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Composite concrete filled steel tube columns are increasingly used for high-rise building structures, owing to their excellent structural performance such as superior load-bearing capacity, high ductility, good energy dissipation and fire behaviour which arises from the combination of the two different materials in the structure. Composite structures exploit the characteristics of steel and concrete; steel with its high tensile strength and ductility and concrete with its high compressive strength and stiffness. In general, concrete filled composite columns with circular hollow sections (CHS) have the advantage over columns with other section shapes due to the circular cross sections providing a uniform confinement to the concrete core.

Circular Concrete Filled Tubes are gaining importance in the construction industry due to their advantages insofar as economy and structural efficiency. Due to the recent developments in concrete and steel technology, the usage of high strength materials in these concrete filled tubes is increasing. The governing American specification (AISC 360-16) classifies these composite members as compact, non-compact and slender sections. The allowed section slenderness (ratio of diameter to thickness ratio) in each classification is related to the material properties (ratio of Young's modulus to yield strength ratio). AISC 360-16 is applicable for steels up to 75 ksi and concretes up to 10 ksi. These limits are lower than current available materials and restricts the usage of highly slender sections. As the strength of these tubes is dependent on local buckling, tests on many combinations of high strength steel and concrete are needed to extend these material limits. This preliminary research work focuses on understanding the local buckling behavior of highly slender sections and the effect of concrete infill and its confinement. The research began by compiling a database that highlighted a gap on tests with highly slender sections and high strength materials. To address this issue, a pilot set of experimental tests were conducted on short circular concrete filled members. An analytical evaluation of these experimental results are performed using 3D finite element analysis models. The critical buckling load is determined using J2 deformation theory, which proves to give a good estimate when compared with the experimental results. The main objective of the work is to determine if a simplified test like the one used in this work could be used for the large experimental study that will be necessary to expend the material limits in AISC 360-16. The limited data developed in this study indicates that the test can provide satisfactory results with a few improvements and refinements. / Master of Science / A concrete-filled tube consists of a large steel circular or rectangular tube filled with concrete. This configuration is widely used in the construction industry, primarily as columns in high-rise buildings. Concrete filled tubes make the best use of both its constituent materials. The strength of the concrete in compression is increased by the confining action of the exterior steel tube, while the relatively thin steel tube is prevented from buckling by the presence of the concrete. Thus, this combination of the two materials, known as composite construction, results in a stronger and more ductile structural member than either a steel only or a reinforced concrete one. The design of these members is currently governed by the American Institute of Steel Construction Manual and Specification known as AISC 360-16. The provisions therein contain specific limits on the strength of both the steel and concrete allowed, as well as on the slenderness (wall thickness to width or diameter) of the tubes permitted. Because stronger steel and concretes are coming into the market and more slender tubes are available, the provisions need updating. This thesis presents a first step in that direction by testing some very slender concrete filled tubes in pure compression. This thesis presents preliminary research work focused on understanding the local instability of thin walled steel tubes and its effect with a concrete infill. The research began by compiling a database of experimental tests on concrete filled tubes. This database highlighted a gap on test results for thin walled tube sections and high strength materials. To address this issue, a pilot set of experimental tests were conducted on short circular concrete filled members. An analytical evaluation of these experimental results was performed using 3D finite element analysis models. The test results and accompanying analyses indicate that simplified test like the one used in this work could be used for the large experimental study that will be necessary to expend the material limits in AISC 360-16. The limited data developed in this study indicates that the test can provide satisfactory results with a few improvements and refinements.

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.

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.

Tesis para optar al grado de Magíster en Ciencias de la Ingeniería, Mención Ingeniería Estructural, Sísmica y Geotécnica / En el presente trabajo de investigación se evalúa el desempeño sísmico y el riesgo de colapso de marcos especiales a momento compuestos (C-SMF), utilizando la metodología de FEMA P695 [24]. Para llevar a cabo este planteamiento como primer punto se realizó el diseño de varias configuraciones de edificios prototipo en 2D de 3, 6, 8, 12 y 16 pisos de altura tomados de una planta típica, ubicados en diferente zonificación sísmica y tipo de suelo, aplicando la norma chilena NCh 433 [25]. Se incluye la no linealidad geométrica (efecto P-Delta) y la no linealidad del material al implementar modelos de plasticidad concentrada basados en fibras utilizando las curvas efectivas de esfuerzo-deformación para dicho fin. Se utilizó el software SAP2000 versión 19.1.1 para llevar a cabo todo el trabajo de investigación. Como segundo punto se realizaron análisis no lineales estáticos (Pushover) y no lineales dinámicos incrementales (Time History) para obtener el factor de sobreresistencia Ω, la ductilidad 𝜇���𝑇���, la intensidad media de colapso 𝑆���̂𝐶���𝑇���, la razón del margen de colapso CMR y así poder evaluar el desempeño sísmico comparando la razón del margen de colapso ajustado calculada ACMR con los valores admisibles propuestos por FEMA P695 [24]. El propósito de esta metodología es determinar los parámetros de respuesta (𝑅���,𝐶���𝑑���,𝛺���𝑜���) para que cuando sean propiamente implementados en los procesos de diseño, las edificaciones resulten con una seguridad equivalente contra el colapso durante un terremoto de manera similar a la seguridad que brindan los actuales códigos de diseño sísmico. Además, busca afirmar que el sistema estructural analizado asegure la protección de vida cuando posea una baja probabilidad de colapso (<10%) al ser sometida al sismo máximo considerado (MCE) con la finalidad de incluir el sistema C-SMF en normativa nacional para resistir fuerzas laterales inducidas por sismos. Un total de 30 diseños y 1373 análisis no lineales efectivos repartidos en 18 modelos seleccionados fueron los que se llevaron a cabo. Acorde a los resultados obtenidos, los factores de desempeño sísmico para uso normativo en C-SMF que se recomiendan son: Ωo=3.0, R*=6.0 y Cd=5.5; esto aplicando las limitaciones impuestas por FEMA P695 [24] y ASCE/SEI 7-16 [20]. La evaluación del desempeño fue satisfactoria para todos los casos individuales y grupales; sin embargo, se debe remarcar que los valores de CMR obtenidos disminuirán (aproximadamente en un 50%) si se realiza un Full IDA, lo que provocaría que el valor de CMR para las edificaciones de 16 pisos no cumpla con el criterio de aceptación. Con este antecedente, a criterio del autor, el sistema estructural C-SMF asegura la protección de vida y puede ser incluido en normativa nacional colocando una limitación en altura de 12 pisos.

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.