Charis J. Gantes

Structural analysis and design of deployable structures

Abstract Deployable-collapsable structures have many potential applications ranging from emergency shelters and facilities, through relocatable, semi-permanent structures, to space-station components. Their main advantages are the small volume they occupy during storage and transportation, and their fast and easy erection procedure. A new concept of self-stabilizing deployable structures featuring stable, stress-free states in both deployed and collapsed configuration shows even higher promise. During the deployment phase these structures exhibit a highly nonlinear behavior. A large displacements/small strains finite element formulation is used to trace the nonlinear load-displacement curve, and to obtain the maximum internal forces that occur in the members of the structure during deployment. The influence of various parameters that affect the behavior of the structures, such as geometric shape, dimensions of the members, cross-sectional properties and kinematic assumptions is being investigated.

Elastic–plastic response spectra for exponential blast loading

Abstract The design of structures subjected to loads due to explosions is often treated by means of elastic–plastic response spectra. Such spectra that are currently available in the literature were computed on the basis of triangular shape of blast pressure with respect to time. In the present paper, response spectra based on an exponential distribution of blast pressure, which is in better agreement to experimental data, are proposed. To that effect, analytical expressions of the solutions of the pertinent equations of motion have been obtained via symbolic manipulation software, and have been used to carry out an extensive parametric study. A comparison of the spectra obtained by the proposed approach to the existing ones, reveals that the commonly used assumption of triangular blast load evolution with time can sometimes be slightly unconservative, particularly for flexible structural systems, but can also be significantly overconservative for stiffer structures.

Influence of equivalent bolt length in finite element modeling of T-stub steel connections

In this paper the development and implementation of a finite element model for simple T-stub steel connections is presented. Material and geometric non-linearities as well as contact and friction have been implemented in the model. The model is validated by comparison with experimental data found in the literature, for configurations exhibiting different failure mechanisms and featuring different bolt preloading levels. The impact of bolt length considered in the model is investigated and is shown to be of primary importance. This issue is representative of the continuously increasing use of advanced numerical analysis, supported by progress in computational mechanics, as a tool for practical design of engineering structures.

NON-LINEAR PANEL FLUTTER IN REMOTE POST-CRITICAL DOMAINS

Abstract The non-linear behavior of an elastic panel subjected to the combination of supersonic gas flow and quasi-static loading in the middle surface is studied. Particular attention is focused on the bifurcation paths in the flutter domain, including their remote parts. A variety of attractors, both periodic and chaotic, are observed. Moreover, non-symmetric stable periodic vibrations have been explored in regions far away from areas of divergence (buckling) instability. The transition from one attractor to the next one is discussed when the control parameters vary continuously. Hysteretic phenomena are studied by comparing the system’s behavior in direct and return paths on the control parameter plane.

Nonlinear dynamic buckling of multi-DOF structural dissipative systems under impact loading

The nonlinear dynamic buckling characteristics of multi-degree-of-freedom (MDOF), dissipative systems under impact loading are investigated. A fully plastic impact due to a striking body falling freely from a given height is postulated. The initial velocities of the system immediately after impact are determined analytically by applying the law of impulse momentum. The resulting response is governed by autonomous highly nonlinear ordinary differential equations (O.D.E.) which can be integrated only numerically. Instead, a qualitative analysis based on energy criteria is performed which allows to readily establish approximate dynamic buckling loads, very good for structural design purposes, as well as lower and upper bound dynamic buckling estimates for step loading and impact loading with given initial momentum. The efficiency and reliability of the proposed method is comprehensively demonstrated through numerous examples of two- and three-degree-of-freedom models.

