Este trabalho aborda o estudo da aderência aço-concreto em pilares mistos preenchidos através de revisão bibliográfica e de investigação experimental. De modo esquemático, a aderência é dividida em três parcelas: adesão, aderência mecânica e atrito, que em conjunto compõem a aderência natural. Adicionalmente, podem ser empregados conectores de cisalhamento, que são dispositivos mecânicos fixados nas superfícies internas dos tubos, como meio de ampliar a resistência ao escorregamento. Foram realizados ensaios do tipo pushout em alguns tipos de modelos de pilar preenchido: modelos simples, com adição de conectores tipo pino com cabeça, com cantoneiras na interface aço-concreto e semelhantes aos anteriores mais elementos de ligação. Com os resultados obtidos traçaram-se comportamentos força x escorregamento do concreto, força x deformações axiais nos materiais e curvas de distribuição de força ao longo do comprimento dos modelos. Concluiu-se que conectores tipo pino com cabeça e cantoneiras são uma excelente alternativa como dispositivos auxiliares na transferência de tensões de cisalhamento na interface aço-concreto. Entretanto, é necessário garantir resistência à ligação para que esses mecanismos sejam mobilizados / This research presents a study of influence of bond on concrete-filled steel tubes by means of a bibliography review and an experimental investigation. The bond strength counts on three mechanisms: adhesion of the concrete to the steel surface, friction and wedging of the concrete core. In order to improve the shear resistance of the steel-concrete interface, shear connectors can be used. A series of push-out tests of rectangular concrete-filled steel tubes was conducted in the specimens without mechanical shear connectors, specimens with stud bolt shear connectors and specimens with angles. They were also tested the corresponding beam-column connections. The results are shown in curves force x slip of concrete, force x axial strain of materials and distribution of axial load to the steel and to the concrete. The results of the tests permitted identify the mechanisms of natural bond and indicated that the stud bolts and the angles are excellent alternatives as auxiliary mechanisms to transfer shear between the concrete and steel in the concrete filled composite column. However, it is necessary to guarantee the connection enough strength in order to permit these resistance be developed

This paper describes the finite element method using ABAQUS to model the axial compressive behaviour of inclined, tapered and straight-tapered-straight (STS) concrete filled steel tubular stub (CFST) columns with square hollow sections. The accuracy of the numerical model was verified by comparing the numerical predictions with experimental study of the 200×200×3.75 RHS filled with C60 concrete with inclined angle of 0-9° and tapered angle of 0-4°. The results show that the compressive behaviours, load vs. strain relationship and failure mode predicted by the numerical simulations were agreeable with experimental results. After the validation, a parametric study was performed with 3 typical steel hollow sections (200×200×3.75 RHS, 300×300×6.3 RHS and 400×400×8.0 RHS) and extended the inclined angle and tapered angle to 0-15° and 0-12° respectively. The parametric study highlights some of the behaviour observed in test and extends the application range. In addition, reduction factor for calculating the axial capacity of this form of CFST columns are proposed.

Concrete-filled steel tubes are gaining increasing prominence in a variety of engineering structures, with the principal cross-section shapes being square, rectangular and circular hollow sections. A recent addition to this range has been that of elliptical hollow sections. The structural response of empty elliptical tubes has been examined in previous studies. In this paper, the cross-sectional axial behaviour of concrete-filled elliptical hollow sections is investigated. An experimental programme comprising a total of 21 test specimens, with three nominal tube thicknesses (4 mm, 5 mm and 6.3 mm) and three concrete grades (C30, C60 and C100) has been performed. The effects of steel tube thickness, concrete strength and constraining factor on elastic stiffness, ductility and ultimate strength were studied. To simulate the effects of concrete shrinkage, the inner surfaces of 6 of the 21 test specimens were coated with grease prior to casting. To investigate confinement effects, a further 6 of the 21 test specimens were loaded through the concrete core only. The results of the tests presented herein were combined with those from previous studies, and compared with existing design provisions for square, rectangular and circular concrete-filled tubes. The design expressions from current European, North American, Japanese, British and Chinese Standards were assessed. On the basis of the comparisons, design recommendations for concrete-filled elliptical hollow sections have been made.

