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I'm currently do my final year project, I use time integration method to solve linear dynamics structure.
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Nice and important question and nice answers. As a maybe late comment (probably you already do not need the answer), there is legalized comment on the integration step in the seismic standard of New Zealand; see the references below:
[1] NZS 1170.5 Supp 1. Structural Design Actions - Part 5: Earthquake Actions. Standards New-Zealand, New Zealand, 2004.
[2] NZS 1170. Structural Design Actions, Part 5: Earthquake Actions-New Zealand. New Zealand, 2004.
according to which the digitization step of the earthquake acceleration is an upper-bound of the time integration step. Still, there are techniques and methods suggested for using integration steps larger than the earthquakes steps. I have recently presented a review on one of them, where you can also see explanations about the others. The reference, also attached, is
A. Soroushian. A technique for time integration with steps larger than the excitation steps: Review of the past addressing the existing challenges and a perspective of the future. In Proceedings of 8th ECCOMAS Thematic Conference on Computational Methods in Structural Dynamics and Earthquake Engineering (COMPDYN2021), Athens, Greece, June 28-30, 2021.
In any way, as also implied in the response of dear Iolanda-Gabriela Craifaleanu , according to reference such as the Structural Dynamics book of Clough and Penzien 1993 and even main references from numerical solution of initial value problems, for the sake of responses sufficient accuracy, the time integration analysis need to be repeated with smaller steps once or by times . . . .
OK. many sincere thanks for your kind attention, and best wishes for all of the people in this question group especially regarding health and happiness . . . . . .
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Dear Researchers Greetings,
I'm trying to model a structure under earthquake loadings, I want to investigate the structure's non-linear behavior.
I went through a lot of papers in the literature. Besides those who rely on finite element method and modeling software (i.e., SAP2000, ETABS, Openssen ...). A few papers use the lumped mass system to represent structural models and introduce a hysteretic restoring force that reflects the inelastic behavior of the structure. This force is calculated based on the hysteretic behavior model presented by Bouc and Wen.
My question is, How accurate are the models based on this approach? If there exists an alternative to this approach please let me know.
Warmest regards.
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Thank you for taking the time to answer my question. I'm only interested in coding the non-linear behavior using the lumped mass system in MATLAB or PYTHON.
This choice is mainly motivated by the willingness to introduce a semi-active or active device afterward, which can't be modeled using SAP2000 or ETABS.
Regards.
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I have developed a particle reinforced metal matrix composite for a specific application that require a static,dynamic and couple field analysis. May I know the procedure to perform these modelling and analysis of MMC products using Ansys or any other specific software.
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Following
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Hello Researchers,
The FEM discretized (meshed) geometry/domain is considered stiffer than the actual geometry/domain due to the assumption of variation of the displacement within each element. This is analogous to the displacement being constrained to vary in a particular fashion within each of the elements. This results in the stiffness of the discretized domain being greater than the actual domain. As the element size decreases (or the number of elements increases), the constraint on the displacement loosens due to the smaller size of the element and hence, the smaller constraint zone. Thus, the stiffness of the meshed domain decreases and approaches that of the actual domain as the number of elements is increased.
Based on the above reasoning, the natural frequencies (on increasing the number of elements) must converge from above to the actual value (i.e. converge from higher values to the actual value).
  1. Can this be considered to be strictly true?
  2. Has any deviation from it been observed (i.e. convergence from below or lower values to the actual value) and if so how can that trend be physically explained/interpreted?
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Dear Jatin,
Not always does a finer mesh result in a more exact solution. A mesh convergence study should always be performed to guarantee the descending trend of the error as the mesh size gets smaller.
Having this verified, yes, a finer mesh reduces the stiffness of the model. Because FE approximates the the PDE solution by forcing the element into specific modes of displacement which yields a stiffer element. But as the element size decreases, the FE solution converges to the analytical solution of PDE.
Eigenvalue can be physically interpreted as how stiff the structure is in the eigenvector direction. So it follows the same pattern as stiffness.
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Jerk is defined as the rate of change in acceleration. But I would like to know some practical applications of Jerk inorder to have better understanding. I kindly request to suggest me some examples.
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The application of jerk in physics have many instances and one of the example I can shot is simple that the jerk is nothing but it is all about the rate at which any objects acceleration changes with the time or with respect to time.
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Of course, set the damping ratio of a structure to 5% will simplify the dynamic calculation. However, we all know it is just an assumption and since structures are quite sensitive to their "real" damping ratio, especially when the frequency ratio is near the unit. I wonder how we can achieve an actual damping coefficient numerically?
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First you should find the mass matrix and the stiffness matrix. Afterwards, you should solve the eigenvalue problem associated with the problem at hand and these specific matrices. The structural damping coefficient is equivalent to the fraction of Im(lambda) over Real(lambda), where lambda is the complex eigenvalue and Im and real, respectively denote imaginary part and real part of the eigenvalue. I have attached a triple of screenshots to my comment for your perusal.
For forced vibration (for example in presence of air dynamic pressure), the solution of the eigenvalue problem is a bit harder than to find a solution to an ordinary eigenvalue problem. For concreteness, in this regard, I have attached a PDF of my own original idea to this comment. The PDF is also available in my ResearchGate project:
Linear Aeroelastic Analysis of Laminated Composite Plates with Fully Elastic Boundaries
Under the title:
Eigenvalue problem for aeroelastic vibrations: The solution
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Hello Researchers,
I am using Gauss Quadrature for numerical integration to obtain the stiffness and mass matrices for a plate element in my FEM code. We know that both these matrices are symmetric. However, I find that due to numerical integration the stiffness and mass matrix turns out to be asymmetric.
Kindly note that the asymmetry is not by any means large. The result of the subtraction of a symmetric matrix from its transpose is a null or zero matrix. If I subtract the stiffness and mass matrix from their respective transposes, the resulting matrix has all the non-diagonal terms of the order 10 to the power of -8 and all diagonal terms are zero (maybe for most cases it can be considered as a zero matrix).
At the point of writing this question, I am suspecting that this discrepancy (i.e asymmetry of the mass and stiffness matrices) is due to the finite precision arithmetic of floating-point numbers. (need your thoughts on whether my suspicions are true)
The end result of not having symmetric stiffness and mass matrices is that the 'eig' function in MATLAB gives incorrect eigenvectors although the eigenvalues are correct.
