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  • 1.
    Dalil Safaei, Seif
    KTH, School of Engineering Sciences (SCI), Mechanics, Structural Mechanics.
    Stiffness and vibration properties of slender tensegrity structures2012Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    The stiffness and frequency properties of tensegrity structures are functions of the pre-stress, topology, configuration, and axial stiffness of the elements. The tensegrity structures considered are tensegrity booms, tensegrity grids, and tensegrity power lines.

    A study has been carried out on the pre-stress design. It includes (i) finding the most flexible directions for different pre-stress levels, (ii) finding the pre-stress pattern which maximizes the first natural frequency.

    To find the optimum cross-section areas of the elements for triangular prism and Snelson tensegrity booms, an optimization approach is utilized. A constant mass criterion is considered and the genetic algorithm (GA) is used as the optimization method.

    The stiffness of the triangular prism and Snelson tensegrity booms are modified by introducing actuators. An optimization approach by means of a GA is employed to find the placement of the actuators and their minimum length variations. The results show that the bending stiffness improves significantly, but still an active tensegrity boom is less stiff than a passive truss boom. The GA shows high accuracy in searching the non-structural space.

    The tensegrity concept is employed to design a novel transmission power line .A tensegrity prism module is selected as the building block. A complete parametric study is performed to investigate the influence of several parameters such as number of modules and their dimensions on the stiffness and frequency of the structure. A general approach is suggested to design the structure considering wind and ice loads. The designed structure has more than 50 times reduction of the electromagnetic field and acceptable deflections under several loading combinations.

    A study on the first natural frequencies of Snelson, prisms, Micheletti, Marcus and X-frame based tensegrity booms has been carried out. The result shows that the differences in the first natural frequencies of the truss and tensegrity booms are significant and not due to the number of mechanisms or pre-stress levels. The tensegritybooms of the type Snelson with 2 bars and prism with 3 bars have higher frequencies among tensegrity booms.

  • 2.
    Dalil Safaei, Seif
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics, Structural Mechanics.
    Eriksson, Anders
    KTH, School of Engineering Sciences (SCI), Mechanics, Structural Mechanics.
    Tibert, Gunnar
    KTH, School of Engineering Sciences (SCI), Mechanics, Structural Mechanics.
    Optimum pre-stress design for frequency requirement of tensegrity structures2011In: Proceeding of 10th World Congress on Computational Mechanics, 2011Conference paper (Other (popular science, discussion, etc.))
    Abstract [en]

    Structures composed of tension and compression elements in equilibrium are denoted tensegrity structures. Stability of tensegrity structures is achieved through introducing initial member forces (pre-stress). The pre-stress design can be seen consisting of three different stages: (i) finding the bases of possible pre-stress states, (ii) finding admissible distributions considering unilateral properties of the elements and stability of the structure, (iii) finding the optimum pre-stress pattern for certain magnitude from compatible pre-stress states. So far, no research has been carried out to connect the three steps, i.e. finding a suitable pre-stress pattern which also considers mechanical properties of the highly pre-stressed structure e.g. its natural frequencies. This paper aims at finding an optimum pre-stress pattern and level of pre-stress for the maximum frequency. The pre-stress problem is on a linear static level where no slackening is allowed. An optimization is performed to find the optimum pre-stress pattern fromthe self-stress modes obtained by a singular value decomposition (SVD) of the equilibrium matrix. The objective function is the first natural frequency of the structure. Finite element analysis is employed for the linear analysis of the structure and a genetic algorithm for optimization i.e., a non-gradient method. The example considered is a double layer tensegrity grid consisting of 29 independent self-stress states. The method is applicable to complex asymmetric three-dimensional structures. The new aspect of this work is a link between the SVD analysis, finite element analysis and genetic algorithm.

  • 3.
    Dalil Safaei, Seif
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics, Structural Mechanics.
    Eriksson, Anders
    KTH, School of Engineering Sciences (SCI), Mechanics, Structural Mechanics.
    Tibert, Gunnar
    KTH, School of Engineering Sciences (SCI), Mechanics, Structural Mechanics.
    Stiffness visualization for tensegrity structures2010In: Proc. NSCM-23, 2010Conference paper (Refereed)
  • 4.
    Dalilsafaei, Seif
    KTH, School of Engineering Sciences (SCI), Mechanics, Structural Mechanics.
    Stiffness modification of tensegrity structures2011Licentiate thesis, comprehensive summary (Other academic)
    Abstract [en]

    Although the concept of tensegrity structures was invented in the beginning of the twentieth century, the applications of these structures are limited, partially due to their low stiffness. The stiffness of tensegrities comes from topology, configuration, pre-stress and initial axial element stiffnesses.