Yes / This paper presents the results of a numerical investigation on strategies for enhancing the fire behaviour of concrete-filled steel tubular (CFST) columns by using inner steel profiles such as circular hollow sections (CHS), HEB profiles or embedded steel core profiles. A three-dimensional finite element model is developed for that purpose, which is capable for representing the various types of sections studied and the nonlinear behaviour of the materials at elevated temperatures. High strength steel is considered in the numerical model, as a possible way to lengthen the fire endurance. The numerical model is validated against experimental results available in the literature for various types of steel-concrete composite sections using inner steel profiles, obtaining satisfactory results. Based on the developed numerical model, parametric studies are conducted for investigating the influence of the cross-sectional geometry and the steel grade of the inner profiles on the fire performance of these composite columns, for eventually providing some practical recommendations.

Este trabalho aborda o estudo de pilares preenchidos flexo-comprimidos e algumas ligações viga-pilar preenchido. Para ambos, são realizadas experimentações com modelos físicos e numéricos, sendo que para os pilares preenchidos um amplo estudo paramétrico foi desenvolvido. Ao todo, foram ensaiados 13 pilares preenchidos flexo-comprimidos, cujos valores de força última são comparados com aqueles provenientes da simulação numérica via programa CFT e, os demais resultados, comentados e discutidos. Aferido, o programa CFT foi utilizado no estudo paramétrico com o intuito de avaliar a influência de parâmetros como resistência dos materiais, excentricidade da força axial, eixo de flexão e relação lado/espessura na capacidade resistente do pilar preenchido. Quanto às ligações viga-pilar, foram ensaiadas 4 tipologias de ligação, sendo duas compostas por chapas de extremidade e parafusos passantes e duas soldadas, sendo uma delas enrijecida por cantoneiras soldadas no interior do perfil tubular. Discutem-se os resultados, sobretudo quanto ao comportamento momento-rotação das tipologias investigadas. As ligações também foram investigadas via simulação numérica e os resultados obtidos permitiram elaborar diretrizes para o aprimoramento de tais simulações. Por fim, são apresentadas as conclusões pertinentes a este trabalho e recomendações para novas pesquisas / This research thesis presents a study of concrete filled steel columns subjected to combined bending and compression and beam-column connections. In both cases, tests were conducted on physical and numerical models, resulting in the development of a wide range of parametric studies for the case of the concrete filled steel columns. 13 concrete filled steel columns were tested under combined bending and compression loads whose ultimate values were compared to ultimate forces obtained from numerical simulations using the CFT computer program. After calibration, the computer program was used in the parametric study with the aim of verifying the influence of parameters, such as the material strength, eccentricity of the axial force, bending axis and breadth/thickness relation, on the ultimate load carrying capacity of the concrete filled steel columns. With respect to the beam-column connections, four types of connections were tested, two of them composed of endplate and bolts, while the other two were welded, with one of them stiffened by an angle steel placed inside the steel tube. The obtained results are discussed, giving more emphasis to the bending moment-rotation behaviour of the tested specimens. The beam-column connections were also investigated numerically. The obtained results permit the laying out of important directives for the improvement of such numerical simulation tools as the one employed in the present thesis. Lastly, pertinent conclusions relative to this thesis and recommendations for future research are presented

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Este trabalho aborda o estudo de pilares preenchidos flexo-comprimidos e algumas ligações viga-pilar preenchido. Para ambos, são realizadas experimentações com modelos físicos e numéricos, sendo que para os pilares preenchidos um amplo estudo paramétrico foi desenvolvido. Ao todo, foram ensaiados 13 pilares preenchidos flexo-comprimidos, cujos valores de força última são comparados com aqueles provenientes da simulação numérica via programa CFT e, os demais resultados, comentados e discutidos. Aferido, o programa CFT foi utilizado no estudo paramétrico com o intuito de avaliar a influência de parâmetros como resistência dos materiais, excentricidade da força axial, eixo de flexão e relação lado/espessura na capacidade resistente do pilar preenchido. Quanto às ligações viga-pilar, foram ensaiadas 4 tipologias de ligação, sendo duas compostas por chapas de extremidade e parafusos passantes e duas soldadas, sendo uma delas enrijecida por cantoneiras soldadas no interior do perfil tubular. Discutem-se os resultados, sobretudo quanto ao comportamento momento-rotação das tipologias investigadas. As ligações também foram investigadas via simulação numérica e os resultados obtidos permitiram elaborar diretrizes para o aprimoramento de tais simulações. Por fim, são apresentadas as conclusões pertinentes a este trabalho e recomendações para novas pesquisas / This research thesis presents a study of concrete filled steel columns subjected to combined bending and compression and beam-column connections. In both cases, tests were conducted on physical and numerical models, resulting in the development of a wide range of parametric studies for the case of the concrete filled steel columns. 