meshed floor or equivalent lateral force method. 

R: I am struggling to understand the difference between consistent, lumped with rotations and lumped without rotations

A: See the slide from the webinar:

How will the choice of mass matrix affect my results? i.e. how can I decide what is appropriate for my building?

The consistent is the most accurate one but requires more resources. The lumped one may not be accurate when you e.g. want to determine eigenfrequencies to avoid resonance and you have no nodes defined along the bar element but this is usually not the case for seismic when you target the global modes rather than the local ones (vibrations of a single column or beam).

R: I do not need to include Z direction for this building according to Cl 4.3.3.5.2 (1).

A: Then you need only 10 modes

Is this image you show here from my model? I am unable to reach 90% mass participation at 99 modes but yet you reach it at 10 modes?! What is the difference in how I am running this model?

Direction Z was excluded

R: I have set up my slabs as shell elements, would I gain any benefit from setting them up as rigid diaphragms?

A: Smaller size of a model and less modes required especially when you have local vibrations of floors. Mind that this can be used only when you can disregard Z direction and you don't need results for floors.

When you say less modes required – does this mean it would help me to get the 90% mass participation if I set up my slabs as rigid diaphragms instead?

It should.

What is the benefit of rigid links vs rigid diaphragm?

The rigid diaphragm is actually modeled with use of rigid links.

 If you find your post answered press the Accept as Solution button please. This will help other users to find solutions much faster. Thank you.

[Artur.Kosakowski]

As in your case part of the vertical reactions is taken by vertical supports of diagrams you should look at the sum of masses in the horizontal directions instead and 

a). include self weigh (case 1) in the calculations

b). mind that in mass table for horizontal directions for lumped mass matrix type the masses assigned top supports don't count.

If you find your post answered press the Accept as Solution button please. This will help other users to find solutions much faster. Thank you.

[Anonymous]

Hi Artur,

Thank you for getting back to me again!

From the youtube tutorial I understood that I needed to keep the Z directions ticked, see image below. Could you please explain why you are now advising otherwise? 

With regards to your response:

As in your case part of the vertical reactions is taken by vertical supports of diagrams you should look at the sum of masses in the horizontal directions instead and

 What do you mean by vertical support of diagram? Do you mean the rigid diaphragms are acting as supports and taking part of the vertical reactions? Surely all the load from the buildings should transfer through the columns and walls to the base supports?

I cannot compare the sum of horizontal masses with the sum of horizontal reactions as I have no horizontal reactions:

a). include self weigh (case 1) in the calculations

In the tutorial I notice that self weight was not included in the load to mass conversion calculations - why was this? In what situation would you leave this out?

b). mind that in mass table for horizontal directions for lumped mass matrix type the masses assigned top supports don't count.

I don't understand what you mean here... Does this just mean I should make sure I do not load supports with linear or nodal loads?

When I look at my seismic loadcases to see the forces generated automatically it is blank:

Thanks,

Rosie

[Anonymous]

Hi again Artur,

I now understand what you were saying about self weight - unless you disregard densitites Robot automatically converts the SW to mass. My UX and UY masses now sum to 93% of the Uz reactions. However, I am still having an issue where my X and Y directions do not add up to the same as the Z direction which I find strange. Why do you think this could be? 

Additionally the forces generated automatically are still blank for the seismic load case as shown in the image above. Do you know why this is? 

How can I find out the base shear? Seems to not be possible for EC8 but I would like to check this against my approximate calculation as model verification.

Thanks,

Rosie

[Artur.Kosakowski]

From the youtube tutorial I understood that I needed to keep the Z directions ticked, see image below. Could you please explain why you are now advising otherwise? 

 I'm reading that in some situations you may unmark it ('is allowed to disregard Z'). In addition in your first post you wrote: 'I do not need to include Z direction for this building according to Cl 4.3.3.5.2 (1).'.

With regards to your response:

As in your case part of the vertical reactions is taken by vertical supports of diagrams you should look at the sum of masses in the horizontal directions instead and

 What do you mean by vertical support of diagram?

Don't use a spellchecker 🙂

I meant diaphragms.

Do you mean the rigid diaphragms are acting as supports and taking part of the vertical reactions? Surely all the load from the buildings should transfer through the columns and walls to the base supports?

The load transfer is correct but their use influences the total UZ mass in the results of the modal analysis result table. 

