Dissertations / Theses on the topic 'CFST columns'

Doctor of Philosophy
Department of Civil Engineering
Hayder Rasheed
The analysis of concrete columns using unconfined concrete models is a well established practice. On the other hand, prediction of the actual ultimate capacity of confined concrete columns requires specialized nonlinear analysis. Modern codes and standards are introducing the need to perform extreme event analysis. There has been a number of studies that focused on the analysis and testing of concentric columns or cylinders. This case has the highest confinement utilization since the entire section is under confined compression. On the other hand, the augmentation of compressive strength and ductility due to full axial confinement is not applicable to pure bending and combined bending and axial load cases simply because the area of effective confined concrete in compression is reduced. The higher eccentricity causes smaller confined concrete region in compression yielding smaller increase in strength and ductility of concrete. Accordingly, the ultimate confined strength is gradually reduced from the fully confined value fcc (at zero eccentricity) to the unconfined value f’c (at infinite eccentricity) as a function of the compression area to total area ratio. The higher the eccentricity the smaller the confined concrete compression zone. This paradigm is used to implement adaptive eccentric model utilizing the well known Mander Model and Lam and Teng Model. Generalization of the moment of area approach is utilized based on proportional loading, finite layer procedure and the secant stiffness approach, in an iterative incremental numerical model to achieve equilibrium points of P- and M- response up to failure. This numerical analysis is adaptod to asses the confining effect in circular cross sectional columns confined with FRP and conventional lateral steel together; concrete filled steel tube (CFST) circular columns and rectangular columns confined with conventional lateral steel. This model is validated against experimental data found in literature. The comparison shows good correlation. Finally computer software is developed based on the non-linear numerical analysis. The software is equipped with an elegant graphics interface that assimilates input data, detail drawings, capacity diagrams and demand point mapping in a single sheet. Options for preliminary design, section and reinforcement selection are seamlessly integrated as well. The software generates 2D interaction diagrams for circular columns, 3D failure surface for rectangular columns and allows the user to determine the 2D interaction diagrams for any angle  between the x-axis and the resultant moment. Improvements to KDOT Bridge Design Manual using this software with reference to AASHTO LRFD are made. This study is limited to stub columns.

This PhD thesis is devoted to contributing to the existing knowledge of FRP (Fibre Reinforced Polymer) strengthened metallic structural members subjected to dynamic impacts. The outcome of this research will be beneficial to design structures for mitigating damage and failure due to lateral impact forces from transportation accidents, explosive attacks or from flying debris. A new series of laboratory tests results along with computer based structural analyses are presented to understand the effectiveness of FRP strengthening of tubular structural members. FRP strengthening found to be a promising sustainable option for minimising structural damage and human casualty due to impact loads.

Abstract : The construction industry is expressing great demand for innovative and durable structural members such as bridge decks and piers, piling, and poles. Many steel-reinforced concrete structures subjected to de-icing salts and marine environments require extensive and expensive maintenance. Fiber-reinforced polymers (FRPs) have recently gained wide acceptance as a viable construction material for repair, rehabilitation, or new construction of the aging infrastructures particularly those exposed to harsh environment conditions. The promising concept of concrete-filled FRP tube (CFFT) system, that may be further reinforced with steel or FRP bars, has raised great interest amongst researchers in the last decade. The CFFT technique has been used successfully in different concrete structure elements such as pier column and girder for bridges and also as fender piles in marine structures. The FRP tube acts as a stay-in-place structural formwork, a noncorrosive reinforcement for the concrete for flexure and shear, provides confinement to the concrete in compression, and the contained concrete is protected from intrusion of moisture with corrosive agents that could otherwise deteriorate the concrete core. Using FRP bars instead of conventional steel bars in the CFFT columns can provide a step forward to develop a promising totally corrosion-free new structural system. Nonetheless, the axial behaviour of FRP bars as longitudinal reinforcement in compression members has yet to be explored, especially for the CFFT columns. To date, most of the experimental investigations performed on FRP confined concrete columns have considered short, unreinforced, small-scale concrete cylinders, tested under concentric, monotonic, and axial load. The slenderness ratio, internal longitudinal reinforcement type (steel or FRP bars), and axial cyclic loading effects on the behaviour of FRP confined concrete long columns, however, have received only limited research attention. To address such knowledge gaps, this study aimed at investigating the behaviour of the CFFT long columns internally reinforced with steel or FRP bars tested under monotonic and cyclic axial loading. A total of ten reinforced concrete (RC) and CFFT columns were constructed and tested until failure. All columns had 1900-mm in height and 213-mm in diameter. The investigated parameters were: i) the effect of internal reinforcement type (steel, glass FRP (GFRP), or carbon FRP (CFRP)) and amount, ii) GFRP tube thicknesses, and iii) nature of loading (i.e. monotonic and cyclic). The effect of the different parameters on the axial behaviour of the tested columns is presented and discussed. The research work presented in this dissertation has resulted in one paper submitted to the Elsevier Journal of Engineering Structures (manuscript ID: ENGSTRUCT-D-15-01381) and one accepted conference paper submitted to the 5 th International Structural Specialty Conference (CSCE 2016), London, Ontario, June 1st - 4th, 2016. The experimental test results showed that the CFFT columns reinforced with GFRP bars exhibited similar responses compared to their counterparts reinforced with steel bars with no significant difference in terms of ultimate axial strength and strain capacities. The GFRP tubes provided significant confinement of the tested specimens attributing to shift the mode of failure from axially dominated material failure to flexural-dominated instability failure. The results also indicated that the plastic strains of the FRP-reinforced CFFT columns was linearly proportional to the envelop unloading strains (εun,env). The relationship depended little on level of confinement, but strongly on the longitudinal reinforcement amount and type, particularly when εun,env > 0.0035. On the other hand, an analytical investigation was conducted to examine the validity of the available design provisions for predicting the ultimate load capacity of tested columns. The results of the analysis were compared with the experimental values. It was found that the ACI 440.R1 (2015), CSA S806 (2012), and CSA S6-06 (2010) design provisions provided higher conservative results for the GFRP-reinforced control specimens than that of steel-reinforced specimen. This might be due to neglecting the contribution of the compressive resistance of the GFRP bars to the axial carrying capacity. Furthermore, for FRP-reinforced CFFT columns, the ACI 440.2R (2008), CSA S806 (2012), and CSA S6-06 (2010) provisions results over the experimental results were an average of 1.68±0.31, 1.57±0.18, and 1.72±0.35 with a COV of 18.4%, 11.3%, and 20.5%, respectively. By considering the confinement codes limits, the CSA S806 (2012) showed better correlation for the ultimate carrying capacity based on the average than the CSA S6-06 (2010) and ACI 440.2R (2008), particularly for specimens cast with tube Type B.
Résumé : L'industrie de la construction exprime une grande demande pour les structures innovantes et durables tels que les tabliers de ponts et les quais, les pieux et les poteaux. Plusieurs structures en béton armé sont soumises à des sels de déglaçage et à des environnements marins qui exigent un entretien coûteux. Les polymères renforcés de fibres (PRF) ont récemment été reconnus en tant que matériau de construction viable pour la réparation, la réhabilitation ou la construction de nouvelles infrastructures vieillissantes en particulier celles exposées à des conditions d'environnement sévères. Le concept prometteur du système de tube rempli de béton PRF (CFFT), qui peut être encore renforcé avec de l'acier ou des barres en PRF, a amorcé un grand intérêt parmi les chercheurs durant la dernière décennie. La technique CFFT a été utilisée avec succès dans les différents éléments de structure en béton tels que les colonnes et les poutres de ponts et aussi comme des pieux pour les structures marines. Le tube en PRF agit comme un coffrage structural sur place, un renforcement non corrosif pour le béton en flexion et au cisaillement en utilisant l'orientation des fibres multidirectionnelle, fournit un confinement au béton en compression, et le béton est protégé de toute intrusion d'humidité des agents corrosifs qui, autrement, pourraient détériorer le noyau de béton (ACI 440. R-07 (2007)). L’utilisation des barres de PRF au lieu de barres d'acier conventionnelles dans les colonnes CFFT peut fournir un pas en avant pour développer un nouveau système structurel. Néanmoins, le comportement axial des barres en PRF comme armatures longitudinales dans les membrures en compression n'a pas encore été exploré, en particulier pour les colonnes CFFT. À ce jour, la plupart des études expérimentales effectuées sur les colonnes en béton confinés de PRF, ont considéré des cylindres en béton, courts, à petite échelle non armés, et testés sous un charge concentrique, monotone, et axiale. Le rapport d'élancement, le renfort longitudinal interne (acier ou barres en PRF), et les effets du chargement axial cyclique sur le comportement des colonnes élancées de béton confinés et en PRF, ont connu une recherche limitée. Pour combler ce manque de connaissance, cette étude vise à étudier le comportement des colonnes élancées CFFT armé en acier ou en barres de PRF testées sous charges axiales monotones et cycliques. Un total de dix colonnes en béton armé (RC) et CFFT été fabriquées et testées jusqu'à la rupture. Toutes les colonnes ont 1900 mm de hauteur et 213 mm de diamètre. Les paramètres étudiés sont les suivants: i) l'effet de type de renforcement interne et la quantité de renforcement, ii) les épaisseurs de tubes PRV, et iii) le type de chargement (monotone et cyclique). L'effet des variables considérées sur le comportement axial des colonnes testées dans le travail expérimental est présenté et discuté. Le travail de recherche présenté dans cette analyse a fait l’objet d’un article scientifique soumis à Elsevier Journal of Engineering Structures (manuscrit ID: ENGSTRUCT-D-15-01381) et un article lors d’une conférence acceptée soumis à la 5ième International Structural Specialty Conference (CSCE 2016), London, Ontario, Juin 1er - 4ième, 2016. Les résultats des essais expérimentaux ont montré que les colonnes CFFT renforcées de barres en PRFV présentaient des réponses similaires par rapport à leurs homologues renforcées avec des barres d'acier sans différence significative en termes de capacité ultime de résistance axiale et de déformation. Les tubes en PRFV fournissent un confinement significatif des échantillons testés attribuant à changer le mode de rupture, c’est-à-dire d’une rupture des matériaux axialement à une rupture d’instabilité en flexion. En outre, l'augmentation de l'épaisseur du tube en PRFV de 2,9 à 6,4 mm améliore les rapports de résistance et de déformation de 25 % et 12 %, respectivement. Les résultats indiquent également que les déformations plastiques des colonnes renforcées de PRF sont linéairement proportionnelles aux enveloppes de tension de déchargement (εde,env). La relation dépend un peu du niveau de confinement, mais fortement de la quantité et du type de renfort longitudinal, en particulier lorsque εde,env > 0,0035. D'autre part, une investigation a été menée pour examiner la validité des dispositions de conception disponibles pour prédire la capacité de la charge ultime des colonnes testées. Les résultats de l'analyse ont été comparés avec les valeurs expérimentales. Il a été constaté que les prévisions de l'ACI 440.R1 (2015), CSA S806 (2012), et CSA S6-06 (2010) ont fourni des résultats conservateurs plus élevés pour les échantillons de contrôle en PRFV que celui de l'échantillon d'acier. Cela peut être dû à la négligence de la contribution de la résistance à la compression des barres de PRFV à la capacité de charge axiale. En outre, pour les colonnes de CFFT renforcées de PRF, les prévisions de l'ACI 440.2R (2008), du CSA S806 (2012), et du CSA S6-06 (2010) étaient de 1,68 ± 0,31, 1,57 ± 0,18 et 1,72 ± 0,35 avec un COV de 18,4 %, 11,3%, et 20,5%, respectivement. En considérant les limites des codes de confinement, le code CSA S806 (2012) a révélé les meilleures prévisions pour la capacité de charge ultime basée sur la moyenne que celui du code CSA S6-06 (2010) et de l’ACI 440.2R (2008), en particulier pour les échantillons réalisés avec des tubes de Type B.

