Dissertations / Theses on the topic 'Concrete-filled steel tubular (CFST) columns'

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.

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.

Concrete-Filled Steel Tubular (CFST) columns are increasingly being used because of their many advantages, including high strength, high ductility, and higher fire resistance than conventional steel or concrete columns of the same size. In order to maximise the advantages of CFST column, composite action of the column should be ensured. In realistic structures, the load is not directly applied to the entire CFST column section and is introduced from the beam-column connection. Simple shear connections, which are usually preferred in constructions, are only connected to the external face of the steel tube and there is an issue about how this load is introduced to the concrete core, through the bond at the steel/concrete interface. There are fundamental errors in the load introduction mechanism assumed in various current design methods. Furthermore, based on this erroneous load introduction mechanism, construction methods, such as placing shear connectors inside the steel tube or using through-column plates, are recommended to ensure complete load introduction. However, these methods are either impractical or uneconomical. The aim of this project, therefore, is to develop a thorough understanding of the load introduction mechanism and to use the new insights to assess design implications, for both ambient temperature and fire safety design. The research has been conducted through physical testing, extensive numerical modelling and detailed analytical derivations. A series of new load introduction tests, in which square CFST columns are loaded through simple fin plate connections, are carried out. These tests are designed to investigate the effects of changing column lengths below and above the connection, the effectiveness of using shear connectors inside the steel tube below the connection (according to Eurocode 4) and using a cap plate on the column top for load introduction into the concrete core. The test results indicate that the connection load is introduced to the concrete core through the column length above and within the connection or the cap plate on top of the column. This is different from the currently assumed mechanism of load introduction which assumes that load introduction occurs from underneath the connection. Below the connection, there is transfer of forces from the steel tube to the concrete core, but the total force in the column remains unchanged. Consequently, using shear connectors below the connection is ineffective in increasing CFST column strength, as has been demonstrated by the tests. The physical tests are supplemented by an extensive numerical parametric study to check whether the conclusions are applicable to different design conditions and to provide data for development of a new design method. The parameters include: section geometry (square, circular, and rectangular), position of load application to CFST column, dimensions of the square column cross-section, steel tube thickness, connection length, column length above the connection, column length below the connection, and maximum bond stress at the steel-concrete interface. The numerical simulation results confirm the experimental observations. Furthermore, the numerical simulation results indicate that the entire column length and the entire perimeter of the steel-concrete interface above and within the connection are engaged in load introduction. Based on the experimental and numerical simulation results, a simple calculation method has been proposed to calculate the column cross-section resistance under compression. According to this equation, the concrete compression resistance to the composite column is the minimum of the plastic resistance or the bond strength within and above the connection. This gives rise to a “concrete strength reduction factor” to account for incomplete load introduction, being the ratio of the load introduced to the concrete core through the interface bond to the concrete plastic resistance. Based on the new load introduction calculation method and using representative values of column dimensions and concrete cylinder strength, it has been demonstrated that complete load introduction can be achieved in almost all practical arrangements of concrete-filled tubular construction. For slender CFST column design, this concrete strength reduction factor should also be used to calculate the CFST column cross-section flexural stiffness. For a CFST column under combined axial compression and bending, the concrete strength reduction factor should be used when calculating the compression force, but should be ignored when calculating the bending resistance because composite action is not necessary for bending of the CFST column. The new load introduction mechanism induces additional compression in the concrete core and possible tension in the steel tube above the connection. Therefore, the concrete core of the column above the connection in multi-storey construction should be designed to resist the additional compression force. For the steel tube, in ambient temperature design, the steel contribution ratio (steel section resistance/plastic resistance of composite cross-section) of the top floor column should be at least 0.25. For fire resistance design, the steel contribution ratio of the top floor columns, those on the floor below the top floor, and those two floors below the top floor, should not be less than 0.5, 0.33, and 0.25 respectively.

This thesis studies the uni-axial behaviour of circular double-skinned concrete-filled-steel-tubular (CFST) columns with external confinement in form of external steel rings. Particular attention is paid to the experimental behaviour of double-skinned CFST columns and theoretical model for evaluating the loadcarrying capacity of un- and ring-confined double-skinned CFST columns. Experimental studies on circular double-skinned CFST columns with various spacing of confinement, concrete strength and hollow ratio were conducted and discussed comprehensively. The mechanical properties of double-skinned CFST columns such as elastic stiffness, elastic strength, load-carrying capacity and ductility are presented. From the result, it is found that the elastic stiffness, elastic strength, load-carrying capacity and ductility are enhanced by installing the external steel rings to the outer tube as external confinement. To verify the effectiveness of external steel rings, the Poisson’s ratios of the double-skinned CFST columns are listed and found to be similar to that of concrete so that a perfect bonding is maintained. To emphasis the excellent performance of double-skinned CFST columns with external rings under uni-axial compression, the load-carrying capacity, elastic strength and elastic stiffness are compared to those of single-skinned CFST columns and reinforced concrete columns. To fill up the gap that no design model is provided in Eurocode 4 (EC4) for confined double-skinned CFST columns, a theoretical model based on the force equilibrium condition is proposed for evaluating the load-carrying capacity of both un- and ring-confined double-skinned CFST columns. The model takes into account the composite action between the steel tubes and core concrete. To verify the proposed model, numerous test results obtained by the author and other researchers are used for comparing the theoretical results. According to the above theoretical model above, a parametric study is carried out to investigate the effect of various geometry and material properties on the load-carrying capacity of double-skinned CFST columns. The confining pressure is expressed in terms of geometry and material factors. A simplified design formula is proposed to facilitate the preliminary design of double-skinned CFST columns with and without external confinement.
published_or_final_version
Civil Engineering
Master
Master of Philosophy

