Analysis On Seismic Reliability For Existing Cooling Towers

Cooling tower is an important part of thermal power plants and nuclear power stations.The continuous increase in installed capacity makes concrete hyperbolic cooling towers tend to be oversized.Therefore,the seismic performance of the cooling tower structure began to be taken seriously.In addition,because of the special service environment(high temperature,high humidity),the cooling tower is faced with more serious problems such as carbonization of the concrete protection layer and corrosion of the steel bar,etc.,as compared with ordinary buildings.This paper aims to reveal the effect of resistance decay on the reliability of existing large-scale cooling towers under earthquake disasters and evaluate the level of seismic reliability of existing cooling towers under different site and different fortification intensity.This paper mainly considers a series of damage problems that are mainly caused by concrete carbonization and steel bar corrosion during the service period of cooling towers.On the basis of the existing carbonization depth prediction model and steel corrosion rate model,the reliability analysis of the cooling tower under earthquake is performed.First,based on the existing research,the failure mode of the cooling tower structure under earthquake is researched,and finally the yield of the X-pillar top bar is considered as structural failure.Then a deterministic analysis of the parameters of carbonized corrosion was made.Parametric analysis shows that the reduction of the cross-sectional area of the rebar caused by corrosion has a significant impact on the seismic response of the cooling tower.Taking into account the randomness of the parameters,a number-point selection method was used to obtain a statistical sample of the longitudinal tendon stress at the critical point of the cooling tower under different conditions during the service period of 100 years.The analysis shows that since the 30 th year,the corrosion of the steel bar has had a major impact on the reliability of the cooling tower structure,and with the passage of time,the failure probability of the structure increased.Based on the seismic wave time history analysis considering the randomness of the parameters of the carbonation corrosion model,the reliability analysis method based on the first surpassing damage mechanism and the Cornish-Fisher approximation approach are applied to obtain the reliability of the cooling tower under different conditions during the service period of 100 years.The results show that the probability of structural failure is less in the 7-degree fortified areas of Site I and Site II;For the 8-degree fortification areas of Site II and the 7-degree fortification sites of Site III,structural failures may occur after 70 years;In the 8 degree fortified areas of Site III,structural failures will occur after 50 years,and structural failures are more likely to occur in the later stages of service.Therefore,there is a need for more safety reserves for cooling towers with poor site conditions in the 8-degree fortification zone.