Determination of the Modal Information of Structures under Impact Loads Using Proper Orthogonal Decomposition

Determining the modal characteristics of structures such as natural frequencies and damping ratios is one of the most important issues in structural engineering. In this regard, providing a low-cost and robust experimental method against all types of noises is very important. In the present paper, a new algorithm for determining the natural frequencies and damping ratios of structures under impact loads using the proper orthogonal decomposition technique is presented. This method uses the vibrational response of the structure to impact loads, without the need to calculate the impact magnitude. One of the strength points of the proposed methodology is the accumulation of laboratory noises in the latest modes. In other words, in the process of calculating the frequencies related to the first few modes, laboratory noise does not enter the calculations and will be aggregated in higher modes that are less important. The feasibility and efficiency of the new method was evaluated using numerical simulations as well as laboratory validation. In this research, four pure numerical models were used to evaluate and validate the accuracy of the proposed algorithm. These models include a simply supported beam, a two-dimensional portal frame, a three-dimensional truss and a clambed-clambed beam. The natural frequencies and damping ratios of the mentioned models were calculated using the new method, then the results were compared with those that obtained from the finite element method. Very good agreement was observed between the results of the two methods. For further investigation, various states such as the effect of the noise on the results, the effect of multiple impact loads and the effect of the number of sensors were also studied. The results of these numerical studies showed that the proposed method is very stable and robust against laboratory noises and has small errors. Acceptable results can be obtained in using a few numbers of sensors (even one sensor) and repetition of the test. Also, the results for multiple impact loads are almost similar to the results obtained from the excited state with a simple impulse load. In this study, in order to further investigate the efficiency of the POD based method, a small-scale laboratory model developed at Shahid Rajaee Teacher Training University and whose modal information has already been calculated using the closed image processing method, was studied. Good agreement was observed between the results of the two methods, so it can be another reason for the efficiency and capability of the new method.Based on various studies, it was concluded that for structures with low damping, the proposed method has acceptable accuracy and with increasing damping, the accuracy of the results gradually decreases. Since conventional structures have a relatively low attenuation, the proposed algorithm is very suitable for determining their modal information. The proposed method due to the availability, cheapness and no need for complex experimental tools can be used as a useful algorithm to determine the modal information of a structure as well as control the results obtained from other experimental methods.