| New materials and new structures are the source power to promote the development of the civil engineering.Since the 21st century,with the rapid development of construction industry in our country,high-rise buildings and super-high-rise buildings are springing up like mushrooms.However,because the concrete structure and the steel structure are difficult to meet the complex functional requirements of modern building structures,the steel-concrete composite structure has welcome its the spring of development.With the continuous promotion of"The Belt and Road"initiative and China’s Marine strategy,the construction industry is facing new development opportunities.As a new building material,stainless steel is favored by more and more architects and engineers wtih its advantages of superior corrosion resistance,durability,low life＿cycle costs and strong environmental adaptability.In recent years,with the continuous development of urbanization construction,the density of urban population increasing year by year,which leads to frequent building fires,seriously endangers the safety performance of building structures,and causes huge losses to the property safety of the society and the people.Therefore,it is urgent to study the fire resistance of engineering structures.The research on fire resistance of stainless steel-concrete composite beams is still lacking,and there are no design codes for stainless steel-concrete composite in both domestic and overseas,which impedes the application of stainless steel-concrete composite beams in the construction industry.Based on the above background,this paper carried out experimental research and numerical simulation on the fire resistance performance of simply supported rectangular section stainless steel-concrete composite beams,revealed its behavior response and failure mode in fire and proposed the calculation theory suitable for the fire resistance of stainless steel-concrete composite beams based on a large number of parametric analysis.Based on the existing test results of mechanical properties of materials at normal and elevated temperature,experimental study was conducted on the fire resistance of 4 stainless steel-concrete composite beams with rectangular section,to investigate the influence of load ratio n on the fire resistance of stainless steel-concrete composite beams,and reveal the mechanical performance and failure mechanism of stainless steel-concrete composite beams in fire.The furnace temperature-time curves,the temperature-time curves of the surface of stainless steel beam and the concrete slab,the mid-span vertical displacement-time curves,the critical temperatures,the residual deformations and the failure modes of all composite beam specimens were obtained through the fire resistance test.The test results show that:(1)The load ratio n is an important parameter that affects the fire resistance of stainless steel-concrete composite beams.With the increase of the load ratio n,the fire endurance and critical temperature of the specimens decrease obviously;(2)The specimens of stainless steel-concrete composite beams showed bending-shear failure in fire.During the whole test process,no out-of-plane instability and local instability of the stainless steel beams were discovered.After the test,all specimens had large rebound deformaiton.Firstly,the finite element software ABAQUS was used to establish the accurate finite element analysis model of the shear connector of cylindrical head stud,and the shear performance of the cylindrical head stud was simulated and analyzed.The accuracy of the model was verified by the existing results of the push out test of the carbon steel cylindrical head studs at elevated temperature and the push out test of the stainless steel studs at normal temperature.Secondly,using the verified shear model of cylindrical head stud,the analysis model of the stainless steel cylindrical head stud at elevated temperature was established,the shear performance of the cylindrical head stud at elevated temperature was analyzed,and the load-slip relationship of the stainless steel cylindrical head stud at elevated temperature was obtained.Finally,based on the above research results,the finite element analysis model of stainless steel-concrete composite beam specimens was established,and the fire resistance of4 test specimens was numerically simulated and analyzed,and the accuracy of the finite element model was verified by comparing the numerical simulation results with the test results.Using the verified finite element analysis model,the parametric analysis was carried out for several influencing factors(boundary condition,load type,load ratio n,concrete slab thickness h_c,slab flange width b_c,steel bar diameter d,steel beam flange thickness t_f,steel beam web thickness t_w,steel beam height h_s,steel beam width b_s,span L,shear connecting degreeη,cold working effect of stainless steel)of the fire resistance of the stainless steel-concrete composite beams with rectangular section.The analysis results show that:(1)The load ratio n is a key external factor affecting the fire resistance of stainless steel-concrete composite beams;Concrete slab thickness h_c and span L are key internal factors affecting the fire resistance of stainless steel-concrete composite beams;The increasing of concrete slab thickness h_c,the decreasing of the load ratio n and span L will all rise the critical temperature of stainless steel-concrete composite beams.(2)The boundary condition has a significant influence on the fire resistance of stainless steel-concrete composite beams,and the restraint on the end of the specimen(especially the axial restraitn)can improve the fire resistance of the composite beams.(3)Load type and cold working effect of stainless steel have certain influence on the mechanical properties of stainless steel-concrete composite beams in fire.Under the same maximum bending moment of the section,the fire resistance of the composite beam under single point loading is the best.Under the same load ratio n,the cold working effect of stainless steel reduces the fire resistance of composite beams.(4)The flange width b_cof concrete slab,the flange thickness t_f,the web thickness t_w and the height h_s of stainless steel beam have limited influence on the mechanical properties of stainless steel-concrete composite beams in fire.The steel bar diameter d,the shear connecting degree r and the width b_s of stainless steel beam have a small influence on the mechanical properties of stainless steel-concrete composite beams in fire.On this basis,the coupling analysis was carried out on the three key factors(load ratio n,concrete slab thickness h_c and span L).The analysis result shows that there is a coupling effect between load ratio n,concrete slab thickness h_c,and span L on the fire resistance of stainless steel-concrete composite beams.Finally,by using the above numerical simulation analysis method,the fire resistance analysis models of simply supported rectangular section ordinary carbon steel-concrete composite beams and rectangular section stainless steel beams under the same conditions were established and the numerical simulation analysis was carried out.The numerical simulation results of the above two beams were compared with that of the stainless steel-concrete composite beam,and the comparison results indicate that:under the same load ratio n,the fire resistance of stainless steel-concrete composite beams is better than that of stainless steel beam,and with the increase of load ratio n,the difference between the two critical temperatures increases gradually;when the load ratio is small(n<0.7),the fire resistance of the two composite beams is close.When the load ratio is large(n≥0.7),the fire resistance of stainless steel-concrete composite beam is not as good as that of ordinary carbon steel-concrete composite beam.Based on"Chinese Code for fire safety of steel structures in buildings"(GB51249-2017)and Eurocode(EN 1994-1-2),through the heat transfer analysis and numerical simulation of the components,a simplified calculation formula for the temperature rising of stainless steel beams and concrete slabs was proposed.Secondly,based on the existing calculation method of ultimate bearing capacity of carbon steel-concrete composite beams at elevated temperature,the parametric analysis and numerical fitting were carried out.The formula of ultimate bearing capacity of stainless steel-concrete composite beams at elevated temperature was obtained.On this basis,considering the influence of temperature additional bending moment on the fire resistance of composite beams,a critical temperature calculation method applicable to stainless steel-concrete composite beams was proposed.This proposal has been verified by application to experimental results and numerical results.The critical temperature calculation method has shown good agreement with tests,which provided an important reference for the follow-up research of stainless steel-concrete composite structures. |
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