Strut, Materials

Read here about physical materials that are suitable for compressive struts in models and tensegrity structures. For general concepts regarding struts, see strut or the various strut types including curved, linear, circular and nucleated.

Terminology: Pipe, Tube, Hose, Rod

In common usage the words pipe and tube are usually interchangeable, but in industry and engineering discipline the terms are uniquely defined. Tube is most often specified by the outside diameter (OD) and wall thickness, but may be specified by any two of OD, inside diameter (ID), and wall thickness. A pipe is a tubular section or hollow cylinder, usually but not necessarily of circular cross-section, used mainly to convey substances that can flow such as liquids or gases (fluids), slurries, powders, or masses of small solids. While similar standards exist for specific industry application tubing, tube is often made to custom sizes and a broader range of diameters and tolerances. Many industrial and government standards exist for the production of pipe and tubing. Both pipe and tube imply a level of rigidity and permanence, whereas a hose (or hosepipe) is usually portable and flexible. Pipe assemblies are almost always constructed with the use of fittings such as elbows, tees, etc., while tube may be formed or bent into custom configurations. For materials that are inflexible, cannot be formed or where construction is governed by codes or standards, tube assemblies are also constructed with the use of tube fittings.

In tensegrity structures, pipe or dowel is usually a compressive member, and hose is a tension member; tube is often used as short for hose.

Struts are also commonly called rods, dowels, or sticks.

Pipe

Pipe are made in many materials including ceramic, fiberglass, many metals, concrete and plastic.

Pipe may be made from concrete or ceramic, usually for low-pressure applications such as gravity flow or drainage.

Pipe Sizes

Pipe sizes can be confusing because the terminology may relate to historical dimensions. For example, a half-inch iron pipe does not have any dimension that is a half inch. Initially, a half inch pipe did have an inner diameter of 0.5 inches—but it also had thick walls. As technology improved, thinner walls became possible, but the outside diameter stayed the same so it could mate with existing older pipe, increasing the inner diameter beyond half an inch. The history of copper pipe is similar. In the 1930s, the pipe was designated by its internal diameter and a 1/16 inch wall thickness. Consequently, a 1 inch copper pipe had a 1-1/8 inch outside diameter. The outside diameter was the important dimension for mating with fittings. The wall thickness on modern copper is usually thinner than 1/16 inch, so the internal diameter is only "nominal" rather than a controlling dimension. Newer pipe technologies sometimes adopted a sizing system as its own. PVC pipe uses the Nominal Pipe Size.

Pipe sizes are specified by a number of national and international standards, including API 5L, ANSI/ASME B36.10M and B36.19M in the US, BS 1600 and BS EN 10255 in the United Kingdom and Europe. Agricultural applications use PIP sizes, which stands for Plastic Irrigation Pipe. PIP comes in pressure ratings of 22 psi, 50 psi, 80 psi, 100 psi, and 125 psi and is generally available in diameters of 6", 8", 10", 12", 15", 18", 21", and 24".

Outside Diameter (OD)

There are two common methods for designating pipe outside diameter (OD). The North American method is called NPS ("Nominal Pipe Size") and is based on inches (also frequently referred to as NB ("Nominal Bore")). The European version is called DN ("Diametre Nominal" / "Nominal Diameter") and is based on millimetres. Designating the outside diameter allows pipes of the same size to be fit together no matter what the wall thickness. For pipe sizes less than NPS 14 inch (DN 350), both methods give a nominal value for the OD that is rounded off and is not the same as the actual OD. For example, NPS 2 inch and DN 50 are the same pipe, but the actual OD is 2.375 inch, or 60.325 mm. The only way to obtain the actual OD is to look it up in a reference table. For pipe sizes of NPS 14 inch (DN 350) and greater the NPS size is the actual diameter in inches and the DN size is equal to NPS times 25 rounded to a convenient multiple of 50. For example, NPS 14 has an OD of 14 inches, or 355.6 mm, and is equivalent to DN 350.

Since the outside diameter is fixed for a given pipe size, the inside diameter will vary depending on the wall thickness of the pipe. For example, 2" Schedule 80 pipe has thicker walls and therefore a smaller inside diameter than 2" Schedule 40 pipe. Steel pipe has been produced for about 150 years. The pipe sizes that are in use today in PVC and galvanized were originally designed years ago for steel pipe. The number system, like Sch 40, 80, 160, were set long ago and seem a little odd. For example, Sch 20 pipe is even thinner than Sch 40, but same OD. And while these pipes are based on old steel pipe sizes, there is other pipe, like gold-flow cpvc for heated water, that uses pipe sizes, inside and out, based on old copper pipe size standards instead of steel. Many different standards exist for pipe sizes, and their prevalence varies depending on industry and geographical area. The pipe size designation generally includes two numbers; one that indicates the outside (OD) or nominal diameter, and the other that indicates the wall thickness. In the early twentieth century, American pipe was sized by inside diameter. This practice was abandoned to improve compatibility with pipe fittings that must usually fit the OD of the pipe, but it has had a lasting impact on modern standards around the world.

Tube

Tube in modern architecture is usually copper or plastic.

Copper tubing is popular for domestic water (potable)plumbing systems; copper may be used where heat transfer is desirable (i.e. radiators or heat exchangers). Copper plumbing tube for residential plumbing follows an entirely different size system from standard pipe sizing, often called Copper Tube Size (CTS). Its nominal size is neither the inside nor outside diameter.

Plastic tubing, such as PVC and CPVC, for plumbing applications also has different sizing standards. Plastic tubing is widely used for its light weight, chemical resistance, non-corrosive properties, and ease of making connections. Plastic materials include polyvinyl chloride (PVC), chlorinated polyvinyl chloride (CPVC), fibre reinforced plastic (FRP), reinforced polymer mortar (RPMP)[2], polypropylene (PP), polyethylene (PE),cross-linked high-density polyethylene (PEX), polybutylene (PB), and acrylonitrile butadiene styrene (ABS), for example. In many countries, PVC pipes account for most pipe materials used in buried municipal applications for drinking water distribution and wastewater mains.

Aluminum pipes are commonly called aluminum tubes ('aluminium' in the UK)[1]. Hollow knitting needles are a cheap source.

Dowel

A dowel is a solid cylindrical usually made of wood, plastic or metal. In its original manufactured form, dowel is called dowel rod. Tom Flemons typically uses wooden dowels of 10 mm diameter [2]. Graham Scarr uses wooden dowels from 3-6 mm diameter with slots cut in each end [3]

Variable Length Struts

The conventional tensegrity model is constructed with fixed struts and adjustable tendons; the tendon lengths are then trimmed or tuned to finalize the structure. de Jong, Gerald and others advocate for the opposite approach, using variable length struts with fixed length tendons such as mylar.

One commonly available material for variable length struts are springy curtain rods, such as http://www.bedbathstore.com/sprinprescur.html

Inventory of Strut Materials

Read here an inventory of materials that are ideal for tensegrity struts, with discussion of their properties and suitability to various tensegrity structures.

Organic Materials

Wood

Wood is commonly used in tensegrity models, though not in public works. See for example Flemons log sculpture.

Some of Snelson's tensegrity structures made of wood include Early X-Piece (1948). He often uses wood