Concrete filled steel tubular (CFST) columns are widely used in building and infrastructure projects throughout the world.Compared with other form of construction CFST columns offer superior structural performance and speed and ease of construction.Design procedures and recommendations provided in most of the design codes are often tedious and complex. There have been attempts to simplify the design procedure by providing a simplified expression to predict the capacity of a CFST under a general loading condition.In this thesis a rigorous analysis procedure was presented for the analysis of CFST beam-columns under general loading conditions.All the analytical results were verified by comparisons with the available test results and current ACI, AISC AND Eurocode 4 design codes. The comparisons demonstrated that the proposed numerical equations are accurate, and slightly conservative. Based on the numerical analysis, a simple and easy to follow calculation procedure was proposed for design of CFST columns under either uniaxial or biaxial bending moment and axial load.
Doctor of Philosophy (PhD)

This thesis proposes two forms of external confinement for concrete filled steel tubular (CFST) columns. The confinement efficiency is studied by examining the axial strength enhancement and ductility improvement of the CFST columns with external confinement. Due to the heavy demand of confining steel to restore the column ductility in seismic regions, it is more efficient to confine these columns by hollow steel tube to form CFST column. Compared with transverse reinforcing steel, steel tube provides a stronger and more uniform confining pressure to the concrete core, and reduces the steel congestion problem for better concrete placing quality. The CFST columns are therefore characterised by higher strength, ductility and large energy absorption before failure. However, a major shortcoming of CFST columns is the imperfect steelconcrete interface bonding occurred at the elastic stage as steel dilates more than concrete in compression. This adversely affects the confining effect and decreases the elastic modulus. To resolve the problem, it is proposed in this thesis to use external steel confinement in the forms of rings and ties to restrict the dilation of steel tube. For verification, a series of uni-axial compression test was performed on some CFST columns with external steel rings and ties. From the results, it was found that the external steel rings could improve both the axial strength and stiffness of the CFST columns significantly. However, the steel ties could not improve either the axial strength or elastic stiffness significantly. The confining efficiency was then investigated by comparing the strength of these confined-CFST columns with the reinforced concrete (RC) columns counterparts with the same concrete and steel volume. It is evident that the axial strength of CFST columns is much higher than the RC columns, which suggests that the application of CFST columns can utilise less construction materials and reduce the demolition waste. A theoretical model is also proposed for predicting the axial strength of ring-confined CFST columns. Comparison between the predicted results and the test results obtained by the author and other researchers shows that the proposed model gives good estimation for both unconfined and confined CFST columns.
published_or_final_version
Civil Engineering
Master
Master of Philosophy

Joint behaviour in fire is currently one of the most important topics of research in structural fire resistance. The collapse of World Trade Center buildings and the results of the Cardington full-scale eight storey steel framed building fire tests in the UK have demonstrated that steel joints are particularly vulnerable during the heating and cooling phases of fire. The main purpose of this research is to develop robust joints to CFT columns that are capable of providing very high rotational and tying resistances to make it possible for the connected beam to fully develop catenary action during the heating phase of fire attack and to retain integrity during the cooling phase of fire attack. This research employed the general finite element software ABAQUS to numerically model the behaviour of restrained structural subassemblies of steel beam to concrete filled tubular (CFT) columns and their joints in fire. For validation, this research compared the simulation and test results for 10 fire tests previously conducted at the University of Manchester. It was envisaged that catenary action in the connected beams at very large deflections would play an important role in ensuring robustness of steel framed structures in fire. Therefore, it was vital that the numerical simulations could accurately predict the structural behaviour at very large deflections. In particular, the transitional behaviour of the beam from compression to catenary action presented tremendous difficulties in numerical simulations due to the extremely high rate of deflection increase. This thesis will explain the methodology of a suitable simulation method, by introducing a pseudo damping factor. The comparison between the FE and the experimental results demonstrates that the 3-D finite element model is able to successfully simulate the fire tests. The validated ABAQUS model was then applied to conduct a thorough set of numerical studies to investigate methods of improving the survival temperatures under heating in fire of steel beams to concrete filled tubular (CFT) columns using reverse channel connection. This study investigated five different joint types of reverse channel connection: extended endplate, flush endplate, flexible endplate, hybrid flush/flexible endplate and hybrid extended/flexible endplate. The connection details investigated include reverse channel web thickness, bolt diameter and grade, using fire-resistant (FR) steel for different joint components (reverse channel, end plate and bolts) and joint temperature control. The effects of changing the applied beam and column loads were also considered. It is concluded that by adopting some of the joint details to improve the joint tensile strength and deformation capacity, it is possible for the beams to develop substantial catenary action to survive very high temperatures. This thesis also explains the implications on fire resistant design of the connected columns in order to resist the additional catenary force in the beam. The validated numerical model was also used to perform extensive parametric studies on steel framed structures using concrete filled tubular (CFT) columns with flexible reverse channel connection and fin plate connection to find means of reducing the risk of structural failure during cooling. The results lead to the suggestion that in order to avoid connection fracture during cooling, the most effective and simplest method would be to reduce the limiting temperature of the connected beam by less than 50°C from the limiting temperature calculated without considering any axial force in the beam.