I would like to know if anyone has encountered such issues and how was it resolved.
I am also attaching a couple of links related to finite precision arithmetic errors below for your reference:
Thank you,
Jatin Poojary
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yes, your diagnosis seems correct since the difference is of order of 1e-8. in computers, no floating point number can be represented exactly. therefore, it's common to use some epsilon value is used to avoid it. alternatively, when writing from scratch, only upper or lower diagonal is saved in the memory for a symmetric array to avoid such issues.
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I am going to model a three-layered circular plate with a magnetorheological core in Abaqus CAE 2016 . is there a way to model the MR material without writing subroutines, simply in the property module for example?
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Have u got the solution even i m trying to model it
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i have vibration data of bridge and process the data with different possible techniques such as FFT, Wavelet decomposition, Empirical mode decomposition, Hilbert Transform, Frequency domain decomposition, spectrograms, etc. but still i am not able to locate exact or pin point frequency of the bridge structure. in the following techniques the results are in a specific range of frequency such as 2.6 to 3.1 Hz. The data showing this specific range and not providing the exact frequency. because in this range any number can be the natural frequency of the bridge structure. so the actual frequency lies in this specific range but i want to go further in depth to figure out the exact value of the frequency in Hz. but the problem is i am not able to find any technique that will give me answer of my curiosity. i recorded data from accelerometers considering normal traffic conditions. there is not closure of bridge. vehicles are passing normal situation.if anyone have any clue about it or still my question is not clear so i can explain further
i attached some of my results
i will be very grateful to anyone who let me find my answer
thanks
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Dear Ali,
the maximum response recorded by your accelerometers is mathematically the CONVOLUTION of the structural response of the bridge (with a peak expected at each natural frequency) and the excitation by the trucks (with a peak at their own suspension frequency) meaning the frequency of the maximum response is a mix of both! Of course the truck's primary suspension frequencies vary from a truck to the next, still in the same range.
In addition the natural frequency of the bridge is load dependant (weight and position of the heavy trucks at the time of the record), and temperature-dependant (meaning day/night, clear day/cloudy and seasonal fluctuations...)
Back to your precise question, you have just NO WAY to determine the exact bridge natural frequencies by this "natural response" method. As suggested, you must close the bridge (however with the same added load than the usual traffic) and use for example a dropped weight to generate a broadband shock (preferably located in consideration of the anticipated mode shape, which allows you to trigger distinctively the first flexural/torsional modes). And still the exact frequency will vary with the previously mentioned load and temperature conditions... Sorry for you, that's just the complexity of structural vibration physics!
Back to Vahid's remark, we could only appreciate the likely natural frequency range of your bridge by knowing its precise construction and span, but the natural frequency of very long bridge spans can even be below 1Hz (remember the ill-fated Tacoma bridge, destroyed by wind gusts at approx. 0.5Hz resonance https://www.youtube.com/watch?v=j-zczJXSxnw). Obviously short bridges have higher natural frequencies but you can interpret it as revealing over-designed structures! The Tacoma bridge example is also a good evidence of the 3D complexity of the mode shapes (something also well evidenced by Eric's study, which provide a very accurate in-situ modal analysis). My guess is that in your case the 2.6 to 3.1 band correspond effectively to the interaction of the trucks suspensions with one of the main natural frequencies of this bridge... in this global 2.8 +/- 0.3Hz band! But don't ask for more precision...
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I mean the use of the SQUG database to assess the seismic resistance of NPP equipment with the specified type of reactor.
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GIP - Generic Implementation Procedure
WWER - water-water energetic reactor
NPP - nuclear power plant
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Hi everyone,
to calculate the prying forces in the column base plate connections, we need to get the contact reaction between the plate and the concrete, but in my model, I have a part in compression and another one in tension. so how can I evaluate the surface of the plate in tension under the cyclic load ?
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Dear Aichouche
If I were you, I would forget the Castem and go for SolidWorks Simulation . it's more easier, faster, and efficient.
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So I am beginner in Random vibration research. I have gone through some theory like ergodicity, stationarity, PSD etc.. related to Random Vibration. I am trying to model a very basic SDOF system subjected to random vibration.
I am trying Python. I have already modelled a harmonically loaded linear vibration system. For that I just needed to solve the equation of motion. But I am confused regarding incorporating randomness. How and where I use
How can I move about ? What are the baby steps that I should take ?
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There are two ways-
1. Spectral simulation method- Find output spectral density (So) using harmonic transfer function and input (force) spectral density (Sf). Then calculate the required mean square response (MS) from the area under Sf curve over required frequency band using the numerical integration of Sf function.
2. Time domain method- First you are required to generate the time-domain force f(t) signal using ‘wgn’ function in the MATLAB and save it as data file. Then solve the equations of single DOF system in by using f(t) as input with MATLAB solvers such as ode45.
The white-noise produced by the ‘wgn’ is essentially the band-limited and max frequency is equal to half of the sampling frequency. The ideal white-noise has infinite variance but ‘wgn’ requires to specify it as follows.
Var of wgn = Spectral Ht of white noise required (N^2/rad/s)* sampling frequency(rad/s)/2
In your case choose the sampling time (dt) such that, dt<=1/(2*10*fn), this ensures that the generated white-noise contains the highest harmonic of frequency 10*fn. Where fn=sqrt(k/m)/(2*pi). The factor more than 10 can be used but it is sufficient for lightly damped single dof system.
Then Sampling freq (Hz) = 1/dt.
3. SIMULINK is convenient for this use and it is not required to save the signal f(t). The blocks ‘Band Limited White Noise’ and ‘WGN’ can be used.
Hope this will help.
Note that. the suggestion presented by Chandrashekhar Dharankar.
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I am interested to perform spectral analysis of a structure under random waves. could anyone suggest me a book or an example that starts from wave spectrum (such as
JONSWAP spectrum , P-M etc) to RAO. A complete example from formulation to numerical evaluation.
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The summary can be accessed from the link below:
Please let me know whether the information that has been gathered in the summary file is informative or not.
If the summary description is not clear enough, any suggestions will be welcome.
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Thanks a lot, Dear Armin Huß
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I would appreciate some insights.
