| As a new cementitious composites,ultra-high performance concrete(UHPC)possesses excellent mechanical properties,including the ultra-high compressive strength,durability,high tensile strength and elastic modulus.It is obvious that using UHPC not only reduce the member size and self-weight of a structure to achive span transcendence,but also reduce cement and energy consumption as well as environmental pollution to achieve sustainable development.However,concrete matrix with ultra-high strength will inevitably lead to large brittleness under compression.UHPC is filled into steel tube to form UHPCFST structures,which not only effectively improve its ductility,but also adapt to the development of modern structure to the directions of super high-rise,super long-span,heavy loading,durability,and subject to extreme harsh environment.The number of strong earthquakes in our country land are largest and account for 1/3 of the world.Therefore,it is of great theoretical and practical significance to research the seismic performance of UHPCFST member to ensure its safety service.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 seismic performance and restoring force model of UHPC filled steel tube columns are systematically researched by the experiment,theoretical analysis and numerical simulation.The main work and the achievements are summarized as follows:(1)A total of 19 UHPCFST specimen columns are designed and fabricated for pseudo static test to investigate failure modes and hysteretic behavior,where the influences of steel tube thicknesses,axial compression ratio,sectional length-to-width ratio and loading patterns are considered.The results indicate that UHPCFST columns mainly fail in two failure modes,namely,the flexural failure mode and the compression-flexure failure mode.As the applied axial load increases,the failure mode of UHPCFSTs changes from the flexure failure mode to compression-flexure mode.The hysteretic curves of UHPCFST columns are plump,the equivalent viscous damping coefficients are within the range of 0.197 to 0.358,the elastic-plastic ultimate drift ratios are within the range of 2.37%to9.84%.The seismic behavior is improved with the increase of steel tube thickness.Under the drift ratio of 2%,the degradation amplitudes of peak load,stiffness and dissipated energy for specimens loaded with 10 cycles are within the range of 15%,whilst the degradation amplitudes reach to 50%under the drift ratio of 4%.(2)The 3D macro and micro combined model of UHPCFST column is established,where the random distribution of steel fibers is considered.The rationality and reliability of the model are validated based on the experimental results.On this basis,the whole stressed process of UHPCFST column under cyclic loading is analyzed,and its working mechanism is revealed.The results indicate that at the beginning of loading,the flange of the column’s foot is firstly yielded,and the plastic zone develops upward.After the peak point,the plastic zone gradually penetrates to the web side,and the plastic hinge is finally formed at the bottom,the whole section of the steel tube is yielded with local buckling and part of the UHPC is crushed.The confinement effect of steel tube is weak at the initial loading stage,but it experiences a remarkable enhancement during the elastic-plastic stage.The confining stress of the steel tube is non-uniform and exhibits alternate phenomenon under lateral cyclic loading,the confining stress of corner point is greater than that of middle point.The confining stress rises firstly and then declines with the increase of the cycle numbers.Besides,the farther away from the bottom section,the later and the weaker the confining effect of the steel tube appears.Except for reducing the transverse expansion deformation of matrix,the steel fibers also prevent the development of the crack in the post-peak stage and delay the damage as well as degradation of UHPC,improving the post-peak ductility for UHPCFST members.There is preferable synergy between steel tube and UHPC,and they can work together until failue.(3)The compressive stress-strain curves of rectangular steel tubes are divided into three classes to consider local buckling according to the width-to-thickness ratio coefficient;the stress and strain values at peak point and ultimate points for the confined concrete constitutive model are modified based on the axial compression test results of rectangular UHPCFTST stub column;the materials are secondary developed in ABAQUS software to estabilish fiber model of UHPCFST considering local buckling.The fiber model is validated by lots of experimental results,including axial compression behavior,eccentrical compression behavior and cyclic behavior of UHPCFST members.The results indicate that the axial compression ratio and width-to-thickness ratio are two key factors affecting local buckling of steel tubes.With the axial compression ratio increasing from 0 to 0.45 and the width-to-thickness ratio increasing from 30 to 60,ignoring the local buckling of the steel tubes will make the members obtain larger horizontal bearing capacity and better post-peak ductility.When the width-to-thickness ratio of steel tube is less than 30,the influence of local buckling can be neglected.The fiber model can well simulate the nonlinear mechanical behavior of UHPC members.(4)Based on the principle of minimum potential energy,the rayleigh-ritz method is adopted to deduce the elastic stiffness of UHPCFST column;three methods for predcting the ultimate moment bending capacity(M_p)of UHPCFST columns are proposed to calculate the peak load(P_m),including strip method,sectional plasticity-simplified-based theoretical method and practical calculation method.Based on the test and FE calculated results,the formulas of five key parameters such as yield load(P_y),elastic stiffness(K_e),peak load(P_m),peak displacement(Δ_p)and descending stiffness(K_d)are revised and proposed.Meanwhile,a data-driven program is also developed to predict the key parameters based on bayesian parameter estimation theory and BP neural network algorithm.Two methods for predicting the P-Δ skeleton curves are thus established.Finally,the calculation formulas of displacement ductility coefficient and equivalent plastic hinge length of UHPCFST members are proposed.The results indicate that the contribution of UHPC in the tension zone to the bending moment should be considered under low axial load;the practical calculation method possesses high precision(1.065)and is very suitable for predicting the ultimate moment bending capacities of UHPCFST members;the P-Δskeleton curves predicted by two methods are in good agreement with the experimental results;the displacement ductility coefficient is only related to the geometric dimensions and axial compression ratio,and decreased with the increases of axial compression ratio and slenderness ratio,and increased with the increase of steel ratio.(5)The cyclic degradation effect is considered to reflect the strength and stiffness degradation caused by cumulative damage.Based on stiffness degradation and maximum displacement oriented hysteretic criterions,the degenerated trilinear P-Δrestoring force model of UHPCFST column is established,and the numerical realization is conducted by MATLAB program.Based on the experimental results in this paper and related literatures,the restoring force model is extensively verified.The inflection points in restoring force model are dealt with by interpolation method to avoid abrupt changes of stiffness in integration in time-domain,and a numerical method,unconditional stability for integration in time-domain under both positive and negative stiffness,is established.The restoring force model is applied to the elastic-plastic time history analysis of UHPCFST structures.The results indicate that the hysteretic curves predicted by the restoring force model agree well with the experimental results;the numerical method can satisfy the unconditional stability for integration in time-domain under both positive and negative stiffness and possesses good robustness;the UHPCFST structures possess excellent seismic performance,it does not fail under rare earthquake and can still bear certain ground motion intensity under moderate and high axial compression ratio,the designed axial compression ratio of UHPCFST frame column is suggested not more than 1.0 in practical engineering.Finally,based on the summary of the whole paper,some suggestions are put forward for further research in this field. |
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