Buckling | EPFL Graph Search

In structural engineering, buckling is the sudden change in shape (deformation) of a structural component under load, such as the bowing of a column under compression or the wrinkling of a plate under shear. If a structure is subjected to a gradually increasing load, when the load reaches a critical level, a member may suddenly change shape and the structure and component is said to have buckled. Euler's critical load and Johnson's parabolic formula are used to determine the buckling stress of a column. Buckling may occur even though the stresses that develop in the structure are well below those needed to cause failure in the material of which the structure is composed. Further loading may cause significant and somewhat unpredictable deformations, possibly leading to complete loss of the member's load-carrying capacity. However, if the deformations that occur after buckling do not cause the complete collapse of that member, the member will continue to support the load that caused i

Postbuckling behavior and imperfection sensitivity of elastic-plastic periodic plate-lattice materials

Fani Derveni

Advances in additive manufacturing have enabled a new generation of materials with advantageous properties inherent to their architecture. Recently, architected materials with periodic arrangements of plates, called plate-lattice materials, have been developed to reach theoretical least upper bounds for stiffness and strength of isotropic materials. This work investigates the buckling behavior of several plate-lattice architectures with relative densities between 0.5% to 25% when subjected to uniaxial compression. Finite element unit cell models with shell elements and periodic boundary conditions are used to simulate the buckling and post-buckling behavior of plate-lattices made from an elastic-perfectly plastic parent material. Five plate-lattice architectures with cubic symmetry are investigated - three anisotropic architectures (simple cubic, body-centered cubic, face-centered cubic) and two isotropic architectures (simple cubic and body-centered cubic combination, simple cubic and face-centered cubic combination). Geometric imperfections of varying amplitude are applied to determine the imperfection sensitivity of these plate-lattice materials, and to calculate their buckling knockdown factors. Consistent with the behavior of the elastic-plastic plate building blocks that comprise the lattice, this investigation reveals that plate-lattice materials are generally imperfection insensitive when their constituent plates become thinner and first bifurcation occurs in the elastic range away from the yield stress. When this occurs, the initial post-buckling behavior is stable and the ultimate strength can be many times higher than the initial buckling stress. (C) 2021 Published by Elsevier Ltd.

ELSEVIER2022

Influence of residual stresses on the buckling capacity of axially loaded steel columns

Gabriele Falconi

Over the past 60-80 years, the design methodology of steel structures comprising conventional steel profiles, such as HEA, HEB or HEM, has remained unchanged. However, during the 21st century, production methods have improved considerably. The aim of this study is to develop the basis for evaluating and refining the design methodology of steel members by considering realistic residual stress distributions generated during the fabrication process. Out of the numerous production methods, in this study we focus on hot rolled wide flange cross sections which are the most commonly used profiles in construction. Design specifications vary from one country to another. A comparison of the Swiss, European, Canadian, and Japanese approaches would help towards understanding the factors influencing the sizing of steel profiles. Simplified methods based on analytical and semi-experimental models have been developed over time to help engineers consider the phenomenon of lateral buckling by ensuring a sufficient degree of safety. Considering that the above design specifications have been developed based on experimental research of steel profiles and materials characteristic of old practices, this study investigates potential updates in the current design practices. To succeed in this challenge, it is imperative that the residual stress distributions of current cross-sectional and material characteristics are explained. Several studies throughout the past years have tried to focus on formulating simplified models that consider residual stress distributions. The accuracy of these models is investigated and recommendations are provided based on the best fits to measured residual stress distributions. These have explicitly been collected from nearly 10 experimental programs leading to more than 30 measured distributions with 𝐻/𝐵 (where 𝐻 is the height of the wide flange section and 𝐵 is the width) ranging from 0.95 to 2.22 (where 𝑡𝑓 is the thickness of the flange) ranging from 6.30 to 40mm. To supplement the knowledge regarding the residual stress distributions of current material specifications and to widen the cross-sectional range of available distributions, five additional wide flange residual stress distribution measurements (i.e., HEA160, HEM500, IPE120, IPE200 and IPE360) are implemented for the sake of this study. These distributions have been precisely determined through destructive laboratory measurements (i.e., cutting method) that are thoroughly explained in this thesis. In the advent of new technology and software development, structural element simulation can be performed in a thorough and precise manner. With the aid continuum finite element analysis (CFEA), the influence of residual stresses on lateral buckling of columns under axial load will be investigated. Eight cross-sectional profiles (i.e., HEA160, HEB500, HEM300, HEM500, IPE120, IPE200, IPE 360 and IPE400) are investigated. The CFEA consider realistic residual stress distributions that have been conducted by RESSLab, EPFL. The ultimate compressive load of these profiles considering varying buckling lengths (𝜆̅𝑘 ranging from 0.49 to 1.93, where 𝜆̅𝑘 is the normalised buckling length) are determined and compared with available design specifications.

