The concept of phase lag i.e. timing between loads is not possible to expresses in Time History Analysis . Am I correct?

The reason for my understanding is that the spectrum definition is as per Force Vs Time, but there is no way by which the timing between the force sets can be given as an input.

I think the concept of phase lag is possible only in Harmonic analysis.

Need advise.

Yes you can do this, simply stagger the pulses in time. For example:<table border="1"><tr><th>Pulse</th><th>X value</th><th>Y value</th></tr>
<tr><td>1</td><td>0.0</td><td>0.0</td></tr>
<tr><td> </td><td>0.001</td><td>1.0</td></tr>
<tr><td> </td><td>0.05</td><td>1.0</td></tr>
<tr><td> </td><td>0.08</td><td>0.0</td></tr>
<tr><td>2</td><td>0.0</td><td>0.0</td></tr>
<tr><td> </td><td>0.001</td><td>0.0</td></tr>
<tr><td> </td><td>0.05</td><td>0.0</td></tr>
<tr><td> </td><td>0.08</td><td>1.0</td></tr>
<tr><td> </td><td>0.1</td><td>1.0</td></tr><tr><td> </td><td>0.2</td><td>0.0</td></tr>
</table>

Richard,

Thanks for your resposne.

Kindly tell me:

a)Why we have only one time history case in Dynamic analysis?

b)The concept of phase lag, how can it be modelled in case of Spectrum analysis.

Regards

A) Too much data, excessive run times, not enough memory on the PC, etc, etc.

B) A spectrum analysis is performed in the frequency domain. The concepts of time and phase don't exist in the frequency domain. The spectrum analysis is a (hopefully conservative) statistical approximation of the true response of the system.

You should take a look at the newsletter article A Survey of Impact Loads which appeared in the December 1998 issue of our Mechanical Engineering News. This will give you a basic understanding of the different types of dynamic analysis, along with the pros and cons of each.

Dear Baburao,

Besides going through the article mentioned by Ray,remeber one more point to differentiate between the frequency and time domain:

The Laplace Transformation approach is taken for the Time Domain Problem , while the Fourier Transformation approach is taken for the frequency domain problems.


A classic application of frequency domain problem:

Measurement of field vibration data where the time domain problem is converted to Frequency domain using the algorithm known as:Fast Fourier Transformation or FFT.

Regards

Ray and Anindya,

Thanks.

The articles available in the NEWSLETTER section does not answer few questions pertaining to Time History or Spectrum analysis.They are:

1)The direction of the force that has to be given in the time history of loading ( i.e the pulse generator table) as well as in the Time History or Force set.If instead of + we give a - , does the program underatnd that the direction of the loading will be in + or in -direction?

2) Will the + or- direction change the Dynamic Load Factor computation?

3)We are specifying the direction of load in Force set definition. Again we are showing the direction of the load in Time History or Spectrum load case definition.What this is shown twice?

Regards

1) The force direction should be specified in the "force set" data. The force magnitude should also be specified here ONLY!!! When defining the "force-time" pulse, use a normalized magnitude of 1.0.

2) No the DLF will not change.

3) The +/- direction of the force should be defined in the "force set". The direction specified in the "load case" is used as an output label only (except for seismic analysis which doesn't have a "force set" option).

Richard,

I could not understand your statement "The +/- direction of the force should be defined in the "force set". The direction specified in the "load case" is used as an output label only".

Can you kindly elaborate?

So in case of a force spectrum method, is it ok to put the sign of the force in the "force set" as it does not have any importance in the "spectrum load cases"

When you perform a "force spectrum analysis", the force vector you define in the force set contains both magnitude and direction. Therefore you don't need to specify the direction on the "spectrum load case" dailog.

Unfortunately the way the dialogs were designed, there must be "some" value in the direction column on the "spectrum load case" dailog. (This directional value is used for seismic analysis only.) In the spectrum output, you will notice that there are three lines of data, the last one showing where the "maximum contributor" came from. The direction of the load reported here is also the value defined on the "spectrum load case" dailog.

Yes, you put the sign on the magnitude specified in the "force set".

Dear Baburao,

This has reference to Ray's reply "no the DLF will not change".

I think you are exploring the dynamic analysis as a learner.So I think you should take it as an exercise and prove that indeed there will not be any change.

Try from Duhamel's integral.In case you don't know about this read Anil Chopra's : Dynamics of Structure, Theory and application to earthquake Engineering.Using the Duhamel's integral prove that DLF will not change.

Regards