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When analyzing reinforced concrete (RC) framed structures under monotonic loading—for example, progressive collapse and pushover analysis—besides flexural deformation, the so-called “fixed end” rotations induced by longitudinal bar slips at the beamcolumn ends connected to the joints can be significant and may result in additional vertical deformations not accounted for in the conventional analysis. Hence, it is important to quantify the deformations arising from the fixed end rotations. In this paper, shortcomings of existing bond stress-slip models are discussed in terms of application limitation and prediction accuracy. A new analytical model based on the bond stress integration along the bar stress propagation length is proposed to predict the bar-slip behavior in RC beam-column joints under monotonic loading. The phenomena of combined axial pullout and transverse dowel action at the joints are considered. The proposed model is validated with experimental studies from published literature and is shown to be simple, yet reliable. Published version

Due to increasing threats from terrorism in the past decades, progressive collapse modeling of buildings is gaining popularity with objective of simulating collapse process of the whole or partial structural system, in order to give useful insight to improve existing design for the structure against progressive collapse. In this thesis, a novel modeling framework for progressive collapse of reinforced concrete (RC) frame structures is proposed. A set of damage assessment criteria to identify and quantify flexural, shear and axial damages and failures of RC members is suggested. This set of damage criteria incorporates axial-shear-flexural interactions of the structure during the analysis and it is capable of tracing cracking and crushing of concrete, yielding and fracture of reinforcement as well as final failures of cross-sections within RC members. Direct member removal algorithm is developed to simulate the process of collapse at member level, based on combined flexural/shear/axial failures of RC members. With developing a specially designed searching scheme, a new algorithm to monitor substructure collapse is also proposed. Locations and magnitudes of impact loads of the collapsed partial structure are also identified and calculated, according to rigid-body kinematics and energy principle. In addition, inelastic and oblique impact effects are properly considered. To efficiently simulate progressive collapse of large-scale buildings, superelement technique which separates the structure into linear and nonlinear zones is employed to reduce the computational time. As buildings in progressive collapse may undergo large rigid-body rotations of the structure, a rigid-body rotation correction to improve conventional superelement formulation is proposed for a more accurate large deformation analysis using superelement. Further, in order to consider nonlinearity spread during progressive collapse analysis, an adaptive superelement algorithm to automatically identify the propagation of nonlinear zone and then update the definition of superelement is proposed. By incorporating direct member removal algorithms as well as model regeneration procedure, an efficient adaptive superelement modeling procedure is developed for progressive collapse of RC frame structures. Numerical examples are given to show the advantages and effectivity of the suggested damage criteria and modeling procedures, through comparison with the results obtained from physical tests or standard nonlinear finite element analysis. Doctor of Philosophy (CEE)

Circular hollow sections (CHS) are widely adopted in fixed jacket and topside structures due to their excellent structural and mechanical properties. Virtually in almost all off-shore structures, one of the main design considerations when using CHS is to optimize the structural joints which are susceptible to fatigue failure due the dynamic and cyclic loading natural of the structures. Recently, partially overlapped CHS tubular K-joints become more and more popular due to their optimum strengths when compared to other alternative joint configurations such as non-overlapped (gapped) or completely overlapped K-joints. However, in the past, few research works were carried out to study the fatigue behaviour of partially overlapped CHS K-joints. In fact, currently, only a very limited information regarding the fatigue life of CHS overlapped K-joints are available. Hence, there is a need to investigate the stress concentration factors (SCF) and the hot spot stress (HSS) distributions of this kind of joints in the current research. Doctor of Philosophy (CEE)

The objective of current research is to numerically investigate the deformation behaviour of reinforced concrete (RC) beam-column framed structures subjected to destructive external loading. Firstly, besides the conventional uniaxial concrete models to predict flexural failures, a unified plasticity concrete model is proposed to accurately simulate shear deformations of beams. Secondly, a three dimensional co-rotational beam finite element is formulated with considerations of material nonlinearities for both steel and concrete. The proposed co-rotational beam formulation is shown to be capable of predicting steel and reinforced concrete framed structures with satisfactory accuracy and efficiency. Thirdly, a component-based mechanical model is proposed to simplify two dimensional RC beam-column joints, where three types of components are considered, viz., the bond-slip component, shear-panel component and interfacial shear component. Analytical models are respectively proposed to reasonably calibrate the bond-slip component and the shear-panel component, and an empirical model is summarized for the interfacial shear component based on extensive experimental results and design regulations. Fourthly, as an integrated system, the proposed concrete models, the co-rotational beam element and the component-based joint model are studied at the system level to show the prediction accuracy, computational efficiency and robustness in numerical algorithms. Advantages and disadvantages of different concrete models are also discussed. Finally, a superelement concept is proposed for structural analysis of large-scale structures. Compared with models without superelement, significant saving in computational cost and satisfactory prediction accuracy can be obtained without any loss in critical information of structural responses. This aspect is particularly crucial for progressive collapse analysis of structures subjected to localized damage. Doctor of Philosophy (CEE)

