Analysis and design of rectangular concrete-filled steel tubular members by considering size effect

The aim of this research was to develop an accurate and versatile FE model for rectangular CFST members by considering the size effect. The developed FE model could then be used to generate numerical data for developing accurate design equations to predict the ultimate strengths of rectangular CFST members. For this purpose, only reliable test data of composite stub columns tested in a displacement-controlled mode were used for calibrating the key parameters of the refined concrete model used in the FE analysis. It was found that the refined FE model could provide reasonable predictions about these specimens in terms of the initial stiffness, ultimate strength and post-peak behaviour. An extensive parametric analysis was then conducted using the refined FE model to generate a numerical database of short columns covering a wide range of geometric and material parameters. The revised design equations incorporating the size effect were suitable for use in the design of rectangular CFST stub columns, which was verified by both numerical and test data. The prediction errors were normally within 10% for both the small and the large columns. Reliability analysis was further performed for rectangular CFST stub columns, indicating that the revised design equations significantly improved the design reliability of large columns. Reliability analysis was further performed for rectangular CFST stub columns, indicating that the revised design equations significantly improved the design reliability of large columns. A parametric analysis was further conducted for slender CFST columns with slenderness ratios varying from 10 to 200. Meanwhile, the geometric and material parameters were also varied in the same ranges as in the previous analyses of stub columns. It was found that the ultimate moment of a CFST beam should be defined on the basis of the curvature instead of extreme fibre strains or deflection at mid-span. After evaluation, the EC4 equations were modified to improve the prediction accuracy of the ultimate strengths of RCFST beams and short beam-columns by incorporating the size effect and/or the local buckling effect. Finally, the proposed equations were used to predict the ultimate strengths of slender beam-columns. A significant improvement in prediction accuracy over that of the existing EC4 approach was also found, and the prediction errors were normally within the 10% discrepancy limit

Étude théorique et analyse non-linière par éléments finis pour la conception de colonnes en béton armé confinées à l’aide de tube en PRF

