Modal parameters of large civil engineering structures such as Modal damping ratios (MDRs) are determined mainly through output-only Modal identi cation. In this paper, MDRs of a double-layer grid were obtained using output-only Modal identi cation. For this purpose, a double-layer grid constructed of a ball-joint system was tested. Through some random tapping on the structure, the acceleration response in multiple locations was measured. The acquired data were processed using output-only Modal identi cation to arrive at MDRs. The MDRs corresponding to the  rst eight modes of the grid were extracted by  ve output-only Modal identi cation techniques, namely Enhanced Frequency Domain Decomposition (EFDD), Curve- t Frequency Domain Decomposition (CFDD), and three di erent methods of data-driven stochastic subspace identi cation. To determine the appropriate model order used in SSI methods, sensitivity analysis was carried out and the resulting number of orders was 200. The proper frequency resolution of 1600 was determined to estimate the MDRs of the grid by EFDD and CFDD. The results showed that the MDRs of the grid obtained from di erent methods were in good agreement with each other. The grid has very low MDRs because the MDRs of the modes measured using di erent methods varied from 0. 06% to 0. 11%.

Although natural frequencies and mode shapes can be accurately measured by dynamic tests, error bounds in values of damping estimation can be large. Since damping strongly influences the design and control of structures, efforts are made to find the damping that has the least error. The random and bias errors are the two important types of error in damping estimation of structures via frequency domain methods. These errors can be reduced by choosing appropriate frequency spacing. In this study, the frequency spacing leading to the least bias and random errors for estimation of Modal damping is determined. The complexity of the damping phenomenon on the one hand and complexity of the structural behavior of a double layer grid, on the other hand, led to this study. For this purpose, a double layer grid constructed from ball joint system was tested. the Modal damping ratios related to the first 6 modes of a double layer grid with the ball jointed system were identified for different frequency spacing via two output only Modal identification techniques; namely enhanced frequency domain decomposition (EFDD), curve fit frequency domain decomposition (CFDD). The Modal damping ratio estimations identified through the two methods were then compared with the results of the input output identification method of Ibrahim time domain (ITD) as the reference value. The results showed that there is an almost linear relationship between the Modal damping ratio estimations and the frequency spacing in each mode. At the frequency spacing of 0. 0625 Hz, the Modal damping ratios obtained from the output only methods showed the least difference (between 0 to 21. 43%) with the reference values. At this frequency spacing, the root mean square deviation of Modal damping estimations between enhanced frequency domain decomposition (EFDD) and curve-fit frequency domain decomposition (CFDD) with the corresponding reference values was 0. 02.

In this research, the Modal parameters of a beam in free-free condition are extracted by performing different experiments in the laboratory. For this purpose, two different techniques are employed. The first methodology is considered as a time domain method in Operational Modal Analysis. The other one is frequency domain impact hammer test which is categorized as an Experimental Modal Analysis method and can be regarded as the most common method in Modal analysis. Checking the results obtained by the two methods, one can notice a distinct inconsistency in Modal damping ratios extracted by each method. However, based on recent publications on the subject, it can be inferred that the time domain methods have better accuracy in identifying damping ratios of structures. In order to confirm the findings, the effect of excitation is examined for each method by altering the excitation tool. For the operational method, it is concluded that changing the excitation tool will not have a noticeable influence on the identified damping ratios, whilst for the Experimental Modal Analysis method changing the hammer tip leads to inconsistent results for damping ratios. This study exemplifies the deficiency of Experimental Modal Analysis methods in their dependency on excitation techniques.

It is impossible to measure the random excitation loads of structures during transportation under road excitations or satellite carriers thrown under environmental excitations in order to identify Modal parameters; thus in this research, our challenge and purpose is to determine the Modal properties of structures without having the input excitation load and, but only by having the response. In the studies have been performed so far in the field of Modal analysis, less attention has been paid to comparing time and frequency methods, especially in the determination of damping. In addition, no sensitivity analysis between different excitations of white noise, sinusoidal sweep and the effect of these excitations on time and frequency domain methods of Modal functional analysis have been performed so far. At first, these methods are implemented and validated on a 5-DOF discrete model (mass, spring, and damper), and finally they are industrially implemented on an actual structure. This paper shows that the decomposition methods are capable to extract the Modal parameters of the primary modes of an actual sample. At the end, a comparison is made between the results and the effectiveness of each method is evaluated.

