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# Modal Analysis - Science topic

Explore the latest questions and answers in Modal Analysis, and find Modal Analysis experts.

Questions related to Modal Analysis

Recently I'm working on crack simulation in modal analysis using ANSYS APDL.

The process is essentially to solve a static problem in the first step, then solve a modal analysis problem based on the results in the first step (with prestress effect on).

In the first step I used XFEM to model the crack. The static problem is solved successfully. When I was trying to do the modal analysis, I got this error:

**An unexpected error ( SIG$SEGV ) has occurred... ANSYS internal data**

**has been corrupted. ANSYS is unable to recover and will terminate.**

**Previously saved files are unaffected. Please send the data leading**

**to this operation to your technical support provider, as this will**

**allow ANSYS, Inc to improve the program.**

Does this mean XFEM is not applicable to modal analysis in ANSYS?

I tried simulating the natural frequencies of a beam to compare with my analytical solution using Timoshenko beam theory. My simulation using COMSOL presents only the transverse vibration frequencies. Is it possible to simulate torsional vibration frequencies? Does anyone know the theory employed in the finite element analysis for eigenfrequencies using COMSOL?

**Hello everyone,**

**I'm working on doing modal analysis of simple 1 dimension beam. I know that ANSYS APDL doesn't deal with specific dimensions. But when I do modal analysis to extract the natural frequencies of the beam, Ansys gives me different results from the results of the analytical solution ( manual ). Also, it gives me different results from results of a published paper.**

**so, my question is : what is the dimension of these results ? and how can I overcome this problem ?**

**Thanks in advance**

I am currently working on my project of crack detection in cantilever beams using an Ansys, and I did modal analysis and harmonic response to find the natural frequencies and frequency response of the deformation.

I have found the maximum amplitude of all 6 modes and how does it really work in crack detection?

amplitude gets lower when frequencies get higher.?

Hello ,

I am performing Modal analysis for ultrasonic cylindrical horn at a frequency 20KHz. After performing the linear perturbation analysis to find the natural frequency of the horn the end result shows a distorted meshed figure. I am unable to understand where I am going wrong.

Any help related to this query would be appreciated.

Technical specifications of the horn.

Diameter : case 1: 2 cm , case 2: 6 cm

length :14.3cm

Natural frequency: 20KHz

material: Aluminium

Thank You so much.

Hello World,

I am trying to perform the modal analysis and harmonic response analysis for forced vibration in rotating cantilever beam through ANSYS. Is there any way to incorporate the rotational velocity and add Coriolis and centrifugal effect for the analysis?

I am trying to do modal analysis of tank using

**Ansys Mechanical**but as the fluid is not having any elastic properties, the Ansys is throwing an error. I tried giving properties in the engineering property step but its not working.Also I found that in some old research papers that they are modelling tank walls as SHELL181 and fluid as FLUID80 elements. Do I have to model the tank according to that way as I am not been able to find these elements in the ANSYS.

I have a model of a building in which I have already defined the structural and non structural masses. Structural masses are defined through the density of the material while the non structural masses are defined as point masses. In this step I want to perform the modal analysis. In the subsequent steps I want to define a static analysis and later a dynamic analysis(earthquake). During the static analysis I want to exploit the gravity load for the structural mass while I want to use distributed pressure for the non structural. But if don't remove the point masses I would consider it twice. In the subsequent step I need these masses for the dynamic analysis. To resume, I have to apply the point masses in the first step, remove from the second and put in the third. Is it possible? When I try to do this from one step it's immediately eliminated from all the steps

I did some measurements with Polytec laser vibrometer instruments to obtain the natural frequencies and mode shapes of the different beams I have in the laboratory.

After doing modal analysis, the mode shapes are not smooth although we did with a dense number of measurement points (we did the measurement in 2D and 1D and for both the mode shapes are not smooth). The material of the beam is concrete.

Who has the same experience?

I look forward to hearing your suggestion.