     The first part of the present work is concerned with finding the magnitude of pre-stress. Its role in stiffness of tensegrity structures is to postpone the slackening of cables. A high pre-stress could result in instability of the structure due to buckling and yielding of compressive and tension elements, respectively. Tensegrity structures are subjected to various external loads such as self-weight, wind or snow loads which in turn could act in different directions and be of different magnitudes. Flexibility analysis is used to find the critical load combinations. The magnitude of pre-stress, in order to sustain large external loads, is obtained through flexibility figures, and flexibility ellipsoids are employed to ensure enough stiffness of the structure when disturbances are applied to a loaded structure.

     It has been seen that the most flexible direction is very much sensitive to the pre-stress magnitude and neither analytical methods nor flexibility ellipsoids are able to find the most flexible directions. The flexibility figures from a non-linear analysis are here utilized to find the weak directions.

     In the second part of the present work, a strategy is developed to compare tensegrity booms of triangular prism and Snelson types with a truss boom. It is found that tensegrity structures are less stiff than a truss boom when a transversal load is applied. An optimization approach is employed to find the placement of the actuators and their minimum length variations. The results show that the bending stiffness can be significantly improved, but still an active tensegrity boom is less stiff than a truss boom. Genetic algorithm shows high accuracy of searching non-structural space.

  • 5.
    Dalilsafaei, Seif
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics, Structural Mechanics.
    Eriksson, Anders
    KTH, School of Engineering Sciences (SCI), Mechanics, Structural Mechanics.
    Micheletti, A.
    Tibert, Gunnar
    KTH, School of Engineering Sciences (SCI), Mechanics, Structural Mechanics.
    Parametric study of various tensegrity modules asbuilding blocks for slender boomsArticle in journal (Other academic)
  • 6.
    Dalilsafaei, Seif
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics, Structural Mechanics.
    Eriksson, Anders
    KTH, School of Engineering Sciences (SCI), Mechanics, Structural Mechanics.
    Tibert, Gunnar
    KTH, School of Engineering Sciences (SCI), Mechanics, Structural Mechanics.
    Application of flexibility analysis for design of tensegrity structures2011In: Proceeding of the 4th Structural Engineering World Congress, 2011Conference paper (Other (popular science, discussion, etc.))
    Abstract [en]

    Tensegrity structures have been the subject of research for many years, but very few of them have been built. One major disadvantage of tensegrities compared to typical trusses is their stiffness, which can be significantly reduced when a cable goes slack. This paper aims to introduce a method for stiffness characterization of tensegrity structures for the following purposes: (i) comparison of the stiffness of tensegrity structures with other truss structures, (ii) comparison of the stiffness of different form-found geometries, (iii) finding the most flexible nodes and the principal flexibility directions and (iv) finding stiffness effects of different pre-stress levels and patterns. The method is based on the flexibility analysis of tensegrity structures and the finite element method is used for the non-linear static analysis of the structure to obtain the flexibility figures which visualize the flexibility for different plane and spatial truss and slender boom tensegrity structures.

     

  • 7.
    Dalilsafaei, Seif
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics, Structural Mechanics.
    Eriksson, Anders
    KTH, School of Engineering Sciences (SCI), Mechanics, Structural Mechanics.
    Tibert, Gunnar
    KTH, School of Engineering Sciences (SCI), Mechanics, Structural Mechanics.
    Flexibility-based pre-stress design of tensegrity structuresManuscript (preprint) (Other academic)
    Abstract [en]

    Tensegritiy structures have been subjects of research for many years, but very few of them have been built. One major disadvantage of tensegrities compare to regular truss structures is their low stiffness. This papers aims to have a new look at the stiffness problem of tensegrity structures. Here, it is assumed that the form-finding step has been completed and the axial stiffness of the elements is known. We introduce a tool for stiffness characterization of a given tensegrity structure for different pre-stress magnitudes. Since the pre-stress has a critical influence on the stability of the structure with a role to prevent or postpone slackening, the magnitude of pre-stress of the structure exposed to large external loads and disturbances are found. Finite elements are utilized in the solution for the non-linear static analysis. The method is based on geometrical interpretation of flexibility of unconstrained nodes. Suggested concept, flexibility analysis, shows promising properties in finding flexible nodes, weak directions of structure, detection of cable elements with higher risk of going slack and better knowledge of influence of various external loads. The authors believe results of this research could help the researchers and designer of better understanding the behavior of tensegrity structures.