[EN] Concrete filled steel tubular columns have many advantages in terms of bearing capacity, aesthetics, execution and fire resistance, thanks to the collaborative work of both materials steel and concrete. The effort made in the last decades to rise a high understanding of their behaviour subjected to different loads and assuming multiple variations has resulted in the wide spread of its use between the designers. Nonetheless, how to solve the connection with I-beams is still a handicap and requires a specific study. One of the most common and popular solution to connect open section steel beams (I-beams) to open section steel columns are endplate connections. In the cases of columns with hollow section, special fastenings are needed, which are able to be tightened from one external side and are denominated blind-bolts. Nowadays, there are several fastener systems that allow these types of connections. The characterization of their response and their capacity to support different loads is the objective of several investigations, where the geometrical definition and the material properties are crucial parameters. Despite the promising results of these connections at room temperature regarding their capability to resist bending moments, their performance is un-known at high temperatures. Therefore, the aim of this thesis is the study of the tensile behaviour of blind-bolts in endplate connections to concrete filled tubular columns at elevated temperatures and subjected to bending moment. Primarily, the research comprises the understanding of the pure thermal transfer problem. The temperature distribution through the connection section is obtained experimental and numerically. The thermal parameters that characterize the connections response are determined through the calibration of the numerical models with the experiments. Secondly, the blind-bolt capacity under pull out and at high temperatures is under analysis. During the fire the temperature increases while connection transmits loads from the beam to the column, the objective of this dissertation is to know how the mechanical response of the pulled blind-bolts changes under these conditions. Thus, the study of the material properties dependent on the temperature and their effect on the connection response is covered by the investigation. Furthermore, the influence of the concrete and the type of fastener is a highlighted aspect through the thermal and the fire analysis. Finally, the reliability of these connections to comply with requirements of 30 minutes fire exposure before the collapse is evaluated. As a result, valuable Finite Element models able to simulate the thermal and thermo-mechanical behaviour of the connection are developed, providing useful behavioural patterns of the blind-bolts. Among the main conclusions, it is noted the temperature reduction due to concrete core in concrete filled columns compared to hollow sections, in the exposed bolt surface means 100ºC less. Conversely, a longer bolt shank of the fastener system embedded in concrete has a negligible effect on the temperature of the resistant part of the bolt. Regarding the fire capacity, the concrete core in the steel tube columns presents significant benefits in terms of fire resistance time and connection stiffness. Besides, the bolt anchorage enhances the stiffness at elevated temperatures, however, the failure of the shank next to the bolt head causes that the anchorage does not mean an improvement on the fire time resistance.
[ES] Las columnas tubulares de acero rellenas de hormigón presentan múltiples ventajas en términos de capacidad de carga, estética, ejecución y resistencia al fuego, gracias a la acción combinada de acero y hormigón. El esfuerzo realizado en las últimas décadas por conocer su comportamiento frente a diferentes cargas y bajo distintos parámetros ha dado lugar a una amplia difusión de su uso entre los diseñadores. No obstante, la forma de resolver la conexión con vigas de sección en I sigue siendo un hándicap y requiere un estudio específico. Una de las soluciones más comunes y populares para conectar las vigas de acero de sección abierta (vigas I) a columnas de acero de sección abierta es la conexión con chapa de testa, que en el caso de sección hueca requiere de tornillos especiales denominados tornillos ciegos, puesto que reciben el par de apriete desde una cara de la sección. En la actualidad existen diversos sistemas de fijación que permiten este tipo de conexiones y cuya respuesta y caracterización es objeto de numerosas investigaciones. En este sentido, la definición geométrica de la unión y las propiedades de los materiales son parámetros cruciales en el rendimiento de la conexión. La presente tesis analiza el comportamiento de los tornillos ciegos en el área traccionada de conexiones de placa de testa a columnas tubulares de acero rellenas de hormigón sometidas a momentos de flexión y a elevadas temperaturas. Las prestaciones de esta solución constructiva para la unión viga-columna tubular, junto con la ausencia de datos relacionados con su comportamiento en situación de incendio la convirtió en el objetivo del trabajo. En primer lugar, la investigación aborda el problema de transferencia de calor, analizando experimental y numéricamente la distribución de temperaturas en la sección de la conexión. En esta parte del estudio se obtienen los parámetros térmicos que caracterizan la respuesta térmica de la conexión a través de la calibración de los modelos numéricos con los datos experimentales. En segundo lugar, se realiza el estudio de la capacidad de los tornillos ciegos para soportar cargas de tracción en situación de incendio, es decir, se analiza cómo cambia el comportamiento de la conexión con sus características alteradas debido a las altas temperaturas. El estudio de las propiedades del material en función de la temperatura y su efecto sobre la respuesta de la conexión constituyen una parte importante de la investigación. Además, se evalúa la influencia del hormigón y el tipo de elemento de sujeción tanto en el comportamiento mecánico como termo-mecánico de la conexión. Por último, se estudia la capacidad de las uniones para cumplir con requerimientos de exposición al fuego de 30 minutos previamente al colapso. Como resultado de este trabajo se obtuvieron modelos de elementos finitos capaces de simular la conexión térmica y termo-mecánicamente, proporcionando patrones de comportamiento de gran utilidad en el diseño de las mismas. Entre las principales conclusiones, se observó la reducción de la temperatura en los tornillos gracias al núcleo de hormigón en columnas de hormigón lleno en comparación con secciones huecas, que ya en la superficie expuesta del tornillo se cuantificaba en 100ºC menos. Por el contrario, los elementos de fijación que presentaban mayor longitud de vástago de tornillo embebida en el hormigón, no generaban un efecto significativo sobre la temperatura de la parte resistente del perno. En cuanto a la capacidad resistente frente a fuego, el núcleo de hormigón supuso una mejora en términos de rigidez y de tiempo de resistencia al fuego. Sin embargo, el fallo de los pernos en una sección próxima a la superficie expuesta redujo el efecto esperado del anclaje del tornillo, que si bien implicaba una mayor rigidez de la conexión, no parecía mejorar el tiempo de resistencia a fuego. Finalmente se planteó la necesidad de
[CAT] Els pilars tubulars d'acer omplerts de formigó (CFT) presenten molts avantatges en termes de capacitat de carrega, estètica, execució i resistència al foc, gràcies a l'acció combinada de l'acer i el formigó. L'esforç realitzat en les darreres dècades per conèixer el seu comportament enfront a diferents càrregues i sota distints paràmetres ha donat lloc a una amplia difusió del seu ús entre el dissenyadors. No obstant això, la manera de resoldre la connexió amb bigues de secció en I, continua sent un handicap i requereix d'un estudi específic. Una de les solucions més comuns i populars per a connectar les bigues d'acer de secció oberta (bigues I) a columnes d'acer de secció oberta és la connexió amb 'chapa de testa', que en el cas de la secció buida requereix de perns especials denominats perns cecs perquè es rosquen des d'una cara de la secció. En l'actualitat existeixen diversos sistemes de fixació que permeten aquest tipus de connexions, la resposta i caracterització dels quals es l'objectiu de nombroses recerques. En aquest sentit, la definició geomètrica de la unió i les propietats dels materials son paràmetres crucials en el rendiment de la connexió. Aquesta tesi analitza el comportament dels perns cecs en l'àrea traccionada de connexions de 'chapa de testa', a pilars tubulars d'acer omplerts de formigó, sotmeses a moments de flexió i a elevades temperatures. Les prestacions d'aquesta solució constructiva per a la unió biga-pilar tubular junt amb l'absència de dades relacionades amb el comportament en situació d'incendi, la van convertir en l'objectiu d'aquest treball. En primer lloc, la recerca aborda el problema de transferència de calor, analitzant tant experimental com numèricament la distribució de temperatures en la secció de la connexió. En aquesta part de l'estudi, s'obtenen el paràmetres tèrmics que caracteritzen la resposta tèrmica de la connexió mitjançant el calibratge del models numèrics amb les dades experimentals. En segon lloc, es realitza l'estudi de la capacitat dels perns cecs per a suportar càrregues de tracció en situació d'incendi, es a dir, s'analitza com canvia el comportament de la connexió amb les seues característiques alterades degut a les altes temperatures. L'estudi de les propietats del material en funció de la temperatura i el seu efecte en la resposta de la connexió formen també part de la recerca. Un contingut important d'aquest treball consisteix en determinar l'influencia del formigó i el tipus d'element de fixació tant en el comportament mecànic com termo-mecànic de la connexió. Per últim, s'estudia la capacitat de les unions per a complir amb els requeriments d'exposició al foc de 30 minuts prèviament al col·lapse. Com a resultat d'aquest treball s'obtingueren models d'elements finits amb capacitat per a simular el comportament tèrmic i termo-mecànic de la connexió, proporcionant patrons de comportament de gran utilitat en el disseny. Entre les principals conclusions, es va observar la reducció de la temperatura en els perns gràcies al nucli de formigó en pilars omplerts de formigó en comparació amb el pilars buits, on ja en la superfície esposada del cargol es quantificava en 100 ºC menys. Pel contrari, els elements de fixació que presentaven major longitud de embeguda en el formigó, no generaven un efecte significatiu en la temperatura de la part resistent del pern. En quant a la capacitat resistent davant del foc, el nucli de formigó va suposar una millora en termes de rigidesa i de temps de resistència al foc. Tanmateix, la fallada dels perns en una secció pròxima a la superfície esposada va reduir l'efecte esperat de la fixació del pern, que si be implicava una major rigidesa de la connexió, no semblava millorar el temps de resistència al foc. Finalment, es va plantejar la necessitat de aprofundir en l'anàlisi incorporant un major rang de paràmetres.
Pascual Pastor, AM. (2015). Fire behaviour of blind-bolted connections to concrete filled tubular columns under tension [Tesis doctoral no publicada]. Universitat Politècnica de València. https://doi.org/10.4995/Thesis/10251/53240
TESIS

