Thesis (MSc)--Stellenbosch University, 2015. / ENGLISH ABSTRACT: Concrete filled double skin tube (CFDST) columns is a new method of column construction. CFDST columns consists of two steel hollow sections, one inside the other, concentrically aligned. The cross-sections of the two hollow sections does not have to be the same shape. Concrete is cast in between the two hollow sections resulting in a CFDST. This study only considers CFDST columns constructed with circular steel hollow sections. The advantages of CFDST construction include: ● The inner and outer steel hollow sections replaces the traditional steel reinforcement that would be used in a normal reinforced concrete column. This reduces the construction time since there is no need to construct a reinforcing cage. ● The steel hollow sections acts as a stay in place formwork, eliminating the need for traditional formwork. This also reduces construction time. ● The steel hollow sections confine the concrete, making it more ductile and increasing its yield strength. The objective of this study is to identify methods that can predict the axial capacity of eccentrically loaded circular CFDST columns. Methods chosen for the investigation are: 1. Finite element model (FEM). A model was developed to predict the behaviour of eccentrically loaded CFDST columns. The FE model uses a concrete material model proposed in literature for stub columns. The aim was to determine whether the material model is suited for this application. 2. The failure load of CFDST columns under concentric loading was calculated using a model obtained in literature. These capacities were compared to the experimental test results of eccentrically loaded CFDST columns to establish a correlation. This study found that the concrete material model used does not adequately capture the behaviour resulting in the axial response of the column being too stiff. The difference between the eccentrically loaded experimental test results and the calculated concentrically loaded capacity showed a clear trend that could be used to predict the capacity of eccentrically loaded CFDST columns. / AFRIKAANSE OPSOMMING: Beton-gevulde dubbel laag pyp (BGDLP) kolomme is ‘n nuwe metode van kolom konstruksie. BGDLP kolomme bestaan uit twee staal pyp snitte, die een binne die ander geplaas met hul middelpunte opgelyn, die dwarssnit van die twee pype hoef nie dieselfde vorm te wees nie. Beton word dan in die wand tussen die twee pyp snitte gegiet. Die resultaat is ‘n hol beton snit. Hierdie studie handel slegs oor BGDLP kolomme wat met ronde pyp snitte verwaardig is. Die volgende voordele kan aan BGDLP toegeken word: ● Die binne en buite staalpype vervang die tradisionele staal bewapening was in normale bewapende-beton gebruik sou word. Dus verminder dit die tyd wat dit sal neem om die kolom op te rig. ● Die staalpypsnitte is ook permanente vormwerk. Dit doen dus weg met die gebruik van normale bekisting, wat ook konstruksie tyd spaar. ● Die buite-staalpypsnit bekamp die uitsetting van die beton onder las. Hierdie bekamping veroorsaak dat die beton se gedrag meer daktiel is en ‘n hoër falings spanning kan bereik. Die doel van die studie is om metodes te identifiseer wat gebruik kan word om die aksiale kapasiteit onder eksentriese laste van BGDLP kolomme te bepaal. Twee metodes was gekies: 1. Eindige element model. ‘n Model was ontwikkel om die gedrag van BGDLP kolomme te voorspel. Die mikpunt was om te bepaal of ‘n beton materiaal gedrag model vanuit die literatuur gebruik kan word om BGDLP kolomme te modelleer. 2. Die swiglas van BGDLP kolomme onder konsentriese belasting was bereken vanaf vergelykings uit die literatuur. Hierdie swiglaste was vergelyk met die eksperimentele toets resultate vir eksentriese belaste BGDLP kolomme om ‘n korrelasie te vind. Hierdie studie het bewys dat die beton materiaal model uit die literatuur kan nie gebruik word om die swiglaste van BGDLP kolomme te bepaal nie. Die model het die gedrag te styf gemodelleer. Die verskil tussen die berekende konsentriese belaste swiglas en die eksperimentele resultate van eksentriese BGDLP kolomme was voorspelbaar en kan gebruik word om die swiglas van eksentriese belaste BGDLP kolomme te voorspel.

