Ultimate Limit State Analysis Research Articles

Optimization design of the jacket support structure for offshore wind turbine using topology optimization method

Jacket support structure is the main support form in medium water depth for bottom fixed offshore wind turbine. Topology optimization is presented in this paper for an efficient method to optimize the jacket structure by altering the structural layout to improve its design, minimize weight and ultimately reduce the cost. Together with size and shape optimization, an innovative jacket structure is obtained with constraints on strength, stiffness, stability, ultimate strength, and natural frequency. The optimization has been solved and the overall mass of the optimized structure decreases by 38.24% compared with the primary structure. Moreover, ultimate limit state analysis, eigenanalysis and buckling are used to evaluate the optimized structure. The results indicate that the proposed optimized method is an effective way to reduce the weight and stress concentration. It also shows that topology optimization provides useful insights for the conceptual design phase and provides a better starting point for the further size and shape optimization.

The ISSC 2022 committee III.1-Ultimate strength benchmark study on the ultimate limit state analysis of a stiffened plate structure subjected to uniaxial compressive loads

This paper presents a benchmark study on the ultimate limit state analysis of a stiffened plate structure subjected to uniaxial compressive loads, initiated and coordinated by the ISSC 2022 technical committee III.1-Ultimate Strength. The overall objective of the benchmark is to establish predictions of the buckling collapse and ultimate strength of stiffened plate structures subjected to compressive loads. Participants were asked to perform ultimate strength predictions for a full-scale reference experiment on a stiffened steel plate structure utilizing any combination of class rules, guidelines, numerical approaches and simulation procedures as they saw fit. The benchmark study was carried out blind and divided into three phases. In the first phase, only descriptions of the experimental setup, the geometry of the reference structure, and the nominal material specifications were distributed. In the second phase, the actual properties of the reference structure were included. In the third and final phase, all available information on the reference structure and measured properties were distributed, including the material properties and laser-scanned geometry. This paper presents the results obtained from seventeen submitted FE simulations as well as details on the experiment. It also presents comparisons of the force versus the displacement curve, failure modes and locations for each phase, among others, and a discussion on the participants’ ability to predict the characteristics of the reference experiment with the information that is available for the phase. The outcome of the study is a discussion and recommendations regarding the design of finite element models for the ultimate state analysis of stiffened plate structures, with emphasis on the prediction of the ultimate capacity, force-displacement curve, and failure mode and location related to access to data, uncertainties and modeling of the material properties, geometric imperfections and distortions, and residual stresses.

Ultimate limit state analysis of a double-hull tanker subjected to biaxial bending in intact and collision-damaged conditions

This study presents a comparison between nonlinear finite element analysis (FEA) and the Smith method of Fujikubo et al. (2012). The objective was to compare the accuracy and computation effort of the two methods for a double-hull tanker under biaxial bending and various ship conditions: intact hull structure, collision-damaged hull structure, newly built condition, and ship hull aged due to corrosion. The results for the non-corroded and intact ship hull structures showed good agreement between FEA, the Smith method and IACS CSR-H for vertical bending loading conditions. For all other bending load combinations, FEA always gave lower ultimate bending moments than the Smith method. The differences between the two methods were larger for the corroded and damaged ship hull structure than for other conditions. Results from ultimate strength analyses of the collision-damaged hull structures showed that both methods captured the expected asymmetric ultimate strength response due to asymmetric damage. A residual strength index calculation showed that the reduction was larger for the FEA than for the Smith method. A procedure is proposed that combines results of a few FEAs with the advantages of the Smith method to generate accurate biaxial bending load interaction curves for different ship conditions.