The application of extended finite element method (XFEM) formulation for nonlinear structural analyses is presented in this thesis. It aims to capture accurately the elastic response of a beam with an internal pin connection and the elasto-plastic response of a beam or a plate structure at a relatively low computational cost by utilizing the XFEM Timoshenko beam and Reissner-Mindlin plate elements. In the XFEM formulation for an internal pin, a step function is employed in the enriched rotation approximation field and an absolute level set function is adopted in the enriched translation approximation field. The enrichment function for a plastic hinge is formulated by using Hermite function over the high gradient zone resulted from the plastic hinge. The Hermite function regularizes the discontinuous enrichment function for an internal pin to be a continuous function with a high gradient zone. The strain fields derived from the enriched displacement approximation fields remain continuous inside an element. As the absolute level set function is constructed by standard finite element shape functions, such an enrichment function is also called ‘local’ enrichment function. The local enrichment function is applied in the XFEM plate element in this thesis. However, it is found that such local enrichment function is not suitable for the formulation of a 9-node quadrilateral plate element. Hence a global enrichment function is proposed. The global enrichment function is constructed on the structure level and independent of mesh scheme. Thus, it is applicable for both triangular and quadrilateral plate elements. Shear locking mitigation method is one of the major concerns in the present XFEM formulation. Two methods, including reduced integration and assumed shear strain methods, are employed to control shear locking in this thesis. In the assumed shear strain method, the mixed interpolation of tensorial components (MITC) technique and the discrete shear gap (DSG) technique are adopted in the XFEM plate elements. Numerical examples are given for different applications including an internal pin in a beam, a plastic hinge in a beam and a yield line in a plate. The numerical results show that the XFEM formulation is able to capture the discontinuous displacement over an internal pin connection and the locally high gradient displacement resulting from a plastic hinge or a yield line. It is also shown that shear locking can be controlled effectively by the reduced integration method and the assumed shear strain method. Doctor of Philosophy (CEE)

Due to the rapid increase of terrorist threats, the ability of a building to mitigate progressive collapse is of key interest to government agencies. Alternate Load Path approach is one of the direct methods to assess and quantify the resistance of building against collapse by evaluating the bridging resistance of the structure under notional removal of major load-bearing elements. This research conducted a systematic study on the structural behaviour and the development of secondary load-carrying mechanism in reinforced concrete (RC) structures starting from simple 2-D RC frames to more complete 3-D frame-slabs. Three series of experimental tests, i.e. 2-D RC frame tests, 3-D RC frame tests, and 3-D RC frame-slab tests under single column removal scenario were conducted to investigate structural behaviour and development of load-resisting mechanisms in each configuration. The specimens were loaded on a single point above the removed column until distinct failure was observed. In 2-D RC frame tests, the effects of horizontal restraint and reinforcement detailing on the behaviour and load-carrying capacity of the double-spanning beam were investigated. In 3-D RC frame tests, the presence of direct and compatibility torsions, as well as the interactions among the 3-D connected beams were studied. Lastly, the contributions of slabs to the 3-D frame substructures were identified in 3-D RC frame-slab tests. From the tests, inadequate restraint (penultimate column removal scenario) and the presence of torsion hindered capacities of beams, especially catenary action. In frame and frame-slabs specimen with adequate restraint and negligible torsion, the significances on the development of catenary action in beams and tensile membrane action in slabs in increasing the progressive collapse resistance of RC structures were identified. Numerical studies by employing fibre and plate elements in the modelling of beams and slabs, respectively, demonstrate the efficiency and reliability of this approach in simulating behaviour and resistance of RC substructures under progressive collapse (involving material and geometric non-linearity). The validated numerical models are employed to carry out further analyses such as multi-storey 2-D RC frames to identify the participation and interaction among each storeys and unequal beams in 3-D RC frames (common in building) to identify the effect of span and reinforcement ratio. Finally, a simplified analytical model is developed to predict the overall load capacity (response) of individual frames and slabs, as well as their combined capacities in frame-slab systems. This systematic study consisting of experimental, numerical, and analytical works of 2-D frames to 3-D frame-slabs is essential in supporting and improving the design approaches in current progressive collapse guidelines. Doctor of Philosophy (CEE)

In this study, a novel and consistent geometrical model and mesh generation technique is proposed for partially overlapped CHS K-joints with and without crack. For the mesh without crack, the geometrical model of welding details is developed both on the chord and brace sides by a new approach such that the cut off requirements by AWS (1996) and API (2000) can be satisfied. Welding parameters are verified and adjusted by the measurement of welding thickness on small and full scale specimens. Furthermore, a general algorithm is developed for the determination of the theoretical and actual intersection points between the braces and the chord, which is a distinctive feature of partially overlapped CHS K-joints. For the mesh with crack, a general model for the inclined crack surface and an unsymmetrical crack front are proposed. Based on the developed geometrical model, a new mesh modelling method is proposed for partially overlapped CHS K-joints. The mesh generator is constructed step-by-step, therefore it is able to produce several kinds of meshes such as surface meshes, solid meshes, meshes with or without welding and crack details. At each step, a particular mesh can be exported depending on the complexity of the particular fatigue problem under consideration. The application range of the mesh generator is extended for special cases of identical chord and braces dimensions as well as large overlapped percentage, which are not commonly covered by other commercial software packages. Most importantly, it is able to generate a solid mesh with welding details and surface crack of any length and locates at either sides of the joint intersection. In the experimental program, two full-scale partially overlapped CHS K-joints were tested under cyclic combined loads in order to gather more information about the fatigue performance of the joints, as well as to evaluate the surface crack model proposed. During the tests, the crack initiation and propagation were monitored. Doctor of Philosophy (CEE)

A new h-version adaptive refinement scheme is developed for the analysis of Thin-walled structures in elastostatic range. The basic concepts of adaptivity for the finite element method are discussed and a brief review of the related theories and techniques is presented. Focus is devoted to three essential ingredients for the whole adaptive refinement scheme: automatic mesh generation, error estimation on the FE solution and adaptive refinement scheme design. Doctor of Philosophy (CEE)