This includes frequency response, direct matrix input, modal effective mass, complex eigenvalue analysis, dynamic optimization, and. May 04, 2016 a common rule of thumb for linear dynamic analysis is that a mode should be included if it contributes more than 12% of the total effective mass. Modal mass participation ensure youre capturing at least 80% of the mass participation or covering the frequency range of interest. Why doesnt mechanica give mass participation factors for a modal analysis. It provides a measure of the energy contained within each resonant mode. Export the joint masses table to excel by selecting analysis results joint output joint masses sum all joint masses to find the total mass square the participation factor table and divide this value by total mass. Till now we are not considering the mass participation factor for targeting the modes. Thus, the sum of all the participation factors in a given direction can not exceed 100%. Hi indeed the mass participation factors for linear displacements along u,v,w only, no rotation yet are awailable in 4. Naturally, the same goes for dynamic analyses with centers of mass shifted. 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. For more information, see dynamic analysis response spectra.
Change the name to with added mass before closing the form with the ok button. When running a dynamic random analysis i know how to get the mass participation factors for x, y, and z. When conducting a frequency or linear dynamics study in solidworks simulation, a highly recommended, bestpractice post processing technique is to investigate both the effective mass participation factor empf and the cumulative effective mass participation factor cempf of the output mode shapes. The modal mass participation ratio is widely used as the metric to determine the relative significance of modes in a modal response spectrum analysis. The effective mass participation factor represents the percentage of the system. In several studies llately i am having a hard time getting to the 0. When you want to perform a linear dynamic problem small displacement analysis with linear. Nov, 2014 is it possible to calculate the modal mass form a modal analysis in creo simulate. Mass participation factor and its role in modal anlysis. Interpreting the modal mass participating ratios table from etabs posted on october 8, 2018 march 14, 2019 by engr dennis mercado i have not been modelling extensively in etabs this recently so this query from a former had my technical engine humming once more. Interpreting the modal mass participating ratios table from etabs.
In dynamic analysis, two related topics, resonance and modal participation. I have been doing some basic modal analysis so far successfully and am now about to embark on dynamic random analysis. Nas102b advanced dynamic analysis using msc nastran in this class, you will be exposed to various aspects of dynamic analysis using msc nastran. A mode with a large empf is usually a significant contributor to the dynamic response of a system. Sep 06, 2012 till now we are not considering the mass participation factor for targeting the modes. The square root in this case of the point mass is the participation factor, since the whole of the mass is participating in the mode. What is the difference between equivalent static analysis. What are the participation factors and the captured. Graphs of dynamic amplification factors vs nondimensional rise time trt exist. Do we have to make sure mass participation is 75% or higher in all 3 directions x, y and z answer.
The higher the captured modal mass, the better accuracy you. This includes frequency response, direct matrix input, modal effective mass, complex eigenvalue analysis, dynamic optimization, and test analysis correlation. Does anyone know how to get the rotational participation factors. This application note emphasizes experimental modal techniques, specifically the method known as frequency response function testing. In the first row of the table we select the load name as surfacing mass. The modal participation factor matrix for mode i at dof j is a6 each coefficient is 1 if the eigenvectors have been normalized with respect to the mass matrix. I presented the results that you can obtain using that software. 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. How many modes should we consider in modal analysis of 7. Understanding mass participation factor results in frequency studies.
What are the importance of modal participation factor in. A common rule of thumb for linear dynamic analysis is to include sufficient modes such that the cumulative mass is at least 80% in the predominant direction of excitation vibration. Robot modal analysis torsional mass participation factor. Understanding mass participation factor results in frequency. Dynamically loaded, the point mass is allowed to bounce happily up and down on the spring in one direction, hence the whole of the mass in theory is participating in the bouncing in that direction. So, it is possible that the excitation could match a natural frequency i. Perhaps the number should be enough so that the total effective modal mass of the model is at least 90% of the actual mass. In solidworks a modal analysis is run up the range of 500hz.
Understanding mass participation factor results in. 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. Dynamic response in nx nastran stress analysis experts. Pdf response spectrum analysis for irregular multistorey. Modal analysis is the study of the dynamic properties of a system in the frequency. By plotting the mass participation you can see just how important each mode is. Mpf in meffmass are modal mass participation factors, not conceptually different, dynamics is stiffness, mass and damping. For example, with dynamics it is also important to ensure that you have a. Mass participation factors mechanical acousticsvibration.
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. What are the participation factors and the captured modal mass. One useful check is to plot the mass participation factors. The mass participation factor tells you how significant the modes contribution is, when it comes to the total dynamic response. I have a similar problem, i have my modal analysis up to 40 modes and the maximum percentage of mass participation i get is 77%, i dont think that by increasing the number of modes more than 40 in order to reach 90% mass partici. Determine the number of modes for linear dynamic using mass participation. If a skipped mode is found, an iteration is run again. May 29, 2009 by plotting the mass participation you can see just how important each mode is.
The universally accepted level of mass participation for a linear dynamic study is 0. Also, is critical to decide the number of modes to use when performing either a modal frequency response dynamic analysis sol111 or a modal transient frequency response sol112 analysis, here a minimum of at least the 80% of the total mass participation factor of the system mass structure is required in the direction of the excitation, then. Dynamic analysis can be used to find dynamic displacements, time history, and modal analysis. I need to know what type of analysis did you conduct and if nonlinear dynamic how did you model the plastic hinges. Nas102b nastran dynamic analysis advanced in this class, you will be exposed to various aspects of dynamic analysis using msc nastran.
