Study On Compression Performance Of Rectangular UHPC Filled High Strength Steel Tube Components

Ultra-high performance concrete（UHPC）is a cement-based material with high strength,high toughness and low porosity developed in the past 20 years,which greatly improves the durability and composite function of concrete,and has low energy consumption and less damage to the environment and pollution.UHPC is weak in ductility,but its brittleness can effectively overcome by placing it in steel tube to form the structure of UHPC filled steel tube.The structures of UHPC filled high-strength steel tube can reduce the structural weight,save the building materials consumption,protect the environment,and improve the economic effect.It is supposed to the needs of modern engineering structures that develop toward high,large span,and heavy loads and withstand harsh conditions.It is expected to be widely used in engineering fields such as tall buildings,ultra-high rise structures,long-span bridges,subway structures,high-speed rail stations,explosion resistant structures and so on in the future.The compression performance of CFST is the basis for understanding the mechanism of interaction and its complex mechanical properties between the steel tube and the concrete,and it is the premise to promote its engineering application.Therefore,it is significant in theory as well as application to study the compression performance of UHPC filled high-strength steel tube columns.With the support of the key program of the Chinese National Natural Science Foundation of Research on the mechanical performance and design theory of Ultra-High Performance Concrete filled steel tube under complex actions（Grant No.51738011）,the compression performance of rectangular UHPC filled high strength steel tube column was systematically explored and studied by the method of experimental research,theoretical analysis and numerical calculation.The main work and the achievements are summarized as follows:（1）Considering the influences of the thickness of steel tube,UHPC strength grade and the aspect ratio,54 groups（108 specimens）of rectangular UHPC filled high-strength steel stub columns were designed and produced,and the axial compression performance of the short column was studied through static tests and numerical simulations.The results showed that the whole process of axial compression of short columns goes through three stages:the elastic stage,the elastoplastic stage,and the failure stage.According to the confinement indexξ,the load-axial deformation curves after the peak load of the test specimens can be divided into three categories:the descending type（ξ<2.3）,the approximate horizontal type（2.3≤ξ<3.0）,and the ascending type（ξ≥3.0）.The test specimens with confinement indexξ≥3.0 suffered waist drum failure,and the test specimens with confinement indexξ<3.0 occurred shear failure.The ductility of the test specimens decreases with the increase of the aspect ratio of the section.The double ductility index:the ductility before the peak load and the residual bearing capacity after peak load should be paid attention to the design of rectangular UHPC filled high-strength steel tube columns.To ensure the ductility of the test specimens,the upper limit of the width to thickness ratio of the plates of rectangular UHPC filled high-strength steel stub column is 0.8 times of the width to thickness ratio calculated by the formula of ordinary rectangular concrete filled steel tube.Considering the economic factor,the confinement index recommended should not be greater than 3.0.（2）Considering the effect of slenderness ratio and the thickness of the steel tube,18rectangular UHPC filled high-strength steel tube specimens with medium and large slenderness ratio were designed and produced,and the axial compression performance was carried out by the method of experimental research and numerical simulation.The results showed that the axial compression process of specimens could be divided into three phases:the elastic phase,the elastoplastic phase,and the instability failure phase.Material strength failure occurred in specimens with the slenderness ratio of 24,and instability failure occurred in specimens with the slenderness ratio greater than 30.There are three types of load-deformation of specimens:approximate sinusoidal half-wave shape,fully asymmetrical shape and inverted S-shape.There is already lateral deflection of the specimen at the beginning of loading,and the second-order effect of deflection of columns with medium and large slenderness ratio is obvious.The bearing capacity,ductility and bending stiffness of the specimens increase significantly with the increase of the thickness of the steel tube.As the yield strength of the steel increases,the bearing capacity and bending stiffness of the specimen increase,but the ductility after the peak load decreases.With the increase of UHPC strength grade,the bearing capacity and bending stiffness of the specimens increase,however the post-peak ductility decreases slightly.As the aspect ratio of the section increases,the bearing capacity,bending stiffness and peak load post-ductility of specimens with medium and large slenderness ratio increase.（3）Taking the main parameters such as confinement index and eccentricity into account,18 rectangular UHPC filled high-strength steel stub columns specimens were designed and manufactured to research on the eccentric compression performance by combining experimental and numerical analysis.The results showed that the whole loading process of the specimens could be divided into three stages:elastic stage,elastoplastic stage and failure stage.The eccentric compression failure mode of the test piece is strength failure.When the test specimen is broken,the specimen is bow-shaped,and the bending pressure side of steel tube is yielded.The degree of deformation is closely related to the confinement index and the eccentricity.The load-deflection curve of the specimen is close to the half-wave sine curve,and the longitudinal strain distribution along the mid-span is basically in accordance with the assumption of the plane section.The peak load and the ductility following the peak load decrease with the increase of the calculated length of the specimen.As the thickness of the steel tube increases,the bending stiffness,the bearing capacity and the ductility after the peak load of the specimen increase.With the increase of the strength of steel,the bearing capacity and bending stiffness of the specimen increase,but the ductility after the peak load decreases.With the increase of strength of UHPC,the bearing capacity and flexural rigidity of the specimens increased,but the peak ductility decreased slightly.As the aspect ratio of the section increases,the bearing capacity,bending stiffness and the ductility of the specimen increase.（4）First,the rationality of the ABAQUS finite element model was verified based on the test results.Then,the working mechanism of the axial and eccentric compression of rectangular UHPC filled high-strength steel tube columns was revealed by modeling with ABAQUS software.Under the vertical load,the confinement effect of the outer steel tube on the core UHPC occurs after the elastic phase of the specimens.The restraint force is first generated at the corner of the section,then develops towards the middle,and finally the constraint rules of strong corner confinement,weak middle confinement,strong short side confinement and weak long side confinement are formed.The stress state of the columns has an important influence on the interaction mechanism between the steel tube and UHPC.The stress redistribution between the steel tube and the core UHPC occurs in the process of loading.（5）Based on the constraint characteristics of rectangular UHPC filled steel tube,the effective constraint coefficient was introduced,and the formula for calculating the axial compression capacity of rectangular UHPC filled high-strength steel stub was established based on the limit equilibrium theory.It can be applied to the rectangular concrete filled steel stub columns composed of f_y≤780MPa and f_ck≤180MPa,and the effect of local buckling of steel tube can be considered by the formula.According to the edge yield criterion,the calculation method of axial compression stability coefficient of rectangular UHPC filled high strength steel tube columns with large slenderness ratio is proposed.The equivalent initial eccentricity coefficient k is suggested to be 0.10.Based on the edge yield criterion,the theoretical calculation formula for eccentric compression of rectangular UHPC filled high-strength steel stub columns is derived.The plastic development coefficientγ_xof cross-section is recommended to be 1.25.A practical calculation method for the eccentric compression bearing capacity of rectangular UHPC filled high-strength steel stub columns is proposed,that is,a simplified expression of two polylines passing through the characteristic point（M_u,A_cf_c）on the N-M correlation curve.Finally,based on the summary of the whole paper,some suggestions are put forward for further research in this field.