What do you mean by mass participation factor in modal analysis. Dynamic analysis of railway bridges using the mode superposition method the mass participation factor as convergence criterion masters thesis in the international masters programme structural engineering andreas karlsson and andreas nilsson department of civil and environmental engineering division of structural engineering. For real systems there is often mass participating in the forcing function such as the mass of ground in an earthquake and mass participating in inertia effects the mass of the structure itself, m eq. Notice when you see these factors that they are associated with a direction x, y, z, etc. Effective mass participation altair optistruct altair. Modal analysis embraces both theoretical and experimental techniques. Jun 08, 2018 hi all, how to output the results for effective mass participation factor in modal analysis is it sufficient to look for the first 6 to 10 modes or a couple of hundred hz say 500 hz. In a previous post understanding the mass participation factor i have talked about the modal analysis in solidworks and how to make the most of it. Other areas are treated in a general sense to introduce their elementary concepts and relationships to one another. Modal participation and missing mass corrections nptel. The fundamentals of modal testing application note 243 3. I enclose also an old paper on how it was implemented in nastran using dmap. But the definition of participation factor no where indicates the number of modes as a parameter.
Altair simsolid modal participation factors the altair blog. Modes having a higher mass participation would be the most significant modes to be focused u. Another is that a cumulative mass participation factor of 0. May 11, 20 priestley et al 1996, among other authors, confirm that a sum of all empf known as cumulative effective mass participation factor, cempf of 80% to 90% in any given response direction can be considered sufficient to capture the dominant dynamic response of the structure. I presume that you have basic understanding of a modal analysis. Also, all values of effective mass are very low and the sum of them do not equal total mass of the structure. I tried to retrieve participation factors after modal analysis with some get and i saw that its not possible since get,mode,pfact gives only rotz dof i use to retrieve participation factor after spectrum analysis and i was thinking that i could do the same after a modal analysis. The student community is a public forum for authorized ansys academic product users to share ideas and ask questions. Interpretation of modal analysis in the first phase of the modal analysis procedure, the vi bration propertiesnatural frequencies and modesofthe structure are computed and the force distribution vector s or rna. Modal is the simplest analysis and the only thing it does is telling you what are the resonance frequencies of your geometry. What is role of mass participating factor while doing. Sep 06, 2012 i am surprise there is no clear cut answer to this till now, as far as i know and i got.
Modal parameters estimation of building structures from vibration. Equation 18 indicates the modal acceleration is, of course, lagging the base acceleration input by 90 degrees. As you said in sap2000 display tables, it gives the participation factors which is true but it. The structure is unconstrained and there is no damping. The participation factor identifies the amount each. Priestley et al 1996, among other authors, confirm that a sum of all empf known as cumulative effective mass participation factor, cempf of 80% to 90% in any given response direction can be considered sufficient to capture the dominant dynamic response of the structure. Long ago ansys listed the equivalent mass which didnt vary with the excitation direction. Mscnastran version 2001 and beyond has the capability to extract modal mass participation directly from nomal modes analysis. This should be performed in combination with understand modal mass participation to determine what modes will likely be significant ensure that modal analysis captures full modal content ensure that you are capturing at least 80% of mass participation or full range of frequencies of interest. Responsespectrum analysis faq technical knowledge base. Modal identification of nonclassically damped structures. For more information, see dynamic analysis response spectra. Tdefine mass vector w 0, 10, 10 determine mass participation by.
The modal participation factors and mode shapes are obtained from the relationship between. On the other hand, regarding the results and if we look at the solution information tap, my main doubt is about participation factors values, as most of them are 0. Robot modal analysis torsional mass participation factor. Example of how to use the mass participation factor in. Modal participation factor variation gmarsh mechanical op 29 dec 11 11. Kim and choi 6 presented a nonlinear static analysis. Topic 6 structural dynamics iii analysis of elastic mdof systems. Number of modes depends on number of degrees of freedom. The modal participation factor i for mode i is mo ii li i 26 the modal participation vector is thus. Mass participation factors mechanical acousticsvibration. To understand this physically, we must first understand that our structures, not unlike the rest of the universe, are very lazy and tend to take the path involving least work. The fixed mass participation ratio may be calculated as follows. Square the participation factor table and divide this value by total mass.
Modal mass coefficient and modal participation factor in. Modal analysis provides an overview of the limits of the response of a system. Dynamic analysis of railway bridges using the mode. The modal participation factors can be easily calculated.
By plotting the mass participation you can see just how important each mode is. The modal participation factors, together with the eigenvectors calculatecl in a real eigenvalue analysis can therefore be used to obtain estimates of what the magnitude of the grid point accelerations will be in a subseqcent forced base response analysis. In simple terms the mass participation factor squared gives you the effective modal mass for that mode for your structure. Till now we are not considering the mass participation factor for targeting the modes. Oct 20, 2015 this page is devoted to frequently asked questions faq related to responsespectrum analysis.
Mass participation factor and its role in modal anlysis. Dynamic analysis of railway bridges using the mode superposition method the mass participation factor as convergence criterion masters thesis in the international masters programme structural engineering andreas karlsson and andreas nilsson. Xansys view topic xansys struct participation factors. Contents restartsindynamicanalysis 91 overviewofrestartsindynamicanalysis 91. Original page is of poor quality the solution to equation 16 for the modal acceleration 6 is where and is the modal amplification in mode 1. Access to the modal participation dialog is from the results area of the modal analysis workbench toolbar. It isnt related to a loading at this stage, only to the geometry. Robot modal analysis torsional mass participation factor thanks pawel i actually suspect the spectra scaling factor to account for spmu was not calculated correctly by robot when i plot the fi being story forces over the height of the structure, it does seem some deviations at upper floors. This is a prerequisite to the response spectra analysis, which uses these frequencies to calculate forces, stresses and deflections in the model. However, the mass participation of subgrade levels may be subtracted from the total mass. This application note emphasizes experimental modal techniques, specifically the method known as frequency response function testing. Its less valuable in any case i can imagine which is why its not readily available. Jan, 2017 i presume that you have basic understanding of a modal analysis.
