Define a time history with different steps

03-Analytical/modelling capabilities
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newman
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Joined: 06 Jan 2017, 18:12

Define a time history with different steps

Post by newman » 09 May 2019, 17:49

Hi Seismosoft.
Please help me regarding an issue. I want to run a Time History Analysis on a structure. The time history is created by three time history including:
first: 20 sec, step=0.005
Middle: 10 sec, step:0.002
End: 15 sec, step:0.004
I create a continuous time history on a notepad file. When I want to add this time history to seismostruct, it ask to me to define a special time step, while my record is created by three records with three steps. I know that I can define three steps in Time History Tab in Seismostruct, but, I think that because of loading the whole curve by only one step (for example, the step of the first record) and consider it for the whole record, is not correct. In the preview of time history for the loaded record, the steps are same as each other. I don't know how the program can consider three steps in time history tab, while the origin time history is loaded based on only one step. Why the program cann't use the times of the notepad file, directly? I really need your help. I have lots of files which need to analyze by SeismoStruct, but I'm not sure that this method is correct.

huffte
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Re: Define a time history with different steps

Post by huffte » 10 May 2019, 11:50

I get the same behavior newman. It appears that the "Time and Acceleration" option is not working properly. Even when I specify that option, the single specified time step (which presumably should be ignored when the said option is selected) is used and the time column is ignored instead.
Tim Huff

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seismosoft
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Joined: 06 Jul 2007, 04:55

Re: Define a time history with different steps

Post by seismosoft » 14 May 2019, 18:04

hello newman,

This is the standard behavior of SeismoStruct. All the records are loaded with a constant time-step. This time step is specified in the dialog box from which you load the record.

Then if you want to discretize the record to different time-steps (e..g first: 20 sec, step=0.005, Middle: 10 sec, step:0.002, End: 15 sec, step:0.004, as in your case) this can be done easily specifying different Time-history Stages on the same module. For instance, you will need 3 different stages in your case:
End of stage 1: 20.00 sec & 4000 steps
End of stage 2: 30.00 sec & 5000 steps
End of stage 3: 45.00 sec & 3750 steps

Seismosoft Support

huffte
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Re: Define a time history with different steps

Post by huffte » 14 May 2019, 21:28

I must confess to a bit of confusion regarding time steps. Isn't the time step defined in the Time History Stages (equal to the total stage time divided by the number of stages) the analysis time increment, which is to be distinguished from the loading time increment which defines the acceleration history?

In other words, if I define an acceleration history which has a 50 second duration and a loading time step of 0.01 seconds, that would be 5001 points defining the load. I could specify a time history stage of 50 second duration with 5001 points, and the analysis would be conducted using an analysis time step of 0.01 seconds, identical to the load time step.

Or, I could define a time history stage of 50 second duration with 10001 points, and the analysis would be conducted using an analysis time step of 0.005 seconds. The loading in this case is identical to the previously defined loading. Only the analysis time step has changed. Presumably, the load (acceleration) value is computed internally at each analysis time step based on the defined load. So, in this second case, interpolation would be required to calculate the load at each analysis increment.

So there may be a bit of confusion on my understanding of the original question from newman. I'll be open to any clarifications or corrections to my understanding if the problem.
Tim Huff

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seismosoft
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Re: Define a time history with different steps

Post by seismosoft » 17 May 2019, 15:46

Hello Tim,

Apart from a single phase, the program also is able to have several different stages in the same analysis, each of which could have its own time step. For instance, if you have a record with the higher intensity between 4 and 6 sec, you could define a first stage from 0 to 4 sec with a time step of 0.01 sec, then you could define a second phase between 4 and 6 sec with a time step of 0.005 sec, and then from 6 to 10 sec with a time step of 0.01 sec again, If the input record has a time step of 0.01 sec, interpolation of the loading is required for the second phase (with the dt=0.005 sec)

Maybe the program is not very intuitive in this aspect, however it allows for great flexibility.

Warm Regards
Stelios Antoniou

huffte
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Re: Define a time history with different steps

Post by huffte » 18 May 2019, 13:28

Thanks very much Stelios. Keep up the great work there.
Tim Huff

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