| Structural vibration test and modal parameter identification are the basic methods to study the dynamic characteristics of structures, which occupy a very important position in the bridge structure safety assessment and health monitoring.The traditional vibration test belongs to contact measurement, in order to fully grasp the vibration characteristics, a lots of sensors need to arrange on the structure,There are the following deficiencies:difficulty of data acquisition, heavy workload of data analysis, long test cycle and consume a lot of manpower and material resources.Therefore, It is very necessary to develop a measurement technology that with high density, non-contact, no load effect, and no loss of equipment, high precision, and can realize automatic recording,able to process data continuously and real-time measurement.The vibration of two span continuous beam is studied in this paper.first,get the video of the vibration of continuous beam and convert video into digital image,Using the orthogonal polynomial fitting method as the image edge detection algorithm,and processing each picture with the same method, get the sub-pixel edge of the continuous beam, then can get the beam the vibration time histories and through the DASP system analysis and processing the measured data, Get the vibration frequency and vibration mode of continuous beam. In this paper, the method is discussed and the experimental verification is carried out.The main work and research results of this paper are as follows:(1)Describes the digital image processing technology research background, the significance and the domestic and foreign research present situation and the trend of development,and comprehensive introduction to the basic principle of Digital ImageCorrelation Method and Digital Image Edge detection Method.Discusses the deficiencies in the existing bridge structure vibration detecting method, summarized and analyzed of the video image measurement technology, compared with the traditional measurement techniques,demonstrates the video image measurement technology is applied to the structural vibration detection,have a number of advantages.(2)By using Midas finite element software to establish the model, analysis and calculation, get the beam three order natural frequency and vibration mode.The field vibration test of continuous beam model was carried out using DASP vibration test and identification system, and get the three order natural frequency and vibration mode.(3)Use of mobile phones and Acer laptops as hardware equipment capture video,using Matlab software run of four modules:the video acquisition and decomposition,image preprocessing, image computation, edge time domain signal read, based on DASP system finish the measured signal analysis, finished vibration test of two span continuous beam model, got continuous beam model of the first and second order inherent frequency and vibration mode.compared the results of modal parameter identification based on Midas finite element software and DASP system, and the reliability and accuracy of the identification of structural vibration modal parameters are demonstrated by the method of video image measurement.(4)Combination with the bridge specification discussed the question of the selection of the natural frequency when calculating bridge impact coefficient.It is shown from the results of the analysis and the test that,for the impact factor,when the effect of positive bending moment caused by impact force is calculated,the fundamental frequency must be used as shown in the General Code,and the 2nd frequency must be used when the effect of negative bending moment caused by impact force is calculated.The selection of the frequency should be combined with the mode shape into account for the specific circumstances. |