Les matériaux composites en polymères renforcés de fibres (PRF) ont été utilisés largement dans le domaine de la construction en génie civil, particulièrement pour les structures exposées à un environnement corrosif. L'utilisation des tubes en polymères renforcés de fibres (PRF) est une technique innovante pour les éléments de structures en béton armé tels que les colonnes, les piliers et les poutres, où les tubes en PRF sont utilisés comme coffrage permanent. Des recherches précédentes ont été effectuées pour comprendre le comportement des colonnes (CFFT) sous chargement axial mais il existe très peu de données concernant le comportement des colonnes en béton armé et renforcées de tubes en PRF sous chargement excentrique. Cette thèse présente des données expérimentales, une analyse théorique approfondie et des recommandations de conception pour colonnes cylindriques CFFT armées de barres d'acier ou de barres en polymères renforcés de fibres de carbone (CFRP). Les colonnes CFFT ont été testées sous un chargement monotone avec différents niveaux d'excentricité. Le rapport d'excentricité (e / D), et le type d'armature longitudinale (barre CFRP versus barre en acier) sont considérés comme des variables pour tous les essais effectués. Le diamètre et la hauteur de chaque spécimen sont égaux à 152mm, 912mm respectivement. L'angle d'orientation des fibres du tube a été principalement dans la direction circonférentielle (± 60 degrés par rapport à l'axe longitudinal). Six barres d'armature (acier ou CFRP) ont été utilisées et sont réparties uniformément dans chaque échantillon. Les résultats de cette étude ont révélé que les échantillons armées avec des barres en PRFC se comportent de manière très similaire aux échantillons armés de barres en acier et atteignent, à toute fin pratique, les mêmes résistances axiales. Le mode de rupture des échantillons de CFFT a été dominé par l'instabilité globale des colonnes ainsi que par la combinaison de la rupture en traction du tube en PRF et des barres en PRFC ou en acier. Les résultats expérimentaux de la déformation ont montré que les barres de PRFC développent des déformations élevées sur les côtés de compression (Valeur maximale en compression -5000 lm) et de traction (Valeur maximale en traction 10,400 gE), ainsi les barres PRFC résistent mieux aux contraintes de la traction et de la compression. En outre, la contrainte de traction longitudinale maximale enregistrée dans le tube en PRF est considérée comme étant une contrainte faible par rapport à celle enregistrée dans la direction circonférentielle du tube en PRF. D'après les résultats expérimentaux enregistrés, le confinement induit par le tube en PRF est moins important dans le cas d'une colonne sous charge excentrée. Des diagrammes expérimentaux d'interaction charge axiale-moment ont été présentés pour déterminer l'enveloppe de rupture des échantillons CFFT armées de barres en acier ou de barres en PRFC. De plus, une analyse théorique a été développée pour calculer les résistances des colonnes CFFT soumises à un chargement excentrique. Une comparaison avec les résultats expérimentaux a été effectuée. Aussi, une analyse théorique basée sur l'approche couche par couche a été développée pour prédire la réponse moment versus courbure des colonnes CFFT armées de barres en acier ou de barres en PRFC. Ces résultats ont été comparés aux résultats expérimentaux des courbes de moment-courbure. Il a été conclu que quelques soient le type d'armature (acier versus PRFC) pour les colonnes CFFT et la valeur de l'excentricité, le comportement moment-courbure de tous les échantillons est non linéaire. Par ailleurs, une étude approfondie a été effectuée sur la rigidité en flexion (El) effective des colonnes CFFT. Cette étude est basée sur une étude paramétrique expérimentale et une simulation théorique. Les équations proposées ont été développées et validées par rapport aux résultats expérimentaux afin de représenter la rigidité des colonnes CFFT armées de barres en acier ou de barres en FRPC. Ces équations sont établies pour deux états-limites : les états-limites de service et les états-limites ultimes. Aussi, une formule précise pour la prédiction du taux d'élancement pour contrôler le mode de rupture par flambement pour les colonnes CFFT armées de barres en PRFC a été proposée. Il a été établi qu'un taux d'élancement égal à 14 présente une valeur sécuritaire pour la conception de ces colonnes CFFT en béton armé. Enfin, un modèle non-linéaire par éléments finis utilisant le logiciel ABAQUS a été développé et présenté sur la base d'un modèle de béton confiné « Lam et Teng» prenant en considération la non- linéarité matérielle et géométrique des colonnes CFFT. Ce modèle permet de fournir un aperçu sur le comportement de la structure et du mécanisme de rupture des colonnes CFFT.Abstract: Fibre-reinforced polymer (FRP) composite materials have been extensively used in the field of civil engineering construction, especially in structures subjected to corrosive environments. One of the innovative techniques for using FRP is the FRP tubes which can be used as structurally integrated stay-in-place forms for concrete members as columns, piles, piers and beams. Extensive research was carried out to understand the behavior of concrete-filled FRP tube (CFFT) columns under axial loading, but comparatively limited research was conducted on the reinforced CFFT columns under eccentric loading. This thesis aims to provide experimental work as well as extensive theoretical analysis and design recommendations of circular CFFT columns reinforced with steel bars or carbon fibre reinforced polymer (CFRP) bars. CFFT columns were tested under monotonic loading with different levels of eccentricity. Test variables included the eccentricity to diameter ratio (e/D) and reinforcement type (CFRP bars vs steel). All specimens measured 152 mm in diameter and 912 mm height. The tubes used is basically filament wound glass fibre reinforced polymer tube (GFRP) with a core diameter of 152 mm and a wall thickness of 2.65 mm (6 layers). The fibre orientation of the tube was mainly in the hoop direction (± 60 degree with respect to the longitudinal axis). Six reinforcing bars (steel or CFRP) were used and distributed uniformly in each specimen. Test results indicate that specimens with CFRP reinforcement (CFRP-CFFT) behaved very similar to their steel counterparts with nearly the same nominal axial forces. Failure of CFFT columns was dominant by overall instability of the columns along with the combination of tensile rupture of FRP tube and CFRP bars or steel yielding. Experimental strain results revealed that the CFRP bars developed high strains on the compression and tension sides, thus CFRP bars contribution was considered effective in resisting tensile and compressive stresses. In addition, the maximum tensile strain reached in the GFRP tube was considered low when compared to the GFRP hoop strain, thus, it was concluded that the confinement induced by the GFRP tube become less significant in the case of eccentrically loaded column. Experimental axial-moment interaction diagrams were presented to indicate the failure envelope of steel and CFRP reinforced CFFT columns. Moreover, a theoretical model was developed to indicate the axial-moment capacities of steel and CFRP reinforced CFFT columns using plane sectional analysis and compared to the experimental results counterparts. Theoretical sectional analysis based on layer by layer approach was developed to predict the moment-curvature response for steel and CFRP- reinforced CFFT columns. These results were compared to the experimental moment-curvature curves and it was clear that regardless of the type of reinforcement and value of eccentricity, all specimens exhibit non-linear moment curvature behavior. An extensive study was conducted on the effective flexural stiffness of CFFT columns, on the basis of experimental parametric study and theoretical simulation. Proposed equations were developed and validated against the experimental results to represent the stiffness of steel and CFRP- CFFT circular columns at service and ultimate loads. Moreover, a theoretical investigation was conducted to propose a more precise formula for the critical slenderness limit to control the buckling mode of failure of FRP-reinforced CFFT columns. It was found that the critical slenderness limit of 14 could be used as a safe value for practical design purposes. The theoretical analysis in this research was carried out using excel. Finally, a nonlinear finite element model using ABAQUS software was presented based on Lam and Teng confined concrete model considering material and geometric nonlinearities of CFFT columns. This model aims to provide insight on the structure behavior and failure mechanism of CFFT columns

