×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

Different result obtained from spectrum and transient analysis

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

RE: Different result obtained from spectrum and transient analysis

(OP)
hi
please some one help me to solve this problem.
it is urgent to me.

thanks

RE: Different result obtained from spectrum and transient analysis

(OP)
hi
please some one help me to solve this problem.
it is urgent to me.

thanks

RE: Different result obtained from spectrum and transient analysis

(OP)
hi
please some one help me to solve this problem.
it is urgent to me.

thanks

RE: Different result obtained from spectrum and transient analysis

Just a suggestion, maybe also post it in the Mechanical Acoustics/Vibration engineering forum, you'll might get some answers from the vib side.

Tobalcane
"If you avoid failure, you also avoid success."

RE: Different result obtained from spectrum and transient analysis

Now that I'm thinking about it, from a vib point of view, a spectrum analysis is different than a transient analysis.  Spectrum analysis you will see how the model will react to a band of frequency (0 to 500hz).  For transient, you will look at a specific frequency (may be 250 hz), but at different amplitudes (Gs).  So, it would make sense that your analysis shows that the larger displacments are in the spectrum analysis because you have "struck" the natural frequency of the model.  For the transient, it looks like you are within 3 octavs of the natural frequency.

Tobalcane
"If you avoid failure, you also avoid success."

RE: Different result obtained from spectrum and transient analysis

Hi,
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

(OP)
  hi and thanks cbrn:

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

Hi,
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

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources