Study On Mechanical Behavior Of High-rise Steel Frame Under High Temperature And Wind Load

The steel products are widely used in the modern high-rise buildings due to its properties,e.g.light weight,high strength,high safety,reliability,fast construction speed and short construction period.However,the fire resistance of steel is poor.It has extremely high risk and great destructive ability when a fire breaks out,the people's lives may be endangered,the economic property can also be damaged.Wind load is one of the main lateral loads in high-rise buildings,so it is important to study the mechanical behavior of high-rise steel structure under high temperature and wind load.In order to reduce the loss of steel structure collapse caused by fire and wind load,it is necessary to understand the fire damage process and degree of steel structure in different positions,and to accurately evaluate the failure temperature.The nonlinear failure study of high-rise steel frame at high temperature and wind load by theoretical analysis and numerical simulation is essential,because of the manpower and material resources requirement for fire experiment and wind tunnel test of full-scale high-rise steel frame.The research is mainly proposed in two aspects: the influence of high temperature caused by fire and wind load on buildings.This article discusses three methods of high-rise building wind-resistant design,which are the static equivalent method include gust factor load method,inertial wind load method and three-component method,and summarizes the strengths and weaknesses of these methods.Besides,the basic theory of the computational fluid dynamics is introduced,including the establishment of equations,the selection of the turbulence models,the treatment methods of near-wall,boundary conditions,computation field,grid arrangement and so on.Then a steel frame is taken as an example to establish a model,and relevant programs are written to simulate ground roughness and atmospheric boundary layer.Then,MATLAB software is used to carry out post-processing on the obtained downwind wind time-history data,and statistical method is used to obtain downwind wind load and compared results of three equivalent static methods,the differences are 14.3%,18.8% and 5.2%,respectively,which verifies the feasibility and reliability of the equivalent static wind load method.Based on the comparison between the experimental results of single-layer steel frame subjected to fire by foreign scholars and the results of explicit dynamic analysis in this paper,the correct mechanical and thermal parameters of steel are selected to determine the load ratio of vertical load and the analysis step time of temperature field.This article makes a systematic analysis on local and global failure of 15th-storey and 4-span high-rise steel frames on fire.The influence of different fire source locations on the axial displacement,vertex displacement and axial force is analyzed by changing the load ratio,and obtains the evolution process of steel frame local and overall collapse.The results show that when the outer column of the frame is caught on fire,it is the most unfavorable situation,and it is easy to cause the overall damage of the structure.The larger load ratio,the lower temperature when the frame is broken.The wind load calculated from three equivalent methods mentioned before is applied to the fire frame.The change curve of vertex displacement and interlayer displacement angle is used to compare the effect of equivalent static wind load on different layers of steel frame subjected to fire,and the influence of the fire layer on the adjacent layers fire resistance is analysed.The results show that the bottom storey and the first two storeys are most likely to collapse due to fire,and the deformation caused by thermal expansion has a great influence on the top storey,so it is necessary to strengthen the constraint.