A reduced-order model for the dynamic and post-buckling behavior of tensegrity structures

To investigate the dynamic behavior of offshore structures to irregular waves, we conducted model tests using frame and column models, and took field measurements of the vibrations of an offshore structure installed in 80 m of water. We also conducted theoretical analyses of the dynamic response of offshore structures to irregular waves, and compared calculated values with the results of model tests and actual field measurements. From these studies to evaluate the dynamic response characteristics of offshore structures to irregular waves, we obtained data necessary for the dynamic design of deep-water offshore structures. INTRODUCTION The design of large offshore structures to be installed in deep water must be based on dynamic analysis, but strict analysis of their dynamic behavior requires frequent use of large computers. To avoid this costly procedure, it is necessary to employ either hypotheses or the approximation method in analyzing phenomena ranging from sea-surface fluctuations to vibration responses. To be analyzed by the approximation method, vibration phenomena must be clearly defined, so we analyzed the dynamic behavior of offshore structures under the influence of irregular waves by conducting model tests. We also conducted field measurements of the vibrations of an offshore platform. Further, we determined the responses by linear analysis using relatively simple models and evaluated the dynamic behavior characteristics by comparing calculated and measured results. The model tests comprised free vibration tests in which column and frame models were tested both in the submerged state and in air and dynamic response tests in which the models were tested using two dimensional irregular waves. In the former tests, the natural periods, damping constants and vibration modes of the models were determined in the latter, the fluid force coefficients of irregular waves and the response displacements and stress range frequency distribution characteristics of offshore structures were obtained. In the field vibration measurements, the natural period and damping constant of the platform were determined from its response power spectra in the tremor state. Our field measurements also revealed wave response characteristics of offshore structures during the passage of a storm center and their vibration characteristics during drilling. In the theoretical analyses, a lumped-mass model was substituted for the structure using the natural periods and damping constants obtained in the model and field tests. Borgman's wave force spectrum1 was used for the wave force spectrum in determining the response values. Then these theoretical response values were compared with the measured values. OUTLINE OF THE MODEL TESTS The column and frame models shown in Fig. 1 were used in the model tests. They were made of Acrylite. In the free vibration tests, plates each weighing about 5 kg were used as a load on the deck of each model.

Evaluation of compression member buckling and post-buckling behavior using artificial neural network

Buckling and Post-buckling Behavior of Beams With Internal Flexible Joints Resting on Elastic Foundation Modeling Buried Pipelines

Building on previous efforts dedicated to educing the flow motions associated with low-angle sound emission from turbulent jets ,2 , the current paper proposes a strategy for the reduced-order dynamical modelling of these motions. The technique comprises a dynamic Ansatz obtained by Galerkin projection of the Navier-Stokes equations onto a set of orthonormal spatial basis functions; however, rather than directly computing the coefficients of the resulting evolution equation, which is only possible when full flow information is available, the coefficients are here identified. The described approach, though validated here using partial flow information from a numerical simulation, is thus applicable in an experimental context. A key objective of such modelling is to obtain a dynamical model with as few degrees of freedom as possible, this being motivated by the need to perform real-time sensing and control. The number of degrees of freedom is dictated by the number of basis functions deemed necessary for accurate and robust reproduction of the dynamics, and this raises the question regarding what kind of basis functions are best suited to a given problem. The empirical eigenfunctions obtained by Proper Orthogonal Decomposition (hereafter POD) are frequently used, largely on account of the optimality property of this kind of decomposition. POD is used in this work; however, rather than applying it to the full flow solution, it is applied to the reduced complexity, sound-producing flow skeleton identified in the previous studies . These studies showed that the number of modes necessary to capture a given percentage of fluctuation energy of the acousticallyimportant coherent structures is an order of magnitude less than the number necessary to capture the same percentage of fluctuation energy of the turbulence. The POD modes used here can thus be considered as acoustically-optimised. A reduced-order model (twenty degrees of freedom) is constructed that reproduces the dynamics of the essential sound-producing flow motions. The approach combines the flow calibration technique of Perret et al. and Tikhonov regularisation such as proposed by Cordier et al. . The model successfully computes the evolution of the flow motions over a short time horizon (about six convective time units), and a further intrinsic stabilisation technique is then implemented in order to enable long-time prediction. Sound radiation is computed by means of the jittering wavepacket source Ansatz of Cavalieri et al., as per Kerherve et al., and good agreement is obtained with the sound field of the original simulation, validating the modelling approach with respect to sound source dynamics.