Following recent events such as the World Trade Center building collapse and the Cardington large scale structural fire research program, the fire behavior of connections has now become an important research subject. This thesis presents the results of experimental, numerical and theoretical studies into the behavior of simple welded fin-plate to concretefilled tubular (CFT) columns loaded by tensile or shear force. Such connections represent a simple single-sided joint solution to steel CFTs which are considered an attractive and robust structural element. Experiments have been performed at both ambient and elevated temperatures against the results of which numerical finite element models have been validated. The ranges of parameters encompassed by the tests include column cross-section shape; column and finplate thickness; concrete in-fill; elevated temperatures and connection lever arm. The observed failure modes include fracture of the fin-plate and tearing out of the tube around the welds. By considering the results of previously published research, the current design method for similar connections under purely tensile load, in CIDECT Guide 9, based on a deformation limit of 3% of the tube width is shown to be inadequate when evaluating the ultimate strength of such connections. By comparing the results from the current test program which failed in the fin-plate with Eurocode guidance for failure of a fin-plate alone under shear and bending load it is shown that the column face influences the overall connection strength regardless of failure mode. Concrete in-fill is observed to significantly increase the strength of connections over empty specimens, and circular column specimens were observed to exhibit greater strength than similarly proportioned square columns. When validating the numerical model against elevated temperature tests it was found that the strength reduction factors suggested by Eurocode for steel at elevated temperature are appropriate. The numerical models developed have been used to perform extensive parametric studies from which simple hand calculation methods have been developed for evaluating the strength of the column component of square CFTs under either tensile or shear load imparted through a fin-plate connection. The simple hand calculation procedures are based upon defining a rigid plate deformation pattern for the connection and then applying the internal work principle. For connections under shear load a method is presented for combining the column failure load with the fin-plate failure derived from existing Eurocode guidance. Both simple hand calculation methods are compared favorably with available test results. A limited number of tests and numerical validation have also been performed for reverse channel to CFT connections loaded in shear at both ambient and elevated temperatures.

