Fabric

Read here about the role of fabric in tensegrity structures, with much information gleaned from the world of tensile structures, where fabrics are routinely deployed.

Overview

A fabric or textile is a flexible material consisting of a network of natural or artificial fibres often referred to as thread or yarn. Yarn is produced by spinning raw wool fibres, linen, cotton, or other material on a spinning wheel to produce long strands. Fabric is formed by weaving, knitting, crocheting, knotting, or pressing fibres together (felt). The words fabric and cloth are used in textile assembly trades (such as tailoring and dressmaking) as synonyms for textile. However, there are subtle differences in these terms in specialized usage. Fabric refers to any material made through weaving, knitting, spreading, crocheting, or bonding. Textile refers to any material made of interlacing fibres. Cloth refers to a finished piece of fabric that can be used for a purpose such as covering a bed.

Fabric in Tensegrity

Fabric can play the role of tension in a tensegrity, as it is a material that has very low compressive strength and is primarily intended to function under tension.

Some tensegrities use a fabric envelope instead of tendons: see membrane.

Fabric may also be sewn in a tube, or sleeve, a cylinder that loosely contains a structural element such as a cable, rod, etc.

See also, Textiles as Tension Technology.

In tensile construction, a field often confused with tensegrity, fabric is a woven or laid cloth made of fiber. Fiber is the basic thread of the material from which the yarns and fabrics are made. Yarn is a number of fibers grouped together to make a thicker strand for weaving. They may be twisted together or parallel to each other. Weaving is the process of making a fabric from yarns passing alternately over and under each other. Warp yarn refers to the long straight yarns in the long direction of a piece of fabric. Weft yarn is the shorter yarns of a fabric, which usually run at right angles to the warp yarns; these are also called the fill yarns. Fill yarns refer to the shorter yarns of a fabric, which usually run at right angles to the warp yarns (that is why they are also called weft yarns). Roll goods is an edge treatment in which the edge of the fabric is folded over on itself and a rope or cord is incorporated in the fold to increase the strength of the clamped fabric. The roll goods rope or cord is not the same as the rope or wire that forms the tensegrity. Keder is a brand name for the solid PVC cord used at a “rope edge”; rope edges provide strength and a surface to evenly distribute fabric tension forces.

Fabric In Tensile Structures

A discussion of the use of fabric in tensile structures: many of these issues are the same for fabric deployed in tensegrity.

Tension

In a tensed fabric structure, the fabric when pre-stressed will assume the form of a curve in space. This curve is often an anticlastic surface, meaning a saddle shaped surface like a potato chip, with positive Gaussian curvature in one principal direction and negative Gaussian curvature in the other. Anisotropy is the feature of fabric wherein the physical properties and behavior are not the same in all directions. A synclastic surface is a surface with positive Gaussian curvature in both principal directions, like a sphere or bubble. The radius of curvature is the inverse of the magnitude of Gaussian curvature at a location on a membrane surface. The magnitude is typically considered in two principal directions. The orientation of the principal directions and their magnitude may vary continuously over the surface.

Surfaces can be combined to outline polyhedral forms. Patterning is the process of defining two-dimensional pieces of fabric, which can be spliced together to form a desired three-dimensional shape. Computer aided design tools are used in the process of patterning; M-Panel is one such tool used in AutoCAD. Node points are the intersection points of the elements used to define the fabric shape in the structural analysis; these are normally given in terms of a three-dimensional coordinate system.

Elongation is the change in lengths of a material sample; normally this is associated with some load or force acting on the sample. In structures where fabric functions as a tension element, elongation does not normally refer to true strain of the fiber elements as in the classical sense; but, rather, normally refers to the “apparent” strain resulting from a straightening out of the crimped yarns in the fabric matrix.

Detension is the process of relieving the tension or stress in a membrane.

Membrane

A fabric deployed exclusively to provide structural tension between struts is called in this encyclopedia a membrane; see membrane.

Equilibrium Shape Of The Fabric

The process of determining the equilibrium shape of a structure is known as form finding or form generation. The equilibrium shape is the configuration that a tensioned fabric surface assumes when boundary conditions, pre-stress level, and pre-stress distribution are defined. Pre-stress is the stress state that exists in the fabric structure when it is not acted upon by service loads; it is usually induced by the boundary conditions of the fabric membrane. The membrane's elastic modulus, or modulus of elasticity, is a key input into this process, being the mathematical description of the membrane's tendency to be deformed elastically (i.e., non-permanently) when force is applied to it. The elastic modulus is the ratio of the change in stress to the change in strain; it is usually defined as a force per unit width of a membrane material. Poisson’s ratio is also a factor, being the ratio of lateral strain to longitudinal strain; this may take a wide range of values due to the deformation characteristics of the woven material.

