Mechanicl Behaviour Of Steel-Reinforced Concrete-Filled Steel Tubular Compressive Columns Under Long-Term Loading

As a new type of composite member,steel-reinforced concrete-filled steel tube（SRCFST）has the advantages of higher bearing capacity and better space ratio compared with ordinary concrete-filled steel tubular（CFST）columns.Therefore,it has been used in the bearing columns of long-span bridge or high-rise building structures.The bridge or high-rise building,as the vital structure,has a long service life,so the SRCFST column as the stress component will suffer sustained load during service period.In addition,due to the different construction technology,the outer steel tube as a vertical support may be against the preload first.During the whole life cycle from building construction to normal service,the mechanical behaviour of SRCFST members should be considered under the combined action of construction load and long-term sustained load.However,there are limited studies on the mechanical behaviour of SRCFST members under the combined load.Therefore,it is necessary to conduct the study on the mechanical behaviour of members during the whole life cycle.In this paper,the mechanical behaviour of SRCFST members under combined preload and sustained load are studied by experimental and numerical methods.The main contents can be drawn as follows:（1）The compression tests of 12 SRCFST columns considering the preload applied on outer steel tube are conducted.The test parameters include preload ratio,load eccentricity and slenderness ratio.The failure mode,deformation process,bearing capacity and ductility of the specimens are studied in detail.The load-deformation curves and the development of strain at each measuring point are compared and analyzed.Using ABAQUS,the finite element（FE）models of compressive loaded SRCFST columns considering the preload effect of steel tube are established,and the model is verified against the existing test results.When the preload on steel tube is applied,the load-deformation curves of SRCFST compression column in whole loading process and the load distribution results of each component are analyzed,and the contact stress between different components,as well as the stress development are studied.Finally,the influences of parameters i.e.,preload ratio,load eccentricity,slenderness ratio,sectional dimension of section steel are analyzed,and the calculation method of bearing capacity influence coefficient for circular SRCFST columns with preload is proposed.（2）8 SRCFST columns considering sustained load are tested and studied.In addition,12comparison specimens without sustained load are also designed.The main parameters are slenderness ratio,steel ratio of section steel and long-term load ratio.The growth of the longitudinal deformation of the specimen over time and the influence of various parameters on the longitudinal deformation of the specimen under sustained load are analyzed.The influence of sustained load on the performance of bearing capacity stage is studied.Moreover,the numerical model is established by ABAQUS to analyze the whole process of the axial compression SRCFST members considering the long-term load.The load-deformation curve of the column,the development and distribution of material stress,and the load distribution of each component are mainly investigated.Additionally,compared with the results without long-term load,the influence of sustained load on SRCFST columns is analyzed.（3）After understanding the mechanical behaviour of SRCFST members considering the preload on steel tube and long-term load respectively,the whole process mechanical behaviour of circular（rectangular）SRCFST columns under combined the preload on the steel tube and the long-term load are experimentally studied.Through the test,the longitudinal strain versus time curves of each specimen in the sustained load stage are obtained.In addition,the bearing capacity failure test analysis of the SRCFST members considering the combined load is carried out.This paper mainly investigates the influence of combined load on the failure mode,load-deformation curve,initial stiffness and other indexes of SRCFST.The test shows that the effect of combined load on the later mechanical properties of members is more significant.（4）To further understand the long-term compression performance of SRCFST columns with the preload on steel tube,a FE model considering the combined action of the preoload and the sustained load is established by using ABAQUS software,and the material constitutive model is reasonably selected.On the basis of model verification,the whole process compression behaviour of the circular（square）SRCFST members under the combined load is analyzed.The load-deformation curve,contact stress between components and strain development of stub and long column are analyzed respectively.The effects of different parameters,such as long-term load ratio,preload ratio,eccentricity,slenderness ratio,material strength and section form of section steel,on the longitudinal deformation in the long-term load stage and bearing capacity influence coefficient are analyzed.（5）Based on the test and numerical simulation,the main parameters affecting behaviour of SRCFST members under combined preload on steel tube and long-term load are obtained.Through the FE model,the influence of preload ratio,eccentricity,slenderness ratio and material strength on the bearing capacity influence coefficient(k_cr)of SRCFST members considering the combined load is analyzed.Parameters analyses based on numerical model are carried out to obtain the calculation method of the k_cr,combined with the calculation method of compressive bearing capacity without considering the whole life cycle,the calculation method of bearing capacity of SRCFST members under combined load is proposed.