Influence Of Initial Imperfections Research Articles

Simplified Method for Calculating the Bearing Capacity of Slender Concrete-Filled Steel Tubular Columns

Concrete-filled steel tubular (CFST) columns are one of the most effective reinforced concrete structures, and improving their calculation is a critical task. The purpose of this study was to develop a simplified method for calculating slender CFST columns, taking into account the effect of lateral compression. The idea of the method is to use the equation of a reinforced concrete column’s longitudinal bending, without taking into account the effect of lateral compression. To take into account the lateral effects, the cross-sectional stiffnesses are corrected based on the analysis of the stress–strain state in the cross-sectional plane using the finite element method. The developed method was implemented by the authors in the MATLAB environment. The approbation of the proposed method was carried out on experimental data for centrally compressed columns of a circular cross-section, as well as eccentrically compressed columns of a circular and square cross-section, presented in two papers. For the centrally compressed columns, we conducted a study on the influence of initial imperfections in the form of eccentricities and initial curvatures on the value of the ultimate load. For the eccentrically compressed columns of the circular and square cross-section, the area of their effective operation was determined.

The influence of initial imperfection on ultimate bearing capacity of cylindrical shells with continuous openings

Four kinds of initial imperfections were analyzed to obtain their influence on ultimate bearing capacity of cylindrical shells with continuous openings. The model was created in the finite element program. After the calculation of the nonlinear ultimate bearing capacity of cylindrical shells with continuous openings considering different kinds of initial imperfection and their amplitudes, the failure mode of the structure was studied and the buckling pressure was achieved. Compared with FEM and experimental results, it is proved that the numerical method used in this paper is reliable and could accurately forecast the ultimate bearing capacity of cylindrical shell with continuous openings.

Influence of Initial Imperfections on PMMA Spherical Shell

This paper focused on the influence of initial imperfection on the critical load of PMMA pressure spherical shell by introducing buckling modes as the initial defect. Four types of pressure spherical shells including complete spherical shell, spherical shell with single penetration, spherical shell with double penetrations, and hyperspherical shell were studied. The distribution of initial imperfections was given through the modal configuration of linear eigenvalue analysis, and then the buckling critical load of the spherical shell was analyzed by using the consistent modal defect method and N-order modal defect method. Results confirmed that: Initial imperfections have a great different influence on pressure spherical shells with varying penetration. It has the greatest influence on the critical load of a spherical shell with double penetration at a 5 mm defect. With the increase of defect amplitude, the critical load of the spherical shell decreases. The analysis results also show that the first-order mode is usually not the worst geometric defect configuration of PMMA pressure spherical shell. And the worst mode may appear in higher-order mode due to the influence of penetration, initial defect amplitude, and different thickness-diameter ratio.

Numerical study on buckling capacity of steel H-beams under non-uniform temperature distribution

PurposeThe strength and stiffness of steel deteriorate rapidly at elevated temperatures. Thus, the characteristics of steel structures exposed to fire have been concerned in recent years. Most studies on the fire response of steel structures were conducted at uniformly distributed temperatures. This study aims to evaluate the buckling capacity of steel H-beams subjected to different loading conditions under non-uniform heating.Design/methodology/approachA numerical investigation was conducted employing finite element analysis software, ABAQUS. A comparison between the numerical analysis results and the experimental data from previous studies was conducted to verify the beam model. Simply supported beams were loaded with several loading conditions including one end moment, end equal moments, uniformly distributed load and concentrated load at midspan. The effects of initial imperfections were considered. The buckling capacities of steel beams under fire using the existing fire design code and the previous study were also generated and compared.FindingsThe results showed that the length-to-height ratio and loading conditions have a great effect on the buckling resistance of steel beams under fire. The capacity of steel beams under non-uniform temperature distribution using the existing fire design code and the previous study can give unconservative values or too conservative values depending on loading conditions. The maximum differences of unconservative and conservative values are −44.5 and 129.2% for beams subjected to end equal moments and one end moment, respectively.Originality/valueThis study provides the buckling characteristics of steel beams under non-uniform temperature considering the influences of initial imperfections, length-to-height ratios, and loading conditions. This study will be beneficial for structural engineers in properly evaluating structures under non-uniform heating conditions.