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No / Concrete filled steel tube (CFST) columns are becoming increasingly popular due to the advantages they offered. They are not only considered aesthetically pleasing but can also offer significant improvement in axial capacity without increases in crosssectional area being required. Elliptical steel hollow sections represent a recent and rare addition to the range of cross-sections available to structural engineers, however, despite widespread interest in their application, a lack of verified design guidance is inhibiting uptake. The use of elliptical steel hollow section with concrete infill is new and innovative, not only provides the advantage mentioned above, but also on the basis of both architectural appeal and structural efficiency. The aim of this paper is to investigate the behaviour of the elliptical CFSTs under axial loading. A total of 12 specimens were tested with wall thicknesses of 4 mm, 5 mm, 6.3 mm and concrete core strength of 30 MPa. This paper reported on the behaviour of concrete filled elliptical hollow sections under axial load. The effect of the wall thickness of the steel section, the bond between steel and concrete and the concrete confinement are presented.

Concrete masonry is a multi-component structural system. In the case of reinforced concrete masonry, the system includes the concrete units, the mortar, the reinforcing steel and the grout fill. Placing vertical steel reinforcing bars in the cores of the concrete units enhances the flexural strength of the wall. The vertical steel, when subjected to compression at moderate strain levels, must be confined to improve its resistance to buckling and to improve the effectiveness of the grout around the reinforcing bar. Based on the well established behaviour of reinforced concrete systems, it seems reasonable to presume that the primary means of enhancing ductility is to provide lateral confining steel at closely-spaced intervals to effectively increase the ultimate compressive strain in the grout. It may be assumed that transverse reinforcement in concrete masonry provides lateral confinement to the core so that the axial compressive strength of the grout is enhanced and the ductility improved. <p>The focus of this study was to investigate the effect of vertical reinforcement and lateral confinement on the axial capacity of short partially grouted concrete masonry walls built in running bond. In order to better understand the structural behaviour of both confined and unconfined concrete masonry, it is important to have some knowledge of the load-displacement behaviour, stress-strain behaviour and failure modes of the masonry walls with different configurations of vertical and lateral reinforcement. <p>An experimental study was performed to investigate the behaviour of partially grouted concrete masonry block walls under axial loading. Three types of test specimens of partially grouted concrete block masonry walls were tested: <p>(1) specimens with a grouted core only; <p>(2) specimens with a grouted core and vertical reinforcement (i.e. no confinement); and <p>(3) specimens with a grouted core, vertical reinforcement and spiral confinement in the grouted cores. In total, thirty short wall specimens were tested to failure. <p>The structural behaviour of vertically reinforced, laterally confined walls was compared to vertically reinforced, unconfined walls, as well as to unreinforced, unconfined masonry walls. The test results indicated that vertical reinforcement of the grouted core did not have a significant positive effect on the failure modes and strength of the short masonry walls. Due to problems with adequate compaction, the lateral confinement provided by the spiral reinforcement had a slightly negative effect on the compressive strength of concrete masonry walls built in running bond. Vertical reinforcement and lateral confinement of the grouted core had some positive effect on the ductility. From a comparison of the ductility for all three types of specimens it was found that both the vertical reinforcement and lateral confinement of the core had a beneficial influence on the post-peak ductility. <p>In general, similar crack patterns and failure modes were observed in all three types of specimens. Vertical cracks that progressed through the end faces of the concrete blocks and mortar joints, suggesting that the lateral expansion of the grouted core contributed to tensile splitting stresses in walls. All walls failed in a compression-tension stress state, which featured spalling away of the block shells and vertical tensile splitting on the end faces.

