رفتار ستون های فولادی با استحکام بالا در دمای بالا Behaviour of restrained high strength steel columns at elevated temperature

1. Introduction In recent years, high strength steels (HSS) are widely used in longspan structures and high-rise buildings primarily due to its merits of high yield strength and good ductility. For example, more than 400 tons of high strength steel are used in the construction of China National Stadium (so-called “Bird Net”), where the opening ceremony of 2008 Olympic Games was held [1]. As there is no a general consensus on the definition of HSS around the world, the yield limits adopted for categorizing HSS are different in various design standards. Generally, a type of steel is referred as high strength steel if its yield strength is not less than 460 MPa. In fact, one most common product of high strength steel in Far East is Q460 steel with the nominal yield strength of 460 N/mm2 . In fire conditions, buckling of steel columns can occurs at a lower magnitude of load than that of at ambient temperature as the result of degradations of strength and stiffness at elevated temperatures [2,3]. In the case of local fire, the fire response or behaviour of a steel column with end restraints is quite different from that of a steel column without end restraints since the thermal expansion in the restrained steel column would result in an additional axial force in the heating phase [4]. In the cooling phase, the load applied on the restrained column may be transferred to the adjacent columns which have not experienced elevated temperature. Literature review [5] demonstrates that there are some fundamental researches on fire response of restrained steel columns, not only by testing, but also by finite element modelling. Li et al. [6,8] conducted a series of investigations on behavior of restrained steel column in fire with carrying out fire tests, finite element simulation and development of a practical design approach. It was found from the investigations that the axial restraint resulted in a lower buckling temperature for the restrained steel columns and the effects of axial restraint to the failure temperature were also related to the load ratio and the axial restraint stiffness ratio. Correia and Rodrigues [9] conducted fire tests on restrained steel columns and the results showed that increasing the stiffness of the adjacent structure members might not lead to a reduction of the critical temperature of a restrained steel column. Correia et al. [10] subsequently proposed a simple approach for fire design for steel columns with thermal elongation being restrained based on the results obtained from a parametric study using finite element software ABAQUS. Yang and Yang [11] carried out fire tests for ten unprotected restrained column specimens. The specimens were loaded by steady-state method and heated up to 500 °C. Craveiro et al. [12] conducted a series of experiments to investigate the behavior of restrained cold-formed steel built-up columns for both closed and open sections at elevated temperature. The results showed that the magnitudes of restraining stiffness and applied load are the important parameters influencing the fire behavior of the columns. In addition, it was found that the 350 °C of limit temperature for class 4 cross-sections stipulated in European code [13] is conservative.