I can not repeat the tests and I may do smoothing. But all papers that used this instrument had the initial mode shapes, smooth.

Since Hinges don't get activated during FNA and I have to provide Links.

I need to know what should be the force deformation values and Effective Damping for Multi linear plastic links at the ends of beams and columns in Nonlinear modal analysis.

I can get the moment curvature curve from section designer but how to input it in a 2 joint link which asks for force deformation values.

So there is a simple beam element on which I was experimenting, trying to find answers to the questions: What is the difference between body load on a constrained assembly (bolted onto a baseplate) and direct nodal load on the constrains? Is there any of these setups that is close to Large Mass Method?

I got some very interesting findings that need explanation. I

**bolded out**the questions that need an answer. Please find the simulation details in the attached images!Findings:

- Free-free setups have many orders of magnitude lower response to the same unit acceleration load than fixed-free setups.

**Is this explainable physically, or is it only a simulation gimmick because of mode participation factors (99.99...% factor for rigid-body mode)?**- Also, when nodal acceleration is applied, if rigid-body mode is included, the response is the same as when omitted.

**Why?**(.f06 was checked and it really uses the selected modes) And also if rigid-body mode is omitted from the body acceleration load type, the response is the same (whereas, when rigid-body mode is included, I got unit response because rigid-body mode dominates).**Why?**- Enforced acceleration on one end of the free-free beam resulted in a fixed-free modal response. So I'm guessing enforced motion works like a fixed constraint in the modal analysis, then in the Frequency Response it works like a load.

**Is this statement true?**- In the fixed-free setups, nodal and body load types resulted in similar responses at the eigenfrequencies, but showed differences elsewhere.

**Why?**Also, nodal load goes to unit response at 0Hz while body load goes to 0 response at 0Hz.**Why?**Sidenote: I also made a reference run with SOL108 Direct Freq. Response with the fixed-free nodal load setup without damping, and got of course similar result to SOL111 same setup.

Thank you if anyone can answer my questions! The 'Help' of NX was not very helpful :(

Hello,

I am studying the performance of 2 wing models (with the same size, constraints, loading and material properties but different material distribution) using Nastran. Wing A is lighter and 24% stiffer than wing B and that is demonstrated in the static analysis (under a distributed load on the top skin).

The two wings have about the same fundamental frequency (34.25Hz and 34Hz). Using the relationship f=sqrt(K/m), the difference between the two frequencies should be significant.

Also, the divergence analysis yields exactly the same displacement for the two wings.

Does any one know what could be causing such results? why the divergence displacement is not smaller for wing A (since it is stiffer) and its frequency is not much higher than wing B?

Thank you in advance for your input.

I am trying to perform a modal frequency analysis on valve by Abaqus, But the answer is zero in the first three modes.Anyone know about this?

In fact, I simulated simplified of this model and without any bolts and the simulation was done and ok. When I add the bolts to the model and apply the interaction between hole and screw , the simulation makes error and the first three value gets zero.

Figure 1 is a simplified model without bolts, which was successfully simulated.

Figure 2 is a model with bolts and nuts that I have trouble simulating.

It is recommended to calibrate the measuring instruments before performing any experiment. Ideally, how often do we need to calibrate an accelerometer? Are there any simple and effective methods for calibrating accelerometers in the laboratory without seeking professional assistance?

Hi,

I'm traying to perform a successive modal analysis using APDL, at the end of every analysis i need to save modal frequencies in a table by APDL commands. Please, is there anyone who can help?

kind regards

I am analyzing the squeal of Disc Brake Assembly using Abaqus and I tried to find the unstable frequencies using complex frequency which caused the squealing noise: I am able to find the unstable modes but they are not forming a stable unstable pair. Also, the participation factor is coming for many modes. I am attaching the brake model for reference. Please help me to find unstable frequencies using Abaqus?

Hi everyone,

I want to extract the global mass and stiffness matrices of an Abaqus modal analysis, so as to post-processing it with Matlab.