Optimal Design of Double-Walled Corrugated Board Packaging.

Designing corrugated board packaging is a real challenge, especially when the packaging material comes from multiple recycling. Recycling itself is a pro-ecological and absolutely necessary process, but the mechanical properties of materials that are processed many times deteriorate with the number of cycles. Manufacturers are trying to use unprecedented design methods to preserve the load-bearing capacity of packaging, even when the material itself is of deteriorating quality. An additional obstacle in the process of designing the structure of paper packaging is the progressive systematic reduction of the grammage (the so-called lightweight process) of corrugated cardboard. Therefore, this research presents a critical look at the process of optimal selection of corrugated cardboard for packaging structures, depending on the paper used. The study utilizes analytical, simplified formulas to estimate the strength of cardboard itself as well as the strength of packaging, which are then analyzed to determine their sensitivity to changes in cardboard components, such as the types of paper of individual layers. In the performed sensitivity analysis, numerical homogenization was used, and the influence of initial imperfections on the packaging mechanics was determined. The paper presents a simple algorithm for the optimal selection of the composition of corrugated cardboard depending on the material used and the geometry of the packaging, which allows for a more conscious production of corrugated cardboard from materials derived, e.g., from multiple recycling.

Some aspects of consideration of initial imperfections in the calculations of stability of thin-walled elements of open profile

Performed analysis of the initial geometric imperfections influence on the stability of the open C-shaped bars. Test tasks were solved in MSC Nastran, which is based on the finite element method. Imperfections are given in different formulations: the general stability loss of an ideal bar, of wavy bulging of walls and shelves, of deplanation of a bar. To model imperfections, has been developed a program which for the formation of new coordinates of the nodes of the "deformed" model, the components of a vector similar to the form of stability loss are added to the corresponding coordinates of the middle surface of the bar. In this way, you can set initial imperfections in the forms of stability loss of the bar with different amplitude. Researches made with different values of the imperfection amplitude and eccentricity of applied efforts. All tasks are performed in linear and nonlinear staging. The conclusion is made regarding the influence of initial imperfections form and imperfection amplitude on the critical force in nonlinear calculations. It was found that the most affected are imperfections, which are given in the form of total loss of stability. It was revealed the influence of the imperfection amplitude on the magnitude of the critical force for such imperfections. The influence of imperfections amplitude given in the form of wavy bulging walls and in the form of deplanations is not affected on the value of the critical force.

Влияние начальных несовершенств тонкостенных цилиндрических оболочек с сыпучим заполнителем на их устойчивость при изгибе