Behavior of LWSCC columns reinforced with FRP bars under axial and eccentric loads

Ces dernières années, l'intérêt s'est accru pour l'utilisation des polymères renforcés de fibres (PRF) dans les structures de génie civil. De nombreuses études ont été menées à ce jour sur l’étude du comportement de poteaux en béton de densité normale armé de barres de PRF. Le béton autoplaçant (BAP) léger peut être d'un grand intérêt pour réduire les charges permanentes, les dimensions des sections et les coûts des projets, en particulier pour les éléments préfabriqués. Cependant, très peu d’études ont été menées sur le comportement structural de poteaux en béton léger. De plus, il n’y a pas d’étude antérieure sur le comportement sous charges axiales et en flexion de poteaux en béton léger armé d’armatures de PRF. Le béton léger peut présenter un comportement plus fragile que le béton de densité normale. En outre, la fragilité du béton peut affecter non seulement le mode de rupture, mais aussi la résistance à la compression des éléments en béton léger. Le présent projet de recherche tend donc à pallier le manque de données expérimentales en proposant l’étude du comportement de poteaux en BAP léger armé de PRF. Il présente ainsi une étude expérimentale visant à optimiser une formulation de BAP léger et à étudier les performances de poteaux circulaires en BAP léger renforcés avec des barres et des spirales en PRF sous charges de compression centrées et excentrées. Les paramètres d’étude sont le taux d’armature longitudinale, l'espacement et le pas des armatures transversales, le diamètre des barres longitudinales, le type d’armature longitudinale, le type de confinement et l'excentricité de la charge appliquée. De plus, ce projet de recherche propose un modèle d’apprentissage automatique pour prédire la résistance de poteaux en béton armé avec des armatures de PRF. Les résultats expérimentaux ont montré que l’utilisation de barres longitudinales de PRFV, de PRFB et de spirales en PRFV n’a pas affecté les performances des poteaux en BAP léger. Le comportement des poteaux en BAP léger de PRFV et de PRFB était très similaire à celui de poteaux en béton armé d’acier. Les patrons de fissuration étaient également assez similaires, à la différence que pour les poteaux en BAP léger, les plans de rupture passaient à travers les granulats légers ou se produisaient à l’interface entre les gros granulats légers et la pâte de ciment. L’augmentation du taux d’armature longitudinale de 2,2% à 3,3% n’a pas beaucoup affecté la résistance des poteaux en BAP léger armé de PRFV testés en compression centrée. Cette augmentation a cependant affecté le comportement post-pic des poteaux. La prise en compte de la contribution en compression des barres de PRFV et de PRFB dans les relations de compatibilité des déformations et d’équilibre des forces a conduit à des prédictions précises des diagrammes d’interaction expérimentaux charge axiale-moment. Enfin, le modèle d'apprentissage automatique XGBoost proposé a montré une excellente performance pour prédire la charge axiale maximale des poteaux en béton armé de PRF.Abstract : In recent years, interest levels have risen in the use of fibre-reinforced polymers (FRPs) in civil engineering structures. Valuable research has been conducted to date on the study of the flexural and axial performance of normalweight concrete columns reinforced with FRP bars. Lightweight aggregate self-consolidating concrete (LWSCC) can be of great interest for reducing dead loads, section dimensions and project costs specially for precast elements. However, very few studies have been conducted on the structural behavior of lightweight concrete columns. In addition, there are no previous studies on the axial and flexure behavior of FRP-reinforced lightweight concrete columns. Lightweight concrete can be more brittle than normalweight concrete. Furthermore, concrete brittleness might not only affect the failure mode, but also the compressive strength of lightweight concrete members. The present study attempts to provide an experimental database on the behavior of FRPreinforced LWSCC columns. This study presents an experimental study aimed at optimising an LWSCC mix and investigating the performance of circular LWSCC columns reinforced with FRP bars and spirals subjected to axial and eccentric loads. The study parameters are the longitudinal reinforcement ratio, the spacing and pitch of the transverse reinforcement, the diameter of the longitudinal bars, the type of longitudinal reinforcement, the type of confinement and the eccentricity of the applied load. In addition, this research project proposes a machine learning model to predict the maximum axial loadcarrying capacity of FRP-RC columns. Experimental results showed that the use of GFRP, BFRP longitudinal bars, and GFRP spirals did not affect the performance of LWSCC columns. The behavior of LWSCC columns was very similar to that of steel-reinforced concrete columns. The cracking patterns were also quite similar, with the difference that for the LWSCC columns, the failure planes passing through the lightweight aggregates or occurring at the interface between the coarse aggregate and cement paste. The increase in the longitudinal reinforcement ratio from 2.2% to 3.3% did not significantly affect the strength of the LWSCC columns tested under axial loads. This increase did, however, affect the post-peak behavior of the columns. Considering the compression contribution of the BFRP and GFRP bars in the strain compatibility and force equilibrium, analysis provided accurate predictions of the experimental P–M interaction diagrams. Finally, the proposed XGBoost machine learning model showed an excellent performance and was suitable for predicting the maximum axial load-carrying capacity of FRP-RC columns

