Dynamic Analysis

[Sohaib Nazar]

Dear friends, 

 

Can any body share some examples on dynamic analysis to calculate earthquake forces on buildings.

Need information how to make number of modes and what should be the scale factor?

 

 

Regards

Sohaib

[Badar (BAZ)]

You can find solved examples in the books such as Dynamics of Structures  by Anil k Chopra, and Seismic and wind design of concrete buildings by S.K. Gosh and  David Fanella.

[UmarMakhzumi]

I have attached tutorials that will show you how to setup ETABS model for dynamic analysis. Go through all tutorials. There are lucid and self explanatory.

OneDrive-2014-03-09.zip

[Suarez]

In modal analysis, I have heard that first 3 modes are important and should be considered. Rest are unimportant. Is that the case?

[UmarMakhzumi]

Depends upon what kind of analysis you are doing and what parameters are you looking for, e.g. If you are checking pump vibrations on a steel skid, it doesnt matter what your modal mass participation is and what your primary modes are. All you care is what modes fall closer to the forcing function that would resonate the pump.

[Ahmed Waqar]

Correct I am wrong. As per UBC # of modes shall be such that it incorporates 90% of participating mass. If participating mass is 90% in first two modes then no need of other modes. If 90% is acheiving in first 6 modes then first 6 modes are sufficient. If 90% is not acheiving then you have to scale your spectrum. In STAAD add missing mass value.

Also, I have worked on STAAD mostly. If you require STAAD model then i can attach it as well.

[UmarMakhzumi]

8 hours ago, Ahmed Waqar said:

Correct I am wrong. As per UBC # of modes shall be such that it incorporates 90% of participating mass. If participating mass is 90% in first two modes then no need of other modes. If 90% is acheiving in first 6 modes then first 6 modes are sufficient. If 90% is not acheiving then you have to scale your spectrum. In STAAD add missing mass value.

 

Your understanding is correct.

The higher mass participation, the important the mode. Once you have figured out which modes have the greatest mass participation, take time to see the mode/ demformed shape and think about how can you improve your structural framing. 

Thanks.

 

[Ahmed Waqar]

Thanks Mr Umar.

[WR1]

On 3/27/2016 at 11:28 AM, Ahmed Waqar said:

If 90% is acheiving in first 6 modes then first 6 modes are sufficient. If 90% is not acheiving then you have to scale your spectrum.

No, scaling is for dynamic base shear. Scaling will not increase mass participation. If 90% is not achieved you have to include more mass, that means increase the number of modes.

[Ayesha]

11 hours ago, Rana said:

No, scaling is for dynamic base shear. Scaling will not increase mass participation. If 90% is not achieved you have to include more mass, that means increase the number of modes.

Good that you clarified, but the original statement also includes - "Correct I am wrong. As per UBC # of modes shall be such that it incorporates 90% of participating mass."

 

Edited August 30, 2016 by Ayesha

[Hussain Abid]

On 3/10/2014 at 3:09 AM, UmarMakhzumi said:

I have attached tutorials that will show you how to setup ETABS model for dynamic analysis. Go through all tutorials. There are lucid and self explanatory.

OneDrive-2014-03-09.zip

Dear umar i chekd ur attachment.it is fairly infoyrmaftive. But there is one difficurlty that when chekig foyr story shears n selecting loads the RSA LOAD is not showing. How everr in the analysis log RSA is shown kindly elaborate the problem. Secondly elaborate the difference between RITZ EIGEN VECTORSand under wat conditions are these used. Plz also. Reference from codes be mentione

Best of regards

[UmarMakhzumi]

Hi Abid,

I am not clear about your first question. For your second question, please see the attachment.

Ritz analysis vs Eigen Analysis.pdf

Cheers!