Ultimate limit state design of reinforced concrete shallow shells under seismic‐type horizontal loads

This paper deals with positive Gaussian curvature reinforced concrete (RC) shallow shells and panel shells, including monolithic connection between the shell itself and its edge elements, such as columns, diaphragms, walls, etc. Connections of such shells with edge elements are subjected to additional bending moments and shear forces due to horizontal loads. Such structures are at particular risk in seismic zones. Mutual work of shell and its diaphragms under ultimate horizontal forces is very important, but research in this field is rather limited. In spite of numerous collapses due to failure in the above‐mentioned connections, there are not enough practical investigations, considering their failure schemes, required for ultimate limit state analysis. Such investigation can be carried out by testing scaled shell models. This study is focused on experimental and analytical investigation of scaled shell models, subjected to horizontal loads that grow up to the failure of the tested specimens. For numerical analysis of the models, limit equilibrium method and mini–max principle are used. It provides a theoretical basis for design of the shell–edge element system to ultimate horizontal loads, considering the limit state of the structure. The edge element type, investigated in this study, is an RC element that has monolithic connection with the shell and able to take horizontal loads. Using the mini–max principle, the real structural load‐bearing capacity in the ultimate state of the structure is calculated without its under‐ or over‐estimation. It is the main reason for improving the accuracy of the analysis. The numerical example demonstrates that analytical results, obtained for the tested models, are close to the experimentally obtained data. The proposed methodology allows more wide application of effective spatial RC structures in seismic regions.

A simplified approach for the ultimate limit state analysis of three-dimensional reinforced concrete elements

Several advanced constitutive models of concrete have been developed in recent decades, some of which are able to reproduce the behaviour of concrete structures with a high level of accuracy. Yet given their complex formulation, their application by most practitioners can entail some difficulties. This paper describes the adoption of an alternative, simplified comprehensible constitutive model for the ultimate limit state analysis of 3D reinforced concrete structural elements.The proposed model is described and the undertaken assumptions are justified. Different uniaxial stress–strain models can be adopted, particularly neglecting the tensile strength of concrete permits to study the goodness of the stress field method and the strut-and-tie method for 3D elements.This model was implemented into a non-linear finite element-based tool developed by the authors. The results of twelve four-pile caps and three socket base column-to-foundation connections are shown. The proposed approach facilitated the identification of the flow of forces and allowed a better understanding of the structural response.

ULTIMATE STRENGTH ANALYSIS OF SHIP STRUCTURES

Subjective and objective uncertainties are imposed on ship structures due to the random nature of the loading environment, inadequate knowledge of physical phenomena associated with loads or deviations in material properties which make reliable predictions of structural response a difficult task. Strength criteria for ships can be established by ultimate strength studies of progressive collapse analysis of finite element models under different boundary conditions with combined geometric and material nonlinearities. Load-Displacement and/or Moment-Curvature curves can be generated and the ultimate load causing failure identified as a multiple of the design load. Ultimate limit state analysis can be carried out for various combinations of parameters to identify the ultimate load factor in each case .

Briefing: Soil parameters, sampling and testing for design to Eurocode 7

Eurocode 7 defines a characteristic strength for an ultimate limit state analysis with specified partial factors and related codes specify how the characteristic strength should be determined from laboratory tests on samples. The note discusses how designers might choose between peak, critical state and residual strengths and how they might determine values from tests on samples of different quality class.

Multi-objective structural optimization of a HAWT composite blade based on ultimate limit state analysis

An extensively automated optimization procedure is presented for a horizontal axis wind turbine (HAWT) blade based on ultimate limit state analysis. Two composite materials named glass fiber reinforced plastic (GFRP) and carbon fiber reinforced plastic (CFRP) are applied with a multi-objective of blade cost and total mass. Laminate layer thickness, material type and orientation angle are tailored for the structural performance and subjected to three design constraints which are derived from analysis of ultimate strength, fatigue failure and critical deflection, respectively. Combining FEM analysis and an evolutionary algorithm, the proposed optimization process has dramatically reduced design cost and improved blade performance.

Required and available moment redistribution of continuous steel–concrete composite beams

