Testing Methods Research On The Natural Vibration Characteristics Of Long-span Variable Cross-section Prestressed Continuous Box Girder Bridges

The continuous box girder bridges combine the characteristics of both the continuous girder bridges and the box section. This kind of bridge structure has many advantages as follows:①The negative moment reinforcement at the middle supporting unloads the sagging moment at the mid-span, further rationalizes the internal force distribution, and realizes the larger span with the same section;②The box section has better torsional rigidity;③There are many advantages such as better seismic capacity, smaller deformation; more smooth and comfortable travelling, fewer expansion joints and so on.④It has better adaptability to bridge style and can be easily built into curved and orthotropic structure, which is better adaptive to urban bridge.⑤Advanced construction methods such as pushing construction method and cantilever construction method can be used. Because of so many advantages, this bridge style is widely used.As the service time goes on, some kinds of disease appear in the continuous box girder bridges. For these diseased bridges, removing and rebuilding is unreasonable and is unallowed in economy. The reasonable solution of the bridge disease is to make clear the bridge existing bearing capacity and to take reasonable measures to fix and reinforce, so as to make the bearing capacity to meet the needs of traffic. The effective method to obtain the bridge bearing capacity is to test the bridge and estimate the using state based on the testing data. Dynamic loading test is to test the bridge natural vibration characteristics based on which the using state is estimated. This testing method has better objectivity and shorter testing time. There is no need to close traffic and no limits of obstacles that bridges cross. Thus, conducting the dynamic loading tests to obtain the accurate structure natural vibration characteristics is very meaningful to estimate the using state of the continuous box girder bridges.With the development of the continuous box girder design theory and the appearance of advanced construction method, the span has greater increases and the section styles are varied. Long-span variable cross-section prestressed continuous box girder bridges are used widely and need to take full account of seismic capacity. So aseismatic design is a key point in designing this kind of bridge. However, the bridge aseismatic design is based on natural vibration characteristics.For prestressed continuous box girder bridges, differences always exist in the exact theoretical calculation and the real value. Therefore, testing researches are needed to get the real natural vibration characteristics to update the theoretical calculation model and provide the basic data for the development of seismic theory.Researchers at home and abroad have done some work on the natural vibration characteristic testing technique of long-span variable cross-section prestressed continuous box girder bridges. However, the research is not deep enough to form theoretical system.Therefore, it's very necessary to conduct testing methods research on the natural vibration characteristics of long-span variable cross-section prestressed continuous box girder bridges. This paper first introduces the computational method of natural vibration characteristics of continuous girder bridge based on Euler-Bernoulli beam . Then calculating the natural vibration characteristics of variable cross-section prestressed continuous box girder bridges by finite element method is presented. Measuring point optimization method for the long-span variable cross-section prestressed continuous box girder bridges is formed and exciting ways are also compared and selected, which combined with the theoretical analysis results form the natural vibration characteristic testing method of this kind of bridge. It also combines random decrement technology and modified ERA method to identify he natural vibration characteristics of long-span variable cross-section prestressed continuous box girder bridges. At last, the main bridge of Changbei overpass verifies the effectiveness of this method. The research conducted is as follows:1. Introduce the computational method of line-cross section continuous girder bridge, and clear the applicability of this method. Introduce finite element method into solving the natural vibration characteristics of variable cross-section and form the general process which provides theoretical support for dynamic feature testing.2. Based on the finite element model, the natural vibration characteristics of the five-span variable cross-section prestressed continuous box girder bridge are calculated. Based on the theoretical calculations, effective and independent method is used to optimize power points of main bridge of Changchun Chuangbei Overpass. Compare thedifferent exciting ways of the bridge and select the exciting way for Chuangbei Overpass considering its dynamic characteristics. According to the measuring point optimization and the selected exciting way, dynamic load testing program is made for Changchun Chuangbei Overpass. And based on the program, dynamic load test is conducted and dynamic time history response needed is collected.3. Combined the characteristics of the dynamic time history response collected, use random decrement technology and modified ERA method to identify bridge's natural vibration characteristics. Use this method to identify the mode shape, frequency and damping ratio of long-span variable cross-section prestressed continuous box girder bridges from measured dynamic responses to achieve high accuracy of recognition results. Based on the identified dynamic parameters, estimate the using state of the bridge.