[EN] Concrete-filled steel tubular columns composed of double-tube sections are a recent development. This type of sections are the result of applying a technique used in the construction of immersed tunnels, by forming a sandwich composite section made up of steel-concrete-steel. Apart from the own advantages of concrete-filled steel tubular (CFST) columns, double-tube columns present other additional advantages which make them of great interest for designers and engineers, such as: reduced weight, higher ductility, improved energy absorption and, above all, an enhanced fire resistance. In turn, the improvements in the manufacturing technique of high strength concrete makes their use in double-tube columns create the suitable synergy for the design of slender structural members. Nevertheless, the fast evolution and development of composite construction has led to a situation where the current design guidance cannot provide updated analytical expressions and recommendations. Give this situation, and due to the innovative nature of this configuration, as well as the complexity of experimental testing, few experimental and analytical works can be found on this typology of columns. This makes the numerical investigation of concrete-filled double-tube columns interesting from the scientific point of view. The mechanical behaviour of concrete-filled double-tube columns is studied in this thesis through a realistic three-dimensional finite element model. The values adopted for the variables of the model are the result of a comprehensive sensitivity analysis. The accuracy of the numerical model is verified against experimental tests and, once the numerical model is validated, an extensive parametric study is developed, with the aim of studying the influence of the main factors which affect the mechanical response of this columns and being able to provide design recommendations. With this information, a comparative study is carried out with the results of the application of the current calculation method for composite columns in Eurocode 4-Part 1.1, in order to evaluate the validity of the method for double-tube columns subjected to axial compression and combined compression and uniaxial bending, infilled with ultra-high strength concrete. The scope of this thesis is limited to slender circular double-tube columns filled with normal and ultra-high strength concrete subjected to monotonic concentric or eccentric axial load of uniform bending moment.
[ES] Las columnas tubulares de acero rellenas de hormigón compuestas de una sección de doble tubo son de reciente creación. Son el resultado de aplicar la técnica utilizada en la construcción de túneles sumergidos, de construir una sección compuesta tipo sándwich, a base de acero-hormigón-acero. Además de las ventajas propias de las columnas tubulares de acero rellenas de hormigón (CFST), las columnas de doble tubo poseen otras ventajas adicionales que las hace muy interesantes para proyectistas e ingenieros como son, por ejemplo: menor peso, mayor ductilidad, mayor absorción de energía, y sobretodo, mejor resistencia a fuego. Por otra parte, la mejora de la técnica de fabricación de hormigones de altas resistencias hace que su utilización en columnas de doble tubo produzca una sinergia idónea para diseñar elementos estructurales esbeltos. Sin embargo, la rápida evolución y desarrollo de la construcción mixta hace que las normas actuales no tengan las expresiones analíticas ni las recomendaciones de diseño actualizadas. Debido a ello y por la relativa solución innovadora de esta configuración, así como la complejidad de realizar ensayos experimentales, existen pocos trabajos experimentales y estudios analíticos acerca de esta tipología de columnas. Es por esto que el estudio numérico de columnas de doble tubo rellenas de hormigón sea, desde el punto de vista científico, interesante. El comportamiento mecánico de columnas de doble tubo rellenas de hormigón se estudia en esta tesis a través de un modelo realista tridimensional de elementos finitos. Los valores adoptados de las variables del modelo son el resultado de un amplio análisis de sensibilidad. La precisión del modelo numérico se verifica con ensayos experimentales y, una vez se dispone de un modelo validado, se desarrolla un extenso estudio paramétrico con el fin de evaluar la influencia de los principales factores que afectan la respuesta mecánica de las mismas y poder establecer recomendaciones de diseño. Con esa información, se realiza una comparativa con los resultados de aplicar el método de cálculo de columnas mixtas del Eurocódigo 4 - Parte 1.1, con el propósito de evaluar la validez del método en columnas de doble tubo comprimidas y flexo-comprimidas rellenas de hormigón de ultra alta resistencia. El alcance de esta tesis queda limitado a columnas circulares esbeltas de doble tubo rellenas de hormigón de resistencia normal y ultra alta resistencia sometidas a carga monotónica centrada y excéntrica de momento uniforme.
[CAT] Les columnes tubulars d'acer emplenades de formigó compostes d'una secció de doble tub són de recent creació. Són el resultat d'aplicar la tècnica utilitzada en la construcció de túnels submergits, de construir una secció composta tipus sandvitx, a base d'acer-formigó-acer. A més dels avantatges propis de les columnes tubulars d'acer emplenades de formigó (CFST), les columnes de doble tub posseeixen altres avantatges addicionals que les fa molt interessants per a enginyers com són, per exemple: menor pes, major ductilitat, major absorció d'energia, i sobretot, millor resistència a foc. D'altra banda, la millora de la tècnica de fabricació de formigons d'altes resistències fa que la seva utilització en columnes de doble tub produïsca una sinergia idònia per dissenyar elements estructurals verticals esvelts. No obstant això, la ràpida evolució i desenvolupament de la construcció mixta fa que les normes actuals no tinguin les expressions analítiques ni les recomanacions de disseny actualitzades. Per aquest motiu i per la relativa solució innovadora d'aquesta configuració, així com la complexitat de realitzar assajos experimentals, existeixen pocs treballs experimentals i estudis analítics sobre aquesta tipologia de columnes. És per això que l'estudi numèric de columnes de doble tub farcides de formigó siga, des del punt de vista científic, interessant. El comportament mecànic de columnes de doble tub emplenades de formigó s'estudia en aquesta tesi mitjançant un model realista tridimensional d'elements finits. Els valors adoptats per a les variables del model són el resultat d'una extensa anàlisi de sensibilitat. La precisió del model numèric es verifica amb assajos experimentals i, una vegada es disposa d'un model validat, es desenvolupa un extens estudi paramètric amb la finalitat d'avaluar la influència dels principals factors que afecten la resposta mecànica de les columnes i poder establir recomanacions de disseny. Amb aquesta informació, es realitza una comparativa amb els resultats d'aplicar el mètode de càlcul de columnes mixtes del Eurocódigo 4-Part 1.1, amb el propòsit d'avaluar la validesa del mètode en columnes de doble tub comprimides i flexo-comprimides emplenades de formigó d'ultra alta resistència. L'abast d'aquesta tesi queda limitat a columnes circulars esveltes de doble tub emplenades de formigó normal i d'ultra alta resistència sotmeses a càrrega monotònica centrada i excèntrica amb moment uniforme.
Pons Aliaga, D. (2016). ESTUDIO NUMÉRICO DE LA CAPACIDAD PORTANTE DE COLUMNAS MIXTAS CON DOBLE TUBO RELLENAS DE HORMIGON [Tesis doctoral no publicada]. Universitat Politècnica de València. https://doi.org/10.4995/Thesis/10251/62209
TESIS