13 concrete filled steel columns were tested under combined bending and compression loads whose ultimate values were compared to ultimate forces obtained from numerical simulations using the CFT computer program. After calibration, the computer program was used in the parametric study with the aim of verifying the influence of parameters, such as the material strength, eccentricity of the axial force, bending axis and breadth/thickness relation, on the ultimate load carrying capacity of the concrete filled steel columns. With respect to the beam-column connections, four types of connections were tested, two of them composed of endplate and bolts, while the other two were welded, with one of them stiffened by an angle steel placed inside the steel tube. The obtained results are discussed, giving more emphasis to the bending moment-rotation behaviour of the tested specimens. The beam-column connections were also investigated numerically. The obtained results permit the laying out of important directives for the improvement of such numerical simulation tools as the one employed in the present thesis. Lastly, pertinent conclusions relative to this thesis and recommendations for future research are presented

[ES] Los pilares tubulares de acero rellenos de hormigón (CFST) son un tipo de elementos estructurales mixtos que han ganado popularidad recientemente debido a su excelente comportamiento estructural y al uso racional de los materiales en su fabricación. Con la introducción de un perfil de acero dentro de una sección CFST, surge un nuevo tipo de sección mixta: los pilares tubulares de acero rellenos de hormigón con perfil de acero interior (SR-CFST), en los cuáles se centra esta tesis. Esta tipología incrementa la capacidad portante a temperatura ambiente de los pilares CFST y mejora su comportamiento a fuego, ya que el perfil interior está protegido térmicamente por el hormigón que lo rodea. Las investigaciones disponibles en la bibliografía sobre pilares SR-CFST aún son escasas, pese a sus significativas capacidades mecánicas tanto a temperatura ambiente como a elevada. En esta tesis se revisa el estado del arte sobre pilares SR-CFST, llegando a la conclusión de que son necesarias más investigaciones para comprender el comportamiento de estas secciones a temperatura elevada. La presente tesis analizará el comportamiento frente al fuego de pilares SR-CFST mediante ensayos experimentales y simulaciones numéricas. Inicialmente, se lleva a cabo una campaña experimental para analizar el comportamiento de los pilares bajo dos escenarios: una primera fase que estudia el comportamiento de los pilares tras su exposición al fuego, y una segunda en la que se analizará el comportamiento termo-mecánico de pilares cortos SR-CFST bajo carga constante e incremento progresivo de la temperatura. Tras ello, se desarrolla un modelo de elementos finitos para extender los resultados obtenidos experimentalmente y estudiar en profundidad el comportamiento de los pilares SR-CFST en fuego. El modelo numérico se valida con los ensayos termo-mecánicos realizados anteriormente, así como comparando sus predicciones con los resultados experimentales extraídos de la bibliografía. Con el modelo desarrollado se realizan una serie de estudios paramétricos para examinar la influencia de los parámetros geométricos más relevantes, así como para analizar el efecto del uso de materiales de alta resistencia en el comportamiento a fuego de los pilares. En base a los resultados obtenidos de los estudios paramétricos, se lleva a cabo una propuesta para determinar de forma sencilla la distribución de temperaturas seccional en un tiempo de exposición al fuego dado. Empleando las temperaturas equivalentes obtenidas y en línea con las directrices del Eurocódigo 4 Parte 1.2, se propone una nueva expresión para evaluar la resistencia plástica seccional de los pilares SR-CFST bajo la acción del fuego. Esta propuesta cubre un campo de aplicación que actualmente no está contemplado en la normativa europea. / [CA] Els pilars tubulars d'acer plens de formigó (CFST) són un tipus de membres estructurals mixtes que han guanyat popularitat recentment degut al seu excel·lent comportament estructural y a l'ús moderat de materials en la seua construcció. Amb la introducció d'un perfil d'acer dins d'una secció CFST, s'obté un nou tipus de secció mixta: els pilars tubulars d'acer plens de formigó amb perfil d'acer interior (SR-CFST), al voltant dels quals se centra esta tesi. Esta tipologia incrementa la capacitat