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.

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.

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 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.

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
正修科技大學
營建工程研究所
102
Two series of full-size experiments were carried out to consider the effect of type of concrete infilling (plain and reinforced) and the load level () on the fire resistance (i.e., time to failure) of Concrete-filled steel tubular columns (CFTC). Each series had two specimens. Series A was consisted of HSS filled with plain concrete, while Series B was bar-reinforced concrete-filled HSS column. The columns had square cross-sections and were filled with one type of concrete. The width of the square columns was 400 mm and the wall thickness was 9 mm. All columns were 3060 mm long. No external fire-proofing was provided for the steel. Each of the CFTC had end plates welded to them in order to transfer the load, and end stiffeners were also introduced to ensure that end conditions did not affect the failure resistance of thermal load. Besides, the furnace, concrete and steel temperatures as well as the axial deformations were recorded until failure of the column. The temperature from the specimen's surface to the inner central core was measured with type K thermocouples located at different depths in four sections of the column. During the whole test, the columns were subjected to a constant compressive load. This load was controlled by a load cell of 19.6 MN, located on the head of the piston of a jack. The applied load corresponded to 28%, 47% and 0.66% of the design value of buckling resistance of the columns at room temperature, respectively. Thermal load was applied on the columns in form of CNS 12514 time-temperature curve in a natural gas-fired large-scale laboratory furnace until the set experiment termination condition was reached. The current failure criterion specified in CNS 12514 is adopted in this study, which is based on the amount of contraction and the rate of contraction. Results from the fire tests indicate that the fire resistance of A1 specimen was 168 minutes, as compared to 50 minutes for A2 specimen. For Series B specimens (i.e., RC-filled columns), the fire resistance of B1 specimen was 111 minutes, as compared to 41 minutes for B2 specimen. As a result, it can be concluded that the higher the axial load level is, the lower the fire resistance results. On the other hand, in the bar-reinforced concrete-filled HSS column, the presence of rebars not only decreases the propagation of cracks and sudden loss of strength, but also contributes to the load-carrying capacity of the concrete core. For example, under the load level of 0.47, the fire resistances of B1 specimen was improved significantly compared to A2 specimen. In addition, the established numerical model was able to reasonably predict the temperature distribution in time history on the specimen cross section.

No
This paper summarises the data from 1819 tests on concrete-filled steel tube columns and compares their failure load with the prediction of Eurocode 4. The full data is given on the website http://web.ukonline.co.uk/asccs2 . The comparison with Eurocode 4 is discussed and shows that Eurocode 4 can be used with confidence and generally gives good agreement with test results, the average Test/EC4 ratio for all tests being 1.11. The Eurocode 4 limitations on concrete strength could be safely extended to concrete with a cylinder strength of 75 N/mm2 for circular sections and 60 N/mm2 for rectangular sections.

No
This paper presents the behaviour and design of axially loaded concrete filled stainless steel circular and square hollow sections. The experimental investigation was Conducted using different concrete cube strengths varied from 30 to 100 MPa. The column strengths and load-axial shortening curves were evaluated. The study is limited to cross-section capacity and has not been validated at member level. Comparisons of the tests results together with other available results from the literature have been made with existing design methods for composite carbon steel sections-Eurocode 4 and ACI. It was found that existing design guidance for carbon steel may generally be safely applied to concrete filled stainless steel tubes. though it tends to be over-conservative. A continuous strength method is proposed and it is found to provide the most accurate and consistent prediction of the axial capacity of the composite concrete filled stainless steel hollow sections due largely to the more precise assessment of the contribution of the stainless steel tube to the composite resistance.

