Research On The Mechanism And Applicability Of CFST String Structure Stiffened RC Beams

The research and development of new composite bridge have been very active in our country,which the stiffened frame bridge has been used by a large number of engineering practice because of its excellent mechanical performance and flexible construction organization plan.In order to make bridge construction self-supporting,improved construction technology,this article presents a new kind stiffened frame structure-CFST string structure stiffened RC beams.The CFST string beam be used to meet the need of pre-use,bear the construction load before bridge completed,then poured external concrete make them work together.Order to understand its mechanism,the finite element model was established for a series of parameter analysis and applicability studied.The main works and research conclusions are follows:(1)The obtained load-deflection curve through two ABAQUS finite element model analysis has considered two complex contact relations between unbonded prestressed tendon,steel tube and concrete is in good agreement with experimental results.(2)The structure failure modes,unbonded prestressed tendon stress increment,the influence of CFST jointly working performance made by different contact relations between steel tube and concrete had be deeply researched based on FEM.The analysis show that the stress and strain development laws conform well with test results,the different contact relations between steel tube and concrete is significantly affected the mechanism of whole model and CFST.(3)Meanwhile,the CFST string structure stiffened RC beams were nonlinear full-range analysed.The analysis show that:with the steel ratio of the steel tube,longitudinal reinforcement ratio,prestressed reinforcement ratio,reinforcement yield strength,strength of outer concrete improve,the bending capacity of members is also improved,but its stress increment of unbonded tendon decreased;the span-depth ratio,prestressed tendon profile,loading method are significantly affected the bending capacity and stress increment of unbonded tendon.With the initial effective prestress improve,cracking load,initial stiffness and ultimate load is also raised,but its stress increment of unbonded tendon decreased.(3)By the theoretical analysis and the regression analysis of FEM results,the stress increment of unbonded prestressed tendon practical calculation formula represented by?0 and h/lo was also proposed combined with the current industry standard "technical specification for concrete structures prestressed with unbonded tendons"(JGJ92-2016).The flexural capacity calculation formulas that considered CFST eccentric compressive were proposed.The calculation formulas agree well with test results.Based on the numerical analysis,a three fold line formula for calculating the reduction coefficient considering the slip ratio(rm)is proposed.(4)Validate the applicability of self-supporting idea with a design example,discussed the effect of initial stress to the mechanical behavior of components.The analysis show that:the CFST string structure stiffened RC beams not only to achieve the construction stage self-supporting,and its ductility improved significantly.With the initial stress improve its initial flexural rigidity slightly raised,but the rigidity has a slight decline when the compression zone steel pipe and prestressed tendons yield.