ABSTRACT: The problem discussed concerns the dynamic response of offshore bottom supported structures to the action of periodic deep water waves when yielding is considered in the structure. Results of mathematical studies show that unbounded response can occur after several near-breaking waves have passed a typical offshore structure. Also, the predicted response of a structure, which is considered to, yield, is significantly affected by the number of degrees of freedom considered in the idealization. INTRODUCTION Background Ocean structure engineers have recently been concerned with the design of offshore tower structures for dynamic behavior. Pursuant to the design procedure, knowledge of the dynamic loads and structural behavior is necessary. The research concerned with the loading on these structures has been pertinent and of high quality. It is surprising that very few tudies have been started with the purpose of finding basic structural behavior, viz. deformation patterns or even failure regimes. The basic simulation has not been made for the case where materials in the structure exceed the elastic limit. The relationship of force on structures to fluid motion due to free gravity waves - considered to be a design load case-has received a great deal of attention (1,2,3,4). Large amounts of experimental data on wave forces have been gathered for shallow water cases where structural motions are small. Unfortunately no data has been gathered for cases where there is structural dynamic wave interaction type response. Theoretical studies have frequently(2,3,S) sought to characterize wave forces using statistical methods. The following comments extracted from Ref.(2) are pertinent: " ... The statistical techniques ... neglect the time sequence of the phenomena as they assume the random phase. Thus, one entire class of information is often thrown away in the analysis of waves. From an engineering standpoint, it is the groups of several periodic waves, which are almost always the highest waves in the wave system, that are most effective in causing structural damage..." These comments are unfortunate because the statistical method makes a valuable contribution to wave force evaluation by characterizing the spectrum of the water surface elevation(6). Clearly a reconciliation of views between the deterministic wave train and surface spectrum approaches is needed. Other studies on wave forces were concerned with effects of structural motions on the forces. The governing equation, originally developed by Morison, et. al.(l), has Deen generalized so that relative fluid velocities and accelerations are related to force on the structure (7,8). In certain cases these modifications will significantly alter the predicted results. Experimental verification of these proposed modifications is needed. Perhaps the most comprehensive representation of structural behavior during application of wave forces was attempted by Edge and Mayer(9). This linear mathematical model was structured to include effects of soil and waves on the tower.

Condition assessment, model updating, and basic studies of large and complex structures subjected to static and dynamic loadings necessitate establishing a reasonably low order model to enhance computational efficiency. An essential requirement for the development of a low-order model is to preserve the main characteristics of the full-scale model within the frequency range of interest or the desired static and/or dynamic response. In this regard, a novel reduced-order model (ROM) is developed based on a full-scale model (FSM) built using commercial finite element analysis software. Utilizing the common lumped mass matrix can introduce mass modeling errors. Hence, a method is proposed to extract the reduced consistent mass matrix model (RCMM) for use in the ROM. The modeling errors in the proposed ROM is studied for comparison with the static and dynamic responses. It can curtail capability of parameter estimation to capture the physical behavior of the structure using the ROM. Modeling error simulations prior to actual field-testing is highly recommended to determine the feasibility of non-destructive tests for successful system identification. The Milad telecommunication tower is the sixth tallest tower in the world, with a height of 435 meters. Using the proposed method, the consistency of the generated ROM based on the RCMM is verified with the Milad Tower’s FSM. In this regard, displacements due to equivalent static wind loads, natural frequencies, mode shapes, slopes of mode shapes, and time history response for earthquake loading are compared. The proposed ROM assessments utilizing RCMM confirm the accuracy of the static and dynamic characteristics.

Functionally graded material (FGM) has an important application prospect in aircraft engineering, especially in smart aircraft. The dynamic behavior of FGM has been widely investigated so far but more work is needed for the porous FGM pipes conveying fluid. In this paper, a sensible pore distribution function related with the volume fraction of metal and ceramic is proposed for the dynamic modeling of porous FGM pipes conveying fluid. The maximum porosity and its corresponding position are taken into account in the present mechanical model. The material properties of the porous pipes are temperature dependent and can be affected by pore distribution. The governing equation of the porous FGM pipe is derived and then the exact solution of post buckling is obtained. The nonlinear primary resonance is determined by the multiple scale method. It is shown that the effect of the pore distribution is very significant on the post buckling behavior and nonlinear primary resonance of the porous FGM pipes. The current work is very helpful in understanding the influence of pore distribution on static and dynamic behavior of pores FGM structures in engineering practice.

