# Tag Info

Accepted

### Do internal / external stresses affect modal frequency?

Consider a small piece cut out of a structure that has non-zero internal stress. To maintain equilibrium, there must be some forces applied to the boundary of the piece. (Of course when it was part ...
• 12.6k

Take the Fourier Transform of the time varying driving force, this will give the frequency content of the driving force. Multiple modes of vibration can be driven at once, and will superpose with each ...
• 1,327
Accepted

### How to know which modes are excited by a given input in modal analysis?

They way to answer this question is transform the dynamics model into modal coordinates, and see what happens to the force term. Suppose we can describe the stiffness and mass properties of the ...
• 12.6k

### What is the frequncy range of seismic vibrations?

The frequency and ground acceleration and other pertinent data are recorded in a seismic design espectra which is unique for each site and varies with the type of the soil on the side. Usually ...
• 22.6k

### Why does the mesh size affect my modal analysis results?

I am not knowledgeable to answer this question but to lead you to read this article, which says "In order to obtain greater number of eigenmodes (and natural frequencies as well) finer mesh of ...
• 8,237

### Why does the mesh size affect my modal analysis results?

I mean the purpose of modal analysis is not to acquire nodal displacements. Displacements are random. I disagree with both of those statements. Part of the reason for performing a modal analysis is ...
• 526
Accepted

### What does multiplicity of a multiple natural frequency mean?

"Multiplicity" usually refers to several different modes with the same natural frequency. For example, ignoring any fault conditions and tolerances, if there are several identical planet gears, each ...
• 12.6k

### How to know which modes are excited by a given input in modal analysis?

Firstly, applying a constant load will not excite any modes. You need a time-varying load to induce vibration (e.g. wind, seismic, blast). Which modes are excited will depend on the frequency content ...
• 2,760

### Negative Eigenvalues in Ritz Solution of First-Order Shear Deformation Theory

The only way you can get negative eigenvalues is by including stress stiffness effects (sometimes called "geometric stiffness"), if there are compressive stresses which would cause the plate to buckle....
• 12.6k

• 386
1 vote

### FRF based material constant optimization

I have only done this sort of optimization using non-commercial software. Rather than trying to fit "the FRF" (i.e. the complete shape of all your measured response functions) I would start by ...
• 12.6k
1 vote

### How to handle modal combination in response spectrum analysis for member forces for member with more than 1 element?

You are correct that the response for each mode must be calculated before applying some modal combination methodology. Instead of thinking about the structure as being discretized into numerous ...
• 2,760
1 vote

### The distinction between lateral mass participation and vertical mass participation

From my understanding, mass participation for each mode is determined by the general mass and the eigenvector of the mode, and eigenvector of the mode usually consists of contribution from all the ...
• 12.6k

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