| This dissertation reviews the development and present state of externally prestressed concrete. Through scientific research project of urban construction "Statically structural behavior experimental study of externally prestressed high-strength concrete three-span continuous beams", the crack behavior, deflection variation, moment redistribution of test beams and flexural rigidity of continuous beams are investigated deeply. From the viewpoint of theoretical analysis, the ductility of externally prestressed concrete is discussed, the analytic model of the stress increment of external prestressing tendon related to continuous beam deflection is established, and stress increment of external prestressing tendon is concluded in the form of different boundary conditions, different loading patterns, different ratios of reinforcement and different span-depth ratios. By analyzing the nonlinear numeric value of three-span prestressed continuous beam under the conditions of various span-depth ratios, various PPR and various concrete strength, the theory of "triple broken lines rigidity reduction, and beam body deflection calculation according to elastic theory" is presented; by analyzing the inherent law between stress increment and deflection variation, the simplified calculation equation of stress increment varying with deflection variation is obtained. Result calculated by this formula is almost as accurate as the result calculated by nonlinear theory. However, calculation by this formula needs less computational effort and is more beneficial to engineering project.
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