7/26/2019 What is the Mode Shape

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What is the mode shape?A mode shapeis a specific pattern of vibration executed by a mechanical system at aspecific frequency. Different mode shapeswill be associated with different frequencies.The experimental technique of modal analysis discovers these mode shapesand thefrequencies. www.AzimaDLI.com.

What are the modes of vibration for?No the modeis the shape of the vibration. !or instance the pitch of a "uitar strin" thatyou pluc# is its lowest natural frequency. The correspondin" modeis the vibrationalshape that you will see from the side a shape that has the lar"est displacement in themiddle and vanisin" at the end point.

What is a normal mode?A normal modeof an oscillatin" system is a pattern of motion in which all parts of thesystem move sinusoidally with the same frequency and with a fixed phase relation.

What is meant by modal analysis?Modal analysisis the study of the dynamic properties of structures under vibrationalexcitation. Modal analysisis the field of measurin" and analysin" the dynamicresponse of structures and or fluids when durin" excitation.

Natural Frequencies and Mode Shapes

One analysis technique for calculating the linear response of structures to

dynamic loading is a modal analysis. In modal analysis, we decompose theresponse of the structure into several vibration modes. A mode is defined byits frequency and shape. Structural engineers call the mode with the

shortest frequency (the longest period) the fundamental mode.

One can thin about the mode shapes as principal a!es of the structure fordynamic loading. "his concept is similar to that of principal directions and

corresponding principal stresses in stress theory. In fact, the mathematicalprocedure to calculate principal stresses and principal directions on the one

hand and natural frequencies and corresponding mode shapes on the other

are identical. In both cases we solve an eigenvalue problem.

#uring dynamic loading, i.e. earthquae, wind or blast loading, not all modes

are e!cited in the same manner. "he e!tent to which dynamic loading e!cites

a specific vibration modes depends on the spatial distribution and the

frequency content of the load.