Tensegrity Structures- Benefits and Applications in Civil Engineering

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The applications of tensegrity structures are employed in both civil and architectural engineering mainly in structures such as dome structures, towers, roofs of the stadium, temporary structures as well as tents. Tensegrity also called as tensional integrity or floating compression, involving a structural principle for the creation of complex systems which comprises of elements which are only in compression or tension. In a detailed manner, it consists of strings which are in tension and bars that are in compression. Strings have the property of being foldable, light but strong. This makes us clear that tensegrity structures gain the potential to be light but strong as well as deployable. The word has been defined by many researchers which are: "A tensegrity system is established when a set of discontinuous compressive components interacts with a set of continuous tensile components to define a stable volume in space". Huge literature on the geometry and architectural appeal of tensegrity structural systems exist, but a lesser study on the dynamics and mechanics of these structures exist. The reasons of tensegrity concept including tension stabilization, efficient structures development, deployable and easily tuneable properties make the need for new attention for this concept.

Examples of Tensegrity Structures Used in Civil Engineering

Certain examples are shown in the figure below.

Needle Tower

American sculptor Kenneth Snelson designed the needle tower located in the Unites States. The tall sculpture is a tapering tower that is made of aluminum and stainless steel. As per the principle, the aluminum tubes push, kept together by the stainless-steel wire threaded through the tube ends.

Munich Olympic Stadium

The Estadio Ciudad de La Plata

The patented Telstar Tensegrity roof concept is employed, for twin peak contour and the plan configuration. It is more like a cable dome structure than to a conventional roof structure. Initial studies for the design of tensegrity grids were carried out by Snelson, but its applications were limited.

Applications of Tensegrity Structures

Applications of  tensegrity structures is appropriate in various areas of civil engineering such as:

• Roof structures
• Bridges
• Smart structures

Roof Structures using Tensegrity Structures

An important example of Tensegrity being employed in roof structures is the stadia at La Plata (Argentina), based on a prize-winning concept developed by architect Roberto Ferreira. The design adapts the patented Telstar Tensegrity roof concept to the twin peak contour and the plan configuration, and consequently, it is more like a cable dome structure than to a conventional roof structure. The first studies for the design of Tensegrity grids were carried out by Snelson, but its applications were limited. For the past few years, the focus has been in the development of double-layer Tensegrity grids and foldable Tensegrity systems.  This kind of grid has its most feasible possibilities in the field of walls, roofs, and covering structures.

Bridges using Tensegrity Structures

The design of double grid systems has resulted in an interest in the application of Tensegrity to bridge construction. A recent achievement in this regard is the Kurilpa Bridge in Brisbane, Australia. It is the world’s largest Tensegrity bridge, which was opened on the 4^th of October 2009.

Fig: Kurilpa Bridge, Brisbane, Australia

A cable-stay structure based on principles of Tensegrity producing a synergy between balanced tension and compression components to create a light structure that is incredibly strong. The bridge is 470m long with a main span of 120m and features two large viewing and relaxation platforms, two rest areas, and a continuous all-weather canopy for the entire length of the bridge. A canopy is supported by a secondary tensegrity structure. It is estimated that 550 tons of structural steel including 6.8 km of spiral strand cable are incorporated into the bridge

Smart Structures

Civil Engineering structures are mostly static. One of the challenging functionality for Civil Engineering structures is an active adaptation to changing demands, such as load variations, temperature variations, settlement of supports and damage occurrence. The concept of active structures involves structures that include both static and active structural elements. Adaptive structures are defined as structures whose performance is controlled by a system composed of sensors, actuators and a computer that provides the ability to learn and improve response to changing environments. The system of Tensegrity structures gains the potential to adapt to change in environments, so they can be equipped with active control systems.

Basic Features of Tensegrity Structures

• Pin-jointed bar frameworks: a Structural group of tensegrity structures is pin-jointed three-dimensional trusses.
• Pure compressive/tensile members: Compression and tension members are only involved in tensegrity structures. The tension elements are cables sustaining only tension.
• Localization of compression: Classic Tensegrity structures the compressive elements are discontinuous. They float in a continuous network of tension elements.
• Pre-stressed structures: The stability of the structure since stabilizes internal mechanisms for which a state of pre-stress or self-stress is required.

These structures are based on the combination of a few design patterns. No structural member experiences a bending moment because of these patterns. Exceptionally rigid structures for their mass and for the cross section of the components are obtained as the final product. The design combinations are as follows:

• Loading members only in pure compression or pure tension, meaning the structure will only fail if the cables yield or the rods buckle.
• Preload or tensional prestress, which allows cables to be rigid in tension.
• Mechanical stability, which allows the members to remain in tension/compression as stress on the structure increases.

Benefits of Tensegrity Structures

Tensegrity as a structural system offers many advantages over conventional structural systems. The benefits offered are elaborated as follows:

• Stabilization of the structure through tension
• Easily tunable
• Deployable
• Efficient Structures
• Reliable modeling
• Perform multiple functions

Stabilization of the structure through tension

A tensile member gains stiffness as it is loaded, unlike compression members. Stiffness is lost in two ways in a compressive member: The forces act exactly through the mass center, in the absence of bending moments in the axially loaded members. There is an increase in the diameter of the central cross section as the material spreads during loading. In the case of tensile members, there is a reduction in its cross-section under load. The bar becomes softer due to the bending motion, due to the presence of bending moments. Here the line of application of force is away from the center of mass. Most materials possess a tensile strength of a longitudinal member is larger than its buckling strength (sand, masonry, and unreinforced concrete are exceptions to this rule). Hence, a large stiffness-to-mass ratio can be achieved by increasing the use of tensile members.

Easily Tunable

Tensegrity structures are deployable. This technique can make small adjustments for fine tuning of the loaded structures or in other words adjustment of a damaged structure. Structures that are designed to allow tuning will be an important feature of next generation mechanical structures, including Civil Engineering structures

Deployable

This feature of Tensegrity structures offers operational and portability advantages. A portable bridge or a power transmission tower made as a Tensegrity structure could be manufactured in the factory, stored on a truck or helicopter in a small volume, transported to the construction site, and deployed using only winches for erection through cable tension. These would save transport costs by reducing the mass required, or by eliminating the requirement of labors for installation.

Efficient Structures

The efficiency of a structure increases with the minimal mass design for a given set of stiffness properties. The arrangement of the longitudinal members in a very unusual pattern for the achievement of maximum strength with small mass highlight tensegrity structures.

Reliable Modeling

Tensegrity structural members are axially loaded. The scientific feature of Tensegrity structures is that while the structure bends with external static loads, none of the individual member experience bending moments. Members that experience deformation in two or three dimensions are harder to model than those members experiencing deformation in one dimension. So, the increase in the use of tensile members is expected to yield more efficient structures.

Perform Multiple Functions

Tensegrity structures perform multiple functions like a load-carrying member of the structure, a sensor (measuring tension or length), an actuator (such as nickel-titanium wire), a thermal insulator, or an electrical conductor. A proper choice of materials and geometry the electrical, thermal, and mechanical energy in a material or structure can be controlled.