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I also worked at chimneys on ANSYS.
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Any suggestions? Provision of restrained braces in the frame.
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This is for braced frame,
Thanks
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hello everyone can anyone please suggest me book on structural dynamics?
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Seismic Analysis of Structures By T. K. Datta
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"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
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The mode shape is not altered by hysteretic damping distributed homogeneously as a material property, it is simply the amplitude of the response to a dynamic force input which is reduced. This corresponds to a slight out-phasing between the applied force and the response acceleration/displacement, that requires, as rightly said by Alessandro, to use complex (vectorial) formulations.
If the damping model is not hysteretic (i.e. an intrinsic property of the material's young modulus), the physics turn far more complex and the mathematisation non-linear (e.g. viscous or frictional damping). In such cases the mode shape is altered and reveals generally amplitude-dependant...
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Dear researchers
As you know, one of the challenges of using nonlinear procedures is to determine the behavior of plastic hinges of members with deformation controlled action that this behavior is assigned to the plastic hinge by a force-deformation curve and its relations using parameters modeling. various researches has shown that the uncertainties in these modeling parameters significantly affect the structural responses.
Also, the acceptance criteria of different performance levels relating to the mentioned force-deformation curve are needed for performance-based design of structures.
There are two questions now:
1- Are force-deformation curves presented in ASCE 41-13 suitable only for nonlinear static analysis (push over)? or also is applicable for nonlinear dynamic analysis?
2- Given that the acceptance criteria presented in ASCE 41-13 are derived based on the mentioned force-deformation relations in this code (a, b and c modeling parameters), what acceptance criteria can be used to evaluate the structure at the IO, LS and CP performance levels if the other force-deformation relations presented in the technical literature (such as Lignos and Hartloper relations for beams and columns of moment frames, respectively) are utilized for concentrated plasticity modeling?
The mentioned curves (Lignos and Hartloper relations) are mostly used in structural modeling to study the structural collapse, in which the collapse is determined by the criteria mentioned in FEMA p-695 and as a result, acceptance criteria in accordance with these behavior curves have not been researched.
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1. Seismic codes suggest simplified force-deformation models in order to estimate the inelastic behavior both in monotonic and cyclic loading. The monotonic F-δ curve is considered as the envelope and the skeleton curve of F-δ loops under cyclic loading. The real inelastic behavior under cyclic loading depends on the material and the dynamic loading, e.g. reinforced concrete under seismic loading. So, stiffness and strength deterioration should be considered under cyclic loading in the concentrated plasticity modeling technique.
2. Uncertainties about deformation capacity are high beyond the point C of the F-δ curve. Even in the Collapse Prevention performance level (before point C), the ultimate deformations shows significant dispersion in experimental cyclic tests (e.g. reinforced concrete). Consequently, appropriate acceptance criteria for different performance levels and for different materials can be found in seismic codes (ASCE 41-13, FEMA, Eurocode, EN 1998-3, etc) or in other technical literature using model safety factors to scale down the proposed mean values to mean plus standard deviation ones.
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Hi,
I am trying to model a free-free body (aircraft) as a stick-model only consisting of beam elements. For certain configurations I observed that my Stiffness Matrix loses singularities / my model has an insufficient number of rigid body modes. I broke the problem down to a simple 2-D Beam configuration shown in the attached picture. The model consists of 5 Nodes with 3 degrees of freedom per Node all connected via Euler-Bernoulli-Beams with the analytical Stiffness Matrix shown in the picture. The left configuration exhibits the expected behaviour having 3 rigid body modes. Introducing the additional stiffening beam element for some reason reduces the number of rigid body modes, as indicated by the higher Mode 3 Frequency and the higher Rank of the Stiffness Matrix. Is such behaviour expected? And if yes, what are ways to workaround this problem other than building the model from 3D solid elements?
Thanks for your help!
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Conclusion: Were indeed purely numerical issues. To my surprise beam length errors and node position errors of a magnitude of 10E-6 [m] resulted in a rank deficient Stiffness Matrix.
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by time series, i mean the structural dynamic responses
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Python Environment for Time Series Forecasting
The Python ecosystem is growing and may become the dominant platform for applied machine learning.
The primary rationale for adopting Python for time series forecasting is because it is a general-purpose programming language that you can use both for R&D and in production.
In this post, you will discover the Python ecosystem for time series forecasting.
After reading this post, you will know:
  • The three standard Python libraries that are critical for time series forecasting.
  • How to install and setup the Python and SciPy environment for development.
  • How to confirm your environment is working correctly and ready for time series forecasting.
Python is a general-purpose interpreted programming language (unlike R or Matlab).It is easy to learn and use primarily because the language focuses on readability.It is a popular language in general, consistently appearing in the top 10 programming languages in surveys on StackOverflow (for example, the 2015 survey results).Python is a dynamic language and very suited to interactive development and quick prototyping with the power to support the development of large applications.Python is also widely used for machine learning and data science because of the excellent library support. It has quickly become one of the dominant platforms for machine learning and data science practitioners and is in greater demand than even the R platform by employers (see the graph below).
This is a simple and very important consideration.
It means that you can perform your research and development (figuring out what models to use) in the same programming language that you use in operations, greatly simplifying the transition from development to operations.
Python Libraries for Time Series
SciPy is an ecosystem of Python libraries for mathematics, science, and engineering. It is an add-on to Python that you will need for time series forecasting.
Two SciPy libraries provide a foundation for most others; they are NumPy for providing efficient array operations and Matplotlib for plotting data.There are three higher-level SciPy libraries that provide the key features for time series forecasting in Python.
They are pandas, statsmodels, and scikit-learn for data handling, time series modeling, and machine learning respectively.
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I am modelling a piezoelectric material bonded on the host structure & correspondingly I have to plot electrical conductance vs frequency plot.
I have been doing the work in Ansys apdl & following this video by Sailesh sir. https://www.youtube.com/watch?v=JEvVicDzYl0&t=116s
In a harmonic analysis of PZT, I need to add rayleigh damping coefficients values (alpha & beta) in software, but I don't know what are rayleigh damping coefficients for concrete.
