Dear All

I’m new in the use of the Forum but I’ve been with Caesar II fore a long. My question is about the difference in results between harmonic, time history and water hammer.

I understand that those methodologies of calculation are not the same but I’ve found that if I choose a time history analysis and Ii define a time-force profile similar to an harmonic one and the frequency selected is close to the natural frequency of the pipe, the results are totally different from the result obtained from harmonic analysis. It seams like if time-history analysis doesn’t recognize the natural frequencies of the pipe.

In other way when you charge a time-force profile for the time history, the DLF convert it in a frequency-force multiplier. But the time “history dialog definitions” admits only time-force. The question is: Is correct to edit the Spectra generated and change it to Time Force and reload it with the real values?

Thanks for your answer and sorry about my English.

Nico.

Time history is the most realistic type of water hammer analaysis possible harmonic ignores the time force history of the actual event.

I think the File name was Table.
In this example "harmonic analysis" we have a Force and a excite frequency.
The results max displacement (say 50 mm) is the same result with a hand calculation (estimate DLF with damping,eigenvalues and excite frequency with a formulare) with static displacement multiplied with the DLF.
That had i do in this example.

Now i think we must also for this example with a correkt time history method become the same result.
The force time must be similar the Force/Time sinus-funktion.
Excite frequency ==> Period (time)
Next i will test this.
All other woud not make sense.

John, Ohliger:
That’s exactly what I did. The point is that if you analyze the loads from harmonic analysis and compares these values with those from the Time History (calculated for both cases for a critical frequency), the values are not the same.
I know that if you apply an harmonica excitation with a frequency equal to the natural frequency of the pipe, then the amplitude of the deformation will increase without control. It is reflected through the increasing values of loads at restraints.
If you look for the loads obtained from Time history you will se different values.
I’ll try to attach the file with my calculations or my results in a way you can see them.
Thanks for your answers guys.
Nico.

John, Ohliger:
That’s exactly what I did. The point is that if you analyze the loads from harmonic analysis and compares these values with those from the Time History (calculated for both cases for a critical frequency), the values are not the same.
I know that if you apply an harmonica excitation with a frequency equal to the natural frequency of the pipe, then the amplitude of the deformation will increase without control. It is reflected through the increasing values of loads at restraints.
If you look for the loads obtained from Time history you will se different values.
I’ll try to attach the file with my calculations or my results in a way you can see them.
Thanks for your answers guys.
Nico.

Thanks, but i controll this self next time in my example.
I will self set all parameters for a right model time history method.

@Nico
In a time history is very imported that the parameter "Load Duration" so big, that the max dynamic answer can calculated.
Also the Force/Time input (time) big enough.
Because => system inertia and increasing dyn. answer with time