Seismic Analysis

[brc01]

I am doing seismic analysis using ANSYS 5.7.
After doing response spectrum analysis to find responses(displacement and stresses)for three INDIVIDUAL spectra (say in X,Y ,and Z directions)using SRSS mathod of combination,how to get the COMBINED RESPONSE DUE ATO ALL THE THREE SPECTRA ?How to proceed in the Postprocessor?please help.

Thanks in Advance
brc01

 

[jharish]

Hi,
This is infact a question back to you. I have a similar problem now. I saw your question when I was searching for seismic analysis.
As far as I know it, the 3 directions are independent in terms of the response and has to be given as such. I've no knowledge of any combination of the 3 response spectra.
In case the 3 have to be combined and there is a way to combine the 3, please let me know.
Thanks in advance,
jharish

 

[brc01]

The three have to be combined coz an earthquake shakes your structure SIMELTANEOUSLY in all the three directions.

Well,COSMOS uses "SRSS:Square root of sum of squares"method to combine these responses to get the net response.I am not sure if anything similar is there in ANSYS.

 

How do you specify the exact position where you want the earthquake to act.How do you input the earthquake data (eg. Northridge) for the spectrum analysis. I have tried using the commands from the ansys eXAMPLE INPUT VM70 input listing and it only applied the earthquake load to one side of my model. Please help

 

[jharish]

The earthquake inout is given at the fixed locations on the component. Say, the component that has to be tested for seismic compliance is bolted to a frame, the seismic input has to be given to the bolts, after giving a suitable constraint while doing the modal analysis. The seismic procedure also depends on the type of input spectrum you have.
Hope this helps.
-Harish-

 

[Drej]

The complete combination of the X, Y and Z directions is carried out in ANSYS using the

lcoper,srss,all

command (subsequent to setting the three loadcases as a loadcase definition - lcdef).

 

[jharish]

Drej,
Ordinary loads and reactions can be combined using the SRSS method. But my question is whether the same procedure can be followed for a seismic analysis. I'm questioning this as all the test reports I've seen come with separate response curves for the 3 mutually perpendicular directions. So the question still stands... Can it be done for a seismic analysis. Sorry for being so persistent, but it's vital for me.
Thanks.
-Harish-

 

[Drej]

If you are performing a SPECTRAL analysis, you MUST combine the three directions after the modal responses have been combined. The combination of modes is performed by inputting the .mcom file in a /post1 session, using /input,filename,mcom. (If you specified CQC combination, the responses in each direction will be CQC'd by inputting this file, but will also SRSS the responses as well at the end - check the list of .mcom file commands by opening it!). Anyway, you must combine the three directions together after this - why? What happens when a seismic event occurs? all three directions are shaken simultaneously - not individually. Once you have this, you will have what is known as the Complete Sesimic Case (but you will need to factor these loads if you are using a code).

Hope this helps i happy ANSYSing

- - D R E J - -

 

[Drej]

If you are performing a SPECTRAL analysis, you MUST combine the three directions after the modal responses have been combined. The combination of modes is performed by inputting the .mcom file in a /post1 session, using /input,filename,mcom. (If you specified CQC combination, the responses in each direction will be CQC'd by inputting this file, but will also SRSS the responses as well at the end - check the list of .mcom file commands by opening it!). Anyway, you must combine the three directions together after this - why? What happens when a seismic event occurs? all three directions are shaken simultaneously - not individually. Once you have this, you will have what is known as the Complete Sesimic Case (but you will need to factor these loads if you are using a code).

Hope this helps - happy ANSYSing

- - D R E J - -

 

[bhawesh]

Hi,
I am doing similar analysis.
Can any body help me as what exactly to look for in Seismic analysis for PSD input viz. the displacement or 1 sigma stress. Also when I go for /post26 my screen shows that the initial memory allocation has been exceeded and the ansys window goes off. I am using Ansys 7.0.I have 108000 elements in all.Has anybody faced a similar kind of problem?

Thanks in advance.
Bhawesh.