Any help would be appreciated. Thank you
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This article will be helpful
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As shown in the figure attached, the turbulent fluid flow around a floating (i.e. partially submerged) cylinder with a rectangular cross-section is to be modelled. Clearly, the forces and torques acting on the cylinder are oscillatory due to the vortex shedding phenomenon. The ultimate goal is to study the cylinder stability on the surface. The question is then how to characterize the fluid forces. Particularly, I have no idea if such a stability analysis could be based on the time-averaged forces or the instantaneous values may play a dominant role. How to characterize and quantify the significance of time-averaged values versus instantaneous values in this specific application?
I would appreciate any comment.
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Dear Prof. Armin Hajighasem Kashani, I suppose the following document could give some ideas on how the question posed by you might be addressed. Let's hope for the participation of a vortex specialist in fluid mechanics in this thread.
Instantaneous and time-averaged flow fields of multiple vortices in the tip region of a ducted propulsor by G. Oweis y S. Ceccio.
The mentioned authors emphasize, I unquote then, that "...an identification procedure is used to characterize multiple regions of compact vorticity in the flow fields as series of Gaussian vortices. Significant differences are found between the vortex properties from the time-averaged flow fields and the average vortex properties identified in the instantaneous flow fields....".
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Greeting dear researchers,
I'm interested in non-linear behavior of structures (mainly buildings) and I'm looking for a model to simulate this behavior using a lumped mass system and solved in state space representation, I came into few papers dealing with such problem (papers are mentioned bellow) however the exact method used is not clear or detailed.
Thanks for reading and answering in advance.
1. Cimellaro, Gian Paolo, Oren Lavan, and Andrei M. Reinhorn. "Design of passive systems for control of inelastic structures." Earthquake Engineering & Structural Dynamics 38.6 (2009): 783-804.
2. Reinhorn , A. M., Oren Lavan, and G. P. Cimellaro. "Design of controlled elastic and inelastic structures." Earthquake Engineering and Engineering Vibration 8.4 (2009): 469-479.
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Dear Mahdi, although you are using control systems that can not be done directly in SAP2000. However, you can use VBA interfaces to do the same in SAP2000. Same things can be done by matlab.
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Protonic defects has been used several times in discussions of protons existance in high temperature proton conducting metal oxides. When I read some articles, I got difficulty to understand what really the proton defects stand for either is for the excess or absence of protons. which one is proper? More challengingly, is when hydrogen defects used instead of proton defects. i.e. Sentence substracted from one article: The structural, dynamical, and electronic properties of ionic defects in ........created by the addition or removal of a proton have been studied using the method of ab initio molecular dynamics. These protonic defects correspond.............. 
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Thank you Artur Braun for your nice discussion. Could you please suggest an article related to your comments. Also, I want to know why protonic conductivity increase due to annealing through moist N2.
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Fracture mechanics
Structural dynamics
Earthquake Engineering
Structural Health monitoring
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Hi, I recommend the following book
(recent trends in fracture and damage mechanics)
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Hello,
I am working on the structural dynamics of a rocket. In a free-free condition of a rocket, the modal effective mass of the fundamental mode is zero since 100% of the mass is participating in rigid body modes. But in a paper (attached below page:53-54), modal effective mass is computed for the fundamental mode considering a rocket in free-free condition. So want to know how to compute the modal effective mass
for the fundamental mode in free-free condition, any related reference would be helpful to understand.
Thank you
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Toan Van Nguyen 1. Because for a continuous elastic solid, there are infinite no. of flexible modes which we generally don't capture while performing the eigenvalue problem. Each of these modes has some mass contributions (much much lower at higher frequencies) which really doesn't make any sense to capture if you're looking for vibration (durability) or noise problems. So in a nutshell, to have the sum equal to exactly 100%, you'll have to capture all the modes of the system which is generally not required for most of the problems out there.
2. This is the paper that I share earlier in my post. It's not my paper :)
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I am trying to validate my drop mass impact test results with ABAQUS/Explicit impact simulation. The specimens were bare and CFRP wrapped CHS steel tubes. I got good agreement in case of bare column. However, in CFRP wrapped columns I'm facing problems in capturing contact force-time response of impact tests. The displacement time curve of CFRP wrapped tube reasonably agreed with tests considering the complexity of the model. Please find here mid span displacement and contact force time history comparisons as attached picture with this thread. I have used continuum shell elements to model CFRP and cohesive elements to model adhesive between the layers.
I'm wondering whether continuum shell is capable of capturing the contact force response such dynamic problems or I have to used 3D solid elements by using VUMAT subroutine. I will appreciate if anyone can provide me with any help on this issue.
Many Thanks.
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Dear Md Iftekharul Alam
Did you find a solution to your problem?
Regards
Ahmed
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In a structural dynamic wave propagation problem, is the displacement at a point a continuous function of time or is it a continuous function of time only after the wave reaches the point?
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Hi Mir Aamir Abbas
I believe that it depends a bit.
For unbounded systems, there would not be much of a response before the wave reaches the receiver position. More on this below.
For bounded systems, one must cater for rigid body modes. These have infinite (or a very high) wave speed.
This may at first sound strange, but once we think about it a little, it becomes obvious (or there could simply not be any wave propagation on planet earth which has a high velocity indeed when viewed from the galaxy centrum).
To exemplify, in the case or airbag firing inside cars, there is a pressure increase at the driver's ear before the acoustic wave reaches it. This took the industry about 20 years to figure out and it can be correctly detected when using both pressure transducers and microphones. The former tracks Line Pressure and Sound, while the microphone only tracks sound.
So, for an unbounded system, the 'rigid body' response would still be there, but it would very low and thus something that can be ignored.
Now, let us complicate matters a bit. There is an amplitude dependent portion in wave propagation that causes the wave front to distort - it is referred to as steepening. Simply put, waves with a high amplitude travel faster than the linear free wave speed. After a distance, the peak catches up with the trough and the wave collapses and the process starts anew.
So, when %LP = 100*Pulsation/LinePressure = 100*Sound/Ambient is high, things may happen faster than we would otherwise expect. This is generally the case whenever LP is low, e.g. in vacuum systems, on the suction side of machines and so on.
More on the differences of pressure and steepening is found here. https://qringtech.com/2010/09/15/wave-steepening-increase-peak-pressure-piping-pumps/
I am sure one can compound the issue further if one would want to do so.