During the last decade, tensegrity systems have been the focus of numerous investigations exploring the possibility of adopting them for planetary landing and exploration applications. Early approaches mainly focused on locomotion aspects related to tensegrity systems, where mobility was achieved by actuating the cable members of the system. Later efforts focused on understanding energy storage mechanisms of tensegrity systems undergoing landing events. More precisely, it was shown that under highly dynamic events, buckling of individual members of a tensegrity structure does not necessarily imply structural failure, suggesting that efficient structural design of planetary landers could be achieved by allowing its compression members to buckle. In this work, we combine both aspects of previous research on tensegrity structures, showing a possible lattice-like structural configuration able to withstand impact events, store pre-impact kinetic energy, and utilize a part of that energy for the locomotion process. Our work shows the feasibility of this proposed approach via both experimental and computational means.

Postbuckling behaviour of stainless steel (SUS304) columns under loading–unloading cycles

This paper addresses the issue of the effect of structural load on structural dynamic behavior, and how an understanding of this behaviour can be used beneficially to reduce the amplitude of response. The processes discussed lead to a new method of adaptively retuning structures through the introduction of known loads, with particular reference to helicopter response suppression. In many applications it is desirable to reduce the response of a structure without the use of damping. This has been the subject of much research with regard to helicopter design. Passive methods exist but do not allow for multiple excitation frequencies, shifting frequencies or structural alterations arising from different flight configurations and loading. Active systems have the advantage that they can respond quickly and continuously to changes in system configuration but they require significant power and higher levels of maintenance than passive systems. Adaptive passive or semi-active systems are able to adapt to flight conditions while using far less power than fully active systems. They generally involve similar techniques to passive methods with some mechanism for tuning the parameters of the vibration absorber. In this paper, numerical and experimental techniques are used to investigate the effect of loading on structural response with a view to implementing a structural load based adaptive passive device. To set the scene for this proposal, a case study of structural loading on a helicopter tailboom is presented, illustrating the extent to which the dynamic response can be affected by loading. Finite element techniques are investigated and the results compared with experimental data. The difficulties in modelling the changes in dynamic performance are discussed with refence to a case study on joint parameterization. It is found that parameters updated for one load case may not apply across an entire loading regime. A preliminary study is then carried out to assess the feasibility of the proposal, and it is demonstrated that such a system can provide improved response with low power requirements across a range of narrow band excitation frequencies. This important result provides a starting point for further research into automated adaptive techniques for response suppression, and critical areas for new work to be focused on are identified.

Nonlinear seismic response and damage evolution of a mountain tunnel: Multi-scale simulation by IBEM-FEM coupled method

Composite materials have revolutionized industries such as aerospace and automotive with their impressive strength-to-weight ratios and customizable properties. Ply angle and fiber orientation are critical factors that significantly impact the dynamic behavior of composite structures, influencing stiffness, strength, and vibrational characteristics. This research delves into the dynamic characteristics of composite plates, explicitly examining how ply angle and fiber orientation affect vibrational behavior. Through finite element analysis (FEA), composite plates with varying ply angles and fiber orientations were modeled to understand their influence on natural frequencies, mode shapes, and dynamic responses under various loading conditions. The study reveals that cross-ply [0/0/0/0] exhibits the highest stiffness and superior stress handling, while the cross-ply balanced laminate [90/0/0/90] demonstrates better vibrational characteristics. The findings highlight the intricate relationships between design parameters and structural vibrational behavior, offering opportunities for optimizing composite structures. Additionally, harmonic analysis showed that a cut-out increases the natural frequency. The results underscore the importance of optimizing composite plate configurations to enhance vibrational characteristics and align natural frequencies with operational requirements, thereby mitigating resonance-related issues.

Experimental study of collapse behavior of single-layer cylinder shells with infilled walls

A main aim of spacecraft design is cost reduction that can either be achieved by reducing the cost of the spacecraft or by lowering the cost to launch. One proposed technology to reduce the mass and therefore lower the launch costs is the multifunctional power structure concept that incorporates the secondary spacecraft power supply into the load carrying structure. Here a short introduction of the dynamic analysis and optimization of such a structure is presented. The manufacture and testing of a multifunctional power panel is discussed in detail and its dynamic response is compared to a conventional honeycomb panel. The multifunctional design successfully combined the structural and power storage functions. It provided a similar dynamic response to conventional spacecraft structures and improved the energy density.

Tensegrity structures are pre-stressed pin-joint structures inclusive tensile members and compressive members. Form-finding is very important for Tensegrity structures need to determine shape and pre-stress value for self-equilibrium. In the form-finding of tensegrity structures, Infomation are usually introduced for initial geometry and pre-determined member types. In this study, Form-finding of tensegrity structures by using genetic easy and versatile form-finding processing requires only a minimal information 'topology'. Genetic Algorithms was determined the type of members. The form-finding of tensegrity structures are found by a full iteration pocedure until a state of self-stress is obtained.