I cannot compare the sum of horizontal masses with the sum of horizontal reactions as I have no horizontal reactions:

You should compare sum of horizontal mass from the table as above with the sum of vertical reactions.

a). include self weigh (case 1) in the calculations

In the tutorial I notice that self weight was not included in the load to mass conversion calculations - why was this? In what situation would you leave this out?

It is included automatically (this is why you don't convert it) unless you mark 'disregard density' in the parameters of the modal analysis.

b). mind that in mass table for horizontal directions for lumped mass matrix type the masses assigned top supports don't count.

I don't understand what you mean here... Does this just mean I should make sure I do not load supports with linear or nodal loads?

I'll handle this as in the answer for your last post.

When I look at my seismic loadcases to see the forces generated automatically it is blank:

You should look at the pseudostatic forces instead.

If you find your post answered press the Accept as Solution button please. This will help other users to find solutions much faster. Thank you.

[Artur.Kosakowski]

I now understand what you were saying about self weight - unless you disregard densitites Robot automatically converts the SW to mass. My UX and UY masses now sum to 93% of the Uz reactions. However, I am still having an issue where my X and Y directions do not add up to the same as the Z direction which I find strange. Why do you think this could be? 

See the attached pictures

How can I find out the base shear? Seems to not be possible for EC8 but I would like to check this against my approximate calculation as model verification.

Check the sum of reactions for horizontal directions.

If you find your post answered press the Accept as Solution button please. This will help other users to find solutions much faster. Thank you.

[Anonymous]

Hi Artur,

Thank you again for getting back to me on my queries!

I am still a little confused on this point:

 I'm reading that in some situations you may unmark it ('is allowed to disregard Z'). In addition in your first post you wrote: 'I do not need to include Z direction for this building according to Cl 4.3.3.5.2 (1).'.

I can disregard Z as per the clause yes, what I am unclear on is the best way to do this. In the tutorial it specifically said if you want to disregard Z do not untick the boxes shown, instead put a 0 in the Direction box for Z. But in your earlier post you said I can disregard this. Why the different approach? Is it important to do it one way or the other?

The load transfer is correct but their use influences the total UZ mass in the results of the modal analysis result table. 

So the reason my Ux and UY do not summate to the UZ is because I have used rigid diaphragms? In the tutorial example rigid links were used but the masses in each direction were equal. 

You should look at the pseudostatic forces instead.

How can I look at this? Why can I not see it in the usual manner like the example online? The summation of the pseudostatic forces in Fx or Fy does not equal the expected base shear. Neither does the sum of the reactions, how can I find this?

Thanks again!

Rosie

[Anonymous]

Hi Artur,

Fantastic thank you. I read that consistent gives more accurate results. Do you have any advice on whether to choose consistent or lumped without rotations in terms of how it will affect my overall results? I am using this model to check my shear wall and element sizes. 

The sum of reactions for horizontal directions does not add up to the value I am expecting for the base shear. I was expecting around 16800kN, and looks like I get around 8000. Should I be looking at the main mode result or the CQC result?

Thanks again!

Rosie

[Artur.Kosakowski]

Do you have any advice on whether to choose consistent or lumped without rotations in terms of how it will affect my overall results? I am using this model to check my shear wall and element sizes. 

For seismic analysis the later should be fine. The prior will increase the model size but the choice is up to you 

The sum of reactions for horizontal directions does not add up to the value I am expecting for the base shear. I was expecting around 16800kN, and looks like I get around 8000. Should I be looking at the main mode result or the CQC result?

For CQC. See also this article.

If you find your post answered press the Accept as Solution button please. This will help other users to find solutions much faster. Thank you.

[Anonymous]

Good morning Artur,

Thanks for clearing up the mass matrix query 🙂

Please could you explain in more detail how I can find the base shear using EC8? The article you reference uses a different code and therefore I do not have these options in my model. In response to my earlier post you said I should check the sum of the reactions for horizontal directions - should I be looking at the reactions for the main mode or for the CQC case or for total Fx generally?

There is one more question still outstanding: 

R: When I look at my seismic loadcases to see the forces generated automatically it is blank:

A: You should look at the pseudostatic forces instead.

R: How can I look at this? Why can I not see it in the usual manner like the example online? The summation of the pseudostatic forces in Fx or Fy does not equal the expected base shear. Neither does the sum of the reactions, how can I find this?