Concrete masonry is a multi-component structural system. In the case of reinforced concrete masonry, the system includes the concrete units, the mortar, the reinforcing steel and the grout fill. Placing vertical steel reinforcing bars in the cores of the concrete units enhances the flexural strength of the wall. The vertical steel, when subjected to compression at moderate strain levels, must be confined to improve its resistance to buckling and to improve the effectiveness of the grout around the reinforcing bar. Based on the well established behaviour of reinforced concrete systems, it seems reasonable to presume that the primary means of enhancing ductility is to provide lateral confining steel at closely-spaced intervals to effectively increase the ultimate compressive strain in the grout. It may be assumed that transverse reinforcement in concrete masonry provides lateral confinement to the core so that the axial compressive strength of the grout is enhanced and the ductility improved. <p>The focus of this study was to investigate the effect of vertical reinforcement and lateral confinement on the axial capacity of short partially grouted concrete masonry walls built in running bond. In order to better understand the structural behaviour of both confined and unconfined concrete masonry, it is important to have some knowledge of the load-displacement behaviour, stress-strain behaviour and failure modes of the masonry walls with different configurations of vertical and lateral reinforcement. <p>An experimental study was performed to investigate the behaviour of partially grouted concrete masonry block walls under axial loading. Three types of test specimens of partially grouted concrete block masonry walls were tested: <p>(1) specimens with a grouted core only; <p>(2) specimens with a grouted core and vertical reinforcement (i.e. no confinement); and <p>(3) specimens with a grouted core, vertical reinforcement and spiral confinement in the grouted cores. In total, thirty short wall specimens were tested to failure. <p>The structural behaviour of vertically reinforced, laterally confined walls was compared to vertically reinforced, unconfined walls, as well as to unreinforced, unconfined masonry walls. The test results indicated that vertical reinforcement of the grouted core did not have a significant positive effect on the failure modes and strength of the short masonry walls. Due to problems with adequate compaction, the lateral confinement provided by the spiral reinforcement had a slightly negative effect on the compressive strength of concrete masonry walls built in running bond. Vertical reinforcement and lateral confinement of the grouted core had some positive effect on the ductility. From a comparison of the ductility for all three types of specimens it was found that both the vertical reinforcement and lateral confinement of the core had a beneficial influence on the post-peak ductility. <p>In general, similar crack patterns and failure modes were observed in all three types of specimens. Vertical cracks that progressed through the end faces of the concrete blocks and mortar joints, suggesting that the lateral expansion of the grouted core contributed to tensile splitting stresses in walls. All walls failed in a compression-tension stress state, which featured spalling away of the block shells and vertical tensile splitting on the end faces.

Dans le cadre du Projet National SOLCYP, le présent travail avait pour but d'étudier la réponse des pieux sous chargements axiaux pour les phases avant, pendant et après cycles. Pour atteindre cet objectif, une étude expérimentale étendue a donc été réalisée par le moyen de deux compagnes d'essais de chargements cycliques sur différents types de pieux instrumentées (battus, forés, vissés) dans deux sites expérimentaux : le site de Merville dans l'argile surconsolidée des Flandres et le site de Loon-Plage près de Dunkerque dans les sables denses des Flandres. Les essais cycliques comportaient des essais en compression, en traction et alternés avec des séries de cycles à forte amplitude conduisant à la rupture cyclique et des séries d'amplitude modérée à très grands nombres de cycles (N>5 000 cycles).L'interprétation de ces essais a englobé, d'une part, une analyse globale du comportement des pieux sous chargements cycliques notamment (i) la réduction de la capacité du pieu; (ii) le nombre de cycles que le pieu peut support avant la rupture (iii) l'évolution du déplacement en tête des pieux (rigidité cyclique). D'autre part, elle a intégré une étude locale en particulier sur la dégradation du frottement latéral et l'évolution de la résistance de pointe du pieu. Cette thèse, basée sur l'étude expérimentale, a permis la formulation des conclusions pratiques sur le comportement des pieux sous chargements cycliques axiaux et a proposé des perspectives pour bien cerner cette problématique / As part of the French National research project SOLCYP, the purpose of this work was study the behavior of the piles under axial loadings for the phases before, during and after cycles.To achieve this objective, an extensive series of static and cyclic axial pile load tests have been carried out in two experimental sites of the North of France: the overconsolidated Flandrian clays (Merville experimental site) and in dense Flandrian sands (Loon-Plage experimental site). Tests were performed on driven closed-ended pipe piles, bored piles and screwed. All piles were instrumented with retrievable extensometers for measuring the load distribution along the pile wall.Cyclic load tests were composed of series of cycles of constant load amplitude. A large range of load histories were applied including series of small amplitude cycles and great number of cycles (N > 5000) and series of large amplitude cycles leading to cyclic failure after a small number of cycles..A large volume of data has been interpreted to describe the effects of axial cyclic loads on the behavior of the piles. The interpretation of these tests included, on the one hand, a global analysis focusing on (i) the potential reduction on the ultimate axial capacity; (ii) the number of load cycles of a given load that the pile can sustain before cyclic failure and (iii) the evolution of displacements of the pile head during cyclic loading (pile stiffness). In addition, it integrated a local study in particular on shaft friction degradation along the pile wall and evolution of the resistance of point of the pile.This thesis, based on the experimental study, allowed the formulation of the practical conclusions on the behavior of the piles under axial cyclic loadings and proposed prospects for determining these problems well

