Study On Cracking Control Of Underground Super-long Prestressed Concrete Roof Considering The Temperature And Shrinkage Effects

For underground super-long structures,the cracking control is of great importance,while the thermal stress and shrinkage stress on the structure is relatively large,so the effect of temperature and shrinkage contractions should be analyzed accurately,then the reasonable cracking control measures should be adopted in order to achieve the purpose of both the economy and the cracks within a reasonable range.First,based on the project of the underground square of Hefei Railway Station,the calculation method of the thermal load,shrinkage equivalent temperature difference and reduction coefficient were studied.Then,the finite element simulations for the effect of this project under temperature rise,temperature drop,shrinkage and prestress were carried on,and the cracking control effect was evaluated considering all factors.Finally,concrete temperatures and stresses of each measuring point from monitoring were analyzed,and some suggestions for design and construction of super-long structure were put forward.The following conclusions can be achieved through researching:(1)The calculation of the minimum and maximum mean temperature of underground concrete structures should adopt the method of soil initial temperature under periodic boundary conditions provided by regulation with considering the influence of structural opening.The calculation of shrinkage of structure should adopt the method provided by Wang Tie-meng.The creep reduction coefficient for shrinkage should take 0.32.The combination value coefficient of concrete shrinkage can take 0.8.The effect of temperature and shrinkage can be analyzed by elastic finite element method with considering the influence of creep and members’ cracks.(2)Cracks in the East and middle regions of this project meet design requirements,and the same number of linear prestressed tendons in two directions of the roof is conservative through taking all factors as a whole.Through the data analysis from monitoring,the temperature is uniform in floor thickness less than 400 mm for roof covering thickness of 1.5m,otherwise it is not uniform,and the mean strain increment after stretching linear prestressed tendons was higher than theoretical value.(3)The crack check of roof with linear prestressing can follow the following steps: First,the reinforcement stress is obtained from the crack width derived from the structure design software with considering the actual amount of reinforcement.Then the reinforcement stress increment is calculated from the roof stress,which is obtained under the combination of temperature,shrinkage and the prestressing effect.Finally,put the total reinforcement stress into the code to check the crack width.The closure stage of the underground concrete structure should choose February to April or October to December according to the climate characteristics of Hefei.