Just my 2 cents
Claes
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I am trying to regenerate the Natural Frequencies of a clamped-clamped viscoelastic core via. ACM (Approached Complex Eigenmodes) method as is in the article : "Linear and nonlinear vibrations analysis of viscoelastic sandwich beams" (DOI: 10.1016/j.jsv.2010.06.012).I am trying to use semi analytical Galerkin method in order to do so. the problem with the use of ACM method for modeling the core my answers are close but when I increase the number of modes to solve the problem in more precise way my answer get worse! and also there is no convergence happening with increasing the number of the modes. what the problem could be? and would it be a better approach to solve the frequency dependent problems which wouldn't be highly costed in solving?
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Hi. There are two ways. the first one is to construct a mechanical model to describe the viscoelastic behaviour (in frequency domain) and include it in the stiffness matrix of the model (In this case, the Kglobal matrix will depend of the frequency and you will need use a recursive algorithm to compute the modal parameters). the second way is to express your model in a time domain and use the FFT.
I describe the fist way in my reseach
Uncertainty propagation analysis in laminated structures with viscoelastic core
WP Hernández, DA Castello, TG Ritto - Computers & Structures, 2016
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I am solving finite element model updating problem. In order to achieve that i am studying inverse Eigen sensitivity method as a updating method. There, i am unable to understand how to calculate sensitivity matrix. Can anyone help me with small numerical example to calculate sensitivity matrix.
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Hi,
First you need to have parameters p that eigen values f depend continously on (aka Young modulus, poisson ratio...). The simplest way to get first order sensitivity matrix s is apply numerical differentiation, such df/dp=s. For this you can use commercial software. Nevertheless, you can also get semi-analytical sensitivity matrix by differentiation of generalized eigen value problem premultiplied by eigenvector. For doing this you need to have an access to finite element stiffness and mass matrices.
Hope it helps a bit.
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Hi everyone , I need to model a moving load in ANSYS. I want to model the movement of a train on a bridge. IS there anyones who can help me with this issue??
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Hi Eduardo
I have the same issue. I need to do transient analysis of moving load of a real train. Would you be able to help me with this?
Regards
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I did my masters in Structural engineering and want to pursue a PhD in EQ. plz, guide about demanding research topic in EQ and Structural dynamics.
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Dear Muhammad,
This could be challenging topics however it is a general one.
1. Analysis of Uncertainty Quantification and Exascale Computing Opportunities and Challenges for Earthquake Engineering.
2. Design, development and characterization study using sustainable materials and rapid monitoring for earthquake engineering
Hope it could work.
Ashish
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Dear Researchers,
I'm looking for a database on corporate governance structures (board composition, CEO duality, independent and non executive directors, etc.) and their change through time for US-based listed companies.
Do you have any suggestion?
I would like to thank you all in advance for your contributes.
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I think BOARDEX is one of the most comprehensive databases on the composition of boards. It covers various areas of the world (however, unfortunately, it's not free).
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hallo, i am doing rescale use acceleration of spectral response target and spectral response from TH(ie kobe) for 0.2T until 1.5T, where T from natural period building.
do it correct for 1 pair earthquake ( UX,UY), then i scale to response target?
or from 7 ground motion i make average, then scale to response target?
because my spectral cant like my target.(attach)
or i must do scale from base shear from result my response spectrum and my time history?
thank you
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ASCE 7-16 develops maximum direction spectra by applying a factor of 1.1 to the geometric mean Ss and a factor of 1.3 to the geometric mean S1. See ASCE 7-16 for additional details.
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Hey,
I am working on the Multi body dynamics of a 300kW VAWT as part of my graduation thesis. I have created a reduced order model of the turbine using SIMPACK (MBD solver from dassault systems). I am facing some difficulties with running modal analysis due to error in modal matrix being singular.
I am reaching out to people who have worked with SIMPACK, hoping to get some help with my model .
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I think Anays Rigid Dynamics will give you all the possible options in comparison to any other tool. Try for it to cross verify the results.
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Regarding design of a dam(or any similar solid monolithic structure), from Structural-Dynamics point of view, with/without computer coding(e.g. Matlab, Python, etc.); how can we compute Modal-Mass and Modal-Participation-Factors?
In addition to that, by which method can we compute the dam response when subjected to any specified horizontal ground acceleration(such as horizontal contribution of the seismic excitation)?
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Dear professor Kabir Sadeghi ,
Many thanks for the outstanding and magnificent response, which I do appreciate very much.
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How to choose a community that would be a candidate for restoration in certain region, and create a community model that addresses the questions regarding composition, interactions, structure, and dynamics ?
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In the research in community when looking for understanding how the dynamic interplay between structure and shape a simple/complex communities interacted to structures, a community genetics perspective, however, may not be necessary or informative for all studies and systems. To better understand when and how intraspecific genetic variation and microevolution are important in community and structural dynamics, it is suggested that future research should focus on three areas: (1) determining the relative importance of intraspecific genetic variation compared to other structural factors in mediating community and structural dynamic properties; (2) understanding the importance of microevolution in shaping structural dynamics in multi-member communities; and (3) associating genetic mechanisms that drive community and dynamic analysis processes.
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Hello, everybody! Recent, I have a problem about the fitting of the material parameters. I use the hardening model that proposed by Asaro in crystal plasticity model, but I don't know how to fit the material parameters(C11, C12,C44, h0, ginf, g0, C, D). 
Who can tell me the method about the fitting of the material parameters ?
Thank you in advance!
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hi I am working on the mechanical behavior of Ti-6al-4v on a microstructure level with crystalline plastic theory. But I have trouble to get the hard parameters. for calibrating the hardness parameters with the experimental model, there is not any experimental result on the micro level . How can I get the hardness parameters of this material? best regrads
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Is there any relationship available to relate brace angles and lateral displacement in steel diagrid building ?
I am looking for possible optimum brace angle which result in low lateral displacement.
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Hi Mridul Rawat
in my paper is the optimum angle would come between 60-70 degrees
link of my paper is
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How do I use mixed frequency ( i.e. a combination of monthly, quarterly and annual data) in a structural dynamic factor model for macroeconomic shocks analysis e.g. Stock and Watson (2012) -  Disentangling the Channels 2007 - 2009 Recession (not for nowcasting or forecasting)?