Axial-Flexural Behaviour of Reinforced Concrete Masonry Columns Confined by FRP Jackets

Confining existing concrete and masonry columns by Fibre Reinforced Polymers (FRP) is a beneficial method for enhancing the column capacity and ductility. The popularity of using FRP for strengthening and upgrading columns is mainly attributed to the high strength and lightweight characteristics of the FRP materials. Using FRP composites reduces additional dead load associated with traditional strengthening solutions and simplify the application in areas with limited access. The goal of this research is to experimentally quantify the enhancement in strength and strain capacity of Carbon FRP (CFRP) confined concrete masonry columns under concentric and eccentric loading. Research on FRP-strengthened concrete masonry columns under eccentric loads is essential to understand the effect of this retrofitting technique on the performance of columns. The experimental data was then used to propose a simplified methodology that predicts the axial force-moment interaction diagram of fully grouted reinforced concrete masonry column strengthened with FRP jackets. The methodology considers short prismatic reinforced concrete masonry columns failing in a compression controlled manner and complies with equilibrium and strain compatibility principles. To achieve the research goals, 47 scaled fully grouted concrete block masonry columns were tested under concentric, eccentric, and bending loading up to failure. Parameters investigated in this research include the thickness of CFRP jacket, corner radius of cross section and the magnitude of eccentricity. The proposed analytical methodology showed a good correlation with the experimental data. Parametric study was carried out to determine the effect of design variables on the axial-flexural interaction of fully grouted reinforced concrete masonry column strengthened by FRP jackets

Nonlinear analysis of circular high strength concrete-filled stainless steel tubular slender beam-columns

Concrete-filled stainless steel tubular (CFSST) slender columns are increasingly used in composite structures owing to their distinguished features, such as aesthetic appearance, high corrosion resistance, high durability and ease of maintenance. Currently, however, there is a lack of an accurate and efficient numerical model that can be utilized to determine the performance of circular CFSST slender columns. This paper describes a nonlinear fiber-based model proposed for computing the deflection and axial load-moment strength interaction responses of eccentrically loaded circular high-strength CFSST slender columns. The fiber-based model incorporates the accurate three-stage stress-strain relations of stainless steels, accounting for different strain hardening characteristics in tension and compression. The material and geometric nonlinearities as well as concrete confinement are included in the computational procedures. Existing experimental results on axially loaded CFSST slender columns are utilized to verify the proposed fiber-based model. A parametric study is conducted to examine the performance of high-strength slender CFSST beam-columns with various geometric and material parameters. It is shown that the fiber-based analysis technique developed can accurately capture the experimentally observed performance of circular high-strength CFSST slender columns. The results obtained indicate that increasing the eccentricity ratio, column slenderness ratio and diameter-to-thickness ratio remarkably decreases the initial flexural stiffness and ultimate axial strength of CFSST columns, but considerably increases their displacement ductility. Moreover, an increase in concrete compressive strength increases the flexural stiffness and ultimate axial strength of CFSST columns; however, it decreases their ductility. Furthermore, the ultimate axial strength of CFST slender columns is found to increase by using stainless steel tubes with higher proof stresses

