peteroznewman
Subscriber
nI am trying to find out the natural frequencies of my main load carrying structure; still is it necessary for me to input these masses in my model while conducting modal analysis?nTo simulate the same frequency in Modal that the fully built machine will vibrate with after a hammer strike, you need all the mass that is hanging from that structure.nCan I also see the stresses within my Harmonic analysis, to see if the structure is failing or not?nHarmonic Response results are highly dependent on damping that is used in the model. If you underestimate the damping, the displacements and stresses will be overpredicted. There are ways to experimentally measure the damping of the structure to know that the model has a good value for damping. However, damping can be frequency dependent, so the first mode might have one value of damping while the 6th mode will have a different value of damping. As mentioned, the largest response is at the excitation frequency that matches the resonant frequency. Most machines with harmonic loads such as machines with high speed rotating parts are operated at a frequency away from any resonant frequency of the structure specifically to avoid a large harmonic response.nHowever, machines with high speed rotating parts often have to pass through a low resonant frequency on the way to the operating frequency. This is a Transient problem and the idea is that the system spends so little time when the excitation frequency matches the resonant frequency that there is no time for a large response to develop before the excitation frequency has passed by the resonant frequency.nthe amplitude of the load applied is very high which may cause some significant deformations within the structure, eventually leading it to failnKeep in mind that Modal analysis, Harmonic Response, Random Vibration and Response Spectrum analyses are all linear analysis methods. That means no nonlinear materials such as Plasticity, no large deflection, and no nonlinear contact can be included in the model. Linear analysis is most often used for performance analysis, and less often for evaluating failure. A full Transient Structural analysis, without any MSUP Modal analysis feeding in, can be used to include all nonlinear effects and can be used to evaluate failure conditions.nWill the response to my applied periodic load differ if I change the location at which the load is applied on my structure?nYes.ncan it be possible for the structure to not fail if the excitation freq is the same or near to the natural freqnEither the amplitude of the load is very small or the damping is very high.nBut if wind is causing the excitation, then how will I get the load that should be inputted into the Harmonic response?nA transient Fluid-Structure Interaction (FSI) analysis can predict the response of a structure to airflow.nhttps://medium.com/curiosityftw/busting-the-misconceptions-of-tacoma-narrows-bridge-collapse-467f830f9cc8nThe use of 'participation factor', 'effective mass', and 'ratio of effective mass to total mass' becomes relevant when deciding how many modes are required to have an accurate Modal Superposition (MSUP) method of calculating the Transient Structural, Response Spectrum or Harmonic Response analyses. n