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Steel-reinforced concrete-filled steel tubular (SR-CFST) columns may be an alternative to improve the fire resistance of concrete-filled steel tubular (CFST) columns, which is limited by the direct exposure of the outer steel tube to the heat source. In SR-CFST sections an open steel profile is embedded within the concrete infill of the CFST section. In a fire situation, this form of construction enhances the performance of the section since the inner steel profile is thermally protected by the surrounding concrete, thus delaying its degradation at high temperatures. In this experimental program, six SR-CFST specimens are tested. First, the specimens are uniformly exposed to high temperatures inside an electric furnace; then the postfire resistance of the columns subject to increasing axial load is tested in a vertical frame. The SR-CFST specimens are grouped into two series comprising circular and square geometries respectively and, for the sake of comparison, the selected circular and square steel tubes utilised the same cross-sectional area of steel. The experimental results show the high ductility of the SR-CFST stub columns after heating, with the circular specimens reaching higher peak loads than the square columns. For each series, the peak load at post-fire increases as the size of the embedded steel profile does. RSI values are similar for both circular and square SR-CFST columns. Data indicate that, for circular columns confinement is still active in the post-fire situation, but the influence of the size of the embedded steel profile is not notable in the RSI.

[EN] Concrete-filled steel tubular (CFST) columns are composite sections that may contribute to reduce the envi-ronmental impact in construction through a more efficient use of resources. By embedding a steel profile inside a CFST section, a new typology is generated, the so-called steel-reinforced concrete-filled steel tubular (SR-CFST) columns, which may enhance not only the structural capacity of these composite columns at room temperature but also their fire performance. This paper focuses on studying the thermal behaviour of SR-CFST columns under fire conditions, for which purpose a two-dimensional finite element model was developed by the authors and validated by comparing the temperature distribution results with experimental tests available in the literature. Subsequently, parametric studies were carried out to analyse the influence of the relevant parameters - cross-section shape, outer tube thickness, inner steel profile dimensions, section factor - over the cross-sectional ca-pacity of SR-CFST columns exposed to a standard ISO 834 fire. Using the data obtained from the parametric studies, a simplified temperature distribution proposal was derived. In the presented proposal, the composite section was divided onto four components (hollow steel tube, concrete infill, inner steel profile web and flanges) and simplified equations and tables were developed through statistical data processing in order to find the representative equivalent temperature of each part of the section. By doing so, the reduced cross-sectional ca-pacity of a SR-CFST column at a given fire resistance period can be easily evaluated by using a single strength and stiffness value for each component of the section corresponding to its assigned equivalent temperature. This simplified approach may be helpful for practitioners in the fire design process of SR-CFST columns, for which the current provisions in Eurocode 4 Part 1.2 do not provide guidance in predicting the temperature field., The authors would like to express their sincere gratitude for the help provided through the Grant PID2019-105908RB-I00 and for the first author's pre-doctoral contract through the Grant PRE2020-093106 funded by MCIN/AEI/10.13039/501100011033 and by "ESF Investing in your future".