Shear-flexure-axial load interaction in rectangular concrete bridge piers with or without FRP wrapping

Doctor of PhilosophyDepartment of Civil EngineeringHayder RasheedRecent applications in reinforced concrete columns, including strengthening and extreme loading events, necessitate the development of specialized nonlinear analysis methods to predict the confined interaction domain of axial force, shear, and bending moment in square and slightly rectangular concrete columns. Fiber-reinforced polymer (FRP) materials are commonly used in strengthening applications due to their superior properties such as high strength-to-weight ratio, high energy absorption and excellent corrosion resistance. FRP wrapping of concrete columns is done to enhance the ultimate strength due to the confinement effect, which is normally induced by steel ties. The existence of the two confinement systems changes the nature of the problem. Existing research focused on a single confinement system. Also, very limited research on rectangular sections was found in the literature. In this research, a model to estimate the combined behavior of the two systems in rectangular columns is proposed. The calculation of the effective lateral pressure is based on Lam and Teng model and Mander model for FRP wraps and steel ties, respectively. The proposed model introduces load eccentricity as a parameter that affects the compression zone size, and in turn the level of confinement engagement. Full confinement corresponds to zero eccentricity, while unconfined behavior corresponds to infinite eccentricity. The model then generates curves for eccentricities within these boundaries. The numerical approach developed has then been extended to account for shear interaction using the simplified modified compression field theory adopted by AASHTO LRFD Bridge Design Specifications 2014. Comparisons were then performed against experimental data and Response-2000, an analytical analysis tool based on AASHTO 1999 in order to validate the interaction domain generated. Finally, the developed models were implemented in the confined analysis software “KDOT Column Expert” to add FRP confinement effect and shear interaction

Development of Finite Element Techniques to Simulate Concrete-Filled Fiber-Reinforced Polymer Tube Structures

This dissertation presents the development of finite-element (FE) techniques to simulate the behavior of concrete-filled fiber reinforced polymer (FRP) tubes (CFFTs) in support of more effective structural design and analysis methods for buried composite arch bridges (BCABs) that use CFFT arches as main structural members. The research includes three specific topics to make contributions in different aspects of the investigation of these complex structures. The first topic is the nonlinear three-dimensional FE modeling of steel-free CFFT splices. For model validation, comparisons were made between the model predictions and control beam and spliced beams with and without internal collars tested by others. The modeling was complex due to the need to capture the nonlinear constitutive response of the confined concrete, simulate concrete-FRP interaction, and explicitly incorporate the splice components. Therefore, the numerical analysis utilized the Abaqus/CAE software package with a modified damage concrete plasticity model to idealize the concretefill. The second topic of this research is the development of a computationally efficient structural FE analysis technique for the second-order inelastic behavior of these CFFT arches that includes initial arch curvature. A curved, planar, corotational, flexibility-based (FB), layered frame element is employed to handle geometric and material nonlinearities. An FRP-confined concrete stress-strain model that explicitly considers the effect of dilation of the concrete core and confinement provide by the FRP tube is implemented. Verification of the FB formulation was carried out for elastic-plastic analysis of a beam and elastic post-buckling analysis of a circular arch. The measured flexural responses of different isolated CFFT arches available in the literature were used to verify the proposed model. The model was shown to accurately predict the load-carrying capacity and ductility of the tested CFFT arches. The model captured arch collapse mechanisms arising from FRP rupture and concrete crushing at the apex of the arches. The third topic is an extension of the planar FB model to three-dimensions and incorporation of a soil-spring model to simulate soil-structure interaction using a recently developed horizontal earth pressure model. The model rigorously incorporates the interaction between axial load and bending effects in the arches and permits the examination of out-of-plane stability and arch deformations due to bridge skew. Parametric studies were conducted to assess the effect of abutment skew angle on the behavior of CFFT arch bridge components, an important practical design consideration