La capacité axiale et latérale des pieux foncés dans du sable de Fontainebleau NE34 ont été étudié à l’aide d’essais sur modèles réduits centrifugés. L’effet de la méthode d’installation, de la densité et de la saturation du sable, du diamètre du pieu, de la géométrie de sa pointe (ouvert /fermé) et de sa rugosité sur la capacité axiale a été étudié. Une augmentation significative de la capacité en traction est observée dans les pieux foncés cycliquement, contrairement aux pieux foncés d’une manière monotone à 100 × g. La saturation du sable dense accélère la formation du bouchon lors de l'installation du pieu. L'augmentation de la rugosité du pieu et de la densité du sable accroissent significativement le frottement latéral des pieux testés. Dans tous les cas, les capacités de pieux sont comparées aux codes de dimensionnement des éoliennes offshore. Une étude paramétrique de l'effet de la méthode d'installation, de l'excentricité de la charge et de la saturation du sable sur la réponse latérale des pieux foncés est ensuite réalisée grâce à l'utilisation d'un pieu instrumentée. Le pieu est chargé d’une manière monotone puis un millier de cycles sont appliqués. Une nouvelle méthode a été développée pour la détermination des constantes d'intégration pour déterminer le profil de déplacement latéral du pieu. La méthode d'installation influence directement le comportement global (moment maximum et déplacement latéral) et local (courbes p-y) des pieux. L'effet de l'excentricité de la charge et de la saturation du sable sur le comportement des pieux est également présenté. Dans chaque cas, une comparaison avec les courbes p-y extraites du code DNVGL est réalisée. / The axial and lateral capacity of piles jacked in Fontainebleau sand NE34 are studied using centrifuge modelling at 100×g. The effect of the installation method, sand density and saturation, pile diameter and pile tip geometry (open or closed-ended) and pile roughness on the axial capacity of piles are firstly studied. A significant increase in the tension capacity is observed in cyclically-jacked piles unlike piles monotonically jacked at 100×g. The saturation of dense sand accelerates plug formation during pile installation. The increase in pile roughness and sand density increases significantly the shaft resistance of the piles tested here. For all the cases, pile capacities are compared with the current design codes for offshore wind turbines. A parametric study of the effect of the installation method, load eccentricity and sand saturation on the lateral response of jacked piles is then realized using of an instrumented pile. The pile is loaded monotonically, then a thousand cycles are applied. A new methodology has been developed for determining of the constants needed in the integration procedure to identify the lateral displacement profile of the pile. The installation method influences directly the global (maximum moment and lateral displacement) and local behaviour (p-y curves) of the piles. The effect of the load eccentricity and sand saturation on the behaviour of the piles is also presented. In each case a comparison with the p-y curves extracted from the DNVGL code is realized.

L’état de l’art réalisé dans le cadre de ce travail a permis d’identifier une grande variété de techniques de réalisation de micropieux avec des comportements effort-déformation difficiles à prédire. Quant aux méthodes de contrôle, du point de vue de l’intégrité il existe deux méthodes développées pour les pieux lesquelles ne sont pas toujours applicables aux cas de micropieux ; et du point de vue du contrôle du comportement mécanique, il existe un essai assez utilisé pour le contrôle de micropieux : l’essai d’arrachement. Cet essai est facile à réaliser et à interpréter, cependant il est onéreux (long temps d’essai) et difficile à mettre en place. Un autre type d’essai, utilisé dans le contrôle de fondations profondes, est l’essai de chargement dynamique. La recherche bibliographique a permis d’identifier les avantages et inconvénients de ce type d’essai adaptés et déjà validés dans le contrôle de pieux. Nous avons donc proposé une nouvelle méthodologie basée sur des essais de chargement dynamique à faible contrainte nécessitant des énergies de battage beaucoup moins importantes que dans les essais de chargement dynamique classiques. Le protocole et dispositif d’essai de la méthodologie ont été calibrés et validés à partir de simulations numériques afin notamment de justifier les choix de l’énergie d’impact, de la masse du marteau et de l’amortisseur à utiliser. Les observations numériques sont ensuite vérifiées expérimentalement sur des micropieux à échelle réelle mis en place dans un site expérimental développé dans le cadre de ce projet. La méthodologie de contrôle en place de la résistance en service de micropieux développée, c’est une méthode originale simple à réaliser, facile à mettre en oeuvre et d’interprétation immédiate. Elle a été validée aussi dans le cas de micropieux à échelle