Экспериментально и теоретически исследовано предельное состояние при изгибе тонкостенных цилиндрических оболочек, имеющих начальные несовершенства геометрии. Изготовлены качественные образцы глубокой вытяжкой в матрице из листа и из трубы точением на токарном станке, имеющие геометрические несовершенства (толщины, диаметров). Материал образцов: алюминиевые сплавы 3004 и Д16Т. Перед испытанием образцы тщательно измерялись и обследовались на наличие дефектов. Изготовлен специальный стенд, к которому жестко одним концом крепились образцы, другой конец был свободным. На свободный конец прикладывалась ступенями поперечная сила до потери устойчивости. Испытывались пустые и заполненные железным порошком образцы. Фиксировались нагрузка и прогиб свободного конца образца индикатором часового типа ИЧ-10. Получены линейные зависимости прогиба от нагрузки, устойчивость терялась в упругости. Численные расчеты выполнялись в программном комплексе «Динамика-3». Для расчетов принимались идеальные образцы. Сопоставив критические нагрузки, полученные экспериментами и численными расчетами, судили о влиянии начальных несовершенств на предельное состояние. Численно рассчитанная критическая сила была всегда больше экспериментального значения. Начальные несовершенства образцов снижали критическую силу от 2,3% до 32,5%. У образцов с небольшими вмятинами в зоне потери устойчивости критическая сила уменьшилась в 1...2 раза. У заполненных железным порошком образцов критическая сила была больше до 40,5%, чем у пустых. The limit state for bending thin-walled cylindrical shells with initial geometry imperfections has been studied experimentally and theoretically. High-quality samples are made by deep drawing in a matrix from a sheet and from a pipe by turning on a lathe, which have geometric imperfections (thickness, diameter). The material of the samples is aluminum alloys 3004 and D16T. Before testing, the samples were carefully measured and examined for defects. A special stand was made, to which samples were rigidly attached at one end, while the other end was free. A transverse force was applied to the free end in steps until it lost stability. Empty and iron-powder-filled samples were tested. The load and deflection of the free end of the sample were recorded by an ICH-10 clock type indicator. Linear dependences of the deflection on the load were obtained; stability was lost in elasticity. Numerical calculations were performed in the Dynamics-3 software package. Ideal samples were taken for calculations. Comparing the critical loads obtained by experiments and numerical calculations, we judged the effect of initial imperfections on the limit state. The numerically calculated critical force was always greater than the experimental value. Initial imperfections of the samples reduced the critical force from 2.3% to 32.5%. For samples with small dents in the zone of loss of stability, the critical force decreased by 1..2 times. In iron powder-filled samples, the critical force was greater by up to 40.5% than in empty samples.

Imperfection study on lateral thermal buckling of subsea pipeline triggered by a distributed buoyancy section

Controlled lateral buckling is triggered by distributed buoyancy section at predesigned sites to release the axial force induced by high temperature and high pressure in subsea pipelines. Due to the larger diameter and smaller submerged weight of distributed buoyancy section, compared to the normal pipe section, imperfections are more easily introduced at the location of distributed buoyancy section. In this study, an analytical model is proposed to simulate lateral buckling triggered by a distributed buoyancy section for an imperfect subsea pipeline, which is validated by test data. Semi-analytical solutions are derived. First, snap-through buckling behaviour is discussed. Then the influence of initial imperfections on buckled configurations, post-buckling behaviour, displacement amplitude and maximum stress is discussed in detail. The results show that there is no snap-through phenomenon for large amplitude of initial imperfections, which appears only when the amplitude of imperfection is small enough. The displacement amplitude increases with the amplitude of initial imperfections, and it first increases and then decreases with wavelength of initial imperfection. Compared to a perfect pipeline, the maximum stress amplifies for relative small wavelength of initial imperfections. Therefore, a large enough wavelength of initial imperfection should be introduced.

FEM-based evaluation of friction and initial imperfections effects on sandwich pipes local buckling

Sandwich pipes have been studied as one option to overcome the high pressure problems in deep and ultra-deep waters. They have become a possible alternative solution for submarine infrastructure due to its thermal insulation capacity. This contribute to preventing the pipeline from clogging due to the difference in temperature between reservoir fluids and water at the bottom of the sea. The pipelines in ultra-deepwater are continually exposed to severe operating conditions, such as the effect of high levels of external pressure that can cause local deformation or even collapse of the pipe. Thus, a greater understanding of the mechanical behavior of sandwich pipes is required. This paper presents a FEM-based evaluation of friction and initial imperfection effects on sandwich pipes local buckling. The non-linear evaluation was carried out in FEM of local buckling of two sandwich pipes, with polypropylene and cement as filled annular material. The influence of initial imperfections and the degree of friction, between the annular material and the steel pipes, as well as geometric variations of the pipe were considered. The numerical simulations results indicate a capacity to withstand ultra-deep waters collapsing pressures, around 3000 m, either for polypropylene or cement filled annular material model. In addition, the results indicate that the collapse pressure is inversely proportional to the increase in annular thickness and directly proportional to the decrease in friction which have an impact and contribution on the carrying capacity of the sandwich pipe. Further research will consider a design of experiments analysis of reported effects for different diameter-to-thickness ratios.