This paper investigates damping ratio in micro-beam resonators based on magneto-thermo-elasticity. A unique aspect of the present study is the effect of permanent magnetic field on the stiffness and thermo-elastic damping of the micro resonators. In our modeling the theory of thermo-elasticity with interacting of an externally applied permanent magnetic field is taken into account. Combined theoretical and numerical studies investigate the permanent magnetic field effect on the damping ratio in clamped-clamped and cantilever micro-beams. Furthermore, the influence of the magnetic field intensity on the frequency of the micro-beams with thermo-elastic damping effect is evaluated. Such evaluations are used to determine the influence of magnetic field on the vibration amplitude of the resonators. The meaningful conclusion is that the magnetic field increases the equivalent stiffness and thermo-elastic damping and consequently the energy consumption of the resonators.

In this paper, an alternative approach in operational Modal analysis is presented, utilizing image processing technique and transmissibility functions. Imaging sensors do not impose additional mass on the structure due to their non-contact nature, while transmissibility functions, independent of excitation type, can directly extract mode shapes. The innovation of this research lies in combining these two techniques to record dynamic responses and identify Modal properties. To capture the temporal response history from video signals, the block-matching method with sub-pixel accuracy was employed. Validation was conducted by recording the response of the tip of a cantilevered steel beam subjected to impact excitation, using a high-speed camera and a laser vibrometer, simultaneously. The RMSE plots in the time domain and the PSD in the frequency domain indicate high accuracy of this method. Using this approach, the displacement time histories of various points on the structure were extracted from the video signals, and the Modal properties, including natural frequencies, damping ratios, and mode shapes, were identified using the transmissibility matrix method. The results obtained from the proposed method were compared with the stochastic subspace identification (SSI) method and analytical solutions. The findings reveal the accuracy of the Modal identification approach introduced in this article. The highest relative error in estimating the natural frequencies of the first and second modes, compared to the values from the laser method, are 0.19% and 0.13%, respectively, and in comparison to the analytical values, they are 0.34% and 1.5%, respectively.

In large civil engineering structures, the output-only Modal identification is the most applicable technique for estimating the Modal parameters such as damping. However, due to no measurement and control of excitation force, the identified parameters obtained by output-only technique have more uncertainty than those derived from the input-output technique. Given the different nature and uncertainties of the two Modal identification techniques, in the present study, the damping related to the first 12 modes of a double-layer grid developed from the ball joint system were identified via the two techniques and compared with each other. For this purpose, a double-layer grid was constructed by pipes and balls with free-free boundary conditions provided for both input-output and output-only experiments. Exciting the grid, its acceleration response was measured at appropriate degrees of freedom. Then, by using these data and performing Modal analysis, involving four different methods of input-output and five different methods of output-only, the natural frequencies and damping ratios of the desired modes were extracted. The results indicated that despite the good agreement between the Modal damping of the grid, as identified by different methods of input-output together and by different methods of output-only together, the results of input-output and output-only methods were different with each other. The damping values through the input-output Modal identification methods were on average 65% higher than the corresponding values of the output-only Modal identification methods.

In the present work, an analytical study is proposed to investigate the flutter behavior of low-aspect-ratio wings in subsonic flow. An equivalent plate model is used for structural modelling of a semi-monocoque main wing, consisting of ribs, skins, and spars. Legendre polynomials are used in the Rayleigh-Ritz method as trial functions, and the first-order shear deformation theory is utilized to formulate the structural deformation. Boundary conditions are enforced by applying proper artificial springs. A doublet point method is used to calculate the unsteady aerodynamic loads. Chordwise pressure coefficient distribution at the tip and root of a rectangular wing oscillating in pitching motion is calculated. Flutter analysis is performed using the k method. Instead of using the computationally expensive finite element method, the proposed approach is intended to achieve purposes of quick modelling and effective analysis in free vibration and flutter analyses of low-aspectratio wings for preliminary design applications. The effects of aspect ratio on the flutter behavior of wings in subsonic flow are investigated. The obtained results are validated with the results available in the literature.