I have already read many comments on RG regarding this question and added the following lines at the end of my Input file to get the mass and stiffness matrix.

*STEP

*MATRIX GENERATE, STIFFNESS, MASS

*MATRIX OUTPUT, STIFFNESS,MASS,FORMAT=MATRIX INPUT

*END STEP

So far, everything is fine.

I have also written Matlab code that works perfectly for 3D solids or shell elements, but not when both solid and shell elements are together.

In my model, I had to use both solid and shell elements in Abaqus. Looking at the mass and stiffness matrices, I saw that there are 3DOF per node for solid elements and 6 DOF for shell elements, so I cannot use the attached Matlab code as the model has 3DOF for solid elements and 6DOF for shell elements.

My mass matrix looks like this:

1,1, 1,1, 5.115770202111807e-10

1,2, 1,1, 4.923414202110917e-12

1,3, 1,1, -5.733085750203833e-15

2,1, 1,1, 1.453330463639921e-11

2,2, 1,1, -2.461707101055460e-12

2,3, 1,1, 2.866542875101822e-15

1,4, 1,4, 4.295511112920469e-14

1,5, 1,4, -4.054846090561977e-14

1,6, 1,4, -8.889774326370160e-16

2,4, 1,4, 1.225209356335630e-14

2,5, 1,4, -1.103358818158063e-14

2,6, 1,4, -4.552490123962178e-16

...

131010,1, 131010,1, 6.793914400596321e-08

131010,2, 131010,2, 6.793914400596321e-08

131010,3, 131010,3, 6.793914400596321e-08

131011,1, 131011,1, 7.423556175182037e-08

131011,2, 131011,2, 7.423556175182037e-08

131011,3, 131011,3, 7.423556175182037e-08

As you see here, some parts have 3DOF and some parts have 6DOF. Some parts are like a lumped and some parts are like a consistent mass matrix.

In this case, how can one use Matlab in order to get a global stiffness and mass matrices from this?

I've been struggling with this problem for a long time and still haven't been able to solve it.

Here, I am attaching both the mass.mtx file and the Matlab code.

Any help would be appreciated.

function [matlab_matrix] = import_matrix(mtx_file)

%============== Import Stiffness Matrix ==============%

abaqus_stiffness_matrix = dlmread(mtx_file);

% merge node number info from column 1 and DOF info from column 2 and

% store in the 1st column of a new matrix

%If number of DOF are 2,multiply by 2.

matlab_nodes(:,1) = 2*(abaqus_stiffness_matrix(:,1)-1)+ ...

abaqus_stiffness_matrix(:,2);

% merge node number info from column 3 and DOF info from column 4 and

% store in the 2nd column of a new matrix

matlab_nodes(:,2) = 2*(abaqus_stiffness_matrix(:,3)-1)+ ...

abaqus_stiffness_matrix(:,4);

% extract the stiffness values from the .mtx file, and store in a double

% length vector

stiffness_values = [abaqus_stiffness_matrix(:,5); ...

abaqus_stiffness_matrix(:,5)];

% create a matrix of the new matlab node numbers, and a vector of indices

% of their position in the abaqus stiffness matrix

[matlab_matrix_indices, abaqus_stiffness_value_index] = unique( ...

[matlab_nodes; matlab_nodes(:,2) matlab_nodes(:,1)], 'rows');

% compile the stiffness matrix using the new node numbering convention

matlab_matrix = accumarray( matlab_matrix_indices, ...

stiffness_values(abaqus_stiffness_value_index), [], @max, [], true);

If there, can you recommend me a good software?

Dear everyone,

I am trying to simulate the modal behaviour of an aircraft wing and its dependency on the temperature load. I have treated the aircraft wing as a catilever beam, with the airfoil cross-section.

However, when I am applying the temperature, it shows no change in the results. Can anyone tell me where I am going wrong?