The use of composite steel-concrete columns and beam-columns in many structural systems is increasing globally due to the intrinsic synergy when these materials are designed and detailed together properly. However, limited test data are available to justify the structural system response factors and comprehensive design equations in current design specifications. This research, through the testing of 18 full-scale, slender concrete-filled steel tube (CFT) beam-columns, attempts to address the latter need. The circular and rectangular CFT specimens tested for this research are by far the longest and the most slender full-scale CFT members tested worldwide. These CFT specimens were subjected to a complex load protocol that includes pure compression, uniaxial and biaxial bending combined with compression, pure torsion, and torsion combined with compression. In addition, data from the hydrostatic pressure on the steel tubes due to the fresh concrete at casting was evaluated. The single most important contribution of this research is the clarification of the interaction between strength and stability in slender composite concrete-filled columns and beam-columns. Parallel to the experimental study, advanced computational analyses were carried out to calibrate material and element models that characterize the salient features of the observed CFT response, such as steel local buckling and residual stresses, concrete confinement, stability effects, strength, and stiffness degradation, among others. Based on the observed behavior, simplified guidelines for the computation of the strength and stiffness parameters for CFT columns and beam-columns are proposed for design purposes.

No
In this work, experimental and numerical investigations were performed on the behaviours of circular concrete filled double steel tubular (CFDST) slender columns and beams, in which the external tube employed stainless steel tube. Eighteen specimens, 12 slender columns and 6 beams, were tested to obtain the failure patterns, load versus deflection relationships and strain developments of stainless steel tube. A finite element (FE) model was developed and verified by experimental results. The validated FE model was then employed to investigate the effects of key parameters, including hollow ratio, eccentric ratio and material strength, on the load-carrying capacity. The load distribution among the components and contact stress between steel tube and sandwiched concrete were also analyzed. Finally, the design methods for CFDST, hollow CFST and solid CFST members with carbon steel external tube respectively suggested by Han et al. (2018), Chinese GB 50936-2014 (2014) and AISC 360-16 (2016) were employed to evaluate their applicability for the circular CFDST slender columns and beams with stainless steel outer tube.
The authors gratefully acknowledge the Shanxi Province Outstanding Youth Fund (No. 201701D211006) and the National Natural Science Foundation (No. 51838008).
The full-text of this article will be released for public view at the end of the publisher embargo on 9th Nov 2021.

Ingeniero Civil
Una columna CFT (“Concrete Filled Tube”) corresponde a un elemento compuesto conformado por un perfil tubular de acero en cuyo interior se coloca hormigón con el fin de aumentar su resistencia y rigidez. El presente proyecto de título trata acerca de la aplicabilidad de columnas compuestas CFT al diseño sísmico de edificios en Chile. Junto con esto se realiza la comparación entre el diseño al utilizar columnas de perfiles doble T y el diseño utilizando columnas compuestas CFT. Para analizar la aplicabilidad del uso de columnas compuestas se realiza el diseño de tres edificios cuyas dimensiones generales en planta son dimensiones observadas en edificios chilenos, diferenciándose sólo en la cantidad de pisos. En el diseño de estos edificios se utilizan paralelamente columnas doble T de acero y columnas compuestas CFT, para así poder comparar los resultados de ambos diseños. En el diseño de los edificios se utiliza la normativa vigente. Luego de haber realizado el diseño de los edificios, se analiza el comportamiento de una de las estructuras diseñadas en base a columnas compuestas CFT mediante dos tipos de análisis: Pushover y Análisis Tiempo-Historia. El modelo involucrado en estos análisis incorpora la no linealidad proveniente de la fluencia del acero tanto de vigas como columnas, así como también la producida por las deformaciones del hormigón en el interior de las columnas CFT. Los resultados obtenidos de estos análisis se comparan con los resultados esperados de acuerdo a la normativa utilizada con el fin de apreciar si los parámetros de diseño empleados son los correctos o deberían ser éstos modificados para obtener resultados consistentes con la filosofía de diseño de la norma chilena para el diseño sísmico de edificios. Los resultados obtenidos en este trabajo de titulación indican que los edificios prototipo con columnas CFT diseñados de acuerdo a la normativa chilena resultan con una sobre resistencia muy alta, lo cual, si bien asegura un buen comportamiento sísmico para un sismo de nivel de diseño, no permite visualizar las ventajas, desde un punto de vista económico, de utilizar este tipo de elementos en estructuras sismorresistentes. En este trabajo de titulación queda en evidencia que el criterio de deformaciones admisibles impuesto por la norma NCh433.Of 96 es demasiado restrictivo para las estructuras analizadas, la relajación de este criterio podría resultar ventajosa económicamente para las estructuras controladas por el corte basal mínimo.

A medida que el hombre ha ido creando nuevas estructuras como grandes edificios. Estos se realizan sin importar que no cumplan con los límites de resistencia impuestos por los materiales tradicionales como son el hormigón armado o las estructuras hechas de perfiles de acero. En éstas estructuras, las columnas forman parte esencial de ellas, ya que a través de éstas se transmiten a la base las acciones a las cuales queda sometida la estructura. Es por esto que para grandes esfuerzos en elementos como columnas, hace un tiempo en países como U.S.A. y Japón, entre otros, se han utilizado desde hace más de veinte años columnas compuestas. Las cuales se encuentran formadas por hormigón armado y además perfiles de acero. Las columnas compuestas pueden ser clasificadas en tres tipos: secciones de acero embebido en hormigón armado (steel reinforced concrete, SRC), secciones circulares de acero rellenas de hormigón armado (concrete filled tuve, CFT) y secciones rectangulares de acero rellenas de hormigón armado (rectangular concrete filled tuve, RCFT). Esta sinergia existente entre ambos materiales entrega distintas ventajas: (1) en columnas CFT o RCFT, el acero incrementa la resistencia y ductilidad del hormigón por su efecto de confinamiento, el hormigón inhibe el pandeo local del perfil de acero y a la vez hace innecesaria la utilización de moldajes; (2) en columnas SRC el hormigón previene el pandeo local del perfil de acero y a la vez sirve como protector del fuego. Ahora, en base a las normativas existentes y las distintas investigaciones desarrolladas en distintos tipos de columnas compuestas. El objetivo principal de la presente memoria de título fue formular recomendaciones de diseño de columnas compuestas de acero y hormigón, sometidas a esfuerzos de tracción, compresión, flexión y corte. Para que puedan ser utilizadas en Chile como recomendaciones de diseño para el ingeniero que desee información más ordenada acerca del diseño de este tipo de columna. El resultado final de diseño propuesto para cada uno de los distintos tipos de columnas sometidas a cada uno de los distintos esfuerzos, fue que las normativas americanas se destacan en sus resultados en comparación con códigos europeos. Y a la vez para aquellos tipos de ensayos en los cuales no se encontró datos experimentales para poder llevar a cabo la comparación, se sugiere al lector diseñar, según el criterio que posea, entre alguna de las dos normas americanas. En base a los resultados obtenidos en la presente memoria de título. Se recomienda llevar a cabo estudios experimentales, de tal forma de obtener en base a datos reales el comportamiento de las columnas compuestas sometidas principalmente a esfuerzos de corte y de flexión pura. Los cuales no se encontraron datos experimentales de algunas columnas para poder llevar a cabo la comparación de cual método sería el apropiado.

