| With the advancement of national strategies such as the “Belt and Road Initiative”,the Yangtze River Economic Belt,the development of the western region,there are a large number of bridges that have been built or to be built in China,and bridge safety issues are becoming increasingly prominent.The safety of the bridge is closely related to the load it bears and its bearing capacity,and the two have significant randomness.The safety level can be further evaluated through reliability studies.At the same time,the load on the bridge during its service period and its own bearing capacity are related to time,and its reliability has a time-varying characteristic.Since the long-span suspension bridge occupies an important position in regional traffic,and its tower is used as the main load-bearing member,it is necessary to study the time-varying reliability of its tower.Based on the Honghe bridge project,the load response and time-varying law of the tower under the action of random traffic flow and wind flow are studied.According to the environmental characteristics of the bridge site area,the degradation of the concrete properties of the tower and the attenuation of the structure's own bearing capacity were studied.On this basis,the stability of the tower under the time-varying characteristics of the load and resistance was studied.The time-varying reliability model of the tower was built,and the time-varying law of the tower reliability during the service period was explored.The main research results of the study are as follows:(1)Based on the measured data,the probability density distribution of vehicle flow parameters such as the vehicle type,the lane,the speed,the distance,and the vehicle weight,as well as wind speed parameters such as the wind speed were statistically analyzed.A random vehicle flow and wind flow simulation program was compiled to quickly generate a random vehicle flow and wind flow sample.(2)The stochastic process characteristics of the load response of the tower under the action of random vehicle flow and wind flow are revealed.The mean value of the load response of the tower tends to converge,and it has been observed that the load response is no longer relevant outside a certain time.Meanwhile,the load response obeys to the normal distribution law,which indicates that the tower load response follows a stable Gaussian random process and has various states of ergodicity.Based on Rice's extrapolation theory,the probability model of the extreme load response of the tower was established,and the time-varying law of the extreme load response was predicted during the service period.(3)The time-varying law of the physical and mechanical properties of concrete under multi-factor sulfate corrosion was explored.During the test period,the mass of concrete samples increased with the increase of test time,and the growth rate gradually slowed down.The variations of the elastic modulus and the compressive strength of concrete samples included growth and decline stages,and the higher the solution concentration,the faster the elastic modulus and compressive strength decrease.Both wetting-drying cycles and cyclic loading will accelerate the sulfate corrosion process of concrete.(4)A compressive strength attenuation model for concrete considering the solution concentration under sulfate corrosion and wetting-drying cycles was established.Based on the similar relationship between indoor test conditions and natural environmental conditions on the bridge site,the time acceleration ratio could be approximated.The time-varying law of the compressive strength of the concrete in the bridge site during the service period was predicted: the compressive strength will drop to 0.77 times the initial strength by 100 years.The thickness of the corrosion damage layer is used to characterize the resistance deterioration of the concrete structure,and the time-varying thickness of the corrosion damage layer of the tower was predicted: the thickness of the corrosion layer is about 0.16 m when the tower of the Honghe Bridge is in service until100 years.This exceeds the thickness of the protective layer of concrete against steel bar,and it is necessary to do protective treatment and maintenance and reinforcement in time.(5)The two types of stability problems of towers under different working conditions were explored,and the influence of time-varying loads and resistance degradation on the two types of stability safety factors of towers was revealed.Under the influence of time-varying loads and resistance degradation during the service period,the two types of stability safety factors of the tower decrease gradually with the increase of the service time.The resistance degradation has a more significant impact on the stability factor of the tower.Compared with the effect of a single time-varying load or resistance degradation,the stability safety factor decreases more significantly under the influence of both.(6)The time-varying reliability model of the tower is established considering the dual time-varying effects of load and resistance degradation,and the time-varying mechanism of the tower reliability is revealed.Under the influence of the time-varying load and the resistance degradation,the reliability index of the tower decreases with the increase of the service time,and the reliability declines rapidly in the initial service period.The reliability index of the left and right towers of Honghe bridge is reduced to4.29 and 4.38 respectively after 50 years of service,while the minimum targeted reliability index of the bridge structure is 4.4 under the 85% guarantee principle.The reliability index after 100 years of service is reduced to 3.10 and 3.16 for the left and right towers respectively.(7)The effect of random variables on the failure probability of the tower is ranked as follows(from the highest to the lowest): concrete elastic modulus,horizontal load at the top of the tower,concrete density,vertical load at the top of the tower,and transverse bridge load at the connection between the tower and beam.The influence of other random variables on the tower failure probability is not obvious.The reliability of the tower can be improved through operations such as ensuring that the strength characteristics of the material meet the required standard during the construction of the tower,reducing the unbalanced horizontal force of the tower,and controlling the traffic flow. |
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