Note: I am using the Beam189 element.

can you please tell how to solve the governing equation to obtain the frequencies to compare with the ansys result

I am analyzing the squeal of simple brake using Abaqus and I tried to find the unstable frequencies using complex frequency which caused the squealing noise; however, all of the modes have nearly zero dampings (lie on the imaginary
axis) and the pairs of modes which become coupled and formed a stable/unstable pair couldn't be found. I attach the brake model in the comment. How can I found some unstable frequencies using Abaqus?

Greetings,

We know that locking in the finite element method (FEM) is a numerical artifact due to the choice of the approximation functions. A couple of implications of locking in a static analysis can be mentioned as follows

- A FEM model of a beam subjected to a point load at its tip can severely underpredict the tip displacement if the FEM model is prone to shear locking.
- A FEM model of a beam acted upon by pure bending moment would develop spurious membrane strains if the FEM model is prone to membrane locking.

*As far as modal analysis is concerned, what is the effect of the locking phenomenon on the determination of the natural frequencies and mode shapes of the system?*

If a model of beam without damping is considered we obtain the real natural frequencies. However, I would like to ask what shall be the nature of the natural frequencies if we consider the rotary inertia of the beam? Whether they will be real or complex?

Performing modal analysis on freely hanged structure using impact hammer method. In this case roving acceleration method is followed and obtained different FRFs at different locations. so here my questions are

1. how to use these multiple FRFs to calculate single FRF of a structure ?

2. can we get identical FRFs from different locations ?

3. how many FRFs can a structure exhibit ?

Hi, I am doing research about operational modal analysis using SSI. Generally, SSI only identifies natural frequencies, damping ratios, and mode shapes. But I am wondering how to estimate modal contribution of each identified mode using SSI. I did not find many papers related to this topic. I hope someone can recommend some work or give me some hints to me.

Thanks!

hello

In order to do my thesis, I need to place the damper I developed at OpenSees on several asymmetric steel benchmarks. For this purpose, to develop asymmetric one- and nine-story structures, I decided to use the method given in the above article, but unfortunately in the article, the structures are parametric and I do not realize this.

Here are some basic questions for me.

One, how the mass and structural members of these structures are defined and modeled.

Second, how can a find period for these structures?

And finally, how is it possible to get the response of these structures under the earthquake record?

My strongest guess is that it is to define stiffness of each structural member as a coefficient of mass or vice versa.

Any help is greatly appreciated.

Take refuge in the right.

Hi guys,

I am trying to perform a modal analysis of a pre-tension 2D array of 4-strut simplex tensegrity plate in Abaqus. Can anyone tell me how to apply pre-tension to the struts and cables for modal analysis in Abaqus?

For now, I was able to perform modal analysis without pretension and did get mode shapes. But only the first two were similar to plate mode shapes and the remaining modes have in-plane or only corner motion. I want to study the influence of pretension in mode shapes and for that, I need to apply pre-tension.

Here I am attaching the abaqus input file

Hello

I am trying to perform a sine sweep test using OROS 38 DAQ, Modal analysis software and The modal shop shaker 2100E11. The DAQ software allows to specifying the peak to peak voltage from 0 to 10V and I am using natural air cooling for the shaker so the sine peak force is 220 N.

Only accelerometers are available to get the data and no force sensors are available. Is there any way to measure approximate values of force corresponding to the given peak voltage in the DAQ system?

Please help me out with this.

Thank you

hi, i'm civil engineer phd

my qustion about how i can include a prestressed in my systeme (dam) for a modal analysis with Ansys apdl

think you

Hi

I had a questiones about the operation of the Riley damping command in Opensees.

First, how does the Rayleigh Damping Command affect the damping in any structural mode?

Second, can the Rayleigh Damping Command be set to affect only certain modes of the structure?

Any help is greatly appreciated

I wish you all the best

I've extracted mass and stiffness matrices from a modal analysis in Ansys. I need to know how the elements of each matrix is related to node ID. Anyone can help me with an ansys command to find each node DOF location in mass and stiffness matrices?