Rectangular Cross Section Beams Calculation on the Stability of a Flat Bending Shape Taking into Account the Initial Imperfections

The article presents the derivation of the resolving equation for the calculation of lateral buckling of rectangular beams. When deriving the basic equation, the initial imperfections of the beam are taken into account, which are specified in the form of the eccentricity of the applied load, the initial deflection in the plane of least stiffness and the initial twist angle. The influence of initial imperfections on the process of beam stability loss is investigated.

Finite Element Analysis on Axial Compressive Behaviors of High-Performance Steel Stiffened Plates in Bridge Application

In order to investigate the mechanical behavior, the buckling performance and failure reason of stiffened high performance steel (HPS) plates, three-dimensional finite element (FE) models considering initial imperfections and residual stresses are compressed with axial force. Various influential factors such as element size, constitutive relations, second order effects of structure and membrane effects in buckling area were discussed to achieve a better validation with experimental data. As load increasing, the plastic strain in the U-rib near the end stiffener increases gradually under the influence of initial imperfections and stress concentration. Local buckling occurs after yielding of the whole section. With the refined model, the sensitivity of stiffened structure to the residual stresses and imperfections are investigated, and parametric studies on the material properties, geometric dimensions are conducted. The results show that the initial imperfections have a major influence on the ultimate capacity while the residual compressive stresses govern the elastic capacity and the ductility. Proper values of the imperfections and residual stresses are suggested for FE stiffened plate simulations. HPS stiffened plates with short length is more sensitive to initial imperfections. The effects of length, thickness and spacing are similar to the ordinary steel plates. Both equations in American and Chinese standards are used to evaluate the ultimate capacity of stiffened HPS plates, and formulas in AASHTO provide a more accurate estimate on capacity and failure mode, while Chinese specification is more conservative comparatively.

Poređenje ponašanja tankih cilindričnih i konusnih ljuski od ugljeničnog i nerđajućeg čelika

Thin-walled cylindrical and conical shells represent one of the most complex structural elements considering their behaviour and susceptibility to buckling. A brief theoretical review including the presentation of different currently available design recommendations is given in this paper. Influence of initial imperfections on nonlinear behaviour of cylindrical and conical shells is also presented through numerical analysis. Shells with different wall thicknesses and different values of initial imperfections, but constant length and diameter of shell are analysed. Numerical analysis includes materially and geometrically nonlinear analysis of cylindrical and conical shells, using experimentally obtained stress-strain relation of carbon steel and stainless steel. Material nonlinearity and initial geometrical imperfections resulted in significantly lower buckling resistance of shells.

Probabilistic Verification of Structural Stability Design Procedures

Introduction: This contribution presents a comparison of three methods of the statistical computation of the design load-carrying capacity of a steel plane frame. Two approaches of the European Standard Eurocode 3 and one stochastic approach are applied. The stochastic approach takes into account the random influence of all imperfections and can be applied to the reliability verification of design according to Eurocode 3. Methods: The columns and beams in the steel frame are modelled with beam elements using the stability solution with buckling length and the geometrically nonlinear solution. The stochastic computational model is based on the geometrically nonlinear solution and on the random influence of initial imperfections, whose random samplings are simulated using the Monte Carlo method. Results and Conclusion: The design load-carrying capacity of the steel plane frame computed using the stability solution with buckling length is in good agreement with the stochastic solution in which the design value is calculated as 0.1 percentile. On the contrary, the geometrically nonlinear solution according to Eurocode 3 gives the lowest (safest) values of design load-carrying capacity.