The effective mass participation factor represents the percentage of the system mass that participates in a particular mode. Dec 03, 2017 introduction what is structure, modal and analysis. Oct 08, 2018 interpreting the modal mass participating ratios table from etabs posted on october 8, 2018 march 14, 2019 by engr dennis mercado i have not been modelling extensively in etabs this recently so this query from a former had my technical engine humming once more. The participation factor itself is the percent of the models total dynamic mass that is deflecting in the shape described by the particular mode. In addition to extracting the natural frequencies and mode shapes, the lanczos and subspace iteration eigensolvers automatically calculate the generalized mass, the participation factor, the effective mass, and the composite modal damping for each mode. Interpreting the modal mass participating ratios table. In the main menu select calculate dynamic analysis.
Refer to the chapter 14 of the wilson book dynamic analysis of structures. How to calculate the relative contribution of each mode in. Example of how to use the mass participation factor in solidworks. Altair simsolid makes it easy to view modal participation factors. What is the difference between equivalent static analysis and. The prescribed curve goes up to a maximum of 500 hz. Other areas are treated in a general sense to introduce their elementary concepts and relationships to one another. Nov 22, 2017 equivalent static method esm and response spectrum method rsm both attempt to quantitatively estimate force due to an earthquake on a structure in terms of a set of static forces applied at specific points. Need help working from home with your bentley software. For example, asce 705 710 is at home, sorry, section 12. This webnote compares two common techniques of dynamic analysis for stochastic inputs, spectral analysis with a known response spectrum, and spectral analysis with an input power spectral density, against full transient solutions. Structural dynamics is a type of structural analysis which covers the behavior of a structure. Advanced parameters displays additional modal analysis parameters for a load case. 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.
How do i calculate the fixedmass participation ratio of the model. 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. Modal analysis is defined as the study of the dynamic characteristics of a mechanical structure. The first analysis is a sturm sequence check which checks the number of modes below a given frequency. Most codes require that at least 90% mass participation factor in each of the lateral directions be obtained for the modeshape extraction used for dynamic analysis. Caesar ii applications guide 2019 service pack 1 caesar ii.
Equivalent static method esm and response spectrum method rsm both attempt to quantitatively estimate force due to an earthquake on a structure in terms of a set of static forces applied at specific points. Cumulative mass the cumulative mass for mode n is the sum of the effective mass factors for modes 1 through n. Minimum design eccentricity edi to be considered during analysis 7. Dynamic loads include people, wind, waves, traffic, earthquakes, and blasts. What are the participation factors and the captured modal. I have been told by my collegues to look out for mass participation factors in my results and that they have to be above 80%, but so far nobody has been able to tell me what they are or what they signify not in a way that i. This program finds the eigenvalues and eigenvectors for a. Pdf response spectrum analysis for irregular multi. Consider a discrete dynamic system governed by the following equation. The objective of the this study is to carry out static and dynamic analysis i.
In a moving mass analysis, the dynamic response of the mass springdamper systems, that represent the vehicle, modify the dynamic forces acting on the underlying structure due to inertia effects. 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. Dynamics of structures, theory and applications to earthquake engineering, second ed. Practical dynamic analysis and design for engineers and. 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. Example of how to use the mass participation factor in.
The equation definitions in this section are taken from reference 1. 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. Another is that a cumulative mass participation factor of 0. I presume that you have basic understanding of a modal analysis. In a moving mass analysis, the dynamic response of the massspringdamper systems, that represent the vehicle, modify the dynamic forces acting on the underlying structure due to inertia effects. The effective modal mass vector for mode i and dof j is a7 the natural frequency results for the sample problem are calculated using the program. Modes having a higher mass participation would be the most significant modes to be focused u. The fixedmass participation ratio may be calculated as follows.
Mass participation does not reach the 80% rule of thumb for a modal analysis. I know how to get mass participation factors from a dynamic random analysis, but this wont work for my present problem since i want the unconstrained modal masses and i dont have a base excitation function. Is it possible to calculate the modal mass form a modal analysis in creo simulate. A modal analysis calculates the frequency modes or natural frequencies of a.
This recommendation was duly noted by my colleague bill reuss, in. Jan, 2017 i presume that you have basic understanding of a modal analysis. Structural dynamics is a type of structural analysis which covers the behavior of a structure subjected to dynamic actions having high acceleration loading. Response spectrum analysis for irregular multistorey structure in seismic zone v. Modal analysis parameters robot structural analysis. This should give you consistent modal participation factors. If the direction of the load is known, examine the mass participation factor and the direction of each mode. Dynamics check modal participating mass ratios youtube. The participation factor indicates how strongly a given mode contributes to the response. One common rule of thumb is that a mode should be included if it contributes more than 2% of the total mass. Natural frequency extraction massachusetts institute of.
Dynamic analysis of railway bridges using the mode. 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. Topic 6 structural dynamics iii analysis of elastic mdof systems. This is a prerequisite to the response spectra analysis, which uses these frequencies to calculate forces, stresses and deflections in the model. How can i show model participation factor for modal. Sturm check sets the algorithm for detecting the skipped eigenvibrations. A common rule of thumb is to capture at least 75% of the modal mass but the missing mass correction will capture the remaining modal mass for an accurate dynamic analysis. It has even been codified in many of the codes of record. High modal participation factors indicate that the mode is easily excited by the applied. Because each mode is characterized by its mass participation, a frequently followed rule is to consider enough modes to reach at least 80% of the total mass participation. Check that this option is selected on the data form and enter a frequency of 45. So if you are trying to judge that, you should look at the individual x y z participation factors. View modal mass coefficient and modal participation factor in sap2000.
282 686 751 635 141 957 778 240 936 1082 528 1359 1331 620 339 324 101 338 158 1110 841 826 98 940 828 1236 386 436 392 864 1023 677 1172 750 1339 1025 1290 546