Sensitivity to local imperfections in inelastic thin-walled rectangular hollow section struts

Mass efficient inelastic thin-walled rectangular hollow section (RHS) struts practically always fail in a combination of local–global interactive buckling and material nonlinearity while also exhibiting high sensitivity to initial imperfections. Nonlinear finite element (FE) models for inelastic thin-walled RHS struts with pre-defined local and global geometric imperfections are developed within the commercial package Abaqus. Using a unified local imperfection measurement based on equal local bending energy, the effects of imperfect cross-section profiles, imperfection wavelength and the degree of localization in the longitudinal direction on the ultimate load and the nonlinear equilibrium path are investigated for four characteristic length struts at different material yielding stress levels. The corresponding most severe imperfection profiles are determined and are found to be qualitatively different to the linear eigenmodes in all cases. Moreover, it is found that the most severe purely periodic imperfections may be used to provide a safe approximation of the ultimate load when the corresponding amplitude is constrained to the manufacturing tolerance level. An extensive parametric study on the wavelength of the most severe periodic imperfection profile is conducted and a relationship for this is proposed in terms of the normalized local slenderness, which compares excellently with the FE results

Geometrically non-linear analysis with stiffness reduction for the in-plane stability design of stainless steel frames

In AISC 360-16, the Direct Analysis Method (DM) has been set as the primary method for the stability design of frames. DM, considering initial global sway imperfection, is essentially Geometrically Non-linear Analysis (GNA) in which tangent modulus is used. The aim of this thesis is to provide stiffness reduction factor formulations for using GNA coupled with tangent modulus approach for the stability design of stainless steel frames. GNA with the proposed stiffness reduction factor is aligned to AISC 360-16 and it is aimed at facilitating greater and more efficient use of stainless steel. In accordance with current design standards, the ultimate limit state for this method is the formation of first plastic hinge, and the adequacy of the method is confirmed through member-based resistance checks.The focus of the thesis is the development of flexural stiffness reduction factor formulation for the in-plane stability design of stainless steel elements and frames with cold-formed square hollow section (SHS) and rectangular hollow section (RHS). The proposed beam-column stiffness reduction factor (tMN) accounts for the deleterious influence of material non-linearity, residual stresses and member out-of-straightness. The use of a GNA coupled with the proposed tMN eliminates the need for member buckling strength checks and thus, only cross-sectional strength checks are required.En la normativa Americana AISC 360-16, el método de análisis directo (DM) se ha establecido como el método principal en la verificación a inestabilidad de pórticos. El DM es esencialmente un análisis no lineal por la geometría (GNA) en el que se consideran las imperfecciones globales únicamente y se utiliza el módulo tangente. El objetivo de la tesis es proporcionar una formulación de rigidez reducida utilizando un análisis no lineal por la geometría para las verificaciones a inestabilidad de pórticos de acero inoxidable. El método propuesto se alinea con el AISC 360-16 y su objetivo es facilitar el uso del acero inoxidable en pórticos. De acuerdo con las normativas actuales, la capacidad última de un pórtico se define al formarse la primera rótula plástica y la verificación se plantea en base a la resistencia de los elementos aislados. El foco de la tesis se centra en el plantramiento de factores de reducción de rigidez a flexión de elementos tubulares de acero inoxidable conformados en frío (SHS y RHS) para el caso de diseño de pórticos cargados en su plano. Se propone un factor de reducción para elementos del tipo viga-columna mn el cual tiene en cuenta los efectos de la nolinealidad del material, las tensiones residuales y la imperfección del elemento. El uso de GNA conjuntamente con tMN permite verificar los diferentes elementos del pórtico solo a resistencia sin tener hacer las comprobaciones correspondientes a pandeo.Postprint (published version

Geometrically non-linear analysis with stiffness reduction for the in-plane stability design of stainless steel frames

