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Stiffness modification of tensegrity structures
KTH, School of Engineering Sciences (SCI), Mechanics, Structural Mechanics.
2011 (English)Licentiate 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.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology , 2011. , viii, 24 p.
Series
Trita-MEK, ISSN 0348-467X ; 2011:02
Keyword [en]
tensegrity, boom, finite element analysis, genetic algorithm, flexibility analysis, active structure
National Category
Engineering and Technology
Identifiers
URN: urn:nbn:se:kth:diva-34039OAI: oai:DiVA.org:kth-34039DiVA: diva2:418858
Presentation
2011-05-26, 10:15
Opponent
Supervisors
Note
QC 20110524Available from: 2011-05-24 Created: 2011-05-24 Last updated: 2011-05-24Bibliographically approved
List of papers
1. Flexibility-based pre-stress design of tensegrity structures
Open this publication in new window or tab >>Flexibility-based pre-stress design of tensegrity structures
(English)Manuscript (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.

 

National Category
Engineering and Technology
Identifiers
urn:nbn:se:kth:diva-34036 (URN)
Available from: 2011-05-24 Created: 2011-05-24 Last updated: 2012-09-04Bibliographically approved
2. Application of flexibility analysis for design of tensegrity structures
Open this publication in new window or tab >>Application of flexibility analysis for design of tensegrity structures
2011 (English)In: 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.

 

National Category
Engineering and Technology
Identifiers
urn:nbn:se:kth:diva-34037 (URN)
Note
QC 20110524Available from: 2011-05-24 Created: 2011-05-24 Last updated: 2012-09-04Bibliographically approved
3. Improving bending stiffness of tensegrity booms
Open this publication in new window or tab >>Improving bending stiffness of tensegrity booms
2012 (English)In: International Journal of Space Structures, ISSN 0956-0599, Vol. 27, no 2-3, 117-129 p.Article in journal (Refereed) Published
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.

Keyword
active structure, boom, finite element analysis, flexibility analysis, genetic algorithm, optimization, stiffness, tensegrity
National Category
Engineering and Technology
Identifiers
urn:nbn:se:kth:diva-34038 (URN)10.1260/0266-3511.27.2-3.117 (DOI)2-s2.0-84863524872 (ScopusID)
Note
QC 20120810. Updated from manuscript to article in journal.Available from: 2011-05-24 Created: 2011-05-24 Last updated: 2012-09-04Bibliographically approved

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