Research On Internal Force Redistribution Of Prestressed Concrete Structures

It is important to apply high strength steel bars to implement the scheme of energy saving and emission reduction in State Council12th Five-Year Plan in China, and to promote the transformation of development mode of steel and construction industries. HRB400, HRB500and HRB600steel bars have been adopted by Chinese codes for Design of Concrete Structures GB50010-2010and Hot Rolled Ribbed Bars of Reinforced Concrete GB1499.2. The mechanical performance of statically indeterminate prestressed concrete structures in which high strength steel bars are arranged as non-prestressing steels is important theoretically meaning. So the following researches were made in this dissertation.(1) Rational evaluation of the stress increment in unbonded tendons is a precondition, and the basis to exactly compute the loading capacity of a prestressed concrete structure. If high strength steel bars were arranged as non-prestressing reinforcements, when the combined reinforcement index and the degree of prestress are constant, stress increment in unbonded tendons will be influenced at both of serviceability and ultimate limit states. That is because of the reduction of steel consumption and plastic rotation, and the incease of yield curvature. But existing formulas of ultimate stress increment in unbonded tendons were mainly established on the basis of simply supported beams, and the factors considered were insufficiency. A moment-curvature nonlinear analysis method was used to develop analytical programs from stress increment in unbonded tendons at serviceability and ultimate limit states in prestressed concrete continuous beams and slabs. Based on the results of model tests and simulation analysis, the equations were proposed to predict the stress increment in tendons at ultimate state, in which the non-prestressing reinforcement index βs, prestressing reinforcement index βp span-depth ratio L/h, loading type, number of loading spans and yield strength of non-prestressing reinforcement are the basic parameters. It can be shown from the results of model analysis that the strength of non-prestressing reinforcement will influnce the stress increment in tendons at serviceability and ultimate limit states, when the combined reinforcement index and the degree of prestress are constant.(2) Formulas of moment redistribution coefficient were mainly proposed based on the plastic rotation of cross section in prestressed concrete structures, after yielding of the non-prestressing reinforcement. But before yielding of the non-prestressing reinforcement, the internal force redistribution will be caused by plastic deformation and occurrence and developing of cracking in tensile concrete in critical section of support. So two-process method was proposed to analyze moment redistribution of statically indeterminate prestressed concrete structures. One process is to determinate the redistribution amplitude αb, which is caused from loading to yielding of non-prestressing reinforcements in critical section of support. The other process is to acquire αa, which is caused from the appearance of plastic hinge to crushing of compressive extreme concrete fiber in critical section of support. The coordinate system of moment redistribution coefficient versus relative plastic rotation θp/h0(the ratio of plastic rotation θp to the effective depth of section h0) was established. And the data were plotted in the coordinate, in which includes lots of testing results of reinforced concrete, bonded prestressed concrete and unbonded prestressed concrete statically indeterminate structures. Based on the vertical intercept of fitting bilinear curve with95%probability, the distribution of αb changing from yield strength of reinforcements was determinated. The expression of αa was proposed, in which the relative plastic rotation θp/h0is an independent variable. The current result will promote understanding mechanism and mathematical expressions of internal force redistribution in statically indeterminate structures.(3) Expressions of αa, in which θp/h0is an independent variable, were converted by nonlinear analysis for the engineering in practice. For reinforced concrete continuous beams, formulas of αa were developed, in which the grade of non-prestressing reinforcement and relative height of compression zone ξ, are independent variables. For bonded prestressed concrete continuous beams, formulas of αa were developed, in which the relative height of compression zone ξ, effective prestress σpe, degree of prestress λand grade of non-prestressing reinforcement are independent variables. For unbonded prestressed concrete continuous beams, formulas ofαa were developed, in which the prestressing reinforcement inde βp, index of non-prestressing reinforcement βs and grade of non-prestressing reinforcement are independent variables.(4) Past investigation on design method of stress increment in unbonded tendons and moment redistribution were mainly applied to linear members. A large number of prestressed concrete two-way slabs have been used in practice, but the investigation of the internal force redistribution of these structures is few. Therefore, the finite element was modeled by ABAQUS soft-ware for unbonded prestressed concrete edge-supported two-way slabs and flat plates. Stress increment in unbonded tendons and moment redistribution of prestressed concrete edge-supported two-way slabs at serviceability and ultimate limit states were studied, which are influenced by the grade of non-prestressing reinforcement and combined reinforcement index in critical section of support. Distribution of stress increment in tendons was studied, based on deflection characteristic of two-way slabs. Calculation formulas of stress incrememt in tendons at two limited states were developed, in which the combined reinforcement index, boundary condition, relative location of tendon and grade of non-prestressing reinforcement were considered. The formula of moment redistribution coefficient in critical section of support in continues two-way slabs was proposed, in which the combined reinforcement index and grade of non-prestressing reinforcement were considered. For prestressed concrete flat plates, stress increment in tendons was studied, which is influenced by the combined reinforcement index and grade of non-prestressing reinforcement. The plates were divided into the column strips and the middle ones. Expressions of stress increment in tendons at two limited states were developed for column strips and middle strips respectively. The results predicted by elastic equivalent frame method were compared to the results of simulated analysis, and moment redistribution coefficient in critical section of support were obtained, which is influenced by the combined reinforcement index and grade of non-prestressing reinforcement. Formula of moment redistribution coefficient was developed, in which the combined reinforcement index and grade of non-prestressing reinforcement were considered. This is helpful to revise design codes.(5) Based on the analysis of finite element, prestressed continuous beams, edge-supported two-way slabs and flat plates were compared. The stress increment in tendons increases with the higher yield strength of non-prestressing reinforcements at the serviceability and ultimate limit states in prestressed concrete continuous beams, edge-supported two-way slabs and flat plates. When the combined reinforcement index and the degree of prestress are constant. Stress increment in tendons of prestressed concrete continuous beams is significantly greater than that of edge-supported two-way slabs and flat plates. Ultimate stress increment in tendons increases with the more rotational restriction of boundary in two-way slabs. The moment redistribution coefficient of critical section in support reduces with the higher yield strength of non-prestressing reinforcements in three types of structures. The moment redistribution coefficients of flat plates are less than that of edge-supported two-way slabs.