a b s t r a c tIn this work, a nonlinear finite element three-dimensional model is presented and validated in order to study the behaviour of axially loaded concrete filled tubular (CFT) columns with circular cross-section exposed to fire. A realistic sequentially coupled nonlinear thermal-stress analysis is conducted for a series of columns available in the literature. The model is validated by comparing the simulation results with the real fire resistance tests. By means of this model, and extensive sensitivity analysis is performed over a wide range of aspects concerning the finite element modelling of the problem under study, including new key factors not studied previously. Based on this sensitivity analysis several modelling recommendations are given in this paper, which will be useful for future research work. The validated numerical model is furthermore employed to study and discuss the Eurocode 4 Part 1-2 simple calculation model, which is deeply analysed in this paper.
This paper describes sixteen fire tests conducted on slender circular hollow section columns filled with normal and high strength concrete, subjected to concentric axial loads. The test parameters were the nominal strength of concrete (30 and 80 MPa), the infilling type (plain concrete, reinforced concrete and steel fiber reinforced concrete) and the axial load level (20% and 40%). The columns were tested under fixed-pinned boundary conditions and the relative slenderness at room temperature was higher than 0.5 in all of the cases. A numerical model was validated against the tests, in order to extend the results and understand the failure mode of such columns. It is the aim of this paper to study the influence in a fire situation of the use of high strength concrete, as opposed to normal strength concrete. The results have shown that for slender columns subjected to high temperatures, the behavior of high strength concrete was different than for stub columns, spalling not being observed in the experiments. Furthermore, the addition of steel fibers was not found very advantageous in slender columns, since no increment in terms of fire resistance was obtained for the columns which used this type of reinforcement. However, the addition of reinforcing bars seems to be the solution in some cases, where the use of external fire protection wants to be avoided in the design of HSS structures, since the reinforcing bars allow the tube to resist a higher axial load.
This paper presents the results of an experimental program carried out on slender elliptical hollow section columns filled with concrete. Given the reduced number of experimental results found in the literature on concrete filled tubular columns with elliptical cross-section, the main objective of this paper is to compare the behaviour of such innovative cross-sections under ambient and high temperatures. The test parameters covered in this experimental program were the load eccentricity (0, 20 and 50 mm) and the type of infill (plain concrete or bar-reinforced concrete). Six room temperature tests were performed, while other six tests were carried out at elevated temperatures, under both concentric and eccentric axial load. Using the results of these tests, the current provisions of Eurocode 4 for room temperature and fire design were assessed, and a specific design proposal developed by the authors was evaluated.
In this work, a nonlinear three-dimensional finite element model is presented in order to study the behaviour of axially loaded concrete filled elliptical hollow section (CFEHS) columns exposed to fire. This study builds on previous work carried out by the authors on concrete filled circular hollow section (CFCHS) columns both at room temperature and in fire. The numerical model is first validated at room temperature against a series of experiments on CFEHS stub columns available in the literature and subsequently extended to study the performance of slender columns at elevated temperatures. The aim of this work is to understand and represent the behaviour of axially loaded CFEHS columns in fire situations and to compare their effectiveness with that of the circular concrete filled tubular (CFT) columns. Parametric studies to explore the influence of variation in global member slenderness, load level, cross-section slenderness and section size are presented. Finally, guidance on the fire design of CFEHS columns is proposed: it is recommended to follow the guidelines of Clause 4.3.5.1 in EN 1994-1-2, but employing the flexural stiffness reduction coefficients established in the French National Annex with an equivalent EHS diameter equal to P/, where P is the perimeter of the ellipse. Relative errorEspinos A, Gardner L, Romero ML, Hospitaler A. Fire behaviour of concrete filled elliptical steel columns. Thin-Walled Struct.
This paper presents the results of an experimental campaign where both the room temperature and the fire resistance of six double-tube concrete filled steel tubular slender columns with different combinations of concrete strength are studied. Firstly, the ultimate axial load of the specimens at room temperature was experimentally obtained and afterwards the fire resistance of such columns subjected to a 20% of their load bearing capacity was measured. Given the reduced number of experimental results found in the literature on slender concrete filled tubular columns with double steel tubular cross-sections, the main objective of this paper is to compare the behaviour of such innovative cross-sections under ambient and high temperatures. The influence of filling the inner ring with concrete on the fire performance of these columns is studied in this paper, as well as the variation of thicknesses of the outer and inner steel tubes. Despite the fact that the tested columns are not covered by the scope of Eurocode 4, the current simple calculation models were applied in this paper in order to assess the validity of the standard to this typology of columns, unsafe results being found.
Previous investigations proved the unsafety of the current design guidelines in Annex H of EN 1994-1-2 for the calculation of the fire resistance of slender concrete-filled steel tubular (CFST) columns. In this paper, a new simplified design method based on the general rules in Clause 4.3.5.1 of EN 1994-1-2 is presented for correcting this inaccuracy. For the development of the method, an extensive parametric study consisting of about 20.500 analysis cases was carried out by using numerical models, which in turn were validated against a wide range of experimental results. The proposed method provides a significant extension over the current EN 1994-1-2 applicability limits, reaching high member slenderness, large eccentricities and being valid for all the commercially available geometries, including elliptical hollow sections. The design proposal is divided into two parts: thermal, where a simplified cross-sectional temperature field can be obtained based on equivalent temperatures for the composite section constituents, and mechanical, where a full method for evaluating the ultimate buckling load in the fire situation is given. The proposed method is valid for axially and eccentrically loaded columns, accounting for eccentricities on both minor and major axis and reaching large eccentricities of e/D = 1. Finally, it is proved that the proposed method provides safe predictions as compared to experimental results and meets the CEN/TC250/SC4 accuracy criteria.
This paper presents the results of an experimental program where the load-bearing capacity at room temperature of 14 slender circular concrete-filled dual steel tubular (CFDST) columns under axial load was analysed. In this study, two specimens were prepared as ordinary concrete-filled steel tubular (CFST) columns to be used as references to evaluate the mechanical contribution of the inner steel tube and its infill in the CFDST columns. In addition, the effect of two types of concrete: normal strength concrete (NSC) and ultra-high strength concrete (UHSC) was assessed. Besides, in order to study the influence of the steel share between the inner and outer tube, different cross-sectional configurations were considered. Since the number of experimental results available in the literature on slender CFDST columns is scarce, this work provides novel results to this research field. The different influence of the steel distribution in the response of the specimens of each series was observed, with no influence in the case of columns with ultra-high strength concrete in the outer ring. Finally, the current provisions of Eurocode 4 for the design of composite columns were assessed by means of the results of these tests, being necessary more test data to extract solid conclusions about their accuracy and reliability.
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