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I am modeling vibrations of a beam first preloaded in a four point bending configuration. The set up is as follows: The bar is first loaded in a four point bending load (static load). A shaker is attached at the center of the deformed bar and generates vibrations (Harmonic perturbation load). So, there are a total of two different loads on the bar. The static load for predeformation and the harmonic perturbation load at the center of the prestressed bar.

I am running a pre-stressed frequency analysis. The issue I am running into is that with the current configuration, the pre-deformation does not seem to change the resonance frequency of the bar at all. However, I noticed that, instead of a separate load at the center, if I apply the harmonic perturbation as well at the point where I applied the static force, the preload seems to affect the vibration and change the resonance frequency.

I was wondering why the former method works and the latter doesn't. Any suggestions?

Thanks
Jyani