Research On The Force Performance Of Ultra-High-Performance Concrete-Filled Circular Steel Tube (UHPCFCST) Stub Columns After High Temperature

Ultra-high performance concrete-filled circular steel tube(UHPCFCST)composite column is a new material combination structure.This combined structural form improves the overall load-bearing capacity and ductility of the member and has excellent resistance to bursting at high temperatures.In this paper,axial compression tests were conducted to study the force performance of UHPCFCST stub columns after high temperatures.The main work and conclusions include the following:(1)The material properties of UHPCFCST stub columns were investigated.High temperature tests were conducted on ultra-high performance concrete(UHPC)cubic specimens and standard tensile specimens of steel under the same conditions as UHPCFCST stub columns.The damage morphology and mechanical properties of steel and UHPC after different high temperatures were tested.The results show that: after 400 °C,the UHPC specimens with singledoped steel fibers are spalling to different degrees,while the UHPC specimens with hybrid fibers still maintain the integrity of appearance;the compressive strength of UHPC specimens shows the law of increasing and then decreasing with the increase of temperature,and the microscopic characteristics of UHPC specimens deteriorate more seriously after 800 °C;the yield strength of steel decreases with increasing temperature after 400 °C.(2)Axial compression performance tests were conducted on 24 post-fire action and 6UHPCFCST stub columns at room temperature.The macroscopic deformation characteristics of the specimens under axial pressure were studied to analyze the effects of temperature,fiber type and steel tube thickness on the strain and displacement states during loading.The results show that: the UHPCFCST stub columns did not burst after high temperature.There is no significant change in the shape of the specimens acting at 25°C–400°C.The damage of the specimens after 600 °C is more obvious,and the damage characteristics of UHPCFCST stub columns with different fiber types do not differ much;With the increase of temperature,the ultimate bearing capacity of the specimen is first increased and then decreased;the temperature has an effect on the ductility of the specimen,and the ductility is better at 800 ℃;the doped fiber has a difference on the curve only in the load drop section,and the ductility of the hybrid fiber specimen is better;the increase of the thickness of the steel tube makes the peak displacement decrease,and the load of the specimen with a small hoop coefficient reaches the peak load and then decreases rapidly.(3)Different domestic and foreign codes and calculation methods were analyzed,and the axial compression load bearing capacity calculation model of UHPCFCST stub columns after high temperature was proposed and its applicability was verified.The results show that: the calculated values of EC4(2004)and Yu equation after the introduction of the material discount factor are in good agreement with the test value;the calculated results of regulations DL/T5085-2021 and DBJ13-51-2003 are larger than the test values and will be biased to unsafe for structural verification.(4)The numerical model of the UHPCFCST stub column after the high temperature was established by using ABAQUS finite element software,and the correctness of the model was verified.The simplified load-axial strain curve of the UHPCFCST stub column after the high temperature was proposed,and conduct a parametric analysis of the factors affecting the ultimate bearing capacity.The results show that: the load-displacement curve obtained from the simulation is in good agreement with the test results;the stress changes in various parts of the specimen after 400 °C were more similar to those at room temperature.The ultimate bearing capacity of UHPCFCST stub columns mixed with steel fibers is higher;the increase of steel tube wall thickness improves the ultimate bearing capacity of the specimens,and the enhancement is more obvious after the action of 800 ℃;The ultimate load capacity increases with the increase in steel grade.(5)Based on this paper and a large amount of data at the domestic and international level,a mature neural network model simulating the load-bearing performance of UHPCFCST stub columns was trained by using MATLAB to establish a BP neural network prediction model using code.The results show that the data predicted using the ANN-6 neural network model have higher reliability,better generalization ability,and higher accuracy of the calculation;the neural network is superior compared to other analytical models.