The Steel Frame Structure's Reliability Research After Fire Cooling

Recently, various types of new building materials have been invented and applied to the engineering field. Among these materials, the steel has attracted tremendous attention and is favored by engineers and users, as the construction market is developing towards a personalized, diversified and innovative direction. This can be because that the steel structure has multiple advantages.However,besides the various advantages, the steel structure performs poorly in some aspects. For example, one major disadvantage is that the steel structure often has a low fire resistance, which restricts its application. In the literature, both domestic and foreign scholars have conducted theoretical research and experimental research regarding the fire resistance performance of steel structure. Nonetheless,research on steel properties after fire is very limited. In this research, in order to give full play to the potentially applicable properties of steel structures, as well as save the timing cost and the economic cost, this paper investigates the reliability properties of steel structures after fire based on the reliability theory.First, this paper analyzes several harmfulness of fire disasters in steel structure based buildings.Specifically, taking the Q235 steel as an example, we study the variation of its major mechanical properties after fire. The mechanical properties we investigated include the material yield strength(fy(T))and material tensile strength(fu(T)). Moreover, we also consider the effect of different cooling methods on the mechanical properties of steel structures.Secondly, we propose to establish the structure function under different limiting conditions. We characterize the structure reliability utilizing indexes such as the the reliability probability, the failure probability. In the calculation of structural reliability utilizing stochastic processes, we consider several commonly used random distributions. We also consider the normal tail approximation method. Further,in this paper, the calculation of structural reliability is briefly introduced. Specifically, the calculation methods such as the two order moments, the two times higher order moment method, the asymptotic integral method, the response surface method, et al, are introduced. Among them, the most basic and commonly used methods, including the center point method and the JC method, are described in details to calculate the structure failure probability. Examples of calculation are also given for illustration purposes. Thereafter, structural reliability calculation utilizing the MATLAB is introduced and illustrated with an example. Some commonly used MATLAB program identifiers and the function statements are also listed. As the structural steel at high temperatures can still be treated as ductile materials, the sectional plastic hinge is used to calculate the steel structure reliability after the cooling process.Finally, the steel portal frame reliability after fire cooling process is computed by the JC method as an example. The most unfavorable failure mode of the portal frame is analyzed, and the structure function under such failure mode is established. Meanwhile, random variables that can affect the reliability of portal frame structure are listed. The influences of different cooling methods on the structure mechanical properties and reliability are considered. The structure reliability/failure state is measured by the target reliability index. The calculation procedure utilizing the MATLAB software as an automated procedure is also implemented.