In AISC 360-16, the Direct Analysis Method (DM) has been set as the primary method for the stability design of frames. DM, considering initial global sway imperfection, is essentially Geometrically Non-linear Analysis (GNA) in which tangent modulus is used. The aim of this thesis is to provide stiffness reduction factor formulations for using GNA coupled with tangent modulus approach for the stability design of stainless steel frames. GNA with the proposed stiffness reduction factor is aligned to AISC 360-16 and it is aimed at facilitating greater and more efficient use of stainless steel. In accordance with current design standards, the ultimate limit state for this method is the formation of first plastic hinge, and the adequacy of the method is confirmed through member-based resistance checks.The focus of the thesis is the development of flexural stiffness reduction factor formulation for the in-plane stability design of stainless steel elements and frames with cold-formed square hollow section (SHS) and rectangular hollow section (RHS). The proposed beam-column stiffness reduction factor (tMN) accounts for the deleterious influence of material non-linearity, residual stresses and member out-of-straightness. The use of a GNA coupled with the proposed tMN eliminates the need for member buckling strength checks and thus, only cross-sectional strength checks are required.En la normativa Americana AISC 360-16, el método de análisis directo (DM) se ha establecido como el método principal en la verificación a inestabilidad de pórticos. El DM es esencialmente un análisis no lineal por la geometría (GNA) en el que se consideran las imperfecciones globales únicamente y se utiliza el módulo tangente. El objetivo de la tesis es proporcionar una formulación de rigidez reducida utilizando un análisis no lineal por la geometría para las verificaciones a inestabilidad de pórticos de acero inoxidable. El método propuesto se alinea con el AISC 360-16 y su objetivo es facilitar el uso del acero inoxidable en pórticos. De acuerdo con las normativas actuales, la capacidad última de un pórtico se define al formarse la primera rótula plástica y la verificación se plantea en base a la resistencia de los elementos aislados. El foco de la tesis se centra en el plantramiento de factores de reducción de rigidez a flexión de elementos tubulares de acero inoxidable conformados en frío (SHS y RHS) para el caso de diseño de pórticos cargados en su plano. Se propone un factor de reducción para elementos del tipo viga-columna mn el cual tiene en cuenta los efectos de la nolinealidad del material, las tensiones residuales y la imperfección del elemento. El uso de GNA conjuntamente con tMN permite verificar los diferentes elementos del pórtico solo a resistencia sin tener hacer las comprobaciones correspondientes a pandeo

Extension of the Overall Interaction Concept to steel open sections and members

La capacité résistante des sections en I en acier laminées à chaud et soudées est étudiée en profondeur dans cette thèse. La résistance en section influencée par le voilement local ainsi que la résistance d'une barre soumise au flambement sont étudiées. Les normes de dimensionnement actuelles reposent sur le principe de classification des sections et la méthode des largeurs efficaces (E.W.M.), ce qui peut conduire à des prédictions de résistance discontinues et des calculs longs et fastidieux, en particulier dans le cas de sections ouvertes non-symétriques ou soumises à des cas de chargement combinés. En raison de ces lacunes, une méthode de conception plus économique et plus simple, l'Overall Interaction Concept (O.I.C.), a été développé. L'O.I.C., qui repose sur l'interaction résistance-instabilité bien connue avec une définition de l'élancement relatif généralisé, abandonne le concept de classification des sections et l'E.W.M., et traite toutes les géométries de section de manière similaire, pour les sections et les membres, et pour des cas de charge simples ou combinés. Compte tenu de ces avantages, le domaine d'application de l'O.I.C. est ici étendu à la conception de profilés en I en acier laminés à chaud et soudés. L'O.I.C. est élargi au Chapitre 1 pour la conception de profilés en I laminés à chaud et soudés dans des situations de chargement combinées ; Le Chapitre 2 se concentre sur la conception de sections en I mono-symétriques sous des cas de charge simples, et le Chapitre 3 traite de l'application de l'O.I.C. au cas des éléments comprimés influencés par les instabilités couplées locales-globales. Dans chaque chapitre, les détails des modèles numériques ainsi que les résultats de validation sont fournis. Des études paramétriques numériques approfondies par le biais de modèles d'éléments finis validés vis-à-vis de résultats expérimentaux sont menées pour étudier l'influence de différentes nuances d'acier, formes de section et divers cas de charge sur la résistance ultime. Sur la base des résultats des éléments finis, des expressions de conception basées sur l'O.I.C. sont proposées. Les prédictions de résistance de l'Eurocode 3, des normes américaines et de la proposition O.I.C. sont comparés aux résultats numériques de référence. Dans l'ensemble, il est démontré que l'O.I.C. délivre des résistances continues et beaucoup plus précises que celles prédites par les normes existantes. Les propositions faites ici serviront de base au développement d'une approche O.I.C plus générale, pour d'autres formes de sections transversales et divers procédés de fabrication.The resistance capacity of hot-rolled and welded steel I-sections are deeply investigated in this thesis. Both cross-section resistance influenced by local buckling and member resistance influenced by global buckling are considered. Current code-oriented design relies on the traditional cross-section classification system and the Effective Width Method (E.W.M.), which may result in discontinuous resistance predictions and lead to long and tedious design calculation processes, especially when it comes to non-doubly symmetric open sections or to combined loading cases. Due to these shortcomings, a more economic and simple design method, the Overall Interaction Concept (O.I.C.), was developed. The O.I.C., which is based on the well-established resistance-instability interaction with a definition of generalised relative slenderness, abandons the cross-section classification concept and the E.W.M. and deals with all cross-section shapes in a similar way for both sections and members, under simple or combined loading cases. Considering these advantages, an extension of its application scope is expected. The range of application of the O.I.C. in this thesis is extended to cover the design of hot-rolled and welded steel I-sections. The O.I.C. is developed in Chapter 1 for the design of hot-rolled and welded I-sections under combined loading situations; Chapter 2 focuses on an O.I.C.-based design of mono-symmetric I-sections under simple load cases and Chapter 3 discusses the application of the O.I.C. to the resistance of compression members as influenced by local-global coupled instabilities. In each chapter, the details of numerical models as well as validation results are provided. Extensive numerical parametric studies through test-validated finite element models are carried out to investigate the influence of different steel grades, section shapes and various load cases on ultimate resistance. Based on the finite element results, O.I.C. design expressions are proposed. Resistance predictions from Eurocode 3, the American Standards and the proposed O.I.C. approach are compared to the reference numerical results. Overall, it is evidenced that the proposed O.I.C. approach provides more continuous and significantly more accurate resistance predictions than existing design standards. The proposals from this research shall serve as a basis for the derivation of a more general O.I.C. approach to other cross-section shapes and manufacturing processes

