Geometrically nonlinear analysis of the stability of the stiffened plate taking into account the interaction of eigenforms of buckling

The aims of this work are a detailed consideration in a geometrically nonlinear formulation of the stages of the equilibrium behavior of a compressed stiffened plate, taking into account the interaction of the general form of buckling and local forms of wave formation in the plate or in the reinforcing ribs, comparison of the results of the semi-analytical solution of the system of nonlinear equations with the results of the numerical solution on the Patran-Nastran FEM complex of the problem of subcritical and postcritical equilibrium of a compressed stiffened plate. Methods. Geometrically-nonlinear analysis of displacement fields, deformations and stresses, calculation of eigenforms of buckling and construction of bifurcation solutions and solutions for equilibrium curves with limit points depending on the initial imperfections. An original method is proposed for determining critical states and obtaining bilateral estimates of critical loads at limiting points. Results. An algorithm for studying the equilibrium states of a stiffened plate near critical points is described in detail and illustrated by examples, using the first nonlinear (cubic terms) terms of the potential energy expansion, the coordinates of bifurcation points and limit points, as well as the corresponding values of critical loads. The curves of the critical load sensitivity are plotted depending on the value of the initial imperfections of the total deflection. Equilibrium curves with characteristic bifurcation points of local wave formation are constructed using a numerical solution. For the case of action of two initial imperfections, an algorithm is proposed for obtaining two-sided estimates of critical loads at limiting points.

RELIABILITY OF CORRODED STIFFENED PLATE SUBJECTED TO UNIAXIAL COMPRESSIVE LOADING

The work is focused on the reliability of corroded stiffened plates subjected to compressive uniaxial load based on the progressive collapse approach as stipulated by the Common Structural Rules for Bulk Carriers and Oil Tankers, employing the limit state design. Two different cases have been investigated. In the first model, the corrosion degradation led to uniform thickness loss, whereas the mechanical properties were unchanged, as given in the Rules. In the second model, the plate thickness degradation was followed by mechanical properties reduction. The uncertainties related to the mechanical properties, thicknesses, and initial imperfections of the corroded stiffened plate were taken into account. Several initial design solutions of stiffened plates, as well as different severity levels of corrosion degradation were investigated. The results show that structural reliability significantly decreases with corrosion development, especially when in addition to the initial imperfections and corrosion plate thickness reduction, corroded plate surface roughness and the changes in the mechanical properties were considered. The uncertainties, their origins and confidence levels are discussed. It was found that non-linear time-dependent corrosion degradation accounting not only for the thickness reduction due to corrosion wastage but also the subsequent decrease of mechanical properties lead to a significant reduction in the reliability index. Additionally, it was defined that the reliability estimate is very sensitive to the uncertainties related to the initial thickness and the spread of corrosion degradation as a function of the time. Incorporating the probability of corrosion detection into the original reliability model introduces additional information about the validity of structural degradation that may lead to a higher beta reliability index estimate compared to the original model.

FULL-SCALE MEASUREMENTS OF WELDING-INDUCED INITIAL DEFLECTIONS AND RESIDUAL STRESSES IN STEEL-STIFFENED PLATE STRUCTURES

Plated structures such as ships and offshore structures are constructed using welding techniques that attach support members (or stiffeners) to the plating. During this process, initial imperfections develop in the form of initial deformations (deflections or distortions) and residual stresses. These initial imperfections significantly affect the buckling and ultimate strength of these structures. Therefore, to assess the strength of welded plate structures, it is very important to predict the magnitude and pattern of welding-induced initial imperfections and their effects on buckling and ultimate strength. To determine the reliability of the prediction methods, it is desirable to validate the theoretical or numerical predictions of welding-induced initial imperfections through comparison with full-scale actual measurements. However, full-scale measurement databases are lacking, as they are costly to obtain. This study contributes to the development of a full-scale measurement database of welding-induced initial imperfections in steel-stiffened plate structures. The target structures are parts of real (full-scale) deckhouses in very large crude oil carrier class floating, production, storage and offloading unit structures. For parametric study purposes, four test structures by varying plate thickness are measured while the stiffener types and weld bead length are fixed. Modern technologies for measuring initial deformations and residual stresses are applied. The details of the measurement methods are documented for the use of other researchers and practicing engineers who welding-induced initial imperfections.

Story Stability of a Structure- A Literature Review

Column is a slender beam, which carries load. Failure pattern of a column varies with different parameters such as buckling, compression, shear and tension. The initial imperfections in a column increases deflection and reduction in load carrying capacity. To accomplish stability, the key engineering elements such as connection and rigidity governs the effective length and width of the members. The researchers, covering the key engineering elements with different loading patterns, established numerous comprehensive studies. Further, advancement in the research were carried out to determine lateral stiffness, inter-story displacement and deflected beam shape under various loading patterns. The present study focuses on various literatures on effective length and governing factors, which determine the stability of the structure.

Numerical and Analytical Investigation of Stability of the Reinforced Plate

The work is aimed at the construction of an algorithm for studying the equilibrium states of a reinforced plate near critical points, using the first (cubic terms) nonlinear terms of the potential energy expansion. Using geometrically nonlinear analysis of displacement, deformation and stress fields, the Eigenforms of buckling were calculated and bifurcation solutions and solutions for equilibrium curves with limit points were constructed depending on the initial imperfections.

Ultimate Strength Behavior of Cracked Ship Panels: a Review

Cracks of various sizes and orientations may arise at many locations under conditions of pitting corrosion, local stress concentration, fatigue and impact loads. It is essential to evaluate the effect of cracks on the ultimate strength and collapse behaviors of various ship structural members, so as to avoid complete structural failure. The assessment of this effect would serve the life-cycle management, maintenance and repair strategies aiming to ensure structural longevity of ships and offshore steel structures. A review of the advances carried out during the last few years, shows that the effect of locked and propagated cracks on the ultimate strength behavior is complex since it cannot be isolated from the effect of initial imperfections and corrosion. Published experimental attempts were a good base, but most literature is based on finite element modeling that proved to be a useful tool to generate different possible damage scenarios. Recent empirical formulae had been proposed to estimate the reduction in ultimate strength due to cracks and other factors; some of these formulae may prove to have a practical use to attain a consequence based crack assessment criteria.