The stability of arches is a classical mechanics and pragmatic engineering problem that has been extensively studied by many researchers over the years. Despite the comprehensive construction and research of arches throughout history, their complex behaviour still presents a challenge to engineers and ensures they are the subject of continual investigation. The problem of arch stability is of contemporary relevance due to the surging popularity of concrete-filled steel tubular (CFST) arch bridges. Hence, due to the inherent complex structural function of arches when coupled with the increasing construction of CFST arches, research into the response and stability of CFST arches under all possible environmental conditions is necessitated. However, investigations into the effects of extreme temperatures on concrete and CFST arches have not been conducted. This thesis presents a comprehensive analytical and numerical investigation into the stability of circular concrete and CFST arches subjected to combined mechanical and thermal loading. Original models are derived for the non-linear prebuckling and buckling analysis including closed-form solutions for the in-plane elastic buckling loads of concrete and CFST arches, and non-discretisation mechanically-based numerical models for their elastic and inelastic analysis prebuckling analysis. Additionally, a numerical methodology to determine the elastic flexural-torsional buckling loads of CFST arches is proposed. Furthermore, a novel fractional viscoelastic creep law is developed for concrete at elevated temperatures in order to analyse the significance of basic creep strain on thermal response and stability boundaries. The fractional-derivative creep law proves to be a robust and compact method of modelling basic creep strain under stress and temperature varying conditions. Finite difference schemes are employed to numerically approximate the fractional derivative and incorporate basic creep into the prebuckling and stability analyses. Finite Element (FE) models are developed to verify the derived models and to also investigate the inelastic buckling strength and fire performance of concrete and CFST arches. The findings of this study provide a detailed understanding of the fundamental thermomechanical behaviour and failure modes of concrete and CFST arches. Consequently, engineers may utilise the results detailed herein to assess and improve the fire resistance of concrete and CFST arch structures. Additionally, the developed creep law has widespread application in the analysis of concrete structures under elevated temperatures. The proposed inelastic numerical models also provide efficient tools for the analysis of other structures such as steel arches and beams.

This paper presents the behaviour and design of axially loaded concrete filled stainless steel circular and square hollow sections. The experimental investigation was conducted using different concrete cube strengths varied from 30 to 100 MPa. The column strengths and load-axial shortening curves were evaluated. The study is limited to cross-section capacity and has not been validated at member level. Comparisons of the tests results together with other available results from the literature have been made with existing design methods for composite carbon steel sections ¿ Eurocode 4 and ACI. It was found that existing design guidance for carbon steel may generally be safely applied to concrete filled stainless steel tubes, though it tends to be over-conservative. A continuous strength method is proposed and it is found to provide the most accurate and consistent prediction of the axial capacity of the composite concrete filled stainless steel hollow sections due largely to the more precise assessment of the contribution of the stainless steel tube to the composite resistance.

High strength thin-walled concrete-filled steel tubular (CFST) slender beam-columns may undergo local and global buckling when subjected to biaxial loads, preloads or cyclic loading. The local buckling effects of steel tube walls under stress gradients have not been considered in existing numerical models for CFST slender beam-columns. This thesis presents a systematic development of new numerical models for the nonlinear inelastic analysis of thin-walled rectangular and circular CFST slender beam-columns incorporating the effects of local buckling, concrete confinement, geometric imperfections, preloads, high strength materials, second order and cyclic behavior. In the proposed numerical models, the inelastic behavior of column cross-sections is simulated using the accurate fiber element method. Accurate constitutive laws for confined concrete are implemented in the models. The effects of progressive local buckling are taken into account in the models by using effective width formulas. Axial load-moment-curvature relationships computed from the fiber analysis of sections are used in the column stability analysis to determine equilibrium states. Deflections caused by preloads on the steel tubes arising from the construction of upper floors are included in the analysis of CFST slender columns. Efficient computational algorithms based on the Müller’s method are developed to obtain nonlinear solutions. Analysis procedures are proposed for predicting load-deflection and axial load-moment interaction curves for CFST slender columns under axial load and uniaxial bending, biaxial loads, preloads or axial load and cyclic lateral loading. The numerical models developed are verified by comparisons of computer solutions with existing experimental results and then utilized to undertake extensive parametric studies on the fundamental behavior of CFST slender columns covering a wide range of parameters. The numerical models are shown to be efficient computer simulation tools for designing safe and economical thin-walled CFST slender beam-columns with any steel and concrete strength grades. The thesis presents benchmark numerical results on the behavior of high strength thin-walled CFST slender beam-columns accounting for progressive local buckling effects. These results provide a better understanding of the fundamental behavior of CFST columns and are valuable to structural designers and composite code writers.

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.

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.

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.