Altair simsolid calculates modal participation, effective mass and cumulative mass factors for each mode in a specified global or local coordinate reference frame. I used the main structure, with 2 point mass at the positions of main wheels as equivalent masses of the total sprung mass of the pod, that is around 128 kg. When the responsespectrum curve is defined, acceleration is typically entered as a fraction of gravitational acceleration 9. The following is a sample executive and case control deck for a normal modes run with modal mass extraction. Jun 08, 2011 unsubscribe from computers and structures, inc sign in to add this video to a playlist. When i am doing a modal analysis, how many resonant modes should i check. Internal forces can be determined by static analysis of the lddb h i l l lf 2, 0 sin i iiwt ji i j ji g d i di f twm xe wtd. Table tool in midas civil offers a variety of powerful builtin functions. Modal mass is independent of directioncoordinate systems as one example. My doubt is that modal mass coefficient and participation factor as per atc 40 are unitless values. The modal mass is actually different than the participation factor masses. An example of the use of finite element analysis in the creation of a lowcost, environmentally sensitive structure is itford farm bridleway bridge 2. I presented the results that you can obtain using that software. The effective modal mass for mode i is 28 for mode 1, 29 30 for mode 2, 31 32 note that 33 34.
Hi logesh, you can add param,effmas,yes in your input deck bulk part or in hypermesh tool pagecontrol cardsparam,effmass,yes. How can i estimate modal participation factor from mode. The modal participation factors and effective mass will be computed and output to the. Participation factor and effective modal mass altair. Quantification of uncertainties and model updating master. The theoretical modal analysis anchors on a physical model of a dynamic system comprising its mass, stiffness and damping properties. Modal mass coefficient and modal participation factor in sap2000. After spending some time yesterday i discovered that i might be able to use modal analysis and extract the mass participation factors to get a measure of the energy contained within each resonance mode. Just to add the generalised mass associated with each mode is same irrespective of number of modes. The dynamic analysis calculates the modes and frequencies of vibration for the model. If modes 3, 6, 8 and 10 look like torsional modes, then this would probably confirm my assumption. Topic 6 structural dynamics iii analysis of elastic mdof. Modal direction factor, modal participation factor and participation vector have been added to the output so as to check various eigenvalue analysis results in a table. Modal analysis is defined as the study of the dynamic characteristics of a mechanical structure.
Modal analysis is defined as the study of the dynamic. Effective mass participation altair optistruct altair forum. Note that the eigenvectors are defined with a free normalization and so the modal parameters have no physical meaning. How can i estimate modal participation factor from mode mass. Scale factor should be specified such that its product with responsespectrum values. There are very few documents where they have written the mass participation factor, and its direction, external excitation direction and mode shape determine the no of modes we have to consider. If so, then rx, ry and rz would likely represent the mass particiaption of the rotational mass moment of inertia. The weighting, often called the modal participation factor, is a function of excitation and mode shape coefficients at the input and output degrees of freedom.
The captured modal mass percentage shows how much of the response is attributed to a particular mode and also shows the mode orientation x, y or z. These properties may be given in forms of partial differential equations. The first three elements describe the physical system. My guess is that ux, uy, and uz represent the mass participation of the translational mass. The captured modal mass is another way of quantifying the importance of the mode and the two are related. Understanding the mass participation factor engineer jau. As a case study, system identification of a baseisolated bridge is undertaken. No dmap is required and it can be used with the normal modes solution sequence. I carried out modal analysis in ansys apdl for mode shape upto 100 modes since i wasnot able to obtain modal participation factor more than 90 % with lesser mode. Dynamic response in nx nastran stress analysis experts. What is role of mass participating factor while doing modal analysis and how can we come to a conclusion that how many number of modes are required for a particular modal analysis. Check the mode shape normalized to unity maximum eigenvector 1. Topic 6 structural dynamics iii analysis of elastic mdof systems equations of motion for mdof systems uncoupling of equations through use of natural mode shapes solution of uncoupled equations recombination of computed response modal response spectrum analysis by example use of reduced number of modes.
I am surprise there is no clear cut answer to this till now, as far as i know and i got. During responsespectrum analysis, loading is applied to the entire structure, though subgrade modes will have much higher frequencies, providing minimal contribution to modal response. The use of modal analysis for structure analysis leads. The participation factor is a measure of the importance of the mode in earthquake type load. For brevity, only a limited number of modes should be included in the analysis. Effective modal parameters to evaluate structural stresses rochus p. Export the joint masses table to excel by selecting analysis results joint output joint masses. The modal participation factor for mode i is 26 the modal participation vector is thus 27 the coefficient vector and the modal participation vector are identical in this example because the generalized mass matrix is the identity matrix. Hi all, how to output the results for effective mass participation factor in modal analysis is it sufficient to look for the first 6 to 10 modes. Apr 25, 2016 what is the dominant mode of a system. But if the first six modes only contribute 60% mass participation, and above 12 th mode also if for mode the mass participation factor is higher should we check with compressor harmonics upto that or not. The use of finite element analysis in the design of. This article what is modal analysis and why is it necessary gives a more detailed answer to your question, but in short.
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