Stress, compensation and hysteresis are all considered as characteristics of the fabric in its deployed configuration. Reinforcement is an additional layer of fabric placed in an area of high stress to protect the main fabric. Wind stress is one example of a significant stress upon the membrane. Flutter is the excessive, uncontrolled movement, usually caused by the interaction between the structure and wind. This occurs when the fabric lacks sufficient pre-stress. In construction, compensation is often applied. Compensation is the operation of a shop fabricating a fabric structure or pieces of the structure smaller in the unstressed condition than the actual installed size, to account for the stretch at pre-stress level. Hysteresis may later distort the form: in fabrics, hysteresis is the failure of fabric to return to its original geometry after the strain-inducing force has been removed. Bias must also be considered: bias is a force oriented at 45-degrees to the warp and fill directions of the fabric.

Fabric Materials

Fabric for tensegrity structures can be composed of various materials. Polytetrafluoroethylene or PTFE, commonly known by its trademark name Teflon, is a coating applied to a fiberglass scrim that produces a high strength tension structure fabric membrane with a life expectancy of thirty plus years. PTFE may also be expanded and woven into a fabric that can be coated with a fluoropolymer to create a high strength architectural fabric. Polyvinyl chloride or PVC, properly mixed with plasticizers for flexibility and applied to a polyester scrim, makes for a high strength and popular tension structure fabric membrane. The life expectancy and cost are proportionally lower than PTFE.

Fabric Panels, Or Membranes

The fabric panels used in tension structures are called a membrane, are are often conceptualized purely as a surface. A fabric membrane is biaxial, meaning that it has two concurrent orthogonal, principal directions. Crimp is the extent of deformation normal to the plane of the fabric that the fill and warp yarns undergo as they are woven together.

Fabric is secured by means of clamps. Sectionalizing is the name of the method of field joining large fabric panels utilizing clamping hardware. A fabric clamp is a device for clamping the edge of a fabric panel, usually a bar or channel shape and made of aluminum or steel. Fabric may also fasten to tensile cable by means of pockets or cuffs. A membrane plate is the metal plate attached to a membrane corner and used to secure the membrane to the frame. A catenary pocket (aka “banana pocket”) is the pocket that is placed at the perimeter of the fabric cover to secure the catenary cable. The pocket has a curve with a ratio that is defined by the fabric patterning, but is typically close to a 1:10 ratio. This means for every 10 feet of length, there will be about a foot of bend to it. Due to the curvature of the shape, the pocket is typically fabricated by sealing together two halves of the pocket together with an overlap of 1” to 2” at the outside edge of the pocket. A cable cuff is an edge treatment in which the fabric is folded over on itself to form a pocket in which a catenary cable can be installed. Weldment is an assembly of several parts joined by welds. In tensegrity structures the weldment is usually steel, for the attachment of cables and/or fabric. If may be free-floating or connected to other membranes.

Two fabrics are joined across a seam. A lap seam is created when the two pieces being joined are overlapped by the width of the seam. A butt seam is created when the two pieces being joined are butted together and joined with a strip twice the width of the seam.

Effects of Moisture, Light

Moisture And Its Effect On Fabric Structures

Fabric naturally absorbs water. Base fabric refers to the uncoated fabric, also known as greige goods. Coating is a material applied to a fabric for waterproofing and protection of the fabric yarns. Coating adhesion is the strength of the bond between the substrate of a fabric and the coating. Wicking is the conveying of liquid by capillary action along and through the yarns of the base fabric.

Light And Its Effect On Fabric Structures

Light transmission is a measure of the portion of light striking a fabric surface that passes through the fabric and into the space to provide daylighting. Light interacts with the fabric. Ultraviolet (UV) degradation is the deterioration of a fabric under long-term exposure to sunlight. Using a top finish on the fabric will help prevent the UV degradation. Such finish (or "topping") also makes the fabric easier to clean (some examples oare Ferrari’s PVDF named “T2”; or Dupont’s PVF named “Tedlar”).

Links and References

Terms from the fabric-based tension structure construction industry were gathered from Wikipedia and the glossary published by Tension Structures, a division of Eide Industries, Inc. available at [[1]]