Combining numerical analysis and engineering judgment to design deployable structures

Abstract Deployable structures are prefabricated space frames that can be stored and transported in a compact folded configuration and then deployed rapidly into a load bearing configuration. The structures are stable and stress-free in the folded and the deployed configuration, but exhibit a highly nonlinear behavior during deployment. Therefore, their design process should include simulation of their response in two phases: in the deployed configuration under service loads, and during deployment. The first phase involves linear analysis while the second one requires a geometrically nonlinear finite element formulation. Both simulations can be very demanding in terms of computer storage requirements as the number of degrees of freedom increases. In addition, the nonlinear analysis is quite expensive because of the large number of load steps that are necessary in order to trace the complete load-displacement path. This paper first describes a set of numerical models that were used to simulate the exact structural behavior using the finite element program ADINA. Then, some simplified analytical and numerical models are proposed that can be applied in the preliminary design stage, or even for final design, in order to obtain approximate but satisfactory results at a much lower cost.

Deployability Conditions for Curved and Flat, Polygonal and Trapezoidal Deployable Structures

Prefabricated, deployable space frames that exhibit self-standing and stress-free states in both the deployed and collapsed configurations are investigated in this paper. This type of deployable structures shows considerable advantages as compared to previous designs that either required external stabilizing or had members with residual stresses in the deployed configuration. Following previous developments for flat deployable structures consisting of units with regular-polygon planviews, this study deals with flat structures made of trapezoidals, and curved structures assembled from regular-polygonal units. First, the general geometric constraints and deployability conditions for these units are formulated, and a methodology for using these constraints as geometric design criteria is presented. Furthermore, additional conditions for the assemblage of single units into larger structures are given. Then, structural analysis issues for this type of structures are discussed. The necessity of nonlinear analysis during deployment is emphasized. Finally, the above geometric design procedures are demonstrated with specific examples.

Modeling, loading, and preliminary design considerations for tall guyed towers

Abstract The inherent nonlinearity in the structural behavior of guyed towers leads to difficulties in their structural analysis, and prevents the formulation of a general-purpose design methodology. As a result, simplifying analysis assumptions regarding the loading and the modeling of structural behavior have to be made, and approximate design methods are used, that are often unjustified, and can lead to disastrous failures. In this paper, the authors first summarize the results of an investigation they carried out on the collapse of a 1900 ft tall guyed tower under ice and wind loads. Based on this investigation, they then proceed to present some structural analysis recommendations relating to loading and modeling concerns. Special emphasis is placed on the importance of ice loading, and on the level of accuracy required in modeling the nonlinear response behavior. Finally, the conclusions drawn from this study are used to formulate preliminary design guidelines. This facilitates a systematic approach for the design of tall guyed towers.

Truncated newton methods for nonlinear finite element analysis

Abstract In the present study procedures for the solution of large-scale nonlinear algebraic discrete equations arising from the application of the finite element method to structural analysis problems are described and evaluated. The methods are based on Newtons method for the outer iterations, while for the linearized problem in each iteration the preconditioned conjugate gradient (CG) method is employed. This combination for the outer and inner iterations allows the use of less accuracy in computing exact Newton directions when far from the solution and the gradual increase in accuracy for the inner loops as the final solution is approached. This technique leads to the truncated Newton methods. Two preconditioning techniques for CG have been described and compared, namely the partial preconditioning and the partial elimination. Both techniques use a drop-off parameter ψ to control the computer storage demands for the extra matrix required. The results of two test examples are very encouraging as they show that the proposed method can be very effective in the solution of nonlinear finite element problems.

Energy-based dynamic buckling estimates for autonomous dissipative systems

The dynamic buckling global response of a nonlinear, 3-degree-of-freedom dissipative model under a step loading of infinite duration is thoroughly discussed. Geometrically imperfect models with symmetric or antisymmetric imperfections losing their static stability through a limit point and an asymmetric bifurcation point, respectively, are considered. Emphasis is given to the combined effect of nonlinearities (geometric and/or material) and damping. Exact, approximate, and lower/upper bound estimates based on energy criteria for establishing the dynamic buckling response of such autonomous models without solving the highly nonlinear initial-value problem are assessed. The reliability and efficiency of the proposed readily obtained estimates is illustrated via numerical simulation, the accuracy of which is checked using energy balance considerations. Certain interesting byproducts associated with a postlimit point bifurcation and breakdown of the symmetry of deformation are also revealed.