Research On Fire Resistance Performance Of Rec-section Q550 High-strength Steel Columns With Axial Compression

New materials,new technologies and new theories are boosters in the development of civil engineering.With the continuous improvement of steel production,continuous improvement of technological level,and continuous innovation of processing technology,driven by strong demand of the construction market,the use of high-strength steel as structural material is a new trend in the civil engineering in recent years.“Standard for design of steel structures”（GB 50017-2017）and the forthcoming“High-strength Steel Structure Design Rules”both present relevant regulations and design provisos for high-strength steel.This provides reliable technical supports for the application of high-strength steel.High-strength steel has obvious advantages in terms of mechanical performance of structural members,building functions,social benefits,and environmental protection benefits.A large number of high-strength steel materials have been used in some key parts of the structure and vital buildings where people gather（such as high-rise office buildings,exhibition halls,etc.）that are related to the national economy and people’s livelihood.However,frequent building fires in recent years have brought serious damage to the safety of building structures,which means the research on fire resistance performance of structures is particularly significant.Similar to ordinary steel,the fire resistance performance of high-strength steel is relatively weak,and there are no relevant fire-resistant design methods for high-strength steel in the domestic and international standards currently.In order to supplement the experimental research and calculation theory of high-strength steel fire resistance performance,and to improve relevant fire safety design provisos for high-strength steel,experimental research and finite element simulation on the fire resistance performance of rec-section Q550 high-strength steel columns with axial compression and without axial constraint were carried out and their behavioral response and failure mechanism under fire were revealed.Finally,corresponding formulas for fire resistance performance of Q550 high-strength steel axially compressed column is given.The mechanical properties tests of 4 specimens at room temperature and 18 test specimens at elevated temperature were carried out on domestic Q550 high-strength steel materials.Among them,the mechanical properties of material at room temperature were determined using static standard tensile test,and the mechanical properties of material at elevated temperatures were achieved by steady state test.The test results show that:（1）when the temperature is 20°C and 100°C,the yielding platform of the stress-strain curve is short,and the multiline string constitutive model proposed by Ban agrees well with the test results.（2）When the temperature is higher than 100°C,the stress-strain curve does not have a yielding platform,and the one-stage constitutive model proposed by Ma et al.are in good agreement with the experimental results.Finally,the two constitutive models mentioned above are used in this dissertation to provide the material mechanical property data for the subsequent finite element analysis.Based on the mechanical properties test results,the experiment investigations of fire resistance performance of 8 rec-section Q550 high-strength steel columns with axial compression and without axial constraint were conducted.The fire test mainly investigated the influence that load ratio n,slenderness ratioλ,width-thickness ratio H/t,and height-width ratio H/B may have on the fire resistance performance of the high-strength steel columns with axial compression and revealed their mechanical performance and failure mode under fire conditions.Through fire test,the furnace temperature-time curve,the surface temperature-time curve,the axial displacement-time curve,the midspan lateral displacement-time curve,the critical temperature and the maximum axial displacement of test specimens were obtained.Fire test results show that:（1）The load ratio n is an important factor that affects the critical temperature of specimens;The increasing of the load ratio n will significantly reduce the critical temperature of specimens;（2）The load ratio n,slenderness ratioλ,width-thickness ratio H/t and height-width ratio H/B have a significant influence on the maximum axial displacement of specimens.The ABAQUS finite element software was used to establish accurate finite element models of the test specimens.Then the mechanical performance of Q550 high-strength steel columns with axial compression under fire was numerically simulated,the whole process of the fire test was reproduced,the failure process and the mechanical performance were investigated.Finally,the numerical analysis results are compared with the test results to verify the accuracy of the finite element model.The parametric analysis was carried out by the verified finite element models to analyze the influence that 6 factors（sectional area A,width-thickness ratio H/t,amplitude of overall bending imperfection e₀,load ratio n,slenderness ratioλand height-width ratio H/B）have on the fire resistance performance of rec-section Q550 high-strength steel columns with axial compression and without axial constraint.The analysis results show that,（1）sectional area A,slenderness ratioλand load ratio n are the vital factors that affect the critical temperature of specimens,and among them the influence of load ratio n is the most significant.（2）It’s whether consider the overall bending imperfection not the dimension of imperfection amplitude e₀ that influence the critical temperature of specimens.（3）The width-thickness ratio H/t and slenderness ratioλwill affect the failure shape of the specimens.When the slenderness ratioλis 10,the specimens only occur local buckling.When width-thickness ratio H/t is not bigger than 21.33 and slenderness ratioλis no less than 50,the specimens only occur global buckling.Otherwise both the global buckling and local buckling occur while the specimens failed.（4）When the slenderness ratioλ=30⁹0,the axial displacement curve of the specimens decreases rapidly after reaching the peak value.The maximum axial displacement of the specimen and the critical temperature correspond to the same time.When the slenderness ratioλ≤20 orλ≥100,The axial displacement curve of the specimen has a brief slow decline after reaching the peak value.The maximum axial displacement of the specimen and the critical temperature correspond to the different time.（5）The sectional area A,width-thickness ratio H/t and slenderness ratioλ,load ratio n and height-width ratio H/B have a significant influence on the maximum axial displacement of specimens.Based on parametric analysis,coupling analysis among three vital factors（sectional area A,slenderness ratioλand load ratio n）was conducted.The analysis results indicate that slenderness ratioλand load ratio n are strongly coupled when slenderness ratioλis small（λ≤50）.According to the failure characteristics of steel columns with different slenderness ratio,the sectional average stressσ_ave and the maximum lateral displacement y_max were used as the discriminating criterions for the critical condition of the high-strength steel columns in fire.The theoretical derivation and numerical simulation analysis were conducted to propose formula for the critical temperature of rec-section Q550 high-strength steel columns with axial compression under different critical conditions.Meantime,three kinds of elastic axial deformation equations（axial compression deformation,axial deformation due to bending and axial expansion deformation）of high-strength steel axially compressed columns under high temperature were put forward through theoretical derivation.Then the equations were modified by the numerical calculation results and the calculation formula for maximum axial displacement of Q550 high-strength steel columns with axial compression was proposed.The two formulas were verified according to the results of fire test,which provided an important theoretical basis for the follow-up study of the fire resistance performance of high-strength steel compressed columns.