Thank you.
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You can use mixed method sampling (MIDAS) to do the analysis or you convert all the variables to the same frequency. You can search for it on google.
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Hello,
A structure is tested for a Random Vibration (say Z axis, 0.1PSD, 20-3000Hz), where I obtained response acceleration from measurement system. Now I wanted to obtain same output(amplitude of modes) by giving Harmonic(sine) sweep as input(in Frequency domain) to Shaker machine. I read some on-line papers explaining conversion/equivalence from PSD to harmonic but none of them are actually working. Can someone please refer some good source?
Thank you,
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The basic assumption of the random vibration is that you can't predict the actual time realisation of the process, only the statistical measures, which assuming a Gaussian process is the standard deviation. So you can only compare a harmonic and random it in an average sense, which is to have the same input power. And that is what the PSD gives you. PSD is usually approximated as the square of the signal's Fourier Transform. So if you know what PSD value your random process has in a fequency bin, then you can work out what amplitude of sine wave would give similar power. But the only way you can relate the two are through the PSD, comparing actual time histories is irrelevant
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I have a multi-storied structure with 3 DOFs at each floor. first 2 translations in X and Y and third being rotation in Z. I am trying to assemble a global stiffness matrix from local stiffness matrices.
I have included local eccentricities in X and Y to account for plan irregularities in the modelling.
This is how my local stiffness matrix looks like.
Ke(:,:,f) = [K1(f) + K1(f+1), 0, -ey(f)*(K1(f) + K1(f+1));
0, K2(f) + K2(f+1), ex(f)*(K2(f) + K2(f+1));
-ey(f)*(K1(f) + K1(f+1)), ex(f)*(K2(f) + K2(f+1)), (ex(f)^2*(K2(f) + K2(f+1)) + ey(f)^2*(K1(f) + K1(f+1))) + K3(f) + K3(f+1)];
Here f and f+1 denote the quantities at floor level f and floor level f+1, ex and ey are the eccentricities in X and Y, K1 is the net lateral force in X and K2 is net lateral force in Y.
I am having trouble in assembling a global stiffness matrix from the local one.Weird part is that, the same code runs perfectly well for regular building with small eccentricities. As soon as I change the plan to an L shape or a C shape where Centre of Mass and Centre of Stiffness drift apart quite a bit, the stiffness matrix changes so much that the natural frequencies are imaginary and the solutions blows up.
Can anyone please guide me?
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Thank you so much Victor and David. I rechecked my global assembly and found an error in one sign of the reactionary force. That was the culprit!
To Victor, yes, I indeed expected eccentricity to have no effect on the nature of the stiffness matrix. It had to be the assembly error.
Thank you so much to all for taking out time to reply to my question. I highly appreciate it!
Regards
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I am Doing project on scale factor. i.e. base shear scale factor. in the manually calculation we dident use any type of 1st scale factor but in etanb 16 we use 1st scale factor as the Ig/2R. also i want to know that why matching is done in static and dynamic analysis.
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Scale factor is Ig/R, not Ig/2R.
As we input values of I and R while defining load patterns for equivalent lateral load procedure(ELF). Unlike ELF in response spectrum analysis u should input these parameters(I & R) by using scale fctor to get inelastic demands.
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Why are horizontal degrees of freedom (DOF) mainly considered in the dynamic equation of motion rather than other DOFs such as vertical and rotational for control of structures?
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In some situations, the horizontal components of earthquake ground motions can excite the vertical degrees of freedom (cause vibration vertical deformation/vibration). This may happen depending on the structural configuration.
For example, consider a fictitious structure, which looks like the letter “F” with fixed support. In this fictitious example, there are two cantilever beams supported on a column.
If this structure is subjected to only the horizontal component of an earthquake, the column deforms and vibrates as a simple vertical cantilever. This initiates the vertical deformation and vibration of the cantilever beams.
With best regards,
Daniel.
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I understand that when solving a static problem, the stiffness matrix is normally singular and that adding specific restraints is what yields a unique solution to the problem. But in dynamic problems, this issue doesn't arise even if the same concept applies, which is the system will have a rigid motion. So, why I don't get the same error when solving dynamic problem. I tried that using SAP2000 by analyzing a frame system without specifying any supports. the static analysis gave errors while the dynamic analysis didn't. So, why?
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The external forces which are unbalanced in statics (owing to lack of some constraints) are balanced by inertial forces in dynamics.
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I have used the Galerkin method in order to acquire the Mass and Stiffness matrices and by calculation of the natural frequencies and comparing them with FEM software, the matrices seem to be valid. I have to say that the assumed mode shapes of the problem are of the Bessel kind. by the use of foresaid matrices and 'ode45' solver of MATLAB when it comes to time dynamic response to the mentioned load, the solution always diverges. the force matrix is already validated by checking the static response of the plate. I have reduced the time step, time span, relative and absolute tolerances and also set the 'NormControl' option to 'on' in order to reduce to solution's errors but unfortunately nothing changed. so what the problem could be? should I change the solver to another one such as Newmark-Beta ? by the way, the Mass matrix determinant is about 0.006 and is a little close to singularity but it haven't caused any trouble in calculating the natural frequencies where we need the inverse of this matrix the same as calculating the dynamic response, however in calculation of time response I have used 'pinv' function for the inversion of the Mass matrix, which is used in such cases to increase the accuracy.
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Dear Mohammad and Lubos thanks.
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I used a frame structure to represent a continuous shell plate and this frame structure was calibrated with a physical model in order to  have the same rigidity as the real one. However I was trying to identify how representative would if a change the thick of the frame elements to represent a shell plate.
Thank you in advance.
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Good answer.. Nathan Madutujuh ... Following
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I am working on rotational seismology, and I want to give rotational ground motion data as a input in SAP 2000, and want to check the torsional mode shapes of structure, and want to study about effects of rotational component of ground motion data, If any other software is available for this purpose then please suggest me that also.
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Hi Ankit Goyal , you may input the rotational ground motion as a rotational acceleration load in SAP2000. To do this, first define a time history function based on the ground motion data you have. Next, define a load case for the acceleration load. Select "Time History" under the load case type. Under "Loads Applied", select "Accel" load type and the rotational degree of freedom under "Load Name". Use your defined function for the rotational acceleration. You may refer to the following websites for more information:
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I have modeled a multi-story building structure equipped with active tendons systems. I am using sliding mode control method to decrease the dynamical responses of building in MATLAB.
