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AlephZero

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The answer depends on how you want to model the damping.

In the general case where you have known physical sources of damping (e.g. dashpots) in the model, you have a 4x4 quadratic eigenproblem and both the eigenvalues and vectors (mode shapes) will be complex. In other words, the motion of the different DOFs in a mode are not in phase with each other.

In practice, for small levels of damping where the physical cause of the damping is not known explicitly, you would use a modal damping model based on the undamped modes and frequencies.

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for a viscous damping on this book (pag815):The answer depends on how you want to model the damping.

In the general case where you have known physical sources of damping (e.g. dashpots) in the model, you have a 4x4 quadratic eigenproblem and both the eigenvalues and vectors (mode shapes) will be complex. In other words, the motion of the different DOFs in a mode are not in phase with each other.

In practice, for small levels of damping where the physical cause of the damping is not known explicitly, you would use a modal damping model based on the undamped modes and frequencies.

http://books.google.com/books?id=AK...sult&ct=result&resnum=1&sqi=2&ved=0CCkQ6AEwAA

I found that from an undamped transfer function, it is necessary to substitute the stiffness coefficients (k) with k-jc, where c is the damping coefficients.

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