| Applies To | |
| Product(s): | AutoPIPE |
| Version(s): | ALL; |
| Area: | Results |
| Date Logged & Current Version | Sept 2020 12.04.00.13 |
Problem:
In the harmonic analysis, the force is input as an amplitude, for example 100N,which is then applied sinusoidally to give 200N peak to peak.
Are the resulting displacements and stresses also given as peak to peak values, or are they amplitudes?
Solution:
Neither and Both.
In this example from AutoPIPE's example model (HRMEXP), refer to the following explanation to understand how the displacement results for node point A5 are calculated for harmonic load case H2.
AutoPIPE looks at the loading function as defined in the Harmonic Load dialog, or an ASCII text file (ex. RPM280.HMF).
During the analysis, the program first runs a modal analysis to determine the mode shape and frequencies of the entire model. Note that the harmonic analysis cannot be run without first performing a modal analysis, thus implying its dependency.
At node point A5, AutoPIPE looks at the first mode shape (ex. Mode Number = 1) to calculate it's frequency ( ex. 7.9379 hz), from the frequency calculations report it reads the respective frequency’s period (ex. 0.126 sec), referring back to the forcing function (ex. RPM280.HMF), and from the forcing function derives the applied force at that time.
This process is repeated for each mode shape. In the end all results are enveloped to determine the maximum or RMS load being applied to a node point. The enveloping method is determined by the ‘Comb Method’ selected in the Harmonics tab of the Dynamic Analysis dialog. Maximum simply takes the largest (unsigned) applied force of all modes used. RMS is the ‘Root Mean Square’ of the applied forces of all modes used. The resulting forces are then applied as a load during the analysis in order to calculate a displacement and rotation for each node point in the model.
Remember that all dynamic results are positive values. To consider the -ve side of this harmonic and (add?) a new combination that considers a -ve 1 scale factor for the dynamic load.
Again, are the resulting displacements and stresses given as peak to peak values, or amplitudes? The program refers to the entire forcing function Peak to Peak and duration. However, only the envelope applied forces are used to determine the displacement .