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I have been reading a text on "Finite Element model updating". This question is not related to the subject of "Finite Element Model Updating" but rather to the subject of "Structural Dynamics" and experimental modal analysis.

My questions is:

1)Let us say I have carried out a free vibration analysis of a system (let us say single storey frame)

2) The text says that,

"N number of observed modes in dynamic tests may not necessarily be the N lowest-frequency modes in practise. In other words, some lower modes might not be detected. For example, some torsional modes are not excited. Further, in case there is damage in the structure, the order of modes may shift because local loss of stiffness from damage may affect some modal frequencies more than others"

I do not understand this concept of mode matching. Can someone put some physical insight. Let us say I have a simply supported beam. I get 5 modes following a modal analysis.

Why is that if I experimentally obtain the first 5 modes, then the numerically obtained modes by FEA not match the measured modes (With respect to the above stated by the author)

With respects

Kajal

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# Mode matching (measured modes and modes obtained from FEA)

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