Frequency response mode of vibration

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Discussion Overview

The discussion revolves around the frequency response mode of vibration for a simply supported beam with a lumped mass, specifically focusing on the comparison of numerical results from Abaqus with an analytical equation for natural frequency. Participants explore how to incorporate mode numbers into their analysis.

Discussion Character

  • Technical explanation, Debate/contested

Main Points Raised

  • One participant presents an equation for natural frequency and seeks guidance on incorporating mode numbers into their analysis.
  • Another participant suggests that the provided "exact" solution may only be an analytic approximation for the first mode and encourages checking which mode it aligns with best.
  • A different participant confirms that the exact solution aligns most favorably with the first mode results from Abaqus.
  • Another participant cautions against assuming the solution is exact, suggesting it is likely derived from a Rayleigh approximation.

Areas of Agreement / Disagreement

There is no consensus on the validity of the "exact" solution, with participants expressing differing views on its accuracy and applicability to modes beyond the first.

Contextual Notes

Participants do not clarify the assumptions behind the analytical equation or the specific conditions under which it may hold true. There is also no resolution on the mathematical steps required to incorporate mode numbers.

roldy
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I'm working on an analysis problem using Abaqus. I would like to compare my results with an exact equation. My problem is modeled as a simply supported beam with a lumped mass at the end. The equation that I'm using for the exact solution is as follows:

k=3EI/L3
ωn=[itex]\sqrt{k/(M + 0.23m)}[/itex]

How do I incorporate the mode number to find what the exact solution is at say mode 2 or 3?
 
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That "exact" solution is probably only an analytic approximation for the first mode. Run the numbers and see which mode it compares with most favorably. I would bet it will be the first mode.
 
The exact solution does compare most favorably with the first mode results from Abaqus.
 
Don't hold your breath on that being an exact solution. It is most likely obtained from a Rayleigh approximation.
 

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