Based on the ultimate limit state analysis, the required moment redistribution to enable full plastic mechanism for continuous composite beams is derived. The composite beams studied are continuous over the internal support and with a uniform section along the beams which are one of the conventional steel structural forms in Chinese construction practice for buildings and medium span bridges. It is illustrated that the required moment redistribution for the beam increases as the ratio of negative to positive moment resistance reduces, but decreases as the span difference, or the difference of load in the two spans increases. A method to assess the available moment redistribution based on the rotation capacity at the notional plastic hinges of a composite beam is developed. The potential moment redistribution in a continuous composite beam is also assessed when the available rotation capacity at the notional hinge fails to satisfy the required capacity capable of a plastic design. For a continuous composite beam to develop full plastic design, the available moment redistribution for the beam should be greater than or at least equal to the moment redistribution required, hence the full moment redistribution from the hogging region to the sagging region in the beam is capable. The derived available moment redistributions agree with the test results and computer simulations, but in a general lower bound of the strength capacity. An example is given and the results are compared with that based on the moment redistribution proposed in the present Chinese design code for steel–concrete composite beams. The present study provide a design approach to assess the load carrying capacity for a continuous composite beam based on the available moment redistribution capable depending on the rotation capacity or the force ratio etc rather than a fixed value of moment redistribution proposed, so that in most cases, an economic design is capable.

Recent research advances on ECBL approach.: Part I: Plastic–elastic interactive buckling of cold-formed steel sections

The objective of this two parts paper is to present some recent developments and applications of erosion of critical bifurcation load (ECBL) approach for the interactive buckling. Two different types of problems are analysed: (1) plastic–elastic interactive buckling which implements into the Ayrton–Perry interaction formula the plastic strength of the stub columns evaluated by means of local plastic mechanism analysis, and (2) elastic–elastic interactive buckling for members with perforations. The first part of the paper analyses the occurrence of local plastic mechanisms in cold-formed steel sections in compression, and how they can be implemented in the ultimate limit state analysis of the members. Actually, the failure of thin-walled cold-formed members in compression always occurs with a local plastic mechanism. Starting from this observation, the authors suggest to use in the interactive local-overall buckling analysis the sectional plastic mechanism strength instead of traditional ‘effective section’. The ECBL approach is used to implement the proposed interactive buckling model. Results are compared with those of other two recent methods, namely the direct strength method and plastic effective width approach. Relevant tests are used to evaluate the three methods. Comparisons with European and American design codes are also presented in the paper.

Ultimate load analysis of plate reinforced concrete beams

In a previous paper, the experimental work carried out on concrete–steel composite beams utilising the ‘encased plate construction’ was presented. The provision of steel plates as shear reinforcement allows very high shear stresses to be safely developed. This is a new concept in the design of reinforced concrete beams. This paper describes the structural behaviour and ultimate limit state analysis of plate reinforced beams with short shear spans. The proposed analytical method is based on the equilibrium of forces and strain-compatibility. This method predicts both the mode of failure and the ultimate limit state capacity. The proposed analysis shows good agreement with the experimental results presented in the previous paper.

Application of linear structural analysis program for steel frame ultimate limit state analysis

This paper describes the feasibility of applying a modified linear structure analysis computer program in the ultimate limit state design of unbraced steel frames. The elastic–plastic hinge zone concept and the linear elastic structural analysis computer program SAP90, are used to simplify the nonlinear steel frame structural analysis model. Most steel structures are designed by plastic theory, called ‘ultimate limit state design’. In practice, most steel structure analysis is performed by linear elastic computer programs that do not take into account either inelastic behavior or calculation of the ultimate strength. Herein, a modified analysis approach is presented and its design implications discussed. The results obtained from the modified approach are provided and compared with both experimental data and the inelastic structural analysis computer program DRAIN-2DX with respect to the ultimate strengths and the lateral displacements of unbraced steel frames. Copyright © 1999 John Wiley & Sons, Ltd.

Optimum Design of Horizontally Curved R/C Slabs

A method for selecting the optimum design variables in the design of horizontally curved two-way reinforced concrete floor slabs is presented. The slab panels are assumed to be uniformly loaded by characteristic dead and live loads and supported on all sides by rigid boundary supports. The optimum values are selected from within the feasible region in the design space defined by the limit state requirements of CP 110 (British Standard Code). Cost of material is treated as the objective function and is optimized subject to the bending behavior and side constraints imposed by the limit-state requirements. For the ultimate limit-state analysis, the bending moments in the curved slab panels are determined from the linear-elastic theory of sector plates. Nonlinear programming problems of high degree are generated by setting the effective depth and the transformed steel ratios as the design variables in both the objective function and the constraints. The sequential linear programming is introduced as an analytical method of nonlinear optimization. The optimum design variables are then determined.