Stress Evaluation For Prestressed Tendons In Concrete Girders With Consideration Of Crack-induced Incompatible Strain

Due to the effect of gravity and long-term loading,the beam of prestressed concrete bridge which in service usually works with cracks.After cracking,the beam will enters a nonlinear state and mechanical properties of in-service bridge may not meet design expectations.Meanwhile,large numbers of bridges may be damaged in major disasters such as earthquakes.It is of great importance for the rescue work to quickly and effectively evaluate the mechanical state of damaged bridge structures by cracks,deflections and other intuitive parameters.Prestressed steel bar as the key force component of a bridge,how to effectively confirm its actual prestress size is the key of the safety assessment of concrete bridge structure.In this paper,the model beam and 30 m prototype beam test are carried out.The structure response laws such as the beam deformation,the prestressed tendon strain and the fracture parameters are obtained.The nonlinear mechanical behavior of prestressed concrete beams after cracking is researched.The main research results are the following aspects:(1)Through experimental studies,the crack mechanism is analyzed and the rule of crack development is obtained.The width and height of crack increases with the increase of load level.And the growth rate of the width and height of crack increased together.When the width of crack is developed to a certain extent,it will be appeared in mid span.The amount,width and height of crack in mid span is more than other areas.The crack spacing decreases gradually and the rate decreases gradually.When the non-prestressed tendon yields,the new crack development slows down and the spacing is basically unchanged.(2)The relationship between the crack width ratio and the bearing rate is obtained by experimental data,which can be used to evaluate the load-bearing capacity of the beam.(3)The development of the height of crack indicates the change of the neutral axis of the beam.The height of the compression zone is solved by considering the beam non-coordinate deformation iteration and the relation curve between the height of compression zone and the height of crack is obtained.(4)On the basis of the theory of crack non-coordinated deformation,combining theoretical analysis with experimental research,the bending stiffness of beam is introduced.Based on the measured deflection and plastic hinge theory,the stress assessment method of prestressed concrete structure prestressed concrete structure is proposed.(5)Compared to monotonic loading beam,the stress of pre-stressing tendons of reload beam is increase under load,which shows that the uncoordinated deformation of beams under multiple loads is greater.(6)The effect of uncoordinated deformation on the stress level of prestressed concrete structures is not negligible.And with the load increases,the effect of uncoordinated deformation becomes more and more obvious.Considered uncoordinated deformation,the prestressed tendon stress is safe and abundant compared with the measured value.It avoids the unsafety consequences of the evaluation of the design specification and provides a simplified method to evaluate the actual state of service bridge structure.(7)With the combination of finite element and experiment,the structural properties of 30 m prestressed concrete box girders were studied.The results of finite element model agree well with the experimental results.Compared the model prestressed tendon stress with the result of the evaluation,the applicability of the prestressed reinforcement stress assessment method to the real beam is verified.