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. A modal analysis calculates the frequency modes or natural frequencies of a. 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. Nas102b advanced dynamic analysis using msc nastran in this class, you will be exposed to various aspects of dynamic analysis using msc nastran. In several studies llately i am having a hard time getting to the 0. 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. Minimum design eccentricity edi to be considered during analysis 7. This is a prerequisite to the response spectra analysis, which uses these frequencies to calculate forces, stresses and deflections in the model. Modal analysis is the study of the dynamic properties of a system in the frequency. Dynamics check modal participating mass ratios youtube.
Mass participation does not reach the 80% rule of thumb for a modal analysis. I presume that you have basic understanding of a modal analysis. 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 presented the results that you can obtain using that software. One useful check is to plot the mass participation factors. 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. It has even been codified in many of the codes of record. Response spectrum analysis for irregular multistorey structure in seismic zone v. Nas102b nastran dynamic analysis advanced in this class, you will be exposed to various aspects of dynamic analysis using msc nastran. Natural frequency extraction massachusetts institute of. The fixedmass participation ratio may be calculated as follows. Consider a discrete dynamic system governed by the following equation. Example of how to use the mass participation factor in solidworks.
When you want to perform a linear dynamic problem small displacement analysis with linear. I need to know what type of analysis did you conduct and if nonlinear dynamic how did you model the plastic hinges. 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. Does anyone know how to get the rotational participation factors. 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. 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. Dynamics of structures, theory and applications to earthquake engineering, second ed. For more information, see dynamic analysis response spectra. 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. Dynamic analysis can be used to find dynamic displacements, time history, and modal analysis. Till now we are not considering the mass participation factor for targeting the modes.
What is the difference between equivalent static analysis. Understanding mass participation factor results in frequency. How do i calculate the fixedmass participation ratio of the model. 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 the main menu select calculate dynamic analysis. Sep 06, 2012 till now we are not considering the mass participation factor for targeting the modes. Another is that a cumulative mass participation factor of 0. Dynamic loads include people, wind, waves, traffic, earthquakes, and blasts.
Modal mass participation ensure youre capturing at least 80% of the mass participation or covering the frequency range of interest. A mode with a large empf is usually a significant contributor to the dynamic response of a system. 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. Mpf in meffmass are modal mass participation factors, not conceptually different, dynamics is stiffness, mass and damping. Naturally, the same goes for dynamic analyses with centers of mass shifted. It provides a measure of the energy contained within each resonant mode. 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.
The equation definitions in this section are taken from reference 1. Why doesnt mechanica give mass participation factors for a modal analysis. Refer to the chapter 14 of the wilson book dynamic analysis of structures. Robot modal analysis torsional mass participation factor. 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 effective mass participation factor represents the percentage of the system. For example, asce 705 710 is at home, sorry, section 12. Structural dynamics is a type of structural analysis which covers the behavior of a structure. 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. By plotting the mass participation you can see just how important each mode is.
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. This application note emphasizes experimental modal techniques, specifically the method known as frequency response function testing. Dec 03, 2017 introduction what is structure, modal and analysis. Jan, 2017 i presume that you have basic understanding of a modal analysis. 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.
Pdf response spectrum analysis for irregular multi. The modal mass participation ratio is widely used as the metric to determine the relative significance of modes in a modal response spectrum analysis. How to calculate the relative contribution of each mode in. 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. Other areas are treated in a general sense to introduce their elementary concepts and relationships to one another. What is the difference between equivalent static analysis and.
Cumulative mass the cumulative mass for mode n is the sum of the effective mass factors for modes 1 through n. Pdf response spectrum analysis for irregular multistorey. The universally accepted level of mass participation for a linear dynamic study is 0. Dynamic response in nx nastran stress analysis experts. Dynamic analysis of railway bridges using the mode. What are the participation factors and the captured modal. For both options the calculations can process a range of vehicle speeds on each execution of the imdplus facility. The participation factor itself is the percent of the models total dynamic mass that is deflecting in the shape described by the particular mode. Modal analysis parameters robot structural analysis. Determine the number of modes for linear dynamic using mass participation. 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. The participation factor indicates how strongly a given mode contributes to the response. Caesar ii applications guide 2019 service pack 1 caesar ii.
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. The first analysis is a sturm sequence check which checks the number of modes below a given frequency. What are the participation factors and the captured modal mass. If a skipped mode is found, an iteration is run again. This program finds the eigenvalues and eigenvectors for a. In the first row of the table we select the load name as surfacing mass.
This recommendation was duly noted by my colleague bill reuss, in. Practical dynamic analysis and design for engineers and. May 29, 2009 by plotting the mass participation you can see just how important each mode is. So, it is possible that the excitation could match a natural frequency i. Interpreting the modal mass participating ratios table. Advanced parameters displays additional modal analysis parameters for a load case. If the direction of the load is known, examine the mass participation factor and the direction of each mode. The mass participation factor tells you how significant the modes contribution is, when it comes to the total dynamic response. Graphs of dynamic amplification factors vs nondimensional rise time trt exist. Modes having a higher mass participation would be the most significant modes to be focused u. 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.
The objective of the this study is to carry out static and dynamic analysis i. Structural dynamics is a type of structural analysis which covers the behavior of a structure subjected to dynamic actions having high acceleration loading. Mass participation factors mechanical acousticsvibration. I have been doing some basic modal analysis so far successfully and am now about to embark on dynamic random analysis. 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 can i show model participation factor for modal. Hi indeed the mass participation factors for linear displacements along u,v,w only, no rotation yet are awailable in 4. Do we have to make sure mass participation is 75% or higher in all 3 directions x, y and z answer. 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. So if you are trying to judge that, you should look at the individual x y z participation factors. 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.
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. 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. Mass participation factor and its role in modal anlysis. In dynamic analysis, two related topics, resonance and modal participation. Sturm check sets the algorithm for detecting the skipped eigenvibrations.
Is it possible to calculate the modal mass form a modal analysis in creo simulate. Example of how to use the mass participation factor in. How many modes should we consider in modal analysis of 7. What are the participation factors and the captured. May 28, 2018 dynamics check modal participating mass ratios.
The prescribed curve goes up to a maximum of 500 hz. Check that this option is selected on the data form and enter a frequency of 45. View modal mass coefficient and modal participation factor in sap2000. Change the name to with added mass before closing the form with the ok button. One common rule of thumb is that a mode should be included if it contributes more than 2% of the total mass. Need help working from home with your bentley software. Altair simsolid makes it easy to view modal participation factors. 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. This includes frequency response, direct matrix input, modal effective mass, complex eigenvalue analysis, dynamic optimization, and. In solidworks a modal analysis is run up the range of 500hz. 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. Nov, 2014 is it possible to calculate the modal mass form a modal analysis in creo simulate. The higher the captured modal mass, the better accuracy you. 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.
Modal analysis is defined as the study of the dynamic characteristics of a mechanical structure. This should give you consistent modal participation factors. 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. 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.
Understanding mass participation factor results in frequency studies. Topic 6 structural dynamics iii analysis of elastic mdof systems. 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. This includes frequency response, direct matrix input, modal effective mass, complex eigenvalue analysis, dynamic optimization, and test analysis correlation. The dynamic analysis calculates the modes and frequencies of vibration for the model. High modal participation factors indicate that the mode is easily excited by the applied. Thus, the sum of all the participation factors in a given direction can not exceed 100%.
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. The effective mass participation factor represents the percentage of the system mass that participates in a particular mode. 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. For example, with dynamics it is also important to ensure that you have a. Altair simsolid modal participation factors the altair blog.
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