| Cable supported bridges include the suspension bridge,the cable-stayed bridge,the halfthrough arch bridge and the bottom-through arch bridge,which are main structural types of medium and large span bridges.As one of the key components of cable supported bridges,cables have transferred the load from the supported girder.Once an abrupt cable-breakage event occurs,the safe operation of cable supported bridges would be affected,and even the subsequent collapse of the whole structure might be triggerred.In recent years,the cablebreakage event on cable supported bridges occasionally occurs as a result of various hazards,such as corrosion,fatigue,vehicle collision,and fire damage,causing serious loss of life and property.With the increase of service time,the risk of the breakage of cables increases.The issue of how to ensure the safe operation of cable supported bridges subjected to an cablebreakage event has drawn wide attention.As a kind of the self-balancing system,the selfanchored suspension bridge’s main cables are directly anchored at the end of the main girder,and thus the main girder bears great compressive force applied by main cables.Once the progressive breakage of hangers occurs,the pre-stressed girder in the self-anchored suspension bridge(SSB)is more likely to buckle,therefore the collapse of the SSB is more serious than that of the ground-anchored suspension bridge.However,there are still few studies focused on dynamic responses of the SSBs subjected to an hanger-breakage event,and the suspension bridge specification lacks of related contents of structural safety design.In order to provide references for the design,maintenance and repair of concrete SSBs,the dynamic responses of the concrete SSBs subjected to the abrupt breakage of hangers are studied.The main works are presented as follows:(1)The numerical analysis method of the suspension bridge subjected to an abrupt hangerbreakage event is studied.Considering the suspension bridges’ unique features,a dynamic analysis finite element model is developed,in which the effect of main cable’s local vibration is fully considered to investigate hanger-breakage induced dynamic responses.In addition,the dynamic analysis method and corresponding procedures are illustrated.The influence of various factors on hanger-breakage induced dynamic responses of the suspension bridge have been systematically studied.The influencing factors include the duration and time-dependent tension loss function of the breakage process,the initial stress and failure position of the broken hanger,the clamp’s mass,the main cable’s elements mesh density,main cable’s physical bending stiffness,modal damping and the damping property the main cable,etc.The results show that the main cable’ local vibration has an un-neglect influence on the dynamic responses of suspension bridges subjected to an abrupt hanger-breakage event.Therefore,the effects of main cable’s local vibration should be fully considered to improve the analysis accuracy.Based on research findings,a general framework is proposed to calculate the hanger-breakage induced dynamic responses of suspension bridges,in which the corresponding requirements of analysis model and calculation method are given.(2)Model experiments and the relevant numerical analysis are conducted to study the main cable’s physical bending and damping properties.A series of bending tests and free vibration tests are conducted on parallel wire cable section models to investigate the influence of various parameters on the main cable’s physical bending stiffness and damping ratio.The parameters include clamp’s length,clamp’s span,cable diameter,radial constrain,etc.A numerical method is proposed to investigate the main cable’s physical bending and damping properties through the equivalent frictional laminated beam model.The numerical results agree well with the experimental results,which validates the practicability of the proposed method.Utilizing the proposed numerical method,parametric analysis is conducted on the actual suspension bridge’s main cables,and the effects of cable diameter,clamp’s span,radial constraint of wire wrapping,tensile stress are considered.Based on the results,a regressive formula is given to calculate the main cable’s physical bending stiffness,and a practical approach is proposed to quantify the main cable’s damping ratio,which meet the needs of key parameters setting in the dynamic analysis of suspension bridge subjected to an abrupt hanger-breakage event.(3)The dynamic amplification factors(DAFs)of hanger-loss induced responses of the concrete SSBs are studied.The proposed numerical analysis method in chapter 2 is adopted to study the effects of load cases and hanger-loss scenarios(e.g.,hangers’ type,the number of broken hangers,etc.)on dynamic responses of the concrete SSBs subjected to an abrupt hangerbreakage event.In order to identify the prominent factors,the effects of structural configurations(e.g.,main cable’s vector-span ratio,bridge span,deck width,hanger span,etc.),section properties of load-bearing components(e.g.,the main girder’s stiffness,the hanger’s axial stiffness and the tower’s stiffness,etc.),and the dead load on DAFs of hanger-loss induced responses of concrete SSBs are systematically investigated.Summarizing the research results,the DAFs are recommended for the hanger’s tensile stress,main girder’s bending moment and main girder’s torsional moment,which meet the needs of using the static method to calculate hanger-loss induced dynamic responses of the SSBs.In addition,some recommendations are proposed to reduce an abrupt hanger-breakage event induced dynamic responses,which improve structural safety.(4)The design safety factor of the concrete SSBs’ hangers is studied considering the influence of the abrupt breakage of hangers.Considering the effects of mechanical properties of corroded steel wires and corrosion distribution inside the cable,a modified series-parallel model is developed to describe the tension-deformation nonlinear constitutive relation of th corroded hanger.The effects of corrosion depth of the steel wire in the first layer,corrosion diffusion ratio and corrosion length ratio on the degraded mechanical properties of hangers are quantitatively investigated.For the combined effects of corrosion and fatigue induced hangerbreakage event,the effects of material nonlinearity and the degradation of corroded hangers’ bearing capacity are considered,and the relationship between the design safety factor of hangers and the demand capacity ratio of the remaining hangers adjacent to the broken hanger is studied.For vehicle collision induced hanger-breakage event,the influence of the breakage process on the safety level of the remaining hangers adjacent to the broken hangers is studied.The research work perfects bridge specifications regarding to the safety verification of hangers’ strength,which provides references for the design,maintenance and replacement of the concrete SSBs’ hangers. |