Yes
Elliptical Hollow Sections (EHSs) have been utilized in construction recently because of their visual appearance as well as the potential structural efficiency owing to the presence of the two principle axes. However, little information currently exists for the design of beam to elliptical column connections, which is an essential part of a building structure. Thus, to ensure the safe and economic application of EHSs, a new research project has been initiated. Rotation behaviour of simply bolted beam to concrete-filled elliptical steel column connections was investigated experimentally. Various joint types were considered and the benefits of adopting core concrete and stiffeners were highlighted. This paper covers the experimental studies and simulation of the connections using the ABAQUS standard solver. Comparisons of failure modes and moment vs. rotation relationships of the connections between numerical and experimental results were given. Good agreement has been obtained and the developed finite element model was therefore adopted to conduct a preliminary parametric study to explore the effect of critical parameters on the structural behaviour of beam to concrete-filled elliptical column connections.

碩士
國立臺灣大學
土木工程學研究所
107
Concrete-filled steel tubular (CFT) columns show not only high strength and high ductility but also exhibit favorable seismic performance. Besides, CFT columns infilled with self-consolidating concrete (SCC) shorten construction time because of its self-compacting characteristics. As a result, these kind of structural members have been widely adopted in high-rise buildings in Taiwan. The primary intent of concrete infill is to increase lateral stiffness of member and delay the local buckling of the steel tubular. However, in the recent future, engineers begin to incorporate concrete in materials subjected to axial load during design. Once concrete is subjected to load, development of concrete creep begins. In order to maintain the equilibrium of forces of CFT section, part of axial load of concrete will be transferred to steel tubular which leads to the growth of steel stress. Furthermore, with the ever-changing nature of technology, concrete compressive strength of SCC has reached up to 90 MPa. In general, the higher the concrete compressive strength is, the lower the water cement ratio is. Concrete with low water cement ratio intensifies the rise of steel stress in CFT columns on account of high autogenous shrinkage. A three-dimensional finite element model of CFT column, which takes account of the phenomenon of concrete creep and shrinkage, is developed to evaluate stress transfer between concrete and steel in ABAQUS. According to recent research, there is a characteristic of high amount of paste in concrete mix designs in Taiwan owing to the soft nature of coarse aggregates. It is not appropriate to directly adopt foreign prediction formulas which will lead to underestimate long term deformation of concrete in the CFT columns. Consequently, this study compares different creep and shrinkage prediction formulas for concrete to gain a better result of the research. The analysis results show that under the condition of initial steel stress of 0.6f_y, the final steel stress of CFT column is probably not qualified according to “Design and Technique Specifications of Steel Structures for Buildings” owing to the long term deformation of infilled concrete whether the load is eccentric or not. In the extreme case of high concrete compressive strength of SCC and high diameter to thickness ratio, the steel stress significantly exceeds the original design value with 0.33f_y. Apart from this, combination of high diameter to thickness ratio and low yield strength of steel should be avoided during design due to the considerable growth of steel stress. However, specifications among countries so far merely focus on the short term loading performances of CFT columns, while its time dependent behavior is deficient. It is suggested that relevant specifications should be revised in “Design Specifications and Commentary of Steel Reinforced Concrete Structures” in Taiwan.

This paper presents the details of an experimental investigation on the behaviour of axially loaded concrete-filled stainless steel elliptical hollow sections. The experimental investigation was conducted using normal and high strength concrete of 30 and 100 MPa. The current study is based on stub column tests and is therefore limited to cross-section capacity. Based on the equations proposed by the authors on concrete-filled stainless steel circular columns, a new set of equations for the stainless steel concrete-filled elliptical hollow sections were proposed. From the limited data currently available, the equation provides an accurate and consistent prediction of the axial capacity of the composite concrete-filled stainless steel elliptical hollow sections.

Yes
This paper investigated the rotation behaviour of simply bolted I-beam to concrete-filled elliptical steel tubular (CFEST) column connections experimentally. Ten different joint assemblies were tested to failure, with a constant axial compressive load applied to the column and upwards concentrated loads at the beam ends. All of the steel tubes were hot-finished and had a cross-sectional aspect ratio of 2. The orientation of the column and the arrangement of the stiffening plates were taken into consideration. Moment versus rotation relationships and failure modes were compared for each joint, highlighting the benefits of using core concrete and stiffeners in these connections.

碩士
國立臺灣科技大學
營建工程系
105
The Concrete-Filled Steel Tubular (CFST) columns possess the advantages of high strength and high toughness, and its seismic ability has been widely affirmed and commonly used in low-rise buildings. However, the steel tube is a new product in Taiwan, and its associated beam-column behaviors attract many engineers’ attentions in practice. The behavior of beam-column connections depends on many factors such as the connection types. In order to further clarify the mechanical behavior of beam-column connections and enhance the interest of engineers. In this study, the largest size of BCR steel tube in Taiwan is used as the subject of the experiment. Five beam-column sub-assemblage specimens are designed, in which the beam-flange-through type beam-column connection is used. The experimental factors are infilled concrete, width and depth of beam. Four of five specimens are designed for joint failure to evaluate the panel zone shear strength which is contributed by steel tube web and concrete. To evaluate the energy dissipation capacity and feasibility of this beam-column connections type, another sub-assemblage specimens is designed in which beam failure is expected. Experimental results show that: (1) Infilled concrete significantly increase the panel zone shear strength. (2) Because the steel tube web doesn’t reach its yielding stress after the crack of welding, the specimens can remain a certain strength at this stage. (3) Increasing the width of beam can effectively increase both shear strength contributed by steel tube web and the concrete. (4) Increasing the depth of beam can increase the early shear strength of steel tube web, and decrease the concrete shear strength. (5) The proposed panel zone shear strength formula is able to provide an effective design for the evaluated beam-column connection.