a temperatura ambient dels pilars CFST i millora el seu comportament a foc, ja que el perfil interior està protegit tèrmicament pel formigó que l'envolta. Les investigacions disponibles a la bibliografia sobre pilars SR-CFST encara són escasses, malgrat les seues significatives capacitats mecàniques tant a temperatura ambient com a elevada. En esta tesi es revisa l'estat de l'art sobre pilars SR-CFST, concloent que són necessàries més investigacions per tal de comprendre el comportament d'estes seccions a alta temperatura. En la present tesi s'analitzarà el comportament a foc dels pilars SR-CFST per mitjà d'assajos experimentals i simulacions numèriques. Inicialment es desenvolupa una campanya experimental per tal d'analitzar el comportament dels pilars en dos situacions: una primesa fase estudia el comportament dels pilars després de la seua exposició al foc i una segona en què s'analitza el comportament termo-mecànic de pilars curts SR-CFST subjectes a una càrrega constant i a un increment progressiu de la temperatura. Seguidament es desenvolupa un model d'elements finits realista per tal d'estendre els resultats obtinguts experimentalment i estudiar en profunditat el comportament dels pilars SR-CFST en foc. El model numèric es valida amb els assajos termo-mecànics realitzats anteriorment, així com comparant les seues prediccions amb els resultats disponibles a la bibliografia. Amb ajuda del model desenvolupat es realitzen una sèrie d'estudis paramètrics per tal d'examinar la influència dels paràmetres geomètrics més rellevants així com l'efecte de l'ús de materials d'alta resistència en el comportament a foc dels pilars. Basat en els resultats obtinguts dels estudis paramètrics, es desenvolupa una proposta per determinar, de forma senzilla, la distribució de temperatures seccional en un temps d'exposició al foc concret. Amb les temperatures equivalents obtingudes i en línia en les indicacions de l'Eurocódi 4 Part 1.2, es proposa un nou mètode per avaluar la resistència plàstica seccional de pilars SR-CFST baix l'acció del foc. Esta proposta cobreix un camp d'aplicació que actualment no està contemplat en la normativa europea. / [EN] Concrete-filled steel tubular (CFST) columns are a type of composite structural members that have gained popularity in recent years owing to their excellent structural performance and rational use of the materials. By embedding a steel profile inside a CFST section, a new and innovative type of composite section is generated: the so-called steel-reinforced concrete-filled steel tubular (SR-CFST) column, which is the focus of this thesis. This typology improves the room temperature capacity of CFST columns, while enhancing their fire performance, as the inner steel profile is thermally protected by the surrounding concrete. Despite their remarkable load-bearing capacity at both room and elevated temperature, investigations on SR-CFST columns under fire are still scarce. The state of the art on the fire behaviour of SR-CFST columns is reviewed in this thesis, proving that further research is needed to completely understand the performance of this type of columns under elevated temperature. The fire performance of SR-CFST columns is analysed in this thesis by means of experimental tests and numerical simulations. First, an experimental investigation is conducted to study the behaviour of these composite columns under two scenarios: the first phase of the experimental campaign is focused on the post-fire performance, while the second phase is conducted to determine the thermo-mechanical performance of SR-CFST stub columns under constant load and increasing temperature. Subsequently, a realistic finite element model is developed to extend the experimental results and analyse in depth the behaviour of SR-CFST columns in fire. The numerical model is validated against the previously performed thermo-mechanical tests, as well as by comparing its predictions with the available experimental results from the literature. A series of parametric studies are conducted to analyse the influence of the relevant geometrical parameters and to study the effect of the use of high-strength materials on the fire performance of the columns. Based on the results of the parametric studies, a simplified proposal is developed to easily determine the cross-sectional temperature distribution at a given fire exposure time. Using the obtained equivalent temperatures and in line with the current provisions in Eurocode 4 Part 1.2, a new design equation is proposed to evaluate the cross-sectional plastic resistance of SR-CFST columns under fire conditions, filling an existing gap in the European design code. / 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’’. Finally, the authors would like to acknowledge the funding for open access charge from CRUE-Universitat Politècnica de València. / Medall Martos, D. (2024). Numerical and experimental research on the fire resistance of composite columns with steel sections embedded in concrete and high strength materials [Tesis doctoral]. Universitat Politècnica de València. https://doi.org/10.4995/Thesis/10251/207130