Dear researchers

What is your opinion about the criterion recommended in seismic codes for determining scaling period, which are used to scale ground motion records?

As you know, the mentioned criterion is the period of the structure’s dominant mode, which has the largest modal participating mass ratio (usually the first vibration mode). Hence, the period of the mode with the second largest modal participating mass ratio is not considered in the scaling process. Consequently, although this criterion usually results in the largest value of scaling period, it is not logical ones.

This is especially important when Tuned Mass damper (TMD) or Base-Isolation system is utilized, which cause the modal properties of the structures to change.

I used a new criterion based on the weighted mean value of the periods for the structures equipped with TMD.

Have you used any criteria other than the criterion mentioned in the seismic codes?

I am running modal analysis using Ansys software. i need to extract the effective mass of each mode using APDL and Workbench

hello everyone,
i'm trying to model a 3D one story buiding with mass IIrregularity, the model has 4 elastic columns and rigid diaphragm. the model has no beam, so i used rigid diaphragm to equal dof of top nodes of columns. i use a model from a reasercher's article, he note that the modal frequences of first three mode is 1.16 , 1.96 , 2.37 HZ respectively. i builded the model in etabs and can verify my model with 0.33% error, but i couldn't verify model in opensees. i trying almost everythings to verify it but Unfortunately all my work was unsuccsesfull. in my reaserch i found that mass moment of inertia is principle parameter in model with mass IIrregularity, so i try figure out how modal analysis run in etabs and opensees and what difference between them. i found that etabs centeralized all masses in center of mass and then make mass matrix with 3 parameters like that
[mass in x direction 0 0
0 mass in y direction 0
0 0 mass moment of inertia about hight axis ]
but i wondering opensees generate mass matix for each node(also we can use center of mass for assigne mass) and also mass matrix for 6 dof model with 3 limited dofs (dofs limited in z direction and rotation about x ,y axis) is 6*6, so i'm confused to understad how opensees derrive mass matix and calculate modal frequences. i try both assigning mass to nodes and center of mass. in first condition i assigned mass moment of inertia like that
mass n "mass in x direction" "mass in y direction" 0. 0. 0. "mass moment of inertia about height axis"
etabs calculate mass moment of inertia with multiple each mass of nodes by distance of node from center of mass then sum these valus and attain mass moment of inertia abuot height axis. also we can use another formula for it. i try all patterns i found but i can't verify my model because i can't understand how opensees run modal analysis and make eigen values. for best understanding i upload my model in opensees and etabs.
I'm so thankful if you can help me.
with best regard

I am trying to simulate response of structure with TLD using ANSYS APDL. I mesh the fluid volume and tanker so that they can have coincident nodes, merge the nodes, flagged on FSI for the interface areas plus I assign SURFACE-To_SURFACE contact between tanker and structure slab (which am not sure if it's the right way to assign connection between tanker and top slab). when i run modal analysis for the fluid filled tanker (with out attaching it to the structure) i find frequency value which is almost equal with Housner's equation, but when i do the modal analysis for the structure+TLD and check the deformed shape the fluid elements penetrate the structure boundary and don't know how to correct it. I would appreciate if any one can give me some recommendation how to solve the problem. For your reference i have attached the snip of the deformed shape.
Thank you in advance

Hello.

I modeled a complex ring in abaqus and with adding below command in keyword, extracted Stiffness matrix with Coordinate format. But output is a .mtx file that listed stiffness for each node (or element). What to do i to assemble this file to reaching to Global stiffness matrix (stiffness matrix for whole part, i mean m(d2x/dt2)+Kx=F)?