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.

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

Concrete filled steel tubular columns have been extensively used in modern construction owing to that they utilise the most favourable properties of both constituent materials. It has been recognized that concrete filled tubular columns provide excellent structural properties such as high load bearing capacity, ductility, large energy-absorption capacity and good structural fire behaviour. This paper presents the structural fire behaviour of a series of concrete filled steel tubular stub columns with four typical column sectional shapes in standard fire. The selected concrete filled steel tube stub columns are divided into three groups by equal section strength at ambient temperature, equal steel cross sectional areas and equal concrete core cross sectional areas. The temperature distribution, critical temperature and fire exposing time etc. of selected composite columns are extracted by numerical simulations using commercial FE package ABAQUS. Based on the analysis and comparison of typical parameters, the effect of column sectional shapes on member temperature distribution and structural fire behaviour are discussed. It shows concrete steel tubular column with circular section possesses the best structural fire behaviour, followed by columns with elliptical, square and rectangular sections. Based on this research study, a simplified equation for the design of concrete filled columns at elevated temperature is proposed.

博士
朝陽科技大學
營建工程系
107
The study provides a technique to quantitatively and quickly assess the debonding range between steel plate and substrates for concrete-filled steel tubular member (CFST member) or reinforced concrete (RC) structure strengthened by steel plate with epoxy as the bonding agent. The interfacial debonding is evaluated by a parameter derived from the group velocity profile of zero-order antisymmetric mode (A0) of the steel plate with substrates. The technique involves single test with a transducer 400 mm away from a mechanical impact source. The spectrogram of the signal is obtained by Short Time Fourier Transformation (STFT) and reassigning technique. Numerical and experimental studies were performed for two layered (steel + concrete) and three layered (steel + epoxy + concrete) composite plates containing debonding crack with different lateral sizes. The effects of impact-sources, and the defect position related to the test line were also discussed. The experimental and numerical results show the different impact sources, steel plate thicknesses, the locations of test line and defect have no decisive influence on the debonding evaluation. The A0-slope is primarily affected by the length of the defect underneath the test path. The extend of debonds was evaluated from the velocity-normalized wavelength (NWL) diagram of A0 dispersion modal vibration extracted from the group slowness spectrogram. The near linear portions initiates from the wave lengths 2 times the steel plate-thickness for steel-concrete composite plate and 4 times for steel-epoxy-concrete composite plate. The relations of A0-slope and defect ratio for 2-layered and 3-layerd composite plates were obtained using fitting formulas of third degree polynomial. By establishing the formulas, in field study, the portion passing the debonding defect underneath the test line can be estimated by A0-slope. In addition, two case studies were proposed. The first case assessed the debonding between steel plate and concrete for CFST columns after fire damage. Another case was aimed for detecting the unfilled part at the gap of the RC floor bonding steel plate by epoxy resin. In the future, after coarse screening of the interfacial condition with the proposed method, more precise location of interfacial debond can be estimated using pointwise A-scan method, such as normalized impact-echo method. Therefore, an automated detection system can be developed for speedy testing.

The purpose of this research is to examine the behaviour of elliptical concrete-filled steel tubular stub columns under a combination of axial force and bending moment. Most of the research carried out to date involving concrete-filled steel sections has focussed on circular and rectangular tubes, with each shape exhibiting distinct behaviour. The degree of concrete confinement provided by the hollow section wall has been studied under pure compression but remains ambiguous for combined compressive and bending loads, with no current design provision for this loading combination. To explore the structural behaviour, laboratory tests were carried out using eight stub columns of two different tube wall thicknesses and applying axial compression under various eccentricities. Moment-rotation relationships were produced for each specimen to establish the influence of cross-section dimension and axis of bending on overall response. Full 3D finite element models were developed, comparing the effect of different material constitutive models, until good agreement was found. Finally, analytical interaction curves were generated assuming plastic behaviour and compared with the experimental and finite element results. Ground work provided from these tests paves the way for the development of future design guidelines on the member level.

碩士
國立臺灣科技大學
營建工程系
105
Concrete-Filled Steel Tube (CFST) often has a better performance than that of conventional steel or concrete columns with same size. In CFST, concrete is confined by steel and its strength and ductility are increased. On the other hand, concrete provides lateral support to prevent steel from local bucking. Thus, CFST often possesses more strength and more ductility. In Taiwan, it is important to ensure that the beam to column connection would not damage when earthquake is occurred. When designing beam to column joint, cost and constructability have to been considered. The purpose of this study is to build an accurate numerical model for future use. In this study, simulation was performed using commercial software ABAQUS. The shear force of the concrete-filled steel tube is provided by steel and concrete. The column shape studied here is square, it is known under the cycle loading, the strut and tie mechanism will be formulated in concrete. However, the shear behavior in steel web is unclear. This study attempts to investigate such behavior through numerical simulation. Results show that the built model is able to provide a satisfy simulation for steel tube specimen (without concrete) up to 4% drift. Results of CFST simulation are relatively inaccurate. Improvement could be achieved if the concrete parameter and contact surface setting in ABAQUS are properly applied .