Different result obtained from spectrum and transient analysis
Different result obtained from spectrum and transient analysis
(OP)
hi dears:
I have a structure and do two seismic analyses with it.
1-spectrum analysis
2-transient analysis
But the thing that is very strange is different result obtained from them.
Stress, nodal displacement, and ......are different.
I expect that at least nodal displacement at top of structure to be the same,
But there are many differences between them, for example:
1-spectrum analysis: maximum x-component of displacement of top node of structure is: 35 cm
2-transient analysis: maximum x-component of displacement of top node of structure is: 3 cm
My seismic data in both analyses are the same and it is El Centro earthquake accelerogram.
But I integrate acceleration to obtain displacement versus time in transient analysis and do Fourier transform to obtain acceleration versus frequency for spectrum analysis.
In transient analysis I set displacement loads at base nodes in x direction and fix y and z directions.
Please someone help me why it happened?
Is it correct and common or I made mistake in some part of my analysis?
Thanks
Regards
I have a structure and do two seismic analyses with it.
1-spectrum analysis
2-transient analysis
But the thing that is very strange is different result obtained from them.
Stress, nodal displacement, and ......are different.
I expect that at least nodal displacement at top of structure to be the same,
But there are many differences between them, for example:
1-spectrum analysis: maximum x-component of displacement of top node of structure is: 35 cm
2-transient analysis: maximum x-component of displacement of top node of structure is: 3 cm
My seismic data in both analyses are the same and it is El Centro earthquake accelerogram.
But I integrate acceleration to obtain displacement versus time in transient analysis and do Fourier transform to obtain acceleration versus frequency for spectrum analysis.
In transient analysis I set displacement loads at base nodes in x direction and fix y and z directions.
Please someone help me why it happened?
Is it correct and common or I made mistake in some part of my analysis?
Thanks
Regards





RE: Different result obtained from spectrum and transient analysis
please some one help me to solve this problem.
it is urgent to me.
thanks
RE: Different result obtained from spectrum and transient analysis
please some one help me to solve this problem.
it is urgent to me.
thanks
RE: Different result obtained from spectrum and transient analysis
please some one help me to solve this problem.
it is urgent to me.
thanks
RE: Different result obtained from spectrum and transient analysis
Tobalcane
"If you avoid failure, you also avoid success."
RE: Different result obtained from spectrum and transient analysis
Tobalcane
"If you avoid failure, you also avoid success."
RE: Different result obtained from spectrum and transient analysis
the question that arises to me is the following:
in transient analysis you excite only the X-direction; are you sure that, in the spectral analysis, the excitation is also confined to this single direction? Crossed-terms in resonance could otherwise give you a very strong X-response caused, for example, by a Z-excitation.
Other thing: during integration of the time-variant acceleration, did you provide correct values for the integration constants? If you don't, you alter the content of harmonics.
Last thing: did your transient analysis have a sufficient duration to capture all the significant response of the system? Would it be possible that you notice an increasing trend in the X-amplitude, so that you can argue that the max displacement has not been reached yet? Or, vice-versa, is the timestep sufficiently fine in order to capture the highest harmonics of interest? Say you have a peak rise from 1 to 35 mm in 10 ms followed by a decrease to 2 mm in another 10 ms: if you have a delta-time of 20 ms it is very unlikely that you will capture the real peak (except if your timestep is exactly "in-sync" with it, but it's a bit randomic!).
Regards
RE: Different result obtained from spectrum and transient analysis
yes,i set Global Y-axis as spectrum direction,but my structure is symmetric, its cross section is circular then i think that there is no diffrent in x or y direction,but in spectrum analysis when i set global y -axis as specrum direction i check y displacement.do u think i have to change spectrum direction to x like transient analysis?
Question 2: my acceleration time data start like this t=0 s -->a=0 ,t=0.02s -->a= 0.003640 i set c1=0 and c2 =0,is it correct?
Question 3: my third frequency is 1.7721 then i use time step size az time=1/20*f =0.28215112and it is Approximately 0.02.is it correct?
thanks
regards
RE: Different result obtained from spectrum and transient analysis
1- no, I don't think so; if your structure is truely symmetric it should be fine as you're doing.
2- I suppose so; with c1=0 and c2=0 you are saying "initial displacement and velocity are null at start time". Being also a=0, your condition is "at rest".
3- if the max freq you are interested in is a bit less than 2 Hz, one cycle lasts for 0.5 s and so, adequately describing it with at least 10 points/cycle, leading to a timepoint interval of 0.05 s. If you use 0.02 s you should be OK...
The last point to verify is "does your transient analysis last sufficiently"? Do you notice an increasing tendency in the displacement response?
Regards