Thermo-mechanical Coupling Response Analysis Of The Box-plate Fabricated Steel Structure Under Fire

The system of box-type steel structure was a new prefabricated steel structure system,the idea for this structure grew out of the superstructure of ships.The box-type steel structure is composed of stiffened steel plate with I and T stiffeners arranged vertically on the plate,which don’t need external frame constraint.The stiffening steel plate wall as the main component of the structure can bear both horizontal and vertical loads,which is the main characteristic different from other structures.Because the structure belongs to pure steel structure,its poor fire resistance makes it urgent to study the fire resistance mechanism,failure mode,fire endurance and other capability under fire,and the economical and reasonable fire resistance design method also should be built,which could provide convincing references for the fire resistance design of box-plate steel structure.The main content and conclusion of this thesis are summarized as follows:1.The relevant literature of domestic and foreign scholars on the characteristics of steel structures under fire in recent years was sorted out,and the characteristics of high temperature materials,the space temperature field,the calculation method of steel structure heating,and the fire resistance of steel structure were systematically summarized.Comparing the values of steel material properties under high temperature in the code from different countries,the most suitable one was determined for the subsequent heat transfer analysis and the mechanical analysis,in which the thermal physical property include density and thermal expansion used constant,the specific heat and heat conduction coefficient with temperature change in the European Code eurocode3 were chosen,the Poisson’s ratio constant,the yield strength and elastic modulus were calculated according to the formula provided in the Technical Specification for Fire Protection of Building Steel structures（GB 51249-2017）,and the stressstrain relationship was calculated using the smooth curve material model proposed in the eurocode3 that does not consider strengthening;2.The fire dynamics models of box-plate steel structure with one and three layers were established.By changing the location of the fire source,seven fire scenes were set up,after that,the temperature measuring points were arranged in different zones of each layer and temperature time curves were drawn.By comparison and analysis,the distribution of indoor air temperature field of box-plate steel structure and the influence of the position of fire on air temperature field were discussed.The results show that the air temperature field has distinguishing feature of stratification and regionality.At the same height of single-layer structure,the part of fire source has the highest temperature,where the temperature of scene1,2 up to 1000℃,scene 3 was about 800℃,and the other parts were under 600℃,at the same time,the temperature increases with the height.In the three-layer structure,the highest temperature was appeared at the floor of fire source,which was almost over 1000℃,where the heating rate was nearly vertical,meanwhile,the other parts had low temperature below 200℃at the same floor.The curves were gentler on the top floor,except the part C had high temperature between 300～400℃,others were about 100℃.When the fire source located on second floor,the temperature of the bottom floor was distributed evenly but low,which maintained at 20℃;3.The fire transient thermal analysis model of box-plate prefabricated steel structure was constructed.After calculating,the time history data of the temperature field about steel members and the whole structure under seven fire scenarios were obtained.The results showed,the indoor temperature of the single-layer box-plate structure increases with the height.In the three-story structure,the temperature of the floor where the fire source was located increases with the height,and the temperature of other floors outside the fire source was lower and more even.The temperature field of the structures was similar to that of the air;4.The thermo-mechanical coupling analysis model of the box-plate prefabricated steel structure was constructed.The mechanical response analysis of key components and key measuring points was carried out.Combined with the structural characteristics of the box-plate prefabricated steel structure,the failure mode of the structure under fire was proved,and the fire resistance limit of seven fire scenarios was obtained.The results showed that,in singlelayer structure,the fire source position had a great impact on the fire resistance endurance,and when the fire source was in the center,the film stress produced after the middle surface yield of the plate could provide the post buckling strength,which effectively improves the fire resistance time.When the fire source was near the wall,the structure was under uneven and redistributed stress,as well as the wall bore too much stress,which caused a great reduction of fire resistance time.For three-story structure,the fire resistance time would be shortened when the fire source was at the bottom,which would achieve earlier when the fire source located in the center position.The fire resistance time of each working condition had reached more than1000 s,and there is no obvious mutation at the maximum displacement in each direction.Above all,it shows that the box-plate prefabricated steel structure has great rigidity,good ductility,excellent fire resistance,scientific stress system and superior overall performance.