Study On Mechanical Properties And Remaining Service Life Evaluation Of Post-fire Cables

In recent years,with the development of transportation systems,as well as increasing transport of flammable and combustible materials,fire incidents in bridges have happened occasionally.Statistics show that fire has become one of the serious threats to bridge safety.Among many bridge structures,cable-supported bridges have been widely used recently owing to its appealing aesthetics and exceptional spanning capacity.As an important artery in the transportation network,it may result in significant economic and public consequences in case of fire.Hangers are key load-carrying element in suspension bridge,which connect girder to main cable.In many cases,the vehicle caught fire would park on the right lane which directly adjacent to the cables.As a result,fire can induce degradation of mechanical and fatigue properties.On the other hand,current bridge codes do not contain any provision on fire resistance requirements.Besides,there is very little research on the fire resistance of bridge structures,and even less on cable supported bridges.Therefore,it is necessary to do some research on residual performance of post-fire hanger to ensure the safety of bridge.In this paper,the mechanical and fatigue properties of hangers after high temperatures were experimental and theoretical studied based on Runyang suspension bridge fire case.The main research contents are as follows:(1)Tensile and fatigue tests were carried out to investigate the effect of high temperature on the strength and fatigue life of steel wire,and the influence of exposure time and cooling methods were discussed.In addition,by the means of the scanning electron microscope(SEM)and energy dispersive spectrometer(EDS),the change of microstructure in the heat and cool process was observed,and the mechanism of the material degradation under fire attack was revealed.(2)Based on the results of tensile tests and parallel system,a statistical model was established to estimate the residual capacity of hanger after heated,and the parallel effect was taken into account.Subsequently,static tensile tests were performed on three small cables each consisting of 19 parallel steel wires to verify the model.Combine with field measured hanger forces,the reliabilities of post-fire hangers were assessed.The results show that the residual capacity of hanger decreases with the increasing temperature.When the heating temperature exceeds 700℃,the reliability would below the specified target value.(3)Extensive fatigue tests are conducted on steel wires after various elevated temperatures and then cooled down,the number of cycles to failure was obtained.These data were statistical analysed based on Weibull distribution,in which the parameters were evaluated.A mathematical model is established between fatigue life of steel wire and hanger.The fatigue life of hanger was calculated using Monte Carlo simulation and order statistics approach,and the results agree with each other well.It is observed that the fatigue life of hanger follows normal distribution,and the its value is far less than the mean of the steel wire,this is due to the growing stress amplitude accelerates the fracture of wires in the hanger.In addition,the fatigue life of hanger is not affected by cable size,however,its variability decreases with increase of number.Finally,two small cables comprising 19 parallel steel wires are tested for verification.(4)According to wind power spectra of bridge field and WIM data,the corresponding wind load and vehicle load models were established.Apply two loads to finite element model simultaneously,the stress time histories of hanger were obtained.On the basis of Miner’s fatigue rule,the remaining service life and time-varying fatigue reliability of hangers after high temperature are evaluated.The results show that the fatigue life of the hanger is more than 20 years when the heating temperature not exceeding 600℃,indicating that the hanger will not suffer from fatigue failure in the design service life.The fatigue reliability is slightly greater than target reliability index 5.2 when the traffic increase rate equal 0 and 0.03,however,it is lower than 5.2 after 18 years when the traffic increase rate equal 0.05.When the heating temperature reaches 700℃,the fatigue life is only more than 2 years,and the fatigue reliability drops rapidly with time.Only about 1 year,the fatigue reliability is lower than 5.2.(5)A fire model was established by using computational fluid dynamics,and the temperature rising process of hanger under various fire scenarios was calculated.Combined with the previous test results,the residual strength of the hanger was obtained.The influence of the distance from the hanger to the fire source was discussed,when the distance is over 5m,the load capacity and fatigue life of the hanger are almost unaffected in case of heavy goods truck fire.(6)The extent of damage after fire was assessed,based on which repair strategy was proposed.The hanger replacement methods were summarized and compared.A novel device was designed and the replacement process was described in detail.This method can obviously reduce the construction quantity and cost.The results of this paper can be used to quantify the extent of damage caused by a specified fire,and judge the residual capacity and remaining service life of hanger.Besides,it also can provide guidance for maintenance or repair strategy to mitigate large fire losses.