| Applies To | |||
| Product(s): | AutoPIPE, | ||
| Version(s): | ALL | ||
| Area: | Warning | ||
| Original Author: | Bentley Technical Support Group |
Problem:
The following warning appears when running a Time History analysis:
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Warning
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W801-58: Smallest timestep for TPCE07 .TIH file = .000617 <.000625002 .
Consider a smaller user defined(non-automatic) analysis timestep.
This may indicate that modal cut-off frequency is not adequate.
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OK
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How to resolve this issue?
Solution:
The following example helps to explain the message received above and how to fix it.
After receiving this error message, create an output report that contains the Analysis summary sub report. In the sub-report find the following:
TIME HISTORY LOAD CASES :
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Number of load cases analyzed : 1 Load case 1 - M1 Time history load file 1 : xxx
Time History Load Type : 5. Imposed Force
Number of TIH files of this type : 9 Time history load file 2 : TPB
Time History Load Type : 5. Imposed Force
Number of TIH files of this type : 4 Time history load file 3 : TPC
Time History Load Type : 5. Imposed Force
Number of TIH files of this type : 3 Analysis duration : 9.0000 sec
Analysis time step : 0.000625 sec (Automatic)
Output time step : 0.000625 sec (Automatic)
Number of time steps : 14400
Modal damping ratio : 0.0200
ZPA : Yes Captured modal mass (%) : 65.24 Added missing mass (%) : 0.00
In this example, the analysis duration was found to be, Analysis time step = 0.000625 sec.
From the warning dialog message above, TPCE07 .TIH file = .000617 <.000625002.
TPC = name of fluid transient
E07 = node point where warning is occurring
TIH = file extension name
0.000617 = time calculated from the TIH file
Open the THL file mentioned above (ex. TPC.THL), search for the node point mentioned in the warning (ex. E07), The following was found:
Existing TPA CONVEYING Line W1004466
3 TIH FILE SCALE DX DY DZ TIME(sec)
B08 TPCB08 1.000 -0.707 -0.707 0.000 0.079889
E04 F TPCE04F 1.000 0.000 -1.000 0.000 0.081416
E07 TPCE07 1.000 0.000 -1.000 0.000 0.082444
TIME AT END OF FLUID TRANSIENT : 0.083061 sec
Calculte duration from node point to node point to end of fluid transient:
B08 to B04 = 0.081416 - 0.079889 = 0.001527 sec
B04 to E07 = 0.082444 - 0.081416 = 0.001028 sec
E07 to End = 0.083061 - 0.082444 = 0.000617 sec
Compare results: Actual value in transient vs Value automatically calculated
0.000617 < 0.000625 was found to be true.
Fix as recommended in warning message, open Dynamic Analysis dialog screen, select the Time History tab> change the following:
Analysis Steps (s) = from Automatic to value smaller than mentioned in warning message, ex. 0.00061 sec
Output Steps (s) = from Automatic to value smaller than mentioned in warning message, ex. 0.00061 sec
Run the analysis, remember it may take a bit of time to process, the application may appear to have stopped, just leave the mouse and keyboard alone until analysis is finished. The Warning message should not have appeared now. continue review with analysis results.
Note:
1. this WIKI does not address the "Captured Modal Mass", typically stress engineers will strive to have between 75% and 100% participating modal mass; suggest appropriate changes to the Model and / or Modal Analysis settings.
2. Again, try to reduce the time step so that it is lower than the smallest time step in the THP file. Try doing that and see if the results improve. Setting a higher cut-off frequency and number of modes for the modal analysis also helps generally to get better results from Time History Analysis.