Study On Bearing Capacity Of Members Of Concrete Simply-supported Beam Bridges Reinforced With GFRP Or GFRP-steel Composite Bars

The problem of steel corrosion in reinforced concrete has always been serious.For bridges,especially those coastal or sea-crossing,the natural environment will accelerate the corrosion of steel bars,so they are confronted with many such issue,especially for piers in tidal zone.Fiber reinforced Polymer(FRP)has many advantages,such as light,high strength and corrosion resistance.In addition,GFRP is cost-effective,and therefore,scholars at home and abroad proposed to replace steel bars with GFRP bars or GFRP-steel composite bars,and carried out corresponding research,including material properties,bond behavior between reinforcements and concrete,flexural and compressive behavior of concrete members.After years of research and accumulation,the material properties and bond behavior had been basically clear,but the design and calculation of bearing capacity of beams and columns still need to be further improved,including the flexural,axial and eccentric capacity of members.The main research contents and achievements were as follows:(1)According to the constitutive models of GFRP reinforcements and GFRP-steel composite reinforcements and the requirements of existing codes,the design strength of them was determined as the stress corresponding to the strain of0.005 and yield stress.According to the design principle of reinforced concrete beam,the balanced relative depth of compressive area,minimum reinforcement ratio,yield and ultimate bending moment of composite reinforced concrete beam s were obtained.Compared with the test results,the yield bending moment was slightly smaller and ultimate bending moment coincided well.Based on the strength equivalence,the design and calculation method of concrete beams reinforced with composite bars is proposed.(2)According to the existing test data,under the peak load of axial compression members,the strain of GFRP reinforcements was 1.2 times of the peak strain of concrete,and the strength of concrete was 1.08 times of its axial compressive strength.According to the stability coefficient of reinforced concrete columns,the accuracy of tangent modulus theory was verified,and then the stability coefficient formula of concrete columns reinforced with GFRP bars was obtained through the theory.In order to obtain the constitutive model of concrete confined by GFRP spiral stirrup,a new variable,equivalent elastic modulus of stirrup,was proposed,and then based on the existing test data and Mander model,the peak stress and strain formulas of confined concrete were obtained.Finally,the calculation formulas of the axial capacity of concrete rectangular and circular section columns reinforced with GFRP bars were obtained,which were in good agreement with the test results.Similarly,for concrete columns reinforced with GFRP-steel composite bars,the calculation formulas of axial capacity was obtained,which were in good agreement with the existing test results.(3)Based on the equilibrium equation and curvature method,the discriminant of failure modes of concrete columns reinforced with GFRP bars under eccentric compression was analyzed and derived,the results of which were the same as the experimental ones.Then,the eccentricity increase factors,considering relative eccentricity,slenderness ratio and strength reinforcement ratio,of rectangular and circular eccentric columns were constructed and fitted by theoretical analysis and the trend of existing test data.In addition,according to the principle of reinforced concrete column,the simplified calculation formula of eccentricity increase factor,considering relative eccentricity and slenderness ratio,was derived,which was in good agreement with the existing test results.Finally,the design and calculation methods of eccentric capacity of concrete rectangular and circular section columns reinforced with GFRP bars were obtained,which were in good agreement with the test results,and in order to facilitate the calculation,the bearing capacity provided with GFRP reinforcements could be ignored when necessary.In the same way,concrete columns reinforced with GFRP-steel composite bars were analyzed,and the design and calculation methods of eccentric capacity were obtained,which were in good agreement with the existing test results,and in order to facilitate the calculation,it was necessary to ignore the secondary stiffness of longitudinal reinforcement and adopt the equal stiffness method for design.(4)Based on the research of this paper,the design of ultimate limit state of typical concrete simply-supported beam bridge was carried out,including concrete simply-supported beams reinforced with GFRP-steel composite bars,concrete piers reinforced with GFRP or GFRP-steel composite bars,which laid the foundation for future engineering application.