This thesis describes a research of the behaviour of the elliptical CFT columns under axial loading. The most substantial part of this research is experimental works conducted on twenty-seven specimens including the hollow stub columns as references. Parameters such as slenderness ratio, uni-axial compressive strength of concrete infill and the aspect ratio were considered to investigate their influence on the behaviour on these columns. The results presented are the first member buckling tests on elliptical CFT columns. Keys results from the tests have been presented and discussed. Parallel with the experimental works, numerical analyses were carried out and verified with the experimental results. Parametric studies were performed following the validation of the numerical models. As there is no design guidance seems to be available in any standard, thus this research provides a review of the existing design standards of Eurocode 4 (EC4) and American Specifications (AISC). The design expressions from these current design provisions for circular, square and rectangular concrete- filled tubes design strengths were used to predict the capacities of elliptical eFT columns. The influences of concrete enhancement, steel reduction due to biaxial effects and column slenderness were all incorporated in design rules of EC4. Based on the experimental, numerical findings the evaluations were made on the design rules of the codes. This investigation was aimed at providing reliable design guidelines for practising engineers to employ the elliptical concrete-infill columns in the construction industry.

This thesis investigates the performance of composite frame structures with smart partially-restrained (PR) concrete filled tube (CFT) column connections through simplified 2D and advanced 3D computational simulations. It also provides a design methodology for new types of innovative connections based on achieving a beam hinging mechanism. These types of connections intend to utilize the recentering properties of super-elastic SMA tension bars, the energy dissipation capacity of low-carbon steel bars, and the robustness of CFT columns. In the first part of this study, three different PR-CFT connection prototypes were designed based on a hierarchy of strength models for each connection component. Numerical simulations with refined three dimensional (3D) solid elements were conducted on full scale PR-CFT connection models in order to verify the strength models and evaluate the system performance under static loading. Based on system information obtained from these analyses, simplified connection models were formulated by replacing the individual connection components with spring elements and condensing their contributions. Connection behavior under cyclic loads was extrapolated and then compared with the monotonic behavior. In the second part of this study, the application of these connections to low-rise composite frames was illustrated by designing both 2D and 3D, 4 and 6 story buildings for the Los Angeles region. A total of 36 frames were studied. Pushover curves plotted as the normalized shear force versus inter story drift ratio (ISDR) showed significant transition points: elastic range or proportional limit, full yielding of the cross-section, strength hardening, ultimate strength, and strength degradation or stability limit. Based on the transition points in the monotonic pushover curves, three performance levels were defined: Design Point, Yield Point, and Ultimate Point. All frames were stable up to the yield point level. For all fames, after reaching the ultimate point, plastic rotation increased significantly and concentrated on the lower levels. These observations were quantified through the use of elastic strength ratios and inelastic curvature ductility ratios. The composite frames showed superior performance over traditional welded ones in terms of ductility and stability, and validated the premises of this research.

[EN] The combination of different materials to form a single structural member to take profit of their individual good characteristics is a successfully established practice in building industry. In concrete filled tubular columns (CFT) the combined action of steel and concrete results in many positive attributes at ambient temperature: high load-bearing capacity with smaller cross-section size, aesthetics, high stiffness and ductility and reduced construction cost. In the last decades, the use of CFT columns in building industry, especially in high-rise buildings, has increased not only because of their positive characteristics at room temperature, but also for their inherent high fire resistance. Besides, CFT sections are greatly versatile given that they admit different types of concrete infill such as plain concrete, bar-reinforced concrete or fiber reinforced concrete; and also a wide variety of shapes. Although the more commonly used shapes are circular, and rectangular, new configurations and shapes are continuously appearing together with innovative materials. The ambient temperature behavior of CFT columns has been deeply studied and, in turn, the investigations dealing with their fire behavior have increased. For its structural analysis, the column can be considered as an isolated member or as a column integrated in a structure interacting with other structural members. The review of the state of the art in the area of CFT columns in fire carried out in the framework of this thesis has pointed out that most works cover the fire response of isolated members and that the existing studies on columns within frames differ in their proposals and conclusions. In this thesis, the fire response of CFT columns is analyzed by means of a fiber beam element model. First, a realistic cross-sectional thermal model is implemented to be integrated in the thermo-mechanical model developed whose accuracy is validated against experimental results after its calibration. Parametric studies are carried out with the aim of investigating the main factors affecting the problem and developing a simple calculation method based on Eurocode 4 and using the concept of equivalent concrete core cross-section. Finally, given the reduced computational cost of the fiber model, the effects of axial and rotational restraint in the fire response of CFT columns are investigated by integrating the heated CFT column within a frame. A parametric analysis is performed in order to draw conclusions about this interaction and contrast the current code provisions. The scope of this thesis is limited to circular CFT columns subjected to concentric axial loads.
[ES] Combinar diferentes materiales en un único elemento estructural para aprovechar las fortalezas individuales de cada uno es una práctica consolidada con éxito en el sector de la construcción. En los pilares tubulares de acero rellenos de hormigón (CFT) la acción conjunta del acero y el hormigón presenta muy buenas cualidades a temperatura ambiente: alta capacidad de carga con secciones pequeñas, buena apariencia, alta rigidez y ductilidad y bajo coste de puesta en obra. En las últimas décadas, el uso de pilares CFT en el sector de la construcción, especialmente en edificios de gran altura, ha aumentado no solo debido a sus buenas características a temperatura ambiente sino también por su inherente alta resistencia al fuego. Además, las secciones de pilares CFT son muy versátiles ya que admiten diferentes tipos de relleno, como hormigón en masa, con armaduras o reforzado con fibras; y también una amplia variedad de formas. Aunque los perfiles tubulares más usados son los circulares y rectangulares, nuevas configuraciones están continuamente en desarrollo junto con nuevos materiales. El comportamiento de los pilares CFT a temperatura ambiente ha sido ampliamente estudiado y, a su vez, las investigaciones sobre su comportamiento a fuego han aumentado. Para su análisis estructural, el pilar puede ser considerado como un elemento aislado o como un elemento integrado en una estructura que interactúa con otros elementos estructurales. La revisión del estado del arte en el área de los pilares CFT sometidos a fuego llevado a cabo en el marco de esta tesis ha puesto de manifiesto que la mayoría de los trabajos cubren la respuesta a fuego de elementos aislados y que los estudios existentes sobre pilares en estructuras difieren en sus propuestas y conclusiones. En esta tesis, la respuesta a fuego de pilares CFT se analiza por medio de un modelo de elementos viga con integración por fibras. En primer lugar, se implementa un modelo térmico realista para ser integrado en el modelo termo-mecánico desarrollado cuya precisión se valida con resultados experimentales después de ser calibrado. Un estudio paramétrico se lleva a cabo con el objeto de estudiar los principales factores que afectan al problema y desarrollar un modelo simplificado de cálculo basado en el Eurocódigo 4 Parte 1-1 y que emplea el concepto de sección equivalente del núcleo de hormigón. Finalmente, dado el reducido coste computacional del modelo de fibras, se investigan los efectos de la restricción axial y rotacional en la respuesta frente al fuego de los pilares CFT integrando la columna expuesta a fuego dentro de una subestructura. Se ejecuta un estudio paramétrico para extraer conclusiones sobre esta interacción y contrastar las prescripciones de la normativa actual. El alcance de esta tesis queda limitado a pilares CFT sin protección externa, de sección circular y sometidos a carga axial centrada.
[CAT] Combinar distints materials en un únic membre estructural per a aprofitar les fortaleses individuals de cada u és una pràctica consolidada amb èxit en el sector de la construcció. En els pilars tubulars d'acer omplerts de formigó (CFT) l'acció conjunta de l'acer i el formigó presenta molt bones qualitats a temperatura ambient: alta capacitat de càrrega amb seccions xicotetes, bona aparença, alta rigidesa i ductilitat i baix cost de posada en obra. En les últimes dècades, l'ús de pilars CFT en el sector de la construcció, especialment en edificis de gran altura, ha augmentat no sols degut a les seues bones característiques a temperatura ambient, sinó també per la seua inherent alta resistència al foc. A més, les seccions de pilars CFT són molt versàtils, doncs admeten distints tipus de farcit, com formigó en massa, amb armadures o reforçat amb fibres; i també una àmplia varietat de formes. Encara que els perfils tubulars més usats són els circulars i rectangulars, noves configuracions estan contínuament en desenvolupament junt amb nous materials. El comportament dels pilars CFT a temperatura ambient ha sigut àmpliament estudiat i, al mateix temps, les investigacions sobre el seu comportament a foc han augmentat. Per a la seua anàlisi estructural, el pilar pot ser considerat com un element aïllat o com un element integrat en una estructura que interactua amb altres elements estructurals. La revisió de l'estat de l'art en l'àrea dels pilars CFT sotmesos a foc, duta a terme en el marc d'aquesta tesi, ha posat de manifest que la majoria dels treballs cobreixen la resposta a foc d'elements aïllats, i que els estudis existents sobre pilars en estructures difereixen en les seues propostes i conclusions. En aquesta tesi, la resposta a foc de pilars CFT s'analitza mitjançant un model d'elements biga amb integració per fibres. En primer lloc, s'implementa un model tèrmic realista per a ser integrat en el model termo-mecànic desenvolupat, i la seua precisió es valida amb resultats experimentals desprès de ser calibrat. Un estudi paramètric es du a terme amb l'objectiu d'estudiar els principals factors que afecten al problema i desenvolupar un model simplificat de càlcul basat en l'Eurocodi 4, Part 1-1 i que empra el concepte de secció equivalent del nucli de formigó. Finalment, tenint en compte el reduït cost computacional del model de fibres, s'investiguen els efectes de les restriccions axial i rotacional en la resposta a foc dels pilars CFT integrant la columna exposada a foc dins d'una subestructura. Un estudi paramètric s'executa per a obtenir conclusions sobre aquesta interacció i contrastar les prescripcions de la normativa actual. L'abast d'aquesta tesi queda limitat a pilars CFT sense protecció externa, de secció circular i sotmesos a càrrega axial centrada.
Ibáñez Usach, C. (2016). FIRE RESPONSE ANALYSIS OF CIRCULAR CONCRETE FILLED TUBULAR COLUMNS AND THE EFFECTS OF AXIAL AND ROTATIONAL RESTRAINTS [Tesis doctoral no publicada]. Universitat Politècnica de València. https://doi.org/10.4995/Thesis/10251/60150
TESIS
Premiado