Experimental and numerical behavior of anisotropic laminated FRP thin-walled web beams under different loading and boundary parameters

Master of ScienceDepartment of Civil EngineeringHayder A RasheedStructural elements made of fibrous composites are increasingly used in aerospace, automotive, civil and marine engineering applications due to their high stiffness and strength-to-weight ratio and corrosion resistance properties. Most of the composite structural elements are thin-walled and their design is often controlled by stability considerations mainly due to slenderness effects. Hence, for thin-walled slender composite beams, lateral torsional buckling (LTB) is the dominant failure mode regardless of the fiber orientations. In this study, combined numerical and experimental investigations for lateral torsional buckling of laminated composite web-cantilever and simply support beams are presented. A total of twelve carbon-fiber beams with six different anisotropic layups having a nominal length to height (/ℎ) ratio of 10 and four glass-fiber reinforced polymer beams with varying /ℎ ratios were experimentally tested for cantilever and simply support conditions respectively. The experimental response is compared against a non-linear numerical solution using Static Riks Analysis (SRA) to compare the predicted vs. actual load-displacement curve. An analytical approach, developed in an earlier study, was used to find the critical buckling load. Eigen value analysis was performed to benchmark the analytical buckling load using Abaqus

Seismic behavior of composite bridge columns

“This study investigates experimentally and numerically the seismic behavior of large-scale hollow-core fiber-reinforced polymer-concrete-steel (HC-FCS) innovative bridge columns as a sustainable approach to endure and rapidly recover from natural disasters such as earthquakes. The HC-FCS column consisted of a concrete shell sandwiched between an outer fiber-reinforced polymer (GFRP) tube and an inner steel tube to provided continuous confinement for the concrete shell along with the height of the column. The columns have a slender inner steel tube with diameter-to-thickness (Ds/ts) ratios ranged between 85 to 254. Each steel tube was embedded into the footing, while the GFRP tube was not embedded into the footing. The HC-FCS columns having a high Ds/ts ratio of 147 and 254 with short embedded length (1.25 Ds) do not dissipate high levels of energy and display nonlinear elastic performance due to severe steel tube buckling and slippage. However, the column with a Ds/ts ratio of 85 combined with substantial embedment length (1.6 Ds) results in a nonlinear inelastic behavior, high-energy dissipation, and ductile behavior. A retrofitting technique for a high Ds/ts ratio HC-FCS column precluding buckling of the inner steel tube was proposed, examined, and approved to be effective. New bond-slip expressions were proposed based on the analytical solution to capture the bond-slip effect between steel and concrete accurately. New design guidelines were proposed for HC-FCS columns in flexural and shear, as well as the column-to-footing connection. The innovative column approved to be easy to construct and repaired with high strength, drift, and resilience connection compared to the conventional bridge columns”--Abstract, page iv