Study On Bearing Capacity Of Concrete Filled Steel Tubular Members With Multilayer Confinement At Different Temperatures

As the application of engineering projects in combination with concrete filled steel tubular(CFST)columns such as industrial and civil buildings,bridges and towers and other structural projects,the requirement of engineering structures of large-span,tower,heavy-load has been increased in recent years.When the bearing capacity of member and the cross-sectional size are increasing,it will cause a series of problems,such as the production processing difficulties of the thickness of steel tube,and the low quality of the concrete pouring,and so on.And concrete-filled steel tubular members with stiffener can be used as a solution.However,The suitable object of the current national standard GB50936 "Technical code for concrete filled steel tubular structures" is only for common CFST,and the general cross-sectional size can not exceed 2000 mm.For the CFST with inner stiffener,there is not a unified static and fire resistance design formula to consistent with the current national standard GB50936.The CFST column with internal confinement member can be seen as the common CFST when the inner steel ratio is equal to zero.According to Professor Zhong Shantong’s ideas of "unified theory",the properties changes by reason of the confinement effect between the concrete and the internal and external steel tube under various load conditions should be completely continuous.Consequently,it is necessary to research the common CFST and the CFST with inner stiffener together,which has an actual engineering and a strong theoretical significance.In this thesis,three kinds of sectional forms of CFST with inner stiffeners are referred to as multi-storey hoop steel tube concrete for mechanical analysis,namely,the forms of inner reinforced,solid inner steel tube and hollow inner steel tube,respectively.Through the establishment of the relevant theoretical model derivation,the relationship between the properties and the variation of the parameters under axial compression and fire are obtained.The relevant uniform consecutive calculating design formulas of CFST with multilayer confinement with three different cross-sections at ambient temperature and high temperature based on the unified theory of CFST are obtained.The validity of the theoretical results is verified by combining the present and existing axial compression static results and fire tests,besides,the finite element calculation and parameter analysis.Finally,based on the compression-bending related equation of CFST members with multilayer confinement at ambient temperature,the unified simplification formula of the compression-bending related equation for the component under fire condition is given and verified.The research contents are as follows:(1)To fill and increase the relevant experimental research,experimental study on axial compression of CFST members with multilayer confinement under ambient temperature and fire condition are conducted.And the testing phenomenon,the load versus deformation curve,the temperature field versus time curve and the axial deformation versus time curves are analyzed in detail.(2)A three-dimensional finite element analysis(FEA)and parameter analysis of CFST members with multilayer confinement under ambient temperature and fire conditions are carried out.After calibration of the FEA model,the influences of important parameters on the temperature field of the composite columns are investigated.(3)The continuous theoretical study of the CFST members with multilayer confinement under ambient temperature is given.Based on the principle of superposition,thin-walled cylinder theory,thick-walled cylinder theory and limit equilibrium method,the bearing capacity calculation formula of reinforced concrete-filled steel tubular(RCFST)members and concrete-filled double steel tubular(CFDST)members are deduced.When the CFST is regarded as a composite material,the stability factor of the column is derived on the basis of the Perry-Robertson formula.And then through the introduction of the "Unified theory" of CFST,the unified design formula of the axial compressive strength and the stability bearing capacity of the CFST members with multilayer confinement is obtained.Finally,compared with the experimental data collected and the FEA results in this thesis,the correctness of the theoretical formula is verified.(4)Temperature distribution of the CFST members with multilayer confinement under fire is presented.Based on the Green function method and the separation variable method,the section temperature field model is analytical analyzed,and the structural form of the temperature field distribution formula of each material is obtained.Then,the semi-analytical solution of average temperature of steel tube,steel bar and concrete are achieved by finite element parameter analysis fitting of temperature field.Finally,the results are compared with the finite element simulation cases and the experimental results,and the correctness of the temperature field distribution prediction is verified.(5)A theoretical study on the continuity of the CFST members with multilayer confinement under fire is given.Based on the semi-analytical solution of the temperature field distribution of each material after verification and the unified formula of static axial compression bearing capacity under ambient temperature,the unified simplified calculation formula for the axial load bearing capacity and the stability bearing capacity of the CFST members with multilayer confinement at any time under fire is presented when the strength reduction factor and the stability coefficient under high temperature are introduced.Finally,the rationality of the formulas is verified by comparison with the collected experimental data.(6)The analytical calculation of fire resistant time and fire protection measures of the CFST members with multilayer confinement under fire are given.The fire-resistant time of the column with and without fire protection layer is obtained.Three kinds of protection measures are given to meet the requirements of the fire resistance limit,that is to say,the fireproof measures with setting the stiffeners,the fire protection layer with using non-intumescent coatings and setting M5 ordinary cement mortar with installing the steel wire mesh,and the cross-sectional area of the stiffener and the formula of the fire protection layer thickness are got under the condition of satisfying the fire-resistant time,respectively.(7)A simplified formula for the calculation of the compression-bending related equation of CFST members with multilayer confinement under ambient temperature and fire is presented.The compression-bending related equation of ordinary CFST members recommended by domestic norms was introduced into CFST members with multilayer confinement and further extended to fire conditions.Finally,the unified formula for the compression-bending related equations of CFST members with multilayer confinement under ambient temperature and fire is obtained.Then the numerical results are compared with the experimental results to verify the rationality of the formulas.In this thesis,the formula of axial compressive bearing capacity,pure bending capacity and compression-bending capacity of the CFST members with multilayer confinement under ambient temperature and fire is applied to the common CFST column,CFST column with inner multilayer steel bars,solid and hollow CFST column with inner multilayer steel tube,and ensure the continuity and unity under ambient and high temperature,which has a good guiding significance for engineering design.