Research On The Dynamic Properties Of Concrete Simply Supported Beam Bridge Based On Vehicle-bridge Coupling Vibration

In order to meet the growing needs of transportation day by day, not only the quantity of each kind of vehicles has a rapid growth, moreover the velocity and load-carrying capacity of vehicles also have a great enhancement. And in recent years, along with the unceasing consummation of computation theories, as well as the construction of new structures and the application of light and high strength materials, the bridge structures become gradually lighter and lighter. The above two factors make the proportion of vehicle dynamic loads bigger and bigger in whole bridge loads. Bridge is forced to vibrate under vehicles'excitation, when the frequencies of exciting force produced by the combined action of bridge rough surface and vehicle speed are close to the natural frequencies of bridge, the resonating of bridge will take place. Therefore, the impact and vibration influence of vehicles load to bridge structures has become one of non-negligible important attributes in the computation and analysis of bridge structures. In the highway bridge design standard, the impact effect of vehicles to bridge are considered by calculating the impact factor of bridge according to bridge type (steel bridge, concrete bridge) and the base frequency of bridge, which cannot reflect the influence of bridge surface roughness, bridge damage, the dynamic performance of vehicles and driving speed to the impact effect. Therefore, conducting synthetic study to the vehicles-bridge dynamic interaction system, in order to analysis and evaluate the dynamic performance of bridge, has a very important theoretical and practical significance to the development, design and construction of bridge structures.As a result of vehicles' existence, the vehicles and bridge formed vehicle-bridge coupling vibration system and vibrates together. The measured bridge frequencies got in the bridge dynamic test, in fact are the vehicle―bridge system's vibration frequency, rather than the natural frequencies of bridges. The measured bridge frequencies of vehicle-bridge system are called loaded vibration frequencies of bridge in the engineering field. Sometimes the difference between the load vibration frequency and the natural frequency of bridge is even bigger than the variation of the natural frequency caused by bridge's damage, so it often causes the precision of bridge condition assessment based on actual modal parameter to be low. Therefore the research on the loaded vibration frequencies of highway bridges under the effect of vehicles is very essential.This paper takes the reinforced concrete simply support beam bridge as study object, uses beam elements and plate elements to establish two-dimensional planar finite element model of bridge, selects 7 degrees-of-freedom three dimensional vehicles model, and considers the roughness of bridge surface. The motion equations of vehicle-bridge coupled system was established, and solved using the iterative method, and then the dynamic response of system can be obtained. A 30m concrete simply support T beam bridge and a two axle truck are taken as examples, to calculate the dynamic response of the bridge under vehicle load. The influence of vehicle speed, horizontal driving position of vehicle on bridge, as well as suspension stiffness and tire stiffness of vehicle to the bridge dynamic response are discussed.The acceleration response of bridge under the excitation of vehicle ware analyzed using FFT, and free decay response of bridge obtained after the vehicle leaving the bridge ware used to identify the modal parameter of bridge using eigen-system realization algorithm. Theoretical calculation method and ANSYS simulation are adopted separately to study the loaded vibration frequency of bridge under the effect of vehicle. The influence of tire rigidness, suspension rigidness, unsprung mass,sprung mass, as well as the vehicle's position on bridge to the bridge loaded vibration frequencies were discussed. This paper has mainly drawn the following conclusion:(1) the DAF of bridge doesn't increases unceasingly along with the increase of vehicle speed, it will present peak value at some speed, as a result of random characteristic of the bridge surface roughness, the vehicle speed corresponding to maximum DAF is changeable; The DAF of each girder changes along with the changing of horizontal driving position of vehicle on bridge, when vehicle driving along one certain girder, the DAF of this girder is smallest. When the bridge surface roughness is considered and the amount of vehicle mass varies at some extend, the DAF of bridge decrease with the increases of the vehicle mass; when the suspension rigidness of vehicle varies at some extend, the DAF of bridge increase with the increases of suspension rigidness; when the tire rigidness of vehicle varies at some extend, the DAF of bridge increases with the increase of tire rigidness first, then tends to be steady.(2) Analyzing the acceleration response of bridge under the excitation of vehicle using FFT directly can identify the natural frequencies approximately, but has certain error. Theoretically, using eigen―system realization algorithm to identify the modal parameter of bridge from free decay response of bridge obtained after the vehicle leaving the bridge, the identified result of frequency and modal shape have very high precision.(3) The change of tire rigidness and suspension rigidness has great influence on the loaded vibration frequency of bridge. The loaded vibration frequency increases, when tire rigidness and suspension rigidness increase at some extend. The change of unsprung mass has great influence on the loaded vibration frequencies of bridge, the loaded vibration frequencies decrease with the increase of unsprung mass. When actual suspension rigidness and tire rigidness of vehicle are considered, the loaded vibration frequency slightly decreases, when sprung mass increase at some extend. When suspension and tire of vehicle are considered as rigid, the loaded vibration frequency decreases obviously with the increase of sprung mass. The influence degree of vehicle position on the loaded vibration frequencies of bridge is related to the modal shape of bridge, the bigger of the value of modal shape at the position of wheel action point, the greater of influence of vehicle position on the loaded vibration frequencies of bridge.