Many Thanks,

Rosie

[Artur.Kosakowski]

Please could you explain in more detail how I can find the base shear using EC8?The article you reference uses a different code and therefore I do not have these options in my model. In response to my earlier post you said I should check the sum of the reactions for horizontal directions - should I be looking at the reactions for the main mode or for the CQC case or for total Fx generally?

There is one more question still outstanding: 

R: When I look at my seismic loadcases to see the forces generated automatically it is blank:

A: You should look at the pseudostatic forces instead.

R: How can I look at this? Why can I not see it in the usual manner like the example online? The summation of the pseudostatic forces in Fx or Fy does not equal the expected base shear. Neither does the sum of the reactions, how can I find this?

See post 2 from http://forums.autodesk.com/t5/robot-structural-analysis-forum/cqc-seismic-forces-in-seismic-combinat...

If you find your post answered press the Accept as Solution button please. This will help other users to find solutions much faster. Thank you.

Many Thanks,

Rosie

---

[Anonymous]

Hi Artur,

Thank you for responding, I have some more questions! 

1. For the base shear I assume I should be taking the sum of all modes in pseudostatic forces?

2. There was a previous question unanswered:

A: I'm reading that in some situations you may unmark it ('is allowed to disregard Z'). In addition in your first post you wrote: 'I do not need to include Z direction for this building according to Cl 4.3.3.5.2 (1).'.

R: I can disregard Z as per the clause yes, what I am unclear on is the best way to do this. In the tutorial it specifically said if you want to disregard Z do not untick the boxes shown, instead put a 0 in the Direction box for Z. But in your earlier post you said I can disregard this. Why the different approach? Is it important to do it one way or the other?

3. I have found the following guidance and I am wondering at which point to apply each:

- The CQC rule for the combination of different modes should be used (EN 1998-1/4.3.3.3.2(3)).

Is that this box?

- The results of the modal analysis in both horizontal directions should be combined by the SRSS rule (EN 1998-1/4.3.3.5.1(2a)).

Where can I apply this?

4. When checking the interstorey drift displacements has robot already taken into account the q factor or do I still need to multiply these results by q?

5. I am looking for Vtot - storey shear values. Is this the Fx and Fy values in the 'Reduced Forces' tab of the Storey tables? What does it mean by Reduced Forces? Is the Fz option on this table equivalent to Ptot - the total gravity load at the storey considered? 

Many Thanks!

Rosie

[Artur.Kosakowski]

1. For the base shear I assume I should be taking the sum of all modes in pseudostatic forces?

Oops. It seems like I missed the attachment here. You should look at the sum of reactions for quadratic combination.

2. There was a previous question unanswered:

A: I'm reading that in some situations you may unmark it ('is allowed to disregard Z'). In addition in your first post you wrote: 'I do not need to include Z direction for this building according to Cl 4.3.3.5.2 (1).'.

R: I can disregard Z as per the clause yes, what I am unclear on is the best way to do this. In the tutorial it specifically said if you want to disregard Z do not untick the boxes shown, instead put a 0 in the Direction box for Z. But in your earlier post you said I can disregard this. Why the different approach? Is it important to do it one way or the other?

These are two different things. You can generate seismic excitation in horizontal directions only (I assume this is what you refer to  as the tutorial) and set the dynamic degrees of freedom as horizontal only (you don't have the Z one when you use diaphragm).

3. I have found the following guidance and I am wondering at which point to apply each:

- The CQC rule for the combination of different modes should be used (EN 1998-1/4.3.3.3.2(3)).

Is that this box?

These are Newmark combinations between the directions of expectations. The CQC or SRSS are between modes.

- The results of the modal analysis in both horizontal directions should be combined by the SRSS rule (EN 1998-1/4.3.3.5.1(2a)).

Where can I apply this?

This is done automatically (program either generates SRSS or CQC combinations depending on your settings). 

4. When checking the interstorey drift displacements has robot already taken into account the q factor or do I still need to multiply these results by q?

 Is this what you are asking for?

http://forums.autodesk.com/t5/robot-structural-analysis-forum/ec8-reduction-problem-of-design-spectr...

5. I am looking for Vtot - storey shear values. Is this the Fx and Fy values in the 'Reduced Forces' tab of the Storey tables? What does it mean by Reduced Forces? Is the Fz option on this table equivalent to Ptot - the total gravity load at the storey considered? 

Correct.

If you find your post answered press the Accept as Solution button please. This will help other users to find solutions much faster. Thank you.