Seismic Analysis Method selection
Seismic Analysis Method selection
(OP)
I am doing a model of a dam(concrete dam) subject to seismic loads, i.e. El Centro earthquake accelerogram record & want to select a method in ANSYS.
Which method must be selected?
1- Transient Dynamic Analysis-full method or
2- Spectrum Analysis - SPRS?
How can I input time steps (say 0.02 sec) Vs earthquake displacement?
* Results must include the history of stress during earthquake.





RE: Seismic Analysis Method selection
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RE: Seismic Analysis Method selection
Thanks for your attention.
Considering your comments, let we have a response spectrum & do a response spectrum analysis, but here is a problem. Response spectrum as you know deals with only the max values, not history of data. For instance, for a seismic loads i.e. El Centro earthquake accelerogram & known damping ratio(say 5%),we have a deformation response spectrum vs natural frequencies. In this curve I didn’t see the history of ground shaking!
So If we apply it to the structure, how we can have a history of results (say stress or displacements)?
Regards
RE: Seismic Analysis Method selection
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RE: Seismic Analysis Method selection
Thanks again.
About Transient Analysis:
Once you replied to a question about Tran. Anal. with the title of "EARTHQUAKE TIME HISTORY" from "civil33 (Structural)" in this forum.
His main question was about the accuracy of his method to modeling earthquake force.
You did emphasis that in "Tran. Anal." ANSYS, didn’t accept accelerogram record (ground acceleration Vs time) directly & it must be convert to displacement Vs time. But if you remember,"civil33" use accelerogram directly by this method:
TIME,T1
ACEL,g1
TIME,T2
ACEL, g2,etc.Considering to the above comments:
Do you confirm this method?
Lets we have a simple structure, say a column. Gravity force, as you know is a "body force" and acting to the whole of the structure. But in a seismic shaking, we have these steps:
1-Ground starts to shake.
2-The structure foundation starts to shake.(The base of the structure suffers some displacements vs time)
3-At last the structure, considering its damping properties, starts to shake (displacements) and it causes a new stress distribution vs time.
In these steps, the ground shaking transfer from the foundation nodes to the structure base nodes & at the next steps transfer from the structure base nodes to the upper structure nodes.
When we apply "g" as a variable vs time (ACEL,g1 TIME,T1 ,...) are we sure to consider the above steps?
How we can model the seismic load "transferring steps"?
Regards
RE: Seismic Analysis Method selection
I think Drej will also confirm this thought we seem to share: I believe that applying time-variant ACEL is not correct.
This would not capture correctly the structure's response because in reality a time-variant displacement (which, twice-derivated, gives the time-variant acceleration) is applied to the foundation nodes, not uniformly over the structure.
For ex., if you deal with an extremely "soft" structure, the nodes "far" from the foundations would not be affected at all by what happens at the foundations, whether if you use the ACEL method you have them be excited anyway.
Transfering an acceleration spectrum to a displacement time-history is not easy: it involves Inverse Fourier Transform, but the problem is how to handle the information about phase (the earthquake's accel spectra that I know don't have info about phase)...
I also had this problem some time ago, Drej gave some very valuable advices, sure you can find the post by searching in this thread...
Regards
RE: Seismic Analysis Method selection
I believe that applying time-variant ACEL is not correct, too.
Also, I read your post & comments titled as, “earthquake simulation with non-linearities and displacement spectrum?”
We have some commands in Spectrum analysis such as (sed,svtype,...) to apply Base nodes DISPLACEMENT vs Frequencies. If I want to use TRANSIENT analysis is there any way to apply DISPLACEMENTS vs TIME (modeling a earthquake shakes on the structure base joints)?
You confirm me if I need a time history results, say stresses changes vs time due to a earthquake, it is unavoidable to apply base nodes displacements vs time by caring out a transient analysis, don’t you?
Regards
RE: Seismic Analysis Method selection
amini06 - search through the archives here for information on time-history seismic analysis using displacements. There is lots and lots of information on this already.
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