[ES] Cada vez es más común el uso de columnas mixtas de acero-hormigón en edificación, debido a las importantes reducciones de sección que permiten. En los últimos años, se han realizado numerosas investigaciones en la tipología CFST (¿Concrete Filled Steel Tubular column¿). Pese a ello, esta tipología se ve comprometida debido a su limitada resistencia al incendio en casos de elevada esbeltez. Con el fin de mejorar dicho comportamiento surge la tipología de columnas SR-CFST (¿Steel Reinforced Concrete Filled Steel Tubular column¿), que consiste en embeber dentro de la sección un perfil de acero estructural. En este artículo se presenta el desarrollo de un modelo de elementos finitos bidimensional que se ha utilizado para analizar el comportamiento térmico de las columnas SR-CFST en situación de incendio. Dicho modelo ha sido validado por comparación con los resultados experimentales que se encuentran disponibles en la bibliografía como los de Zhu et al. y los de Dotreppe et al., entre otros. Se hace uso del modelo numérico para llevar a cabo estudios paramétricos con el fin de discernir qué influencia tienen diferentes parámetros sobre la resistencia a compresión de la sección a temperatura elevada. Algunos de estos parámetros son la forma de la sección (cuadrada o circular), las dimensiones del perfil interior, el espesor de la pared del tubo de acero o la humedad del hormigón. Teniendo en cuenta los resultados que se obtienen del estudio paramétrico, queda como trabajo futuro el desarrollo de una propuesta de distribución de temperaturas simplificada para cada una de las partes que conforman la sección, de cara a facilitar el cálculo de este tipo de secciones mixtas a temperatura elevada., Los autores agradecen el apoyo proporcionado a través de la Ayuda PID2019-105908RB-I00 y por el contrato predoctoral del primer autor a través de la Ayuda PRE2020-093106 financiadas por MCIN/AEI/ 10.13039/501100011033 y por FSE Invierte en tu futuro.

[EN] An experimental study was carried out to evaluate the residual mechanical properties of steel reinforcements after their exposure to elevated temperatures. In order to reproduce real loading conditions, specimens were simultaneously subjected to a load level and a thermal cycle consisting of heating up to a target temperature and cooling back to room temperature. The target temperature was a percentage of the critical temperature, i.e., the temperature at which a specimen breaks when it is subjected to a constant tensile load and an increasing temperature at a constant rate. After that, the residual mechanical properties were obtained submitting the specimens to tensile testing until failure. Critical temperatures for each load level were obtained by means of transient-state tests. In parallel, unloaded specimens were subjected to the same thermal cycle for comparison purposes, and for the same reason steady-state tests were also performed. Results indicated that the residual mechanical behaviour of reinforcing bars depended not only on the maximum temperature reached, but also on the load level that was bearing during the thermal cycle. Depending on this combination of variables, the code requirements for a given ductility class of a reinforcement may not be satisfied in a post-fire situation. Considering the lack of information in this issue, the obtained data can be helpful in the assessment of reinforced concrete structures after a fire., The authors would like to express their sincere gratitude to the Spanish Agencia Estatal de Investigacion (AEI) for the help provided through the Project PID2019-105908RB-I00 and also to the European Union through the FEDER funds.

[EN] In recent years, innovative solutions have been developed with the purpose of increasing both the load-bearing capacity and fire resistance of concrete-filled steel tubular (CFST) columns. One of these solutions are the so-called concrete-filled dual steel tubular (CFDST) columns. In this configuration, the inner steel tube is thermally protected by the outer concrete ring and therefore its degradation is delayed, which may thus help resisting the applied load for a longer period of fire exposure time. An alternative solution that might be interesting to evaluate under fire conditions is to embed an inner steel profile inside the CFST section, such as an HEB profile. In addition to the development of new types of composite sections, the use of high strength steels (HSS) in construction is increasing, owing to their excellent mechanical properties, which may also be used for an enhanced fire resistance. In the same line, stainless steels (SS) are also becoming widely used in construction due to their beneficial characteristics such as corrosion resistance, high ductility or good aesthetics. Besides, the degradation of their strength and stiffness at elevated temperature is slower than for carbon steels, hence SS may withstand higher temperatures without significantly altering their mechanical properties. This paper aims at obtaining strategies for enhancing the fire resistance of concrete-filled steel tubular columns with the use of innovative solutions (CFDST sections or embedded HEB profiles) combining high performance steels such as HSS or SS. Taking advantage of the improved mechanical properties of these steels at elevated temperatures and using the appropriate thickness ratio between the outer and inner tube or embedded steel profile, it may be possible to attain the required fire resistance without the need for external protection. A three-dimensional finite element model is used in this paper for conducting parametric studies with the different section types and material combinations, in order to evaluate the interest of using HSS and SS for improving the fire performance of steel-concrete composite columns., The authors would like to express their sincere gratitude to the Conselleria d'Educació, Investigació, Cultura i Esport of the Valencian Community (Spain) for the help provided through the project GV/2017/026.

