EUROSTEEL 2011, August 31 - September 2, 2011, Budapest, Hungary COMPARISON OF THE BEHAVIOUR OF CONCRETE FILLED HOLLOW AND PARTIALLY ENCASED STEEL COLUMNS SUBJECTED TO FIRE Tiago A.C. Pires a,c , António M. Correia a,b , João Paulo C. Rodrigues a , José Jéferson Rêgo Silva c a Faculty of Sciences and Technology of the University of Coimbra, Dept. of Civil Engineering, Portugal b Polytechnic Institute of Coimbra, Dept. of Technology, Portugal c Federal University of Pernambuco, Dept. of Civil Engineering, Brazil INTRODUCTION Composite structures combine the advantages of steel and concrete structures like high load-bearing capacity, possibility of use small cross-sections and enhanced fire performance. In this way it is a good solution for strength and fire resistance. This paper presents a comparison between two types of composites columns largely used in building construction: composite columns with concrete filled hollow sections (CCHS) and partially encased steel columns (HEA). The behaviour in fire situation of composite columns has been studied by several authors for years (Kodur, 1999, Han et al., 2003 and Malhotra et al., 1964), but the majority of these studies do not consider the restraining to their thermal elongation. The behaviour of the columns inserted in a building structure and subjected to fire is different from when isolated. Thermal restraint promoted by the surrounding structure plays a key role in the stability of the column. There are many ways in which the column can interact with the surrounding structure, including the restraining to its thermal elongation, change of the column bending stiffness relative to the adjacent structure, and increasing of P-Δ effect due to its lateral deformation. The surrounding structure induces additional axial forces and moments on the column those varies with temperature and are dependent of the degree of axial and rotational thermal restraint (Ali et al. 1998 and Neves, et al., 2002). In 2000, Rodrigues et al. published the results of a series of 168 fire resistance tests on compressed steel elements with restrained thermal elongation. Parameters such us the slenderness of the surrounding structure, eccentricity of the load, type of end supports and restraining stiffness, were tested. It was showed that for the case of pin-ended elements with centred loading as higher is the stiffness of the surrounding structure smaller is the critical temperature. The buckling of the elements with centred loading occurred suddenly while the ones with eccentric loading occurred in a very gentle way. In 1999, Valente and Neves presented a numerical study using the FEM program FINEFIRE, to analyze the influence of the axial and rotational restraint on the critical temperature of steel columns. It was showed that the increasing on axial restraint diminishes, in general, while the rotational restraint increases, the critical temperature of steel columns. Huang et al. (2007) published a study about the effects of the axial restraint on the behavior of unprotected encased I-section composite columns subjected to fire. They concluded that the axial restraint reduces the column fire resistance since it increases the internal axial force. In these tests all columns failed by flexural buckling and had concrete spalling. In a comparison between the results and the calculated methods proposed by EN1994-1-2 they concluded that these methods are too conservative. In 1999, Kodur presented the results of a series of 75 fire resistance tests on CCHS columns [1]. The results suggested that the fire resistance of CCHS columns is between 60 and 120min. Reinforced concrete and steel fibre reinforced concrete CCHS columns presented more than 180min of fire resistance. The failure mode of these columns occurred by global buckling especially for the cross sections with diameter smaller than 203mm. Malhotra and Stevens (1964) presented the results of 14 residual fire resistance tests on encased steel stanchions with free thermal elongation. The results show that the concrete cover has a significant effect on the fire resistance. It was registered that the load level had influence on the fire resistance of the stanchions, the ones tested with higher load levels presented smaller fire resistances than the others with a low load level.