El empleo de estructuras mixtas de acero-hormigón ha experimentado un aumento creciente en las últimas décadas, dado qu permiten un mejor aprovechamiento de las características de los materiales empleados, en comparación con estructuras de hormigón armado y acero trabajando de forma independiente. Este trabajo de investigación se centra en el estudio y análisis de pilares mixtos de tipo perfil tubular de acero, CFT (Concrete Filled Tube), rectangular o cuadrado, rellenos de hormigón de alta resistencia. Este tipo de pilares cuenta con ventajas respecto a otros pilares mixtos como el hecho de que el perfil de acero sirve de enconfrado para el núcleo de hormigón y a su vez, cuenta con suficiente capacidad resistente como para soportar las cargas exixtentes en fase de construcción. Todo ello permite un importante ahorro, tanto económico, como en las plazos de ejecución. Otras ventajas de estos pilares es que el perfil tubular confina el núcleo de hormigón y este a su vez evita que se produzca el pandeo local del tubo de acero, permitiendo así su plastificación y máximo aprovechamiento. Estas ventajas, proporcionan a los soportes ductibilidad característica muy importante frente a la acción sísima, de ahí que el uso de este tipo de pilares se haya extendido en países como Japón, Estados Unidos y China. Es importante, también, destacar el aumento de resistencia al fuego del sistema en comparación con un perfil de acero hueco. A partir del análisis del estado actual del conocimiento se ha planteado una campaña experiemental que cubre los aspectos no estudiados hasta el momento, y que constituyen los casos más habituales en la práctica real: soportes esbeltos o semiesbeltos sometidos a carga axial y diagrama de momentos no constante. En total se han efectuado 78 ensayos sobre pilares mixtos. Los resultados experimentales se comparan con las previsiones de las normativas de referencia más importantes para el cálculo de pilares mixtos: EC4, AISC 2010, BS5-400:2005, AS5100-6, DBJ13-51, etc, comprobando su validez para el tipo de pilar estudiado. Finalmente se ha planteado un modelo de cálculo simplificado basado en el método de amplificación de momentos y se ha ajustado el factor de rigidez a flexión de los pilares, así como un factor de conversión a diagrama de momentos constantes.
Hernández Figueirido, D. (2012). Estudio experimental del pandeo de perfiles tubulares rectangulares de acero, rellenos de hormigón de alta resistencia, bajo carga axial y diagrama de momentos variables [Tesis doctoral]. Editorial Universitat Politècnica de València. https://doi.org/10.4995/Thesis/10251/14724
Palancia

This paper is an attempt to study the behavior of axially loaded concrete filled steel tubular (CFST) stub columns with special-shaped cross-sections, i.e. triangular, fan-shaped, D-shaped, 1/4 circular and semi-circular. A total of forty-four specimens including CFST stub columns and reference hollow steel tubular stub columns were tested. The effects of the changing steel tube wall thickness and the infill of concrete on the behavior of the composite columns were investigated. The results showed that the tested special-shaped CFST stub columns behaved in a ductile manner, and the composite columns showed an outward local buckling model near the middle section. Generally, the failure modes of these five kinds of special-shaped specimens were similar to those of the square CFST stub columns. Finally, simplified model for predicting the cross-sectional strength of the special-shaped CFST sections was discussed and proposed.

No
This paper presents a series of test results of elliptical concrete filled steel tubular (CFST) beams and columns to explore their performance under bending and compression. A total of twenty-six specimens were tested, including eight beams under pure bending and eighteen columns under the combination of bending and compression. The main parameters were the shear span to depth ratio for beams, the slenderness ratio and the load eccentricity for columns. The test results showed that the CFST beams and columns with elliptical sections behaved in ductile manners and were similar to the CFST members with circular sections. Finally, simplified models for predicting the bending strength, the initial and serviceability-level section bending stiffness of the elliptical CFST beams, as well as the axial and eccentric compressive strength of the composite columns were discussed.

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.

This paper presents a non-linear finite element model (FEM) used to predict the behaviour of slender concrete filled steel tubular (CFST) columns with elliptical hollow sections subjected to axial compression. The accuracy of the FEM was validated by comparing the numerical prediction against experimental observation of eighteen elliptical CFST columns which carefully chosen to represent typical sectional sizes and member slenderness. The adaptability to apply the current design rules provided in Eurocode 4 for circular and rectangular CFST columns to elliptical CFST columns were discussed. A parametric study is carried out with various section sizes, lengths and concrete strength in order to cover a wider range of member cross-sections and slenderness which is currently used in practices to examine the important structural behaviour and design parameters, such as column imperfection, non-dimension slenderness and buckling reduction factor, etc. It is concluded that the design rules given in Eurocode 4 for circular and rectangular CFST columns may be adopted to calculate the axial buckling load of elliptical CFST columns although using the imperfection of length/300 specified in the Eurocode 4 might be over-conservative for elliptical CFST columns with lower non-dimensional slenderness.

no
Tapered concrete filled double skin steel tubular (CFDST) columns have been used in China for structures such as electricity transmission towers. In practice, the bearing capacity related to the connection details on the top of the column is not fully understood. In this paper, the experimental behaviour of tapered CFDST stub columns subjected to axial partial compression is reported, sixteen specimens with top endplate and ten specimens without top endplate were tested. The test parameters included: (1) tapered angle, (2) top endplate thickness, and (3) partial compression area ratio. Test results show that the tapered CFDST stub columns under axial partial compression behaved in a ductile manner. The axial partial compressive behaviour and the failure modes of the tapered CFDST stub columns were significantly influenced by the parameters investigated. Finally, a simple formula for predicting the cross-sectional capacity of the tapered CFDST sections under axial partial compression is proposed.

Rectangular thin-walled concrete-filled steel tubular (CFST) slender columns under axial and eccentric loads may undergo local and global interaction buckling when exposed to fire. Computational studies on the fire and post-fire behavior of rectangular short and slender CFST columns including local buckling effects have been extremely limited. This thesis presents new computational models for predicting the responses of rectangular and square CFST short and slender columns under fire exposure and after being exposed to fire. The models incorporate important features, which include local and global interaction buckling, air gap between the steel tube and concrete, concrete moisture content, emissivity of exposure surfaces, initial geometric imperfections, second-order, and material nonlinearities at elevated temperatures. Computational models are formulated by using the fiber approach for simulating the fire resistance, fire behavior and post-fire performance of rectangular CFST short and slender columns loaded concentrically and eccentrically. The progressive local buckling of steel tube walls at elevated temperatures is included in the formulation by using the local and post-local buckling models proposed. Computer simulation procedures sequentially coupling the nonlinear thermal and stress analyses are developed. The temperature distribution within a CFST column exposed to fire is determined by the thermal analysis. The modeling procedures capture the axial load-strain behavior, axial load-deflection responses, and fire-resistance of loaded CFST columns exposed to fire. Numerical solution algorithms implementing Muሷller’s method are developed to solve the nonlinear equilibrium equations of loaded CFST columns under fire exposure. The existing experimental and numerical results are utilized to validate the fiber-based computational models, which are employed to study the fire and post-fire responses of CFST short and slender columns with various important parameters. It is shown that the computational models are capable of predicting well the responses of rectangular CFST short and slender columns exposed to fire and after being exposed to fire. The computed results on the fires resistance and fire and post-fire behaviors of CFST rectangular columns with local buckling effects are given in the thesis for the first time. The research findings presented provide a better understanding of the fire and post-fire performance of short and slender CFST columns incorporating local buckling, and are valuable to structural designers and composite code writers.