*STEP, name=exportmatrix

*MATRIX GENERATE, STIFFNESS

*MATRIX OUTPUT, STIFFNESS, FORMAT=MATRIX INPUT

*END STEP

I am trying to model the combined effect of a static and dynamic load on a cantilever beam. I have calculated the dynamic response as follows

1) Natural frequency and mode shape extraction (modes 1-5), see image

2) Steady-state modal analysis gives the steady-state response across a range of frequencies. I am looking for deflection with direct model damping of 2%. If I am not mistaken, this form of analysis require a load independent of time and essential applies harmonic loading to each of the natural frequencies and calculates the response at each frequency.

I am unable to apply a static load at this stage (steady-state modal dynamics) to observe the combined effect. See image for the deflection as a function of frequency. This is precisely the plot I want if I was just considering dynamic response. But surely, this would change significantly with a static load.

I tried setting up a static load in a step prior to the frequency and steady-state modal dynamics step but that was of no use.

Any tips on how to do this.

FYI, I am actually trying to model a chassis experiencing cyclic and static loading from an engine and am using this to fully understand the problem.

Is there any software or excel sheet to calculate the mineral proportions (modal % of minerals) based on bulk-rock analysis. Your kind help is highly appreciated.

Hi

I know the procedure of modeling modal analysis to obtain natural frequencies(I mean using Linear perturbation step>>frequency and without defining any other loading....) but the problem is all examples I have seen is just for model made of just one material(most examples for cantilever beam) but my model is consist of two or more material( 2layer soil and layer of polymer between these soil layers), for obtaining natural frequencies in this case, my exact question is: I have to run modal analysis for each material separately or all together?!

I should mention that the polymeric membrane part does not have any type of support and its just in interaction with soil( normal and tangential behaviour), and if I run the modal analysis for this part alone natural frequencies for polymeric membrane is zero. while the soil is fixed at bottom and sides and by running modal analysis once for soil alone and another time soil with polymer,the natural frequencies are the same for both conditions. so i'm willing to know any advise and special point to obtain natural frequencies procedure for model consist of two or more materials in abaqus.

Hello,

I tried to perform a response spectrum in my structure. However I can't proceed because of the error (

**ERROR: Data file file.db does not exist for RESUME**). My modal analysis run successfully but I am getting this error when performing response spectrum. How can I fix this error?I am currently using ANSYS 2020 R2 student version.

Thanks

If I have a rotor on two bearings, under what conditions should I use pre-stressed modal analysis and when should I run a rotordynamic analysis i.e. using the rotordynamics options in a modal analysis?

When I heat the structure and run the analysis, the deformation by temperature occurs, and we want to re-run the modal analysis on the deformation results.

It takes a long time to enter boundary conditions at once and run simulations.

Currently, ET is using 227.

If there's a way to solve it by using commands, could I know how to use?

Hello there! I'm new in using Ansys APDL. I'm trying to do the modal analysis of a simple unimorph piezoelectric cantilever beam, but the result is not what i expect to achive.

I'm wondering that there are some inputs that i miss because of my poor capacity in using APDL, so i'm asking for some suggestions: are there any specific commands that i have to use? I follow a scheme like:

1. Add materials in element type (solid45 for brass, solid5 for pzt, circu94 for the electric part that i don't need for this first analysis)

2. Choose the material props thar i've found on datasheets

3. Modeling the cantilever (2 volumes one on the other): 4.85x1.0x0.4 mm brass and 4.85x1.0x0.5 mm for pzt

4. Glue the volumes

5. Attribute the materials and mesh the model with mapped mesh (0.5mm per edge)

6. Put the boundary conditions on the blocked area (pzt+brass) UX=0, UY=0 and UZ=0

7. Solve modal analysis

This is the result of the first mode:

I'm so upset! I hope someone could help me!

Thanks!

It's one of the Modal analysis identification techniques and also a time domain method.

Dear all

I am trying to perform a modal analysis of a beam yet I am keeping recieving the following warning:

''The model dimensions in the solver unit system were determined to be very large. This may lead to numerical accuracy issues. Check results carefully.''

How to possibly fixe that?!!

I've done the Modal Analysis followed by Structural analysis in Ansys Mechanical & now I want to export my model in State Space Form. So that i can use it in Matlab.