2021

Stabilität von Tragelementen aus Glas

Andreas Luible

Stability of load carrying elements in glass The increasing demand in modern architecture for more slender and lighter structures requires the use of new construction materials. Glass, a material that has been used for a long time in windows as a filling material, has much to offer in this regard due to its very high compressive strength and transparency. For this reason, there is a growing trend to extend the use of glass sheets to load carrying elements such as columns, beams and panels. Due to their high slenderness and high compressive strength, such elements tend to fail because of instability (i.e. column buckling, lateral torsional buckling or plate buckling). At the moment little knowledge exists about the load carrying behaviour of glass structural elements, and existing design methods for other materials (i.e. steel) have been found to be unsuitable for direct transfer to the design of glass panels. With this in mind, the main objectives of the current thesis are: The study of the load carrying behaviour of glass elements which may fail due to lack of stability by means of laboratory tests and analytical and numerical models, as well as the study of the main influencing parameters. Discussion of possible design methods for glass elements which may fail due to lack of stability for the three main stability problems (column buckling, lateral torsional buckling and plate buckling) and proposition of possible aids for design such as buckling curves. The main influencing parameters (dispersion of the glass thickness, initial deformation) on the load carrying behaviour of glass elements which may fail due to lack of stability have been measured and are evaluated herein using statistical methods. The breakage stress, the thermal prestress and the effective tensile strength are defined and explained. Existing models to determine the tensile strength of glass are discussed. The column buckling behaviour of single layer and laminated safety glass is studied by means of column buckling tests, which are compared to analytical and numerical models. The models are used to study the influence of the main parameters, particularly the shear connection due to the interlayer (PVB) in laminated safety glass, on the load carrying behaviour and buckling strength of glass elements. On the basis of this study different possible design methods for column buckling of glass elements in compression are proposed and discussed. It is shown that a second order stress analysis is the most appropriate method for glass. As a further simplification, the cross section of a laminated safety glass structural element can be modelled as a monolithic cross section with an effective thickness. Analytical and numerical models for the lateral torsional buckling of glass beams are also verified by a comparison to test results. Along with a study of the main parameters, different methods to determine the lateral torsional buckling strength are discussed, and it is shown that buckling curves for lateral torsional buckling should be developed for glass beams using a slenderness ratio based on effective tensile strength. As a result of numerical simulations, recommendations for the future development of lateral torsional buckling curves of glass beams are given. The column buckling behaviour of single layer and laminated safety glass is also studied by means of column buckling tests, analytical and numerical models. It is shown that glass panels have a large post critical load carrying capacity but the way the loads are introduced into the panels, as well as the buckling shape, have an important influence on the plate buckling capacity. A design method with buckling curves using a slenderness ratio based on effective tensile strength seems applicable for the design of glass panels. As a result of numerical simulations, recommendations for the future development of plate buckling curves for plate glass elements under compression are given.

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