博士
國立臺灣大學
土木工程學研究所
90
Abstract The objective of this research is to investigate the seismic performance of rectangular RC columns retrofitted by external jackets for the bridge and building columns under low and high axial stresses, respectively. The research also focuses on the mechanical behavior of the double-skinned concrete filled steel tubular (DSCFT) columns with a large diameter-thickness (D/t) ratio. For the retrofit of rectangular RC columns, this research focuses on the seismic retrofit to prevent lap-splice or shear failure of bridge columns designed according to the pre-1987 Taiwan standards. In this research, the octagonal steel-jacketing techniques improving the seismic performance of rectangular reinforced concrete bridge columns have been developed. Effectiveness of using the elliptical and rectangular steel jackets on seismic retrofit of rectangular columns is also assessed and validated by the tests. Tests conducted on the 0.4-scale specimens confirm that seismic performance of rectangular RC bridge columns can be significantly and equally enhanced by elliptical or octagonal steel jacket. Rectangular steel jacketing can improve shear strength, but its deficiency in improving seismic flexural performance is evidenced. Test results of the full scale specimen indicate that the proposed octagonal steel jacketing scheme can successful prevent the lap-splice failure for real bridge applications. Tests confirm that a retrofit scheme excellent in performance but with a smaller cross-sectional area than that in the elliptical jacketing has been successfully developed. Axial compression test results for square RC columns incorporating various kinds of jacketing schemes and Taiwanese construction practice in the placement of stirrups are also presented. The jacketing schemes include circular, octagonal and square shapes. The jacketing materials vary from steel plate to carbon fiber reinforced polymer (CFRP) composites. It is found from the monotonic axial load test results that the failure mode of the benchmark non-retrofitted specimen is identical to the real damage cases observed in the 1999 Chi-Chi Taiwan earthquake. The benchmark specimen developed its design strength but a non-ductile failure mode occurred soon after the peak load was reached. Among the retrofitted specimens, the steel jacketed specimens exhibit not only greatly enhanced load carry capacity but also excellent ductility performance. Test results show that CFRP sheets are effective in increasing the column axial strength, but the sheets could fracture suddenly in high strain conditions due to their brittle material characteristics. Test results indicate that CFRP sheet wrapping in general is not as effective as steel jacketing in improving the axial ductility capacity of RC columns. However, the proposed octagon-shaped CFRP wrapping scheme exhibits an improved performance compared to rectangular-wrapped columns using the same layers of CFRP sheets. Tests confirm that all octagonal steel or CFRP jacketed specimens have axial loading capacities more than 2 times the nominal capacity. In the study of DSCFT columns, the diameter-thickness (D/t) ratio and the hollowness ratio were chosen as main parameters in designing the specimens. A total of 18 specimens were tested under varied combinations of axial and flexural loads, and two specimens were tested under a combination of constant axial load and cyclically increasing bending for comparison. Test results concluded that the DSCFT columns can effectively provide strength and deformation capacity even with a large D/t ratio. The DSCFT columns can have an optimal strength performance if the applied axial load is less than 25% of the axial capacity.

Yes
This paper presents the behaviour of circular concrete-filled double-skin steel tubular (CFDST) stub columns compressed under concentric axial loads. To predict the performance of such columns, a finite element analysis is conducted. Herein, for the accurate modelling of the double-skin specimens, the identification of suitable material properties for both the concrete infill and steel tubes is crucial. The applied methodology is validated through comparisons of the results obtained from the finite element analysis with those from past experiments. Aiming to examine the effect of various diameter-to-thickness (D/t) ratios, concrete cube strengths and steel yield strengths on the overall behaviour and ultimate resistance of the double-skin columns, a total of twenty-five models are created to conduct the parametric study. In addition, four circular concrete-filled steel tubes (CFST) are included to check the dissimilarities, in terms of their behaviour and weight, when compared with identical double-skin tubes. A new formula based on Eurocode 4 is proposed to evaluate the strength of the double-skin specimens. Based on the comparison between the results derived from the analysis, the proposed formulae for the concrete filled double-skin would appear to be satisfactory.

碩士
國立交通大學
土木工程系
90
This study investigates the beam-column behavior of high-strength concrete-filled tube. Twenty-eight specimens were tested to study the effects of the width-to-thickness ratio (B/t= 40 and 70), the axial load level (P/Pn = 0.0, 0.2, 0.3, 0.4, 0.5), cross section shape (round or rectangular), and proposed stiffening scheme on the stiffness, ultimate strength, and ductility of CFT columns. Moreover, sectional analysis method with different confined concrete models was also used to compute moment to curvature curves with the comparison to the experimental results. The experimental results indicate that the proposed stiffening scheme can significantly enhance the ultimate strength and ductility of square CFT columns. Current specifications (AIJ, EuroCode4) overestimate the ultimate strength, Mu, of the specimens with B/t = 70 but underestimate that for B/t = 40. The ductility of the specimens significantly decreases with the increase of the applied axial load level. The results obtained from sectional analysis method could well match with the experimental results by choosing appropriate parameters in Saenz’s concrete model.

碩士
國立臺灣科技大學
營建工程系
107
The calculation of the punching shear strength of the CFT column-slab is not clear due to the difference in material. The purpose of this study is to get the punching shear strength of the CFT column-slab and how to effectively increase its strength. In this study, three kinds of reinforcing members were proposed, which were arranged around the tube of the same section to improve the punching shear strength of the CFT column-slab. (a) It is reinforced with steel plate; (b) It is reinforced with which made of angle steel and stiffener like column capital; (c) It is reinforced with which made of channel steel and stiffener like invisible drop panel. In this study, the load test of the six column-slab specimens was completed. According to the test results, the following conclusions and recommendations can be obtained: The experimental results show that: (1) The punching shear strength of the CFT column-slab is greater than the RC column- slab in the same column section. (2) This study proposes that all three kinds of reinforcing members can effectively improve the CFT column-slab punching shear strength. (3) According to the recommendations for the reasonable assumption of the bearing area, the CFT column-slab and which arranged the reinforcing member can be evaluated conservatively by the formula of code. (4) The coefficient of shear friction between the CFT tube and slab is greater than 0.6.

博士
國立中興大學
土木工程學系
94
ABSTRACT In AISC Specifications, the evaluation of CFT (Concrete-Filled Tube) columns is performed in the same way as for ordinary structural steel compression members, using the same formulas but with values of Fy and E being modified. For normal-weight concrete, the use of in-filled concrete strength fc´ in AISC-LRFD (1999) has been limited to 21  55 MPa (3  8 ksi), the new limit of 21  70 MPa (3  10 ksi) is used in the new 2005 AISC-LRFD Specification. For light-weight concrete, the limit is not less than 28 MPa (4 ksi), and the revised limit of 21  42 MPa (3  6 ksi) is applied in the 2005 AISC-LRFD Specification. A total of 66 specimens including 5 HSS and 61 CFT columns are investigated through the tests with various concrete strengths. The CFT design strength evaluated by AISC Specifications as compared with the test results are in good agreement. The design strength of a CFT column with higher strength concrete estimated from the 2004 EC 4, 2004 AS-5100, or 2001 CSA S16-01 seems to be less conservative and the ACI 318-05 is more conservative as compared with the new 2005 AISC-LRFD. The study finds that the 1999 LRFD tends to penalize these CFT columns with higher concrete strength 61.4 ~ 84 MPa (8.9 ~ 12.2 ksi). The new 2005 AISC Specifications is more justifiable based on the results concluded from this study. For CFT columns with light-weight concrete, the removal of the 4 ksi limitation (LRFD, 1999) and changing the limitation to 3 ksi is suggested in this study. Light-weight concrete strength limit of 21  42 MPa (3  6 ksi) as specified in the new 2005 AISC Specifications will move the CFT design strength closer to the actual level. The design strength of a CFT column with light-weight concrete estimated from the 2004 EC 4, 2004 AS-5100, or 2001 CSA S16-01 seems to be less conservative and the ACI 318-05 is more conservative as compared with the new 2005 AISC-LRFD.