Everything works well except loading of input vectors, I've followed guidelines from the link below:

example:

my query is that the given link provides accessibility to define DOFs in ux, uy & uz. However, I want to Include variable forces in x,y & z direction that will be controlled by my Control Algorithim in Matlab. how can I add forces as input?

The link also says if i don't define any input array it will define all the Loading vectors in the system as Inputs(by Default). But it's not working as it says.

Note: I've defined "Nodal forces" using "Named Selection"

Hey there!

I guess it should be straightforward, but i can´t find out how to extract the eigenvectors from a simple free-free (no bondary conditions) modal analysis in ANSYS workbench . Does anyone know how can i extract these eigenvectors from ANSYS workbench? Maybe using the Command window... I've searched in the folder with the output files and i can't find the eigenvectors anywhere. But i'm sure the program creates these at some point in the solution to obtain the mode shapes and natural frequencies.

Thank you all!

I created a finite element model of steel truss bridge and tried to do a modal analysis to generate the natural frequencies and mode shapes. Unfortunately, i don't get deformed shapes, but rather the SAP 2000 software will add more constraints at the middle spans of the bridge. Furthermore, negative Eigen values are also generated. Please i need help. Find the screen shot of the bridge before the modal analysis and after the modal analysis.

I did two simulations with the same geometry one is Quad dropped (4 nodes/elements) and the other is kept (8 nodes/elements). Maintaining all bc and settings the same , i get different frequencies in modal analysis in Ansys. For example 100 Khz vs 125 Khz , the shapes are correct but the frequencies is totally different .

Any ideas why is that ?

I am performing modal analysis of a Free-Free 2D beam (1 x 0.02 m) and I am trying to compute the mode frequencies and mode shapes up to the lowest 32 frequencies. Here I observed that the mode shapes started to repeat, such as

1,2,3 are rigid modes

4,5,6,7,8,9,10,12,13,14,16,17,19,20,21,23,24,26,27,29,30,32 are elastic modes

11,15,18,22,25,28,31 modes are similar to 2,4,5,6,7,8,9 modes

I used FENICS and verified in ABAQUS and got the same results.

**Is this a phenomenon or a simulation error?**

I will be using this mode data to perform Fluid-structure interaction using the modal expansion method.

**Can anyone suggest what is criteria for choosing the number of mode shapes for this method?**

**Should I include the repeated modes in the modal expansion method?**

**Also, can anyone refer me to some books or articles related to the modal expansion method and modal analysis?**

For details, please see the following studies.

Dear all,

In ansys mechanical, I performed static structural analysis with adaptive convergence and I want to retrieve the same convergent mesh automatically generated tu use it in modal analysis. Is there any way to do it? Thanks.

Hi All

I want to simulate an ambient vibration to be used as an input to a Numerical Model. I want this to represent the vibrations occurred to the building through the wind, vehicle/human movement, etc.

The sole purpose of simulating this ambient vibration is to input it to the numerical model in order to find out the natural frequencies, mode shapes and damping ratios of the building through Operational Modal Analysis

Thank you

I need some help, I have run block lanczos modal analysis in ansys APDL.It shows that the 'solution is done ' . However, there is no any result listed in result summary. How to overcome this problem?

Hello,

For the last weeks I have been comparing modal analysis results for ANSYS, Inventor and analytically. For solid square and rectangular beams the results were quite similar, however, now that I use a C channel beam the results from ANSYS are totally wrong. Does someone know why this is? I have sent the command file, which I used in ANSYS mechanical APDL.

The formula used is f = (K/(2*pi*L^2))*sqrt(E*I/m)

From the formula the first four eigenfrequencies should be: 14.8, 93.08, 261.03 and 511.91. Inventor showed similar results, but ANSYS fourth eigenfrequency is only 90.11. If someone knows why this is the case, please tell me. I have already checked the weight, Moment of Inertia, length etc.