réelle en conditions maitrisées dans le site expérimental et non maitrisées dans trois chantiers réels de micropieux. Finalement, le progrès dans les recherches, a permis le développement d’une méthodologie de contrôle de la résistance limite de micropieux. Pour cela nous proposons d’utiliser un modèle analytique de prédiction du comportement effort-déformation alimenté par les résultats de l’essai de chargement dynamique à faible déformation et d’un essai complémentaire de reconnaissance des paramètres à la rupture du sol. L´objectif de ce développement est de proposer une prédiction de la résistance limite du micropieux testé sans compromettre son intégrité. L’application de la méthodologie a montré qu’elle donne des résultats très proches de ceux observés dans les essais statiques. / The review of the state-of-the-art of micropiles produced as part of this work allows to identify a wide variety of construction methodologies which have a stress-strain behavior difficult to predict. As for the testing methods, there are methods for assessing the condition of piles or shafts but they are not always applicable to the micropiles; and in terms of the control of mechanical behavior, the most used load testing micropile is the uplift static load test. The static load test is easy to perform and interpret, however the reaction system is cumbersome and slow to erect, therefore this test is time and cost consuming. Another type of test that can be used in the control of deep foundations is the dynamic load testing. The literature review helped to identify the advantages and disadvantages of this type of test that has been adapted and validated in control of pile behavior. The new methodology proposed in this work is based on low-strain dynamic test requiring a smaller impact energy than a high-strain dynamic test. The testing protocol and loading device of the methodology have been calibrated and validated by numerical simulations in order to justify the choice of the impact energy, the weight of hammer and the cushion that will be used. The numerical observations are then verified by experiences in full-scale micropiles installed in an experimental site developed as part of this project. The developed in situ control methodology of the resistance in service of micropiles is an original method, easy to carry out, simple to execute and with an immediate interpretation. This methodology has been validated on full-scale micropiles in contained conditions and also in uncontained conditions in three real projects of micropiles. Finally, the advances in the investigations have led to develop a methodology for control the micropile’s ultimate resistance. We propose to use an analytical model for predicting the stress-strain behavior, which must be supplied with the results of the low-strain dynamic micropile test and of a complementary geotechnical test in order to examine the parameter of soil failure. The goal of this development is to provide a method for predict the micropile’s ultimate resistance without compromising its integrity. The results of the application of the new method to the experimental micropiles are in close agreement with the results observed in the static load test. / La revisión del estado del arte en relación a micropilotes, realizada en este trabajo, ha permitido identificar una gran variedad de técnicas de construcción, donde la predicción de sus comportamientos mecánicos, es una tarea difícil de realizar. Con respecto a los métodos de control, desde el punto de vista de la integridad, existen actualmente métodos no siempre aplicables a los micropilotes, y desde el punto de vista del control del comportamiento mecánico, el ensayo más utilizado es el ensayo de arrancamiento. El ensayo de arrancamiento es de fácil realización e interpretación, sin embargo es oneroso, por su larga duración y la necesidad de disponer de equipos muy pesados.Otro tipo de ensayo, que es utilizado para el control de fundaciones profundas en general, es el ensayo de carga dinámica, cuyas ventajas y desventajas han sido identificadas en la investigación bibliográfica.La nueva metodología propuesta, motivo de este trabajo, está basada en los ensayos de carga dinámica a baja deformación, donde se necesita una energía de impacto mucho menos importante que en el caso de los ensayos clásicos de carga dinámica.El protocolo de la metodología y su dispositivo de ensayo han sido calibrados y validados a partir de simulaciones numéricas con el fin de, principalmente, justificar la elección de la energía de impacto, de la masa del martillo y del amortiguador a utilizar. Las observaciones numéricas han sido, luego, verificadas experimentalmente sobre micropilotes a escala real, instalados en un sitio experimental desarrollado en el marco de este proyecto.La metodología de control in situ, de la resistencia en servicio de micropilotes, desarrollada en este trabajo, es un método original, simple de realizar, con un sistema fácil de instalar y de interpretación inmediata. Ella ha sido validada, en el caso de micropilotes a escala real en condiciones controladas en el sitio experimental, y no controladas en el caso de tres proyectos reales.Además, el avance en las investigaciones, dio lugar al desarrollo de una metodología de control de la resistencia límite de micropilotes. Para ello proponemos utilizar un modelo analítico de predicción del comportamiento esfuerzo-deformación alimentado por resultados del ensayo de carga dinámica a baja deformación y de un ensayo complementario de reconocimiento de los parámetros de ruptura del suelo. El objetivo de este desarrollo es de proponer un método de predicción de la resistencia límite de micropilotes sin comprometer su integridad. La aplicación de la metodología ha mostrado que los resultados son muy cercanos a los observados en los ensayos de carga estática.