 

[Hussain Abid]

Sir a lot of thnx for ur elaborative reply to my 2nd querry. My 1st question was that when we add RSA from define menu then in static load case data what LOAD TYPE should we assign to RSA as i m using version 9. 7. 0. I hv seen video tutorials abt RSA analysis bt those tutorials are abt 15th version and inthat RSA is assigned RESPONSE SPECTRUM as load type whereas in 9. 7. 0 this load type is not included. Hope u understand my querry

[Ayesha]

Just now, Abid Qasmi said:

Sir a lot of thnx for ur elaborative reply to my 2nd querry. My 1st question was that when we add RSA from define menu then in static load case data what LOAD TYPE should we assign to RSA as i m using version 9. 7. 0. I hv seen video tutorials abt RSA analysis bt those tutorials are abt 15th version and inthat RSA is assigned RESPONSE SPECTRUM as load type whereas in 9. 7. 0 this load type is not included. Hope u understand my querry

The tutorial is for ETABS 9.7. The post is dated 2014 at which time ETABS v15 wasn't out.

[Hussain Abid]

Thnx for reply. Btwhen i look for story shears fom TABLES and then select earthquake and sectral loads thn only EQX and EQY are shown whereas SPEX and SEPCY are not shown. Hope u understand

[WR1]

Yes spec x and spec y are not shown in load cases but in load combination.

[Hussain Abid]

Ok but in the related tutorial specx n specy are shown in load case for scalling purpose rathr than. Inm opinion RSA will also be defi ed in static load case dropbox also and unless it is done spectral acclertion will not b displayed in tables. For defining RSA LOAD wat will be its type of load in the dropbox this is wat i want to make clear sir. 

[UmarMakhzumi]

@Abid Qasmi,

I would love to help you with it but the only problem is that I don't have ETABS installed, my computer is non-windows (so won't be able to do an off license installation). However, I would suggest that you try to experiment with the options and post your feedback here, and it may help resolve your issue. How about you define RSA load as earthquake. I believe that is the option in ETABS.

Thanks.

[Hussain Abid]

On 3/9/2014 at 11:26 PM, baz said:

You can find solved examples in the books such as Dynamics of Structures  by Anil k Chopra, and Seismic and wind design of concrete buildings by S.K. Gosh and  David Fanella.

Dear BAZ i have gone through the book but in that there isnt any example based on R.S.A w.r.t UBC 97.plz suggest a book its download link too that contains RSA based on UBC 97.

regards

[Hussain Abid]

On 9/4/2016 at 4:33 PM, Abid Qasmi said:

Ok but in the related tutorial specx n specy are shown in load case for scalling purpose rathr than. In my opinion RSA will also be defined in static load case drop box also.And unless it is done spectral acceleration will not b displayed in tables. For defining RSA LOAD wat will be its type of load in the dropbox this is wat i want to make clear sir. 

Dear experienced fellows, i'm still unable to find the reply to the the above mentioned query that in R.S.A what is the LOAD TYPE that we assign to RSA in ETABS when we include it from DEFINE MENU for inserting  THE SPECTRAL LOAD.

[Waqas Maqsood]

On 9/4/2016 at 4:33 PM, Hussain Abid said:

Ok but in the related tutorial specx n specy are shown in load case for scalling purpose rathr than. Inm opinion RSA will also be defi ed in static load case dropbox also and unless it is done spectral acclertion will not b displayed in tables. For defining RSA LOAD wat will be its type of load in the dropbox this is wat i want to make clear sir. 

@Hussain Abid   you cannot define RSA in static load cases, as RSA in dynamic loading.

Instead after you define the "response spectrum function" you should then follow it by defining "response spectrum cases".  Hope its clear enough.

 

[jay olleta]

On 3/27/2016 at 3:28 PM, Ahmed Waqar said:

Correct I am wrong. As per UBC # of modes shall be such that it incorporates 90% of participating mass. If participating mass is 90% in first two modes then no need of other modes. If 90% is acheiving in first 6 modes then first 6 modes are sufficient. If 90% is not acheiving then you have to scale your spectrum. In STAAD add missing mass value.

Also, I have worked on STAAD mostly. If you require STAAD model then i can attach it as well.

Good day. Assuming that 90% of the mass participation is achieved. But the Base shear from dynamic analysis is greater than the base shear from static analysis. Do I need to scale it down? Thank you

[UmarMakhzumi]

Yes, please scale it down.