Research Of Structural Dynamic Characteristics Of Multi-Tower Super-Large Suspension Bridge Based On Health Monitoring System

Multi-tower suspension bridges are constructed by adding one or more middle towers on the basis of traditional double-tower suspension bridges.The addition of middle tower has a great influence on the structural static and dynamic stress calculation and material selection of the bridge.The mechanical characteristics of middle tower are different from side tower,which forms the "middle tower effect' of the overall stiffness change of the structure.At present,the structural dynamic characteristics of suspension bridges with multiple towers and the stress of key steel towers are seldom mentioned in literature.Taizhou Bridge is a typical three-tower two-span suspension bridge.It was opened to traffic in November 2012.Spanning the Yangtse River between Taizhou City and Yangzhong City,it is the world's first kilometer-length multiple-tower suspension bridge with two 1080-m continuous main spans and two 390-m side spans.The middle pylon is designed as an inverted Y-shaped steel structure in consideration of the rigidity of the middle tower and anti-sliding concerns between the main cables and saddles.The restraint of the main cable on the top of the middle tower is weaker than that on the side tower.Under extreme conditions,when one main span is fully loaded and the other main span is empty,the maximum difference of cable forces between the two sides of the tower will occur,and the main cable will most likely slip between the main saddles,which will pose a great risk to the stability of the structural system,In addition,the load carried by the steel box girder is transmitted to the main cable through the sling,and then to the anchorage system at both ends through the main cable.Therefore,the operation safety status of the tower and both the main cables and suspension cables are the key factors for the structural health monitoring of Taizhou Bridge and this type of super-large suspension bridges.Vibration and strain are the two most important indicators for evaluating the structural safety of large bridges.The former pays more attention to the analysis of structural state of components from the overall point of view and is insensitive to local damage,while the latter pays more attention to the acquisition of structural features from the local point of view.Concerning the structural safety of Taizhou Bridge,a long-term environmental and structural health monitoring system(ESHMS)was instrumented to monitor the bridge's environmental and structural condition and assess the bridge's structural health performance after its completion.ESHMS consists of 275 sensors,which will focus on monitoring the stress changes of the steel tower and the vibration and tension force changes of the cables of Taizhou Bridge to ensure that the bridge is in a stable structural state.In this paper,a dynamic stress signal extraction method based on the empirical mode decomposition(EMD)method was proposed for monitoring the stress change of steel tower,and the method was applied to the collected data.According to the extracted dynamic stress data,the stress changes of steel tower were analyzed and counted from the points of the same height and different heights.A statistical method of stress spectrum based on the rain-flow counting and a joint distribution model based on daily stress spectrum were proposed for fatigue analysis of steel tower structures.Then,the fatigue life of key structural joints under certain failure probability was predicted based on the Miner criterion.To meet the needs of structural vibration monitoring,the wireless sensor networks and wireless smart sensor nodes were designed and applies to monitor the vibrations of main cables and suspension cables of Taizhou Bridge.Through its on-board computing and wireless communication capabilities,wireless smart sensors significantly improve the performance of structural health monitoring system.Based on the built-in cable force assessment algorithm,structural damage caused by extreme loads and cumulative environmental erosion can be detected and evaluated in time.Relevant results show that the vibration state and tension force of the main cable and suspension cable are in a stable state.The innovations and key issues to be solved in this paper include:(1)A method of extracting structural dynamic strain data based on empirical mode decomposition and structural frequency characteristics was proposed and applied to the analysis of structural strain data of the steel tower,which solves the problem of the inaccuracy caused by manual intervention in traditional methods.Based on the dynamic strain data and statistical analysis method,it is concluded that the stress in the middle herringbone steel tower is larger in the middle and smaller in the two ends with height.(2)Based on the rain-flow counting method,the dynamic strain data of steel tower was transformed into daily stress spectrum.Combined with typical distribution characteristics,the optimal distribution model was selected by using the AlC-based model evaluation method.A joint distribution model with 5 MPa as the demarcation point was established.Based on the Miner criterion,a fatigue cumulative damage and fatigue life calculation method based on failure probability was proposed.The fatigue life of Taizhou Bridge under certain failure probability was obtained.(3)By deploying wireless sensor networks and nodes on the main cables and slings,the cable force values were obtained by linear fitting between the structure frequencies and orders based on the classical string theory,thus realizing the real-time intelligent monitoring of the cable force.Traditional structural health monitoring has the problem of heavy construction and light management.Through the combination of theory and practice,this paper systematically studies the structural dynamic characteristics of Taizhou Bridge in operation state,comprehensively evaluates the structural safety status of the middle steel tower and cables of Taizhou Bridge,which reveals the structural dynamic characteristics of multi-tower suspension bridge.The methods and conclusions presented in this paper can provide scientific guidance for the design and operation management of multi-tower suspension bridges.