Calculation Of Long-term Stress Losses In Prestressed Concrete Members

Abstract:Precise calculation of long-term stress losses in prestressed steel is the premise of calculating accurately the effective stress in prestress steel and the foundation of analyzing the long-term performance of prestressed concrete structures. Overestimate or underestimate of long-term stress losses can possibly fail to meet the requirements of serviceability limit state. The existed methods, included in national or international codes, to calculate long-term stress losses are approximate approaches and the precision of analysis needs to be improved. Based on the equilibrium of internal force and compatible deformation in section, accompanied with the study of natural relaxation, the formula to calculate the reduction coefficient for natural relaxation in prestressed steel due to concrete shrinkage and creep is deduced. Numerous factors affecting reduction coefficient have been analyzed resulting in time-dependent curves between natural relaxation and factors. When the natural relaxation is introduced, the unified approach to calculate the long-term stress losses in prestressed steel caused by concrete shrinkage, creep and relaxation of steel is established, which considers the effect of sectional characteristics, steel arrangement, and steel ratio etc. in sections. The time-dependen reduction coefficient and total long-term stress losses in prestress for the typitical simplely supported highway and railway prestressed concrete girders are analyzed, and the calculated results are compared with the theoretical values given by the domestic and foreign specifications. The comparison shows that the proposed unified calculation formula for effective long-term prestress in prestressed steel based on the concept of reduction coefficient for natural relaxation bears agreeable precision.