[EN] The mechanical response of concrete-filled steel tubular (CFST) columns subjected to pure compression or uniaxial bending was studied in depth over the last decades. However, the available research results on CFST columns under biaxial bending are still scarce and the lack of experimental tests for this loading situation is evident. At the same time, the design provisions in Eurocode 4 Part 1.1 for verifying the stability of CFST columns under biaxial bending make use of a simplistic interaction curve, which needs to be revised. This paper presents the outcome of a numerical investigation on slender CFST columns subjected to biaxial bending. Eccentricities differing in minor and major axis, as well as varying end moment ratios are considered in the numerical model. A parametric study is conducted for assessing the current design guidelines of EN1994-1-1. Different aspect ratios, member slenderness, reinforcement ratios and load eccentricities are studied, covering both constant and variable bending moment distribution. The numerical results are subsequently compared to the design provisions of EN1994-1-1, showing that the current interaction equation results overly conservative. An alternative interaction equation is developed by the authors, leading to a more accurate yet conservative proposal., The authors gratefully acknowledge "Conselleria d Educacio, Investigacio, Cultura i Esport" of the Valencian Community (Spain) for funding the project GV/2017/026. The authors would also like to thank Leibniz Universitat Hannover for providing the financial support of the research stay of Maximilian Mund at Universitat Politecnica de Valencia.

Slim‐floor beams are a well‐known and cost‐effective solution that permits a significant reduction of floor thickness, and are increasingly used in industrial and commercial buildings. Since only their lower flange is exposed to fire, slim‐floor beams may also achieve higher fire resistance, in comparison to other types of composite beams that are not fully embedded in the concrete floor. Simplified models are available in Eurocode 4 Parts 1‐2 to evaluate the temperature distribution for partially encased and non‐encased composite beams. However this standard does not provide any simplified model to evaluate the cross‐sectional temperature field of slim‐floor beams. In this sense, different proposals have been evaluated in recent years in order to provide simplified models for temperature evaluation. The currently available models in the literature have shown their accurate behaviour providing results on the safe side, and are recommended for use in practice. Finally, this work shows that slim‐floor composite beams can provide good performance during a fire event. Specifically, 60 min of standard fire rating can be achieved for load levels lower than 0.5–0.6. Improved behaviour to achieve 90 or 120 min of standard fire exposure may also be reached by using innovative solutions, advanced materials or external protection. Die Slim-Floor-Bauweise ist eine erprobte und wirtschaftliche Lösung. Sie ermöglicht unter anderem eine erhebliche Redu-zierung der Deckenstärke und wird zunehmend in Industrie- und Gewerbegebäuden eingesetzt. Darüber hinaus erzielen Slim-Floor-Träger, da sie größtenteils in die Decke integriert sind und somit nur die untere angeschweißte Platte direkt einem Brand ausgesetzt wird, einen wesentlich höheren Feuer-widerstand als etwa klassische Verbundträger. In Eurocode4 Teil1-2 finden sich zwar vereinfachte Methoden zur Bestim-mung der Bauteiltemperaturen von Verbundträgern mit und ohne Kammerbeton, jedoch gibt diese Norm keine vereinfachte Methode zur Bestimmung der Temperaturen eines Slim-Floor-Querschnitts an. In den letzten Jahren wurden vereinfachte Modelle zur Bestimmung der Temperaturverteilung von Slim-Floor-Querschnitten entwickelt. Diese derzeit verfügbaren Mo-delle liefern Ergebnisse auf der sicheren Seite und können für eine praktische Anwendung durchaus empfohlen werden. In diesem Artikel wird gezeigt, dass Slim-Floor-Träger während eines Brandereignisses eine exzellente Tragfähigkeit besitzen. Insbesondere können für Ausnutzungsgrade von 0,5 bis 0,6 die Anforderungen an die Feuerwiderstandsklasse R60 erfüllt wer-den. Die Feuerwiderstandsklassen R90 und R120 können durch Anwendung innovativer Lösungen, durch verbesserte Materia-lien oder auch passiven Brandschutz erfüllt werden.