     

  • 8.
    Dalilsafaei, Seif
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics, Structural Mechanics.
    Eriksson, Anders
    KTH, School of Engineering Sciences (SCI), Mechanics, Structural Mechanics.
    Tibert, Gunnar
    KTH, School of Engineering Sciences (SCI), Mechanics, Structural Mechanics.
    Improving bending stiffness of tensegrity booms2012In: International Journal of Space Structures, ISSN 0956-0599, Vol. 27, no 2-3, p. 117-129Article in journal (Refereed)
    Abstract [en]

    There is a high interest in employing lightweight, low-cost, deployable structures for space missions. Utilization of tensegrity structures in space application is limited, due to their low stiffness, while a number of high stiffness-to-mass truss booms have been launched. This paper aims to describe and improve the bending stiffness of tensegrity booms. Tensegrity booms of Snelson and triangular prism type are selected for the study. These structures are excellent samples of class 1 tensegrities, with a single state of self-stress and one mechanism, and class 2 tensegrities, with multiple states of self-stress and mechanisms. The stiffness modification procedure includes three steps: (Step 1) developing a strategy for a fair comparison of tensegrity booms with a high performance truss boom. A genetic algorithm is employed to find the optimum cross-section areas of the boom elements. Sources of low stiffness of tensegrities are discussed. (Step 2) an effort is made to find the optimum placement of actuators for improving the stiffness of the tensegrity booms. (Step 3) a genetic algorithm is utilized to calculate their optimum actuation. All three stages have been performed based on a link between non-linear finite element analysis and a genetic algorithm. The genetic algorithm shows high accuracy of searching non-structural space, and also dealing with above steps. Results indicate that the stiffness of tensegrity booms is highly improved by activating the structures.

  • 9.
    Dalilsafaei, Seif
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics, Structural Mechanics.
    Eriksson, Anders
    KTH, School of Engineering Sciences (SCI), Mechanics, Structural Mechanics.
    Tibert, Gunnar
    KTH, School of Engineering Sciences (SCI), Mechanics, Structural Mechanics.
    Sensitivity analysis of tensegrity booms due to member loss2011In: Proceedings NSCM-24, 2011Conference paper (Other academic)
  • 10.
    Dalilsafaei, Seif
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics, Structural Mechanics.
    Tibert, Gunnar
    KTH, School of Engineering Sciences (SCI), Mechanics, Structural Mechanics.
    Design and analysis of tensegrity power lines2012In: International Journal of Space Structures, ISSN 0956-0599, Vol. 27, no 2-3, p. 139-154Article in journal (Refereed)
    Abstract [en]

    Overhead transmission power lines have undergone very small aesthetic and technical changes over time. Studies on mitigation of the electromagnetic field shows that utilizing a helix configuration is an effective way to reduce the electromagnetic field. This study proposes to use tensegrity structures as power lines. Tensegrity structures are composed of tension and compression elements in equilibrium. Modules, simple units with a certain rotation, are connected together to design of overhead power lines with considerable electromagnetic field reduction. A form-finding method enables the design of various free-form configurations. A parametric study is performed to investigate the influence of the module dimensions on the stiffness of the power line. A design algorithm was used for determining the optimum size of elements and the pre-stress level. The selected baseline structure was able to tolerate the wind and ice loads in severe conditions with a 50 times reduction in electromagnetic field. Finally a sensitivity analysis is performed to show the effects of element loss or damage.

  • 11.
    safaei, Seif
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Eriksson, Anders
    KTH, School of Engineering Sciences (SCI), Mechanics, Structural Mechanics.
    Micheletti, A.
    Tibert, Gunnar
    KTH, School of Engineering Sciences (SCI), Mechanics, Structural Mechanics.
    Study of various tensegrity modules as building blocks for slender booms2013In: International Journal of Space Structures, ISSN 0956-0599, Vol. 28, no 1, p. 41-52Article in journal (Refereed)
    Abstract [en]

    This study investigates the structural performance of long and slender tensegrity booms. Previous studies show that tensegrity structures are generally more flexible than conventional trusses or space frames. The aims here were (i) to quantify how much more flexible eleven different tensegrity booms are, when compared to state-of-the-art truss booms, (ii) to find a general explanation for this. The performance criterion used for the comparison was the first natural frequency of the boom. A finite element program with truss elements was used to compute the natural frequencies around the initial prestressed configurations. The results show that tensegrity booms have between one and three orders of magnitude lower natural frequencies than truss booms. It is concluded that for the best performing tensegrity booms, the bending stiffness is independent of the level of pre-stress and the number of infinitesimal mechanisms as the bending stiffness is given mainly by the material stiffness of the tension elements and not the geometric stiffness as the infinitesimal mechanisms are not activated by bending. Thus, whereas the level of pre-stress and the presence of infinitesimal mechanisms play major roles for the stiffness of some tensegrity structures, the axial stiffness and orientation of tension elements are most important for the studied slender booms.

1 - 11 of 11
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