| Reinforced concrete aqueduct,as an efficient water conveyance structure that spans river valleys and roads,has important strategic significance for balancing the allocation of national water resources.During the service period of the aqueduct,it is affected by water loads,freezethaw,temperature differences,and environmental corrosive media.Both concrete and steel bars tend to deteriorate,and their safety gradually decreases.Their safety is directly related to time.The reliability analysis method is currently one of the important methods for structural safety evaluation,and it is also the theoretical basis for the formulation of current structural design specifications.Conducting time-varying reliability analysis of thin-walled aqueducts has become the basis for scientific evaluation and reinforcement of aqueducts.This article mainly considers the effects of large temperature differences in natural environments,introduces the time-discretization method and Copula theory,calculates the time-varying reliability and system reliability of rectangular aqueduct structures,and conducts systematic analysis,laying the foundation for the safety evaluation and life prediction of rectangular aqueduct structures.The main work and conclusions of this paper are as follows:(1)The relevant regulations and requirements for the bearing capacity of hydraulic structures and the target reliability of normal service limit states have been established in the relevant regulations,and design and safety evaluation standards have been determined.The basic principles and functional functions of the time-discretization method have been Write MATLAB calculation programs based on three methods,using the Monte Carlo method as an approximate accurate solution,to analyze and compare the calculation accuracy of the "dynamic" reliability method and the time discrete method.Taking the time-varying reliability calculation of the bearing capacity of the side wall of the aqueduct structure as an example for analysis and calculation.The results indicate that within the designed service life,the maximum error of the "dynamic" reliability method is nearly twice greater than that of the time discrete method.Moreover,in the later stages of service,the results of the time-discretization method for structural safety evaluation are more conservative,highlighting the applicability of the timediscretization method for reliability calculation and safety evaluation of aqueducts.At the same time,it is confirmed that the "dynamic" reliability method has the disadvantage of low accuracy.(2)Based on the design requirements of the aqueduct structure,the bearing capacity and crack resistance function of the four parts in the vertical and horizontal directions of the rectangular channel structure with and without tie rods were established.Combined with the time-discretization method function,a time-varying reliability calculation model of the rectangular aqueduct structure based on the time-discretization method was obtained.Based on a typical engineering aqueduct,the time-varying reliability of the aqueduct was calculated for two types of cross sections with and without tie rods.Taking the ultimate bearing capacity of the aqueduct body as the research object,the influence of the start time of steel corrosion on the time-varying reliability of the rectangular aqueduct body structure was analyzed.The results indicate that the bearing capacity and crack resistance time-varying reliability of the aqueduct body with or without tie rods are similar.The control state is obviously longitudinal bearing capacity,and under the dual influence of the attenuation of steel and concrete strength,the timevarying reliability of longitudinal bearing capacity increases at a faster rate in the later stage of service.The mid-span part of the bottom plate with tie rods and the longitudinal part without tie rods have relatively high reliability in resisting cracking and time-varying.Overall,all parts have not fallen below the target reliability during their service life,indicating a high level of structural crack resistance and safety redundancy.In addition,the delay in the rusting time of the steel bars is synchronized with the predicted service life of the structure.Under the 30 year starting rust condition,the predicted service life is about 60 years,65 years later under the 35 year starting rust condition,and 70 years later under the 40 year starting rust condition.Therefore,appropriately extending the start time of steel corrosion is crucial for ensuring structural safety and lifespan.(3)Taking the rectangular aqueduct structure without top plate as the basic research object,considering the stress of the aqueduct structure,the bearing capacity of the reinforced concrete structure and the characteristics of the serviceability limit state,it is considered that the negative temperature condition is more dangerous for the aqueduct body,and only has an impact on the serviceability limit state of the aqueduct body.Based on our group’s latest research results on the distribution of temperature gradient mode of rectangular aqueduct,it is calculated by structural mechanics method that the vertical distribution of longitudinal temperature self stress presents tension at both ends and compression in the middle as a whole,and the maximum value appears at the top of the side wall.After the addition of temperature stress,the reliability of crack resistance in all three parts of the transverse direction rapidly decays,and the decay rate increases with time.(4)Based on the Copula theory,a time-varying reliability calculation model for structural systems based on the Copula function was established,and the time-varying reliability calculation process for rectangular aqueduct body structural systems was provided.Considering the edge distribution of the ultimate bearing capacity of the four parts of the structure,including longitudinal,side walls,bottom plate ends,and mid-span of bottom plate,the binary correlation is analyzed to obtain the time-varying joint failure probability and reliability of the rectangular aqueduct body structure series system.The results are compared with the commonly used wide and narrow boundary methods.The results show that the correlation between the longitudinal and transverse parts of the aqueduct body is extremely low,and it can be considered that the longitudinal bearing capacity of the aqueduct body is independent of and does not affect the bearing capacity of other parts.The correlation between the side wall of the aqueduct and the middle of the bottom plate span is slightly higher than the combination of longitudinal and other parts.The calculation results in this article accurately fall within the wide boundary,but are similar to the narrow boundary method and tend to be conservative.The method has certain applicability.The control failure modes of time-varying reliability of structural systems include individual failure modes and joint failure modes of side walls and bottom slab spans. |
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