Study On The Fire Resistance Of Steel Tube Reactive Powder Concrete Columns Under Eccentric Loa

Square steel pipe concrete column is an efficient composite structure,which can give full play to the synergistic effect of steel and concrete to improve the flexural load capacity and stiffness.The structure has the advantages of easy construction and high fire resistance,and is suitable for high-rise and super high-rise buildings.As a new type of square steel tube concrete column,square steel tube RPC column not only meets the requirements of resource conservation and environmental protection,but also can effectively prevent structural damage and human casualties caused by fire.However,due to the exposed steel pipe,the fire resistance design of this structure faces challenges and problems.In this thesis,the fire resistance test of eccentric compressed members is carried out with square steel tube RPC columns as the research object.The purpose of this thesis is to reveal the influence law of eccentric loading on the fire resistance of square steel tube RPC columns through the test method,and to provide theoretical basis and technical support for the design and application of this structure.The specific research contents and conclusions of this thesis are as follows.(1)Six square steel tube reactive powder concrete column specimens with different loading angles and eccentric distances were fabricated and fire tests were conducted to measure the axial displacement,section temperature and damage pattern of the specimens,and the fire resistance limit of each specimen was calculated.(2)Using the finite element analysis software ABAQUS,a numerical model of the steel tube activated powder concrete column under eccentric loading at high temperature was established and its fire resistance performance was analyzed.The temperature field distribution,axial displacement variation and fire resistance limit of the column were obtained through the model calculation and compared with the test results to verify the accuracy and reliability of the model.(3)Based on the validated model,the fire resistance limit of square steel pipe reactive powder concrete columns under eccentric loading was investigated,considering two influencing factors of concrete compressive strength and height-to-width ratio.Using gray correlation theory,the degree of correlation between these two factors and the fire resistance limit was quantified,and an optimization formula was established accordingly to reveal the influence law of eccentric loading on the fire resistance performance.(4)On the basis of ensuring the calculation accuracy and determining the influence law of parameters,the nonlinear finite element full-process analysis method of steel tube activated powder concrete columns under eccentric load in fire is studied and applied to the design of repair and reinforcement of square steel tube activated powder concrete structures in actual engineering.The main conclusions are as follows:(1)Concrete materials subjected to high temperatures undergo irreversible physical and chemical changes,resulting in lighter color,oxidation of steel fibers,surface cracking and moisture loss,which weaken the strength and toughness of concrete.Axially compressed specimens show a brittle damage mode under high temperature conditions,while eccentrically compressed specimens exhibit a bending destabilization damage mode,the degree of which is positively correlated with the eccentricity distance,while it has no significant effect on the loading angle.(2)The specimens were subjected to high temperature,and the temperature of each measurement point varied consistently with time,decreasing along the section temperature,and the rate of temperature increase also decreased,and the characteristics of displacement variation with time were related to the eccentricity distance,compared with the axial compression specimens,the displacement of the specimens under eccentric loading changed steeply,indicating that the eccentricity distance affected the force state and damage mode of the specimens.(3)The fire resistance limit of the specimens is influenced by a variety of factors.Among them,the eccentricity has the greatest effect on the fire resistance limit,and the loading angle has the least effect on the fire resistance limit.This indicates that the eccentricity affects the stability and load capacity of the specimen.(4)The reliability of the model was verified in terms of both temperature field and axial displacement-time curves by comparing the finite element simulated and experimental values of steel tube activated powder concrete columns below high temperature.The model can reflect the mechanical property change law of this type of column and has certain reference value.The comparison results show that the simulated and experimental values are more consistent and the fire resistance limits are similar.(5)Using gray correlation theory,the correlation between the fire resistance limit of square steel tube activated powder concrete columns and two influencing factors,eccentricity distance and loading angle,was investigated,and the influence of eccentricity distance on the fire resistance limit was the largest(correlation of 0.644),followed by the loading angle(correlation of 0.607).