Using the sliding mode control law in the state space form of the systems as dZ/dt=A*Z+Bu*U+Br*ag, I can calculate the sliding mode control "force" as U=-inv(P*Bu)*(P*A*Z+Br*ag)-eta*sign(P*Z).
I wonder if I need to consider the actuator properties of active tendon system in order to obtain the voltage/current signal. If so, considering my work as a numerical study, I would appreciate any comments or references on this issue.
Regards,
Keyvan
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Hi Keyvan Aghabalaei Baghaei, based on the info from Smart Structures: Innovative Systems for Seismic Response Control, if you assume that the K-bracing is rigid without elastic deformation, then the actuator-piston displacement Δa(t) = – z(t). I expanded the dynamics for Eq. (5.12) added the electrical system for the actuator.
There are five state variables, namely
z(t) = structural displacement,
f(t) = actuator force,
c(t) = servo valve-piston displacement,
θ(t) = servomotor position,
i(t) = servomotor armature current.
The 6th-order linear model of the building structure with servo valve-controlled hydraulic actuator as Active Tendon System is given by:
M*z''(t) + C*z'(t) + K*z(t) – ag = f(t)
f'(t) = (2*β*A2/V)*Δa'(t) + (β*A*KV*√(2Ps)/V)*c(t)
c(t) = N*θ(t)
J*θ''(t) + b*θ'(t) = Km*i(t) = Tm(t)
L*i'(t) + R*i(t) + Km*θ'(t) = Vm(t)
where
Tm = servomotor torque
N = gear ratio (assume that c(t) is proportional to θ(t))
J = moment of inertia of the rotor
b = motor viscous friction constant
Km = motor torque constant
L = electrical inductance
R = electrical resistance
Vm = voltage source applied to the motor's armature
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We're using the Cross Multiscale Sample Entropy to real data recorded during a low intensity seismic event for damage detection purpose.
We're obtaining good results with Spectral Entropy (SE) and we want to compare them with the ones obtained with the Cross Multiscale Sample Entropy (CMSE).
The problem is related to the length of the time series: we can use the SE on the whole recorded signal (16,000 samples), while we cannot employ the CMSE. In fact the CMSE works only on a very short portion of the signal (3,000 samples).
Consequently with the CMSE we’re encountering computational issues due to the long time series, as well as problems related to the presence of seismic input.
We know that the CMSE is independent of the time series length when the number of data points is larger than 700-1000 samples. But this is true for stationary problems.
We didn’t find anything applied in civil engineering problems with this conditions.
Does anyone know how to overcome this problem?
Thanks!
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Please share me the best answer might you get...
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I want to start working on the generalized differential quadrature (GDQ) method to solve the problem of the mechanical behavior of the beams, the plates and the shells.
Dear colleagues , I hope to give some suggestions (papers, books, etc.) and simple examples for work on this method.
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I want to find ductility of structure by using pushover analysis in ETABS 2015. Can anybody tell me the procedure for doing same.
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Good question.. Please share me the best answer might you trust...
Regards…
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I had solution for sandwich beam with viscoelastic core. The properties of viscoelastic material is taken in the form as
E = E ' + i * E " or E = E ' ( 1 + i * eta ) ; where eta (loss factor) = E " /E, where E ' = Storage Modulus, E " = Loss Modulus.
I would like to verify the solution using ANSYS. But I couldn't find the way to define the E' / E" / eta in ANSYS APDL.
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Formulate the problem mathematically and seek answers using analytical or numerical techniques, if you have issues with ANSYS. You can also use other programs/software.
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Dear researchers,
the evaluation of the structural damping can be accomplished using either classical method such as Rayleigh damping or Modal damping these methods assume a proportional damping related to the mass and rigidity, or non-classical methods that are more complex and costly.
In this question I want to know which method is recommended and among the non-classical methods which one is the most complicated and which one is the simplest.
Warm regards.
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Please see this article, it may be useful for you
Mode decomposition method for non-classically damped structures using acceleration responses
By: J.-S. Hwang, S.-H. Shin and H. Kim
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I want to explore the dynamics of parametrically excited (nonlinear) beam whose material's Young's modulus (E) and loss tangent (tan(delta)) are known. It is convenient to use the simple Kelvin-Viogt model in the evaluation nonlinear dynamics. Hence, I want to find out viscoelastic coefficient (eta) in the model,
sigma = E*(epsilon)+eta*(d/dt(epsilon))
where, sigma is time varying stress, epsilon is time varying strain. d/dt is differentiation with respect to time.
Please, give me some suggestions for the calculation of viscoelastic coefficient or any other alternative modelings using loss tangent and Young's modulus.
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This is a calibration for a Viscoelastic Material using a fractional derivative model. There is a experimental data for the storage and loss modulus.
In your problem you must define a inverse problem Ymod = Kelvin_Voigh(parameters), and try reduce (a minimization problem) the error between the experimental response Yexp and the model response.
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On what parameters the selection of continuation method depends ?
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We have proposed a numerical method (denoted as MEM) for solving the
Nonlinear Dynamic problems. If you would like, you can view our published articles in this field entitled "dynamic analysis of SDOF systems using modified energy method". By the way, the presented idea is also generalized to MDOF systems in another study, which is available on my research-gate account.
if you need more information, please let me know.
Regards,
Jalili
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SeismoStruct and Opensees programs.
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sir,
hiw can I define a hinge and roller support in seismostruct?
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task description: an inverse numerical-experimental approach is used to determine orthotropic material properties.
Experimental determined eigenfrequencies are used to fit the FEM model with the help of an optimization method the discrepancies were minimized to a minimum and homogenized material properties are determined.
Question: So as this is an undertermined problem how the determined orthtropic material porperties could be improved to be physically true and not only mathematically fitted?
thanks
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Normally cell case is made of aluminum alloy to reduce mass.
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Hi everyone.
This might seem a bit vague but i need some direction. i have run classical molecular dynamics simulations using different potential parameters from literature and want to compare which is the best at simulating the properties of calcium ions in water. I have radial distribution functions, angular distribution functions and vacf data as well as the mean residence times in the first and second hydration shells. I need a way to compare this data with my reference data obtained using ab initio md.
please let me know if you can point me in the right direction.