The use of composite construction has drawn more and more attention in recent decades. This thesis contains a number of journal articles which aim to enrich the knowledge of the performance of concrete filled tubular columns when subjected to blast loading. Experimental investigations are used in conjunction with numerical analysis to provide a thorough assessment of the blast-resistance of concrete filled tubular columns. The first chapter mainly focuses on the experimental study on concrete filled tubular columns under blast loading. A large-scale blast experimental program is carried out on concrete filled double-skin steel tube (CFDST) columns. The blast experiment aims to examine the blast-resistance of ten CFDST specimens, including five with square cross-section and the other five with circular cross-section. The parameters that are investigated during the blast experiment include: cross-sectional geometry, explosive charge weight and magnitude of axial load. After the experiment, several damaged test specimens are then transported back to the laboratory for residual axial load-carrying capacity tests. The proposed CFDST columns are able to retain more than 60% of its axial load-carrying capacity even after being subjected to close-range explosion. As blast experiments are often costly and associated with potential safety concerns, numerical tools have been adopted by more and more researchers. In the second chapter of the thesis, numerical approaches in modelling the dynamic behaviour of concrete filled steel tube (CFST) columns and CFDST columns under blast loading are presented. The numerical models are validated against the results of the blast experiment as described in the first chapter and good agreement is achieved. Parametric studies on the effect of column dimensions and material properties are also discussed through intensive numerical simulations. In the last chapter, a numerical method to generate pressure-impulse diagrams for CFDST columns is proposed which uses a damage criterion involving the residual axial load-carrying capacity. Based on the numerical method, pressure-impulse diagrams for different column configurations are derived and analytical expressions of deriving pressure-impulse diagrams for CFDST columns are also developed through regression analysis.
Thesis (Ph.D.) (Research by Publication) -- University of Adelaide, School of Civil, Environmental & Mining Engineering, 2017.

碩士
國立中興大學
土木工程學系
92
The AISC analysis of composite columns is done in the same way as for ordinary structural steel compression members, using the same equations from AISC Chapter E, but with values of Fy, E, and r that have been modified through the use of numerical coefficients c1, c2 and c3 . These coefficients c1, c2 and c3 are used to transform plain concrete into equivalent steel with the limitations that the compressive strength of concrete must be ranged between 3 and 8 ksi. Because the high strength of concrete has been developed in the past decade, the use of high strength of concrete is getting popular. This study is intended to see if AISC composite column formulas can still be applied through a series of axial compressive tests when the concrete strength is increased exceed the upper limit of 8 ksi. There are 22 CFT specimens consisting of four groups with fc' = 4, 9, 10 and 12 ksi. According to the axial compressive tests, it can be concluded that the average ratios of Ptest / Pn are 1.43, 1.46, 1.54 and 1.59 for fc' = 4, 9, 10 and 12 ksi, respectively. The result shows that the LRFD formulas of high-strength CFT columns with 8≦fc'≦12 ksi is fit and the formula of calculating the buckling strength of CFT columns is also on the favor side of practicing engineers. A proposed linear superposition model also provided to predict the actual buckling strength of CFT columns. The results are quite encouraging when the strain ε is smaller than εfc' , and the compressive strength of CFT columns is in striking increase due to the confined effect of concrete in the CFT column when ε is larger than εfc'..

碩士
國立成功大學
土木工程學系碩博士班
91
This thesis uses ABAQUS to analysis and study concrete filled steel tubes (CFT) .And see the behavior when it is loaded by bending moment under axial force. We use the high-strength concrete (HSC) and compare between the experiment data and analysis result. By this way we can understand the CFT behavior under the condition. Steel tube can provide confinement to the concrete, and concrete has softening phenomenon. Further, we wish to understand the influence of mechanics behavior between different geometry section. In the thesis we separate specimen to six types. They are circular section (the ratio of diameter over thickness 40 and 70), square untied section (the ratio of width over thickness 40 and 70) , square untied section (the ratio of width over thickness 40 and 70) .In the thesis of Wu IM(2001) ,he had the simulation for circular section and square untied section. In the thesis of Chen CL (2002), he added twenty-two specimens to simulate. So in the thesis, we use their approach of simulation, and use HSC instead of normal-strength concrete (NSC). In the diagram of moment —curvature, we can compare the result with the ratio of diameter (width) over thickness 40 and 70. The strength of the ratio of diameter (width) over thickness 40 is better than The strength of the ratio of diameter (width) over thickness 70. If the structure is the same ,the more axial force, the worse toughness. The toughness of circular section is best, the next is the square tied section, the next is square untied section. The confinement of the ratio of diameter (width) over thickness 40 is better than the confinement of the ratio of diameter (width) over thickness 70. If the structure is the same ,the more axial force, the better confinement in the ratio of diameter (width) over thickness 70, but the worse confinement in the ratio of diameter (width) over thickness 40. The reason is the influence of concrete’s brittleness. The confinement of circular section is best, the next is the square tied section, the next is square untied section. From the result, we can know when specimens are loading by bending moment ,the lower part is acted by tension and influence the effect of the confinement obviously. Square untied section have geometry discontinuity and have the phenomenon of local buckling. In order to improve the problem, we use the approach of octagonal tied. From the result, we can know the phenomenon of softening . With the increment of the axial force, the concrete softens obviously.

博士
國立臺灣大學
土木工程學研究所
88
Concrete filled tube (CFT) column usually possesses high axial load and high ductility characteristics that distributes to the contribution of confinement by steel tube. However, for a seismic structural system with the CFT columns, it is necessary that beam-to-column connections possess sufficient strength and ductility. In traditional internally or externally stiffened connections for CFT columns, the complete penetration welds were used to connect beam flange to column or diaphragm. Past research experience and earthquake damages indicate that fractures on these connections frequently occurred at the complete penetration welds or the adjacent base material. The premature fractures are the key factor for the poor plastic rotation capacity of connections. In addition, internal diaphragms in the steel tube column reduce the effectiveness of concrete filling work. Past test results on steel beam to RC column joints using internal clamped diaphragm connection show that beam plastic rotation can exceed 0.03 radian without any fracture occurring in the fillet weld or base material. Related analytical study illustrated that stiffness of beam-to-steel tube column connection using external clamping diaphragm is higher than the stiffness of connection with internal diaphragm. It clears the concept under most engineers’ mind that the connection’s stiffness with external diaphragms is always less one using internal diaphragm. In this dissertation, the beam-to-CFT column connections using external clamping diaphragms (ECDs) including reentrant and extended configurations are assessed. There are thirteen full scale beam-to-CFT column connection specimens that include three ones with extended ECDs and ten ones with reentrant ECDs. Experimental failure modes indicate that special end return fillet welds between diaphragm and beam flange effectively prevent the tearing fracture of the fillet weld. Further, if the difference in thickness between the diaphragm and the beam flange is too excessive, the diaphragm bending deformation is more difficult to conform with the beam deformation. The possibility of tearing fractures of the weld will increase and result in reduction of plastic rotation capacity of the connections. Test results also indicate that the stiffness of extended ECD connection details is smaller than the ones with the reentrant ECDs. Therefore, the connections using external reentrant ECD are not only more economical but also perform better than the ones using extended ECDs. The experimental results demonstrate that properly constructed connections using the reentrant ECD details can provide a plastic rotation greater than 3% radian, even up to 5% radian and at least 2.31% radian. Simultaneously, Based on nonlinear dynamic analysis results for a six-story building with CFT column MRF, the demand plastic rotation of 1.89% radian for the building subjected to earthquake records of Taipei basin with PGA of 250gal. In addition, the well proportioned beam-to-CFT column connections with ECD details can stably develop beam plastic moment strength with an average strain-hardening factor of 1.16. For practical designs, a proposal value of strain hardening factor for the connection is at least 1.2. Further, the “fracture-yielding-yielding” block shear model, proposed in this paper, can exactly predict the fractured block shear strength in beam flange. In addition, test and analytical results also find that the utlimate shear strength of panel zone in the CFT column may not be directly superposed by the ultimate shear strengths of concrete and steel tube’s webs. This is because the shear strength of concrete has been fully developed before the average shear deformation of panel zone reaches the yield shear strain of steel webs. Moreover, the shear strength of panel zone is almost provided by steel web as the average shear deformation of panel zone reaches the yield shear strain of steel webs.

