Need help with seismic analyses

[Jummybear]

Hi,

I am not a pro in seismic analyses because we dont perform them in our everyday job. Now i have a chance to try out such analyses for training in one project. I have questions, hope to get answers also 🙂

1. What is the difference between Modal+Seismic and Seismic (Equivalent Lateral Force Method)? As i understand second one is simplifed method and dont need modal analyses to run.  Can i read about simplifed method in details?

2. I will go through complicated or more detailed way, so to make a new type of seismic analyses i need to run modal analyses first.

3. In modal analyses parameters i should choose nalyses mode: modal, seismic or seismic (pseudo mode). What is the difference between modal and seismic? As i can understand from help, then seismic analyses select vibration modes not in appropriate order and with seismic mode those modes will be set which are important (with large mass paricipation factor). How Robot define which ones are not important and which are important? I did some test on my model: i created 2 modal analyses with 2 different analyses types: modal and seismic. But results are the same and moreover robot switched itself seismic mode to modal mode...

4. Its recommend to obtain 90% of mass participation in seismic analyses but i cant obtain it in first 200 modes. Should i just increase the number of modes?

Updated: i have tried with only 1 modal analyses and seismic mode in parameters but robot switched it also to modal mode...

[t.sautierr]

Hi,

I am not a pro in seismic analyses because we dont perform them in our everyday job. Now i have a chance to try out such analyses for training in one project. I have questions, hope to get answers also 

1. What is the difference between Modal+Seismic and Seismic (Equivalent Lateral Force Method)? As i understand second one is simplifed method and dont need modal analyses to run.  Can i read about simplifed method in details?

Read the Eurocode 8 for details or find literature about it.

2. I will go through complicated or more detailed way, so to make a new type of seismic analyses i need to run modal analyses first.

Yes

3. In modal analyses parameters i should choose nalyses mode: modal, seismic or seismic (pseudo mode). What is the difference between modal and seismic? As i can understand from help, then seismic analyses select vibration modes not in appropriate order and with seismic mode those modes will be set which are important (with large mass paricipation factor). How Robot define which ones are not important and which are important? I did some test on my model: i created 2 modal analyses with 2 different analyses types: modal and seismic. But results are the same and moreover robot switched itself seismic mode to modal mode...

Modal is just modal : nothing to do with seismic. seismic uses the results of modal analysis in ordre to calculate force, parameters of the seismic analysis can be set when defining the analisys type.

Ususally what one do : modal analys + seismic, rarely pseudo mode or equivalent lateral (lateral can be done by hand calculations so not very useful as you use a computer).

4. Its recommend to obtain 90% of mass participation in seismic analyses but i cant obtain it in first 200 modes. Should i just increase the number of modes?

Number of modes required is largely linked to the way you model your project, if you have got plenty of local modes, it can be because you did'nt model properly.

Example : to simulate diaphragm effect, one can use diagonal bars -> mass can be neglected in order to to have local modes for theses elements.

Hint : use concentrated / no rotation matrix in the modal analysis if you study a common building structure, it will avoid part of this effect.

Updated: i have tried with only 1 modal analyses and seismic mode in parameters but robot switched it also to modal mode...

[Jummybear]

Thank you for reply.

Just ordered EC8 and already reading it.

Modal is just modal : nothing to do with seismic. seismic uses the results of modal analysis in ordre to calculate force, parameters of the seismic analysis can be set when defining the analisys type.

I know, but then i have a question why this option (modal mode/seismic mode) is active in advanced parameters of modal analyses and why its dont work properly? Or probably i am doing something wrong?

Number of modes required is largely linked to the way you model your project, if you have got plenty of local modes, it can be because you did'nt model properly.

Example : to simulate diaphragm effect, one can use diagonal bars -> mass can be neglected in order to to have local modes for theses elements.

Hint : use concentrated / no rotation matrix in the modal analysis if you study a common building structure, it will avoid part of this effect.

Already done. My project is a 3D steel frame, without slabs or wals. Only bars. I will try with different mass matrix type and will look at results. Thank you.

[t.sautierr]

You can post your model if you want.

What's wrong when you define modal +seismic?

[Jummybear]

I'll send you PM, I dont want to share current the model here.

One more question. I performed modal cals, picked main modes for signed combinations in X Y Z direction and have assigned them to seismic cases. After that i press ok and robot generate lot of combination by the code (about 60 and these are only seismic comb). There are linear and dynamic seismic combinations also. Now i should calculate all of them and then combine once more seismic linear combinations with other (non seismic combinations). Am i right? Can i delete dynamic seismic combinations adter calculation and leace only linear combinations? Any ideas how to reduce file size? 🙂 RT with results is about 1gb...

[Jummybear]

One more question: should I input eccentricity in modal analyses or in seismic analyses?

[Artur.Kosakowski]

The later is the simplified approach with the following limitations:

1. The modal analysis performed is not exact since it does not take into account the mass eccentricities. Therefore, the mode shapes are not the same as calculated with mass eccentricities.

2. The eccentricity of the mass is then considered "statically" in order to get the forces in the members. Therefore, the forces in the members are not exact.

3. This method provides the same base shear for all eccentricities.

4. This method does not provide base shear values in x and y directions . For example, when considering earthquake in x direction the y component is equal to 0 and vice versa.

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.

[Jummybear]

So its better to define eccentrilicity in modal analyses as i thought. Correct me if i am wrong but i dont need to enter eccentrilisity twice in modal and in seismic analyses? What about deleting dynamic seismic combinations? In some toturials it was done after analyses. From the other hand i can add dynamic seismic analyses as an auxilary case and then going on with static analyses.

[Artur.Kosakowski]

I'd say that this is going to be your decision based on your knowledge what kind of results you need for a given 'task' and the size of the seismic results (the fact if they slow down the access to the other ones when kept in the file) 

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.

[Jummybear]

I want to get the most accurate results. Did an experement with my model:

1. Modal analyses + 5%ecc - calc - seismic - calc

2. Modal analyses - calc - seismic + 5%ecc - calc

The difference in results (forces/reacton) is about 2 times...

[Artur.Kosakowski]

If you mean that you compare these two methods then as it was already said the definition of the eccentricity in modal analysis is the more accurate solution.