Thank you.
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Dear Sahra,
I think you need to better formulate your problem. So, at first you have to work out a physical model. Let us start by telling we have water as a medium e,g, solvent in which we have calcium ions dissolved in it. Is this right? Is there any density to the solved calcium ion? This will determine the inter ionic distance rii. There will be a microscopic model of the charge distribution in the electrolyte. The problem can be solved using Coulomb law. It is an electrostatic problem. It could also be solved macroscopically using Poisson equation.
These are some thoughts on your problem that can guide you to find your way?
One advice. The definition of the problem is half of the solution.
Best wishes
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I am working on macromolecular complex structure and dynamics analysis. The complex has RNA molecule. What is the appropriate way to perform NMA on RNA-protein complex? Also, is there any size(nt) limit of RNA structure to do the NMA analysis. 
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I am currently modeling a simple truss with Newmark Method (time integration method) on SAP 2000, I have read some of article about process of time integration method.
But since I try to compare the "show plot function", it has different shape with the time history input. (please take a look to the picture below)
Anyone who ever use linear time integration method specially newmark method. I hope you have time to help me.
Regards,
Ichsan
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Dear Bowei,
Thank for your response!
I have solved my problem..
I just change the mass source.. and the graphic shape almost same.
Best regards,
Ichsan
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Hi, everyone.
there are many papers about river plume. But each paper looks from a different facet such as salinity, temperature distribution, density, currents and etc. I need a book to know river plume system basically and comprehensively. can any one offer the book?
thanks a lot.
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 Thanks a lot.
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I'm going to do research in this field, so any contribution is appreciated.
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The gentlemen before me gave good advices, however we already know how to build safer buildings and to prevent deaths, now it's time to focus on the ways to reduce overall risk and reduce dollars and downtime losses
In my opinion the real deal here are probabilistic seismic risk analyses for critical infrastructure. Seismic PRAs exist for many years for nuclear power plants and they are slowly progressing into large dams, bridges, petrochemical plants etc. However there are many other critical infrastructures prone to earthquake damage in the IT, financial, commercial, public health etc. These infrastructures house a great number of very expensive equipment which is usually sensitive to vibrations and need adequate fragility and vulnerability assessment. 
Good luck!
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I found that Newmark method is widely used in the research of structural dynamics and nonlinear dynamics. What is the advantage of this method over Runge-kutta method when solving structural or nonlinear dynamic equations?
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1.) Newmark method is for solving second-order ODEs directly while Runge-Kutta methods are for solving first-order ODEs.
2.) Maximum order of accuracy for Newmark method is second-order. There are several variants of RK method with different orders of accuracy, up to fourth-order.
The only advantage of Newmark method over RK method is that one can solve second-order ODEs directly without having to convert them into state-space which is a must for RK methods. This means fewer DOFs, hence lower computational cost, when compared to RK methods.
Having said that, I think, Newmark beta is outdated. Though it is widely used (I don't know why, because there are several methods better than Newmark method), it has certain disadvantages which make the Newmark method less attractive. I recommend using recent methods with improved properties, for example, generalised-alpha methods.
Go through my blog post to know more about the disadvantages of the Newmark method.
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Hello,
can any one explain to me how the gain matrix K in the picture attached is obtained? normally we have u(t)=-1/2 R-1BTP x(t) instead of this one in equation (4) .
The whole paper is also attached.
Thank you
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Thank you Dr Hala for sharing your data.
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When the continuous systems such as string/beam vibrate in an viscous environment, we know that the natural frequencies of the system become damped natural frequencies and there exists a decay constant associated with each of the mode. 
Are these decay constants (or damping factors) different for every mode or same? i.e. given an initial condition corresponding to specific modes, will the system decay down in same amount of period?
The question is raised due to this:
When string vibrates in viscous environment, the equation can be written as,
rho*w,tt+c*w,t-Tw,xx=0.
In this case when we discretize the equation using Galerkin's approach, the damping matrix becomes diagonal with every element same for standard boundary conditions.  
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Hello Pranav Lad
  It is better to answer your question from the assessment.of free vibration of a sdof system governed by the ode
         MUtt  + CUt + KU =0                                                                                        The solution to this equation is   U(t) = A*exp(-ωdt)Sin(ωdt +α)                                   where  ωd = ωn*sqrt(1-D2)   and ωn = circular natural frequency and D is the damping ratio = C/Cc.   The decay factor is exp(-ωdt).   The higher the frequency the faster the decay and the more the damping. Thus, the decay rate will vary with frequency.  As is well known the viscous damping coefficient for a sdof system  is defined by                             c = 2DMωn                                                                This idea can be extended to strings and beams. 
 I hope this answer will be useful to you.  
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I am designing a 110 storey building for different innovative structural systems, I have carried out the elastic design and the design is governed by wind loads for drift considerations. Now I want to carry out the performance based design of these systems , I did try pushover analysis in MIDAS Gen software for response spectrum as per IS-1893(2002) , but my capacity is much greater as compared to the seismic demand.Thus all the systems are still in the elastic region. Now I want to carry out a pushover for Wind loading, is it possible ? Can I carry out time history analysis for Wind loading ? Please help.
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yes abbas you are right
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i have modeled the RC beam with simply supported BC and used concrete damaged plasticity model to account for the nonlinearities in concrete, i have got the frequency value for the case of beam under its own weight only, i want to add loads to it to get values of frequencies for cases.
i have used a static general step to define the added load, knowing that this step is before the frequency step, but the results of frequency are not even close to literature!
i'm attaching two models one without load which has a first frequency of 43.79 cycle/sec which is equal to 267 rad/sec , that is close to the value in literature, my problem is when i add the load in the other model, i added here for example 800lb =3558.577 on each hanger of the beam , cause the loading is applied on two hangers as shown below, the frequency is not even close, it's almost the same as the case without loading.
i have attached the paper "by Jerath and shibai" i'm getting information from to model
i have also attached the beam scheme noting i have used the reinforcment as in series 1
i have used a CDP model based on a python code attached here that is based on a paper by carriera and chu also attached ! 
can anyone help, please?