Failure analysis of crankshafts used in maritime V12 diesel engines

Abstract Maintenance of equipment requires constant monitoring of the components that constitute a mechanical system, as well as the monitoring of the conditions of service, among others. One first indication of failure in a crankshaft is given by the low-pressure value of the lubrication circuit. This is mainly due to the accumulation of debris in the lubrication channels, which causes the oil filters to be clogged. As such, this will cause poor lubrication of the crankshaft, which can consequently cause its catastrophic failure, and frequently originates damage propagation to other components of the engine, namely crankcase, bearing shells, connecting rods, pistons and other mechanical parts. In the 4-stroke internal combustion maritime V12 diesel engine herein studied, there has been a frequent failure of crankshafts. The seven cases of failure of the crankshafts reported in the last 25 years are presented in the paper and several causes of failure were listed. From those, the influence of initial imperfections in the material was discussed in the article, as well as the influence of the loadings applied to the crankshaft. Hence, the theoretical dimensioning of the crankshaft was firstly assessed assuming the conservative Soderberg criterion and the crankshaft model was then analysed using the Finite Element Method, during a complete combustion cycle, at several stress concentration regions. The Rainflow cycle counting method was applied to determine the stress cycles induced during functioning and a stress-life equation was then used to estimate the fatigue lifespan of the crankshaft. Additionally, a modification to the crankshaft's geometry was suggested and a significant reduction of the induced stresses was obtained.

Influence of Initial Imperfection on Buckling Load of Membrane Aeroshell

Influence of Initial Imperfection on Buckling Load of Membrane Aeroshell

Numerical study on lateral buckling of fully bonded sandwich pipes

Sandwich pipes (SP) are promising subsea pipelines for deep-water applications. Thus, their lateral buckling response is a significant design aspect. The temperature field of SP denotes the load in lateral buckling analysis, and it is calculated using the heat transfer differential equation in this study. A general temperature field formula is proposed for an arbitrary multilayer SP, and this formula is verified by finite element models. Another finite element model based on the fiber shell element is then constructed to investigate the lateral buckling response of SP. They are validated by existing experimental and numerical results. The evolution of the lateral buckling profiles and of stress distribution are studied in consideration of nonlinear pipe-soil interaction and nonlinear material constitutive law. Moreover, the influence of initial imperfections and pipe-soil interaction on lateral buckling response is examined. Finally, a formula is approximated for a SP with a half-wave sinusoidal imperfection profile on the basis of dimensionless analysis and numerical results. This formula is convenient for engineering applications.

12.07: Numerical assessment of stayed austenitic steel columns

ABSTRACTThe use of prestressed steel stayed columns systems in construction has numerous advantages related to its light weight, load bearing capacity and assembly process flexibility. A practical application of these structural systems could be recently observed in Brazil for the erection of the Rock in Rio III world stage. In this event the use of prestressed stayed carbon steel columns were used for temporary shoring, optimizing the erection sequence. The main stage had a 82 meters span with a 40 m clear height and a 4500 m2 area. Two stayed column were used, both with a 20 m length. Up to the present the majority of investigations of these stayed systems have been focused on carbon steel columns. In view of these facts the present paper aimed to present an investigation of the structural behaviour of columns made with the austenitic stainless steel. Numerical models were developed in the Ansys software, focusing on the structural system load carrying capacity and the optimal level of prestress to be used. The optimal level of prestress to be used was also determined considering the influence of initial imperfections. The results confirmed that the use of prestress in the cables significantly enhance the ultimate loads and ductility capacity of the columns.

Influence of initial imperfections on ultimate strength of spherical shells

Comprehensive consideration regarding influence mechanisms of initial imperfections on ultimate strength of spherical shells is taken to satisfy requirement of deep-sea structural design. The feasibility of innovative numerical procedure that combines welding simulation and non-linear buckling analysis is verified by a good agreement to experimental and theoretical results. Spherical shells with a series of wall thicknesses to radius ratios are studied. Residual stress and deformations from welding process are investigated separately. Variant influence mechanisms are discovered. Residual stress is demonstrated to be influential to stress field and buckling behavior but not to the ultimate strength. Deformations are proved to have a significant impact on ultimate strength. When central angles are less than critical value, concave magnitudes reduce ultimate strengths linearly. However, deformations with central angles above critical value are of much greater harm. Less imperfection susceptibility is found in spherical shells with larger wall thicknesses to radius ratios.