碩士
國立臺灣科技大學
營建工程系
101
Concrete filled steel tubular (CFT) columns have advantages in strength and ductility. Even under fire attacks, the core concrete could maintain its axial load capacity and thus the strict requirement for fire proof may be liberated. Furthermore, recycling of steel tubes is relatively easy. Typical CFT columns are in the form of square tubes or circular pipes as required by architectural restrictions. Unlike the widely-used box-section columns, the use of circular CFT columns has been limited due to the complexity of the connections to such columns. Recently, skylines in modern cities continue to rise because of urban renewal. The columns in the lower stories in a high-rise building have to sustain high axial load and bending moment. CFT columns have advantages over conventional l and RC columns because the steel tube serves as formwork and offers superior confinement to the infilled concrete, thus improving its strength and ductility under high axial load. However, the complex design and detailing for moment connections have to be further improved, simplified, and verified with experiments. This research proposed a beam-flange-through-type beam-column joint connection for CFT columns and tested four beam-column joint specimens to examine the effect of infilled concrete, beam flange stiffeners, and width of beam on the joint shear strength. Construction of the specimens showed that the proposed connection details are practical and easy to be implemented. Cyclic loading test results showed that the infilled concrete significantly increases the joint shear strength. The use of beam-flange stiffeners and increasing the beam width also have significant contribution to joint shear strength. Current shear strength provisions in the SRC code can be conservatively used to estimate the shear strength of the proposed beam-column joint. However, the shear strength contribution from concrete is significant under-estimated. This research proposed a strut-and-tie joint shear strength model for concrete joint shear strength. Comparison with the test results showed that the proposed model can accurately estimate the shear strength contribution from concrete of the proposed beam-column joint. However, the shear strength contribution from concrete is significantly under-estimated. This research proposed a softened-strut joint shear strength model for concrete joint shear strength. Comparison with the test results showed that the proposed model can accurately estimate the shear strength contribution from concrete of the proposed beam-column joint. Based on experimental obervations and analytical studies, modification to the current code provision on joint shear strength contribution from concrete is proposed. Moreover, the upper and lower limits on the width of the beam flange are proposed to address the constructibility issue related to concrete infilling and shear transfer from the beam to the joint. Furhermore, design suggestions on the beam flange stiffeners are proposed.

no
The behaviour of circular concrete-filled steel tubes (CFT) with various concrete strengths under axial load is presented. The effects of steel tube thickness, the bond strength between the concrete and the steel tube, and the confinement of concrete are examined. Measured column strengths are compared with the values predicted by Eurocode 4, Australian Standards and American Codes. 15 specimens were tested with 30, 60 and 100 N/mm2 concrete strength, with a D/t ratio from 22.9 to 30.5. All the columns were 114 mm in diameter and 300 mm in length. The effect due to concrete shrinkage is critical for high-strength concrete and negligible for normal strength concrete. All three codes predicted lower values than that measured during the experiments. Eurocode 4 gives the best estimation for both CFT with normal and high-strength concrete.

博士
國立臺灣大學
土木工程學研究所
95
From researches, the deformation capacity of the conventional beam-column connections is seriously inadequate. The brittle failure usually occurs at the welding zone. The design of bolted beam-column connections is that welding is carried out at the shop and bolting at the site, thus the guality of the welding can be assured. Moreover, wing plates are welded at beam ends in bolted beam-column connections to make sure that plastic hinges are away from the welding zone. Researches proved that the bolted beam-column connections have excellent seismic resistance satisfying seismic design codes of Taiwan and the US as long as the width-to-thickness of columns, end-plate thickness and wing-plate width suffice for the corresponding limits. In this dissertation, the finite element program ABAQUS is adopted to analyze the influences of wing plates, end plates and stiffeners between flanges to bolted connections, and the seismic hehavior of bolted beam-column connections. Through the convergence analyses collocating the analysis procedure “Riks”, the numerical load-displacement envelope curves are close to the experimental envelope curves. From numerical results, it is proved that the plastic hinge is away from the welding zone as long as the wing-plate width and length are allowable from caculating the design formulas; the brittle fracture of the end plates can be prevent as long as the end-plate thickness is allowable from caculating the design formulas and the upstanding ribs are used; the occurrence of the plastic hinges can be delayed as long as the stiffeners are welded between flanges. For uni-directional bolted connections of rectangular CFT structures, numerical results prove that the design of width-to-thckness ratio of the steel tube can change the main energy-dissipation component. The energy is dissipated by beams when the width-to-thckness ratio is smaller. The energy is dissipated by the panel zone when the width-to-thckness ratio of the steel tube is larger. The energy can be dissipated by the panel zone and the beam at the same time when the width-to-thckness ratio of the steel tube is critical. For bidirectional bolted connections of rectangular CFT structures, numerical results prove that the strength and stiffness of the panel zone can be improved by the pre-stresses of the bidirectional bolts, thus the main energy-dissipation component is the beam. For circular CFT structures, numerical results prove the feasibility of the bolted beam-column connections. Overall, the bolted beam-column connection is certainly a kind of excellent seismic resistant connection.

碩士
國立交通大學
土木工程系所
93
This study aims to investigate application of post-tensioning technology in structural building systems to prevent unexpected brittle failure of steel welded beam-to-column moment connections, as observed during the 1994 Northridge earthquake. Unlike the moment connection, the post-tensioned beam-to-column connections are joined through the use of high-strength strands, without welding. This study investigates the effects of parameters of the post-tensioned connections with energy dissipating bars on the cyclic behavior. The parameters studied include the diameter and the length of energy dissipating bars, the initial post-tensioning force, and the amount of the post-tensioning strands. The behavior of post-tensioned steel beam-to-CFT column connections with energy dissipating bars has been investigated analytically and experimentally. Experimental results demonstrated that the connections possess self-centering response, gap opening and closing at the beam-to-column interface, energy dissipation characteristics, and no inelastic or residual deformation. Moreover analytical predictions correlated well with the experimental results.

碩士
國立臺灣科技大學
營建工程系
101
Concrete filled steel tubular (CFT) columns have advantages in strength and ductility. Even under fire attacks, the core concrete could maintain its axial load capacity and thus the strict requirement for fire proof may be liberated. CFT columns have advantages over conventional type and RC columns because the steel tube serves as formwork and offers great confinement to the filled-concrete. As a result, strength and ductility under high axial load will be improved. However, the complex design and detailing for moment connections have to be further improved, simplified, and verified with experiments. This research proposed a continuous beam flange type of the steel beam to circular CFT column connection. Four beam-column joint specimens are tested to examine the effect of filled-concrete, beam flange stiffeners, and width of beam flange on the joint shear strength. Construction of the specimen shows that the proposed connection details are practical and easy to be implemented. Cyclic loading test results show that the filled-concrete significantly increases the joint shear strength. The use of stiffener plates and increase of the flange width also have significant contribution to joint shear strength. In this research, the proposed shear strength calculation of the CFT beam column joint is based on the strength superposition principle of steel tube strength and concrete strength. The method for calculate the steel shear strength is based on the Von Mises yield criterion. While, the method for calculate the concrete shear strength is based on softened strut model with confinement effect consideration. The critical face of nodal zone is at the face which is in contact with the beam flange. Moreover, a method to evaluate the area of this face is affected by beam flange width, stiffener dimension, and column thickness. Furthermore, the prediction shear strength values are close to the test results, it means that this proposed method can be used to accurately evaluate the joint shear strength.

Concrete-filled double steel tubular (CFDST) columns are high-performance composite columns, which have increasingly been used in high-rise composite buildings and bridges as well as in strengthening conventional concrete-filled steel tubular (CFST) columns. The additional confinement provided by the inner circular tube in CFDST columns considerably improves their strength and ductility compared to CFST columns. However, research studies on the behavior of CFDST columns have been very limited and no design rules are given in current design codes. This thesis presents experimental and numerical studies on the fundamental behavior of circular and rectangular CFDST short and slender columns subjected to axial compression, combined axial load and bending, and preloads. Experiments on the behavior of square CFDST short columns with circular inner tube, circular CFDST short columns with circular inner tube and rectangular CFDST short columns composed of inner rectangular tube loaded concentrically and eccentrically are undertaken. Fiber-based mathematical models are developed for predicting the structural responses of CFDST short and slender columns under various loading conditions. The formulations of the mathematical models consider the influences of concrete confinement, geometric and material nonlinearities, and local buckling. New confining pressure models are proposed based on test results for ascertaining the compressive and residual strengths of confined concrete in CFDST columns, and incorporated in the mathematical models. The highly dynamic nonlinear equilibrium equations of CFDST columns under eccentric loading are solved by the efficient computer solution algorithms, which are developed based on the inverse quadratic method. The validations of the numerical models are made by comparisons with experimental results. The influences of various geometric and material parameters on the behavior of CFDST columns are examined. The results obtained from experimental and numerical studies are used to propose design equations. This research makes significant contributions to the knowledge by adding new test results on CFDST short columns to the database. The numerical models developed provide researchers and structural designers with accurate and efficient computer simulation and design tools, which lead to safer and more economical designs of composite structures. The design equations proposed can be utilized to design CFDST short and slender columns under various loading conditions.