Study On Fatigue Behavior Of P.P.C Beams With Mixed Reinforcement

The fatigue behavior of partially prestressed concrete (P.P.C) flexural members with mixed reinforcement is different due to its special reinforcement details and mechanical behavior in comparison with conventional reinforced or prestressed concrete members. Constant-amplitude fatigue load is generally used in the traditional fatigue design for concrete structures home and abroad, which requires that the structural members not occure fatigue failure under the constant-amplitude fatigue loading. But actually concrete members in practice engineering are subjected to random-amplitude fatigue loading with large amplitude fluctuation. To preferably reflect the real mechanical behavior in the fatigue design and analysis for P.P.C members, experimental study and theoretical analysis on seventeen P.P.C T-beams are carried out under constant-amplitude and random-amplitude fatigue loadings. The thesis is an important part of the project of Doctoral Specialty Foundation of Ministry of Education - Study on the fatigue behavior in normal cross-section of P.P.C beams with mixed reinforcement (No. 20020141026), the contents of which include eight chapters as follows.In the first chapter, the significance of this study and the relevant literatures are expounded. Meanwhile, the historical and present background of the research on fatigue of P.P.C beams is introduced systematically. Based on the existing problems, the objectives of this paper are put forward.In the second chapter, the experimental programs including the mechanics properties of materials, details of beam specimens, apparatus of static and fatigue test are introduced. Moreover, the test procedure, the test parameters and the test data acquisition method are determined. Then the basic test results are provided. Finally, fatigue failure pattern, fatigue life and the effect of fatigue loading on the postcyclic static strength are investigated.In the third chapter, fatigue tests of post-tensioned bonded P.P.C T-beams with both prestressed strands and HRB400 ordinary reinforcement are carried out. Based on the test results, the stress redistribution between prestressed and non-prestressed reinforcement in P.P.C members versus number of cycles under fatigue loading is studied, and a method considering the different bond behavior of both steel types is proposed for calculating stress increment of reinforcing steel. The calculation results fit favorably with the experimental results.In the fourth chapter, based on the fatigue loading spectrum of reinforced concrete crane beams established through practical survey, a random-amplitude fatigue loading spectrum fortest has been obtained by appropriate simplification. Fatigue tests of P.P.C beams under the random-amplitude fatigue loading spectrum have been achieved through MTS fatigue machine. The fatigue lives under random-amplitude are compared with those under constant-amplitude. Then three typical cumulative damage rules are applied to calculate the fatigue lives of P.P.C flexural members under random-amplitude fatigue loading, and results obtained by these three rules are compared. It is suggested that modified Corten-Dolan cumulative damage rule or relative Miner rule can be used to estimate the fatigue life of P.P.C beams under random-amplitude fatigue loading. Finally, the calculation formula for equivalent constant-amplitude stress range based on relative Miner rule is derived according to equal damage degree principle. The formula is easy to apply, and it has the same precision as relative Miner rule.In the fifth chapter, residual strains of concrete in the compression zone versus number of cycles under constant-amplitude, two-stage variable-amplitude and random-amplitude fatigue loading are investigated respectively, based on fatigue tests of P.P.C beams. A calculation formula considered the effect of stress gradient in the compression zone is proposed, and the calculated results fit favorably with the experimental results.In the sixth chapter, a full-range nonlinear analysis method that can account for the interaction between concrete and steel and variable amplitude repeated loadings is proposed in this paper. The calculated results indicate that the method can describe the fatigue damage process of prestressed concrete beams with accepted accuracy and can contribute to establish a more reasonable prediction method for fatigue life.In the seventh chapter, experimental study and therotical analysis are performed on the propagation rule of maximum crack widths of P.P.C. members under constant-amplitude and random-amplitude fatigue loadings, and then the calculation method for the propagation rule of maximum flexural crack widths under constant-amplitude fatigue loading is proposed.In the eighth chapter, the main achievements of this paper are summarized and the further research of this field is pointed out.