[EN] This paper presents a review on the latest advances in the field of steel-concrete composite structures exposed to fire, studying the recent research in the area and developments of the design codes. The paper focuses in particular on concrete-filled steel tubular columns and slim-floor beams, topics where the authors have carried out extensive research during the last years. The more recent experimental and numerical studies are presented, as well as the currently available design methods for the calculation of isolated members in the fire situation. The use of advanced materials, such as high strength steel and concrete, stainless steel, lightweight concrete or geopolymer concrete is considered for the enhancement of the fire behaviour of concrete-filled steel tubular columns and slim-floor beams.

Concrete-filled steel tubular (CFST) columns are being increasingly used in construction, attracting the attention of designers owing to their high load-bearing capacity, which can be accomplished with reduced sectional dimensions. However, the associated high slenderness of such columns may affect their fire performance. Innovative solutions are therefore sought for improving the performance of this typology of composite columns in the fire situation. This paper aims at designing strategies for enhancing the fire resistance of conventional CFST columns through innovative solutions - such as double-tube sections or embedded HEB profiles - in combination with the use of high strength steel (HSS) at the inner profiles and stainless steel (SS) at the outer tube. For that purpose, a numerical model is developed and a comprehensive parametric study is presented, where the interest of using HSS/SS for improving the fire performance of steel-concrete composite columns with innovative sections is evaluated. Los autores agradecen el apoyo de la Conselleria d’Educació, Investigació, Cultura i Esport de la Generalitat Valenciana por la financiación proporcionada a través del proyecto GV/2017/026.

[EN] This paper summarizes the latest technical and scientific progresses on steel-concrete composite structures exposed to fire, presenting the recent research carried out on this subject and the progress of the design codes. In particular, this review focuses on concrete-filled steel tubular columns and slim-floor beams, topics where the authors have carried out extensive research during the last years. The more recent experimental and numerical studies performed by the authors as well as those available in the literature are presented, along with applications where these composite elements have been used in practice. The use of advanced materials, such as high strength steel and concrete, stainless steel, lightweight concrete or geopolymer concrete is considered for the enhancement of the fire behaviour of concrete-filled steel tubular columns and slim-floor beams. Finally, the currently available design methods for the calculation of isolated members at elevated temperatures are reviewed and the recent progress of the code provisions for the fire design of these composite elements is presented., The authors would like to express their sincere gratitude to the Spanish "Ministerio de Economia y Competitividad" for the help provided through the Project BIA2015-67192-R and to the European Union through the FEDER funds.

[EN] This paper presents a numerical investigation on the mechanical performance of concrete-filled dual steel tubular columns of circular section subjected to concentric axial load. A three-dimensional numerical model is developed and validated against a series of experimental tests. A good agreement is obtained between the experimental and numerical results, both in the peak load value and in the ascending and descending branches of the load-displacement curves. By means of the numerical model, a parametric study is carried out to investigate the influence of the main parameters that determine the axial capacity of double-tube columns, such as the member slenderness, inner and outer steel tube thicknesses and the concrete grade - of both the outer concrete ring and inner core -, including ultra-high strength concrete. A total number of 163 numerical simulations are carried out, by combining the different parameters. Specific indexes are defined (Strength Index, Concrete-Steel Contribution Ratio, Inner Concrete Contribution Ratio) to help rating the relative mechanical performance of dual steel tubular columns as compared to conventional concrete-filled steel tubular columns, and practical design recommendations are subsequently given., The authors would like to express their sincere gratitude to the "Conselleriad' Educacio, Investigacio, Cultura i Esport" of the Valencian Community (Spain) for the help provided through the project GV/2017/026.