初期不整が軸方向圧縮力と単調逆対称曲げモーメントを受ける角形鋼管柱の弾塑性挙動に与える影響

It is well know that initial imperfections (e.g. bow-shaped imperfection of member, bow-shaped imperfection of plate and residual stress) have an influence on the performance of the columns. Most studies regarding this issue are investigated under single axial load or single shear force. In reality, the columns of steel structures are subjected to axial load and bending moment. Therefore, it is necessary to clarify the influence of the initial imperfections under combined loading. The influences of initial imperfections on the performance of square steel tubular columns under compressive axial force with one end monotonic bending moment were investigated by means of FEM in our previous study. As mentioned before, the columns of steel structures are subjected to axial load and bending moment, and the moment diagram is antisymmetric bending moment distribution or bending moment distribution close to antisymmetric. Therefore, it is not enough to clarify one end bending moment distribution. Also, in the previous study, the influence of the residual stress in the circumferential direction was not considered despite it exists in the member. In this study, the influences of initial imperfection on the performance of the square steel tubular column which is subjected to a compressive axial force with monotonic antisymmetric bending moment are investigated. The bow-shaped imperfection of the member (global imperfection) and continuous bow-shaped of the plate (local imperfection) are selected as geometrical initial imperfections and these imperfections are described by trigonometric function to reproduce easily. The residual stress in the longitudinal direction and the residual stress in the circumferential direction are selected as material initial imperfections. Firstly, the influence of initial imperfection on the performance and on the elastic-plastic behavor is investigated individually. Secondly, the influence of combined initial imperfections on the performance of the columns and on the elasto-plastic behavor is investigated; comparison between the experimental results are shown. Firstly, followings were found from the analysis results where initial imperfection is introduced in the member individually. 1) When the ultimate state was determined by local buckling, the global initial imperfection had a small impact on the performance. When the ultimate state was determined by bending deformation in in-plane, the direction of global imperfection had an impact on the performance as well its amplitude. 2) The amplitude of the local imperfection had a great impact on the performance of the columns. The maximum bending moment and the deformation capacity significantly decreased when its maximum amplitude exceeded 10% of permissible deviation. 3) When the ultimate state was determined by bending deformation, the plastic deformation capacity decreased with the increase of the amplitude of the residual stress in the longitudinal direction; initial stiffiness gradually decreased with the increace of its magnitude. 4) Regardless of the ultimate states, with the increase of the amplitude of the residual stress in the circumferential direction, the maximum bending moment and plastic deformation capacity decreased. Secondly, followings were found from the analysis results, where combined initial imperfections are introduced in the member. 5) In comparison with analysis results which were introduced the initial imperfection individually, the influence of the combination of initial imperfections on the variation trend of the performance was not confirmed. The local imperfection had the most significant influence on the perfomance in the imperfections that are considered in this study. 6) Validation of the numerical simulation was also performed; initial imperfections that could simulate test results in reasonable accuracy was shown.

Numerical modelling of steel plate girders at normal and elevated temperatures

The main goal of this study is to increase the knowledge on the behaviour of steel plate girders subjected to shear buckling at both normal and elevated temperatures. Hence, numerical models were duly validated with experimental tests from the literature. Experimental tests on steel plate girders with different configurations were numerically reproduced, showing a good agreement between numerical and experimental results. Afterwards, applying the validated numerical models, sensitivity analyses on the influence of initial imperfections were performed. Different values for the maximum amplitude of geometric imperfections were considered and residual stresses were also taken into account. Finally, the effect of the end supports configuration was also studied aiming to understand the strength enhancement given by the rigid end support at normal temperature and evaluating if that strength enhancement is maintained in case of fire.