After taking the frequency spectrum of an impulse force (from a shaker, sampled from a vibration analysis test), there are noticeable peaks at 50 and 150 Hz (presumably due to some electrical/electronic artefacts or some mismatch impedance between the shaker's coil and its structure). This specific experiment aims to study the effect of frequency resolution on the spectrum.

When the frequency resolution is smaller (same bandwidth, or F_max - F_min, is used, and more spectral/FFT lines are used), these sudden peaks start to appear. It should be noted that the original input signal is the same. The question is why this happens?

Thanks for your help

"I have mode shapes and eigen frequencies from FE simulation. I have modal matrix and eigen frequencies corresponding to each mode. The mode shape is mass normalized. I am required to correct each mode for damping, i.e to add 1% modal damping to each mode. do you know how to add 1% damping to each mode. what should I do

Structural dynamics, Vibration, modal analysis

Hi, I am currently simulating pipe using modal analysis.

How can I export a text file that includes node location for Ansys 2020? Currenlty if I export the total deformation text file it only shows node number and deformation value. Kindly help!

Dear researchers

I modeled a specified structure with the same properties (mass and stiffness) (apply mass through defining nodal masses instead of calculating from gravity loads) in both Etabs and Sap2000 software and observed differences between the modal results of the structure (the value of natural frequencies). After various investigation (all factors affecting the stiffness of the structure), I have found that the mentioned differences are arisen from difference between the mass matrices created in Sap2000 and Etabs software.

Since, unlike SAP2000, it is not possible to output the mass matrix and nodal masses of the structure in Etabs, the reason for the difference in mass matrices cannot be identified.

In your opinion, what is the reason for the difference in the mass matrices created in Sap2000 and Etabs? And which one is more correct?

Hi,

For a rectangular plate of a given dimension and material, we have the expression for modal density as n = Area/(4*pi) *((rho*h)/D)^(0.25) , where D is transverse stiffness of the plate, rho is the density of the material, h is the plate thickness.

This gives a constant number.

1. What value of modal density is high?

2. If I look at the FRF for a rectangular plate, I can see that as the frequency increases, the resonance peaks are closer to each other. Is this related to the modal density? If yes then why is modal density a constant number?

regards,

Vachan

**For IUGS classification(QAPF) modal(volume% ) analysis of the plutonic rock is necessary.**

**In thin section how can it be done?**

Hello,

I am trying to export Mass and Stiffness matrices using Ansys Workbench via Modal analysis modular, with APDL commands inserted. However, there is no .matrix file generated. Could anyone please tell me what I'm doing wrong? The figure illustrating the procedure as well as the commands is attached.

Thank you.

Rui Wang

The displacement mode shapes are determined by running the modal analysis on AnsysWorkbench. How we can have the strain mode shape and the modal strain energy using ansysWorkbench. Example: cantilevered beam.

I am working on a brake squeal problem and for the unstable modes I am getting a pair of stability readings in Hz (where one is positive and the other is of the same magnitude but opposite sign) which I dont know how to interpret. I understant that the real part of the eigenvalue obtained is the represents the stability but I don't understand how you can show this as a reading in Hz.

Dear researchers

The modeling of P-Delta for the structural elements (specially columns) reduce the stiffness of the structure and consequently increase the period of vibration. Now, these changes are negligible when the structure and P-Delta effects are modeled in OpenSees software. Instead, the mentioned changes are more noticeable when modeling is perform in the softwares such as Etabs and Sap2000.

How to apply P-Delta effects is the reason for difference changes due to P-Delta modeling in different softwares.

Now with all these interpretations, what is the most accurate method for modeling P-Delta effects? In other words, In which software are stiffness changes calculated more correctly due to modeling of P-Delta effects?

As you know, In Etabs and Sap2000 software, P-delta effects are applied to the structure by defining a gravity load combination, while this procedure is performed in OpenSees using geometric transformation (second-order P-Delta effects) and its effects on stiffness matrix.