Design Study On Prestress Losses Of Shrinkage-creep And Control Of Flexural Cracking For Prestressed Concrete

With the high structural performance, prestressed concrete structure has been applied in civil engineering for as long as 100 years. And it will be improved and applied more widely as the material technology, the design technology and the construction technology of prestressed concrete have been experiencing radical developments. This thesis presents a study on two problems of design technology of prestressed concrete: the calculation method of the prestress loss caused by shrinkage and creep of concrete, and the crack control method of prestressed concrete structure under serviceability limit state. The study and analysis included in the thesis is listed as below:1) Firstly, theoretical formulas used to calculate prestress loss caused by shrinkage and creep of concrete are studied based on linear creep theory and superposition principle. The EN 1992 model, which has high accuracy and suitable for design, is chosen to calculate ultimate values of shrinkage and creep of concrete, and the EN 1992-calculated values are compared with those ultimate values obtained through ACI 209 model and MC 90 model, which is adopted as the new standard by the Ministry of Transport of China. Then simplified formulas used to calculate prestress loss are derived from the calculated ultimate values.2) The regulations for crack control and durability design of the standards both inside and outside of China are discussed and compared to study the limit values for crack width. A practical method to relax the limit of the maximum crack is recommended, in case that the thickness of concrete cover meets the requirements of durability design. Based on the calculation fomulas of Code GB50010-2002 for calculation of crack width and theory of analysis of cracked prestressed concrete sections, 1000 sections with various forms of prestressed concrete beams are calculated and analyzed through compiling programs, then the limit values of nominal tensile stress for design are modified to simplify crack control calculation.3) The maximum bar diameters and maximum bar spacing for crack control under conditions of different steel stress and different limit of the maximum crack are analyzed on the basis of foreign literature and related codes, then a crack control method without direct calculation of crack width is proposed.4) A case study is conducted on designing schemes for a prestressed concrete truss transfer story and sections of prestressed concrete beams of 7 practical projects, to study the applicability and effectiveness of the methods proposed in the thesis.5) Suggestions are recommended for calculation of prestress loss caused by shrinkage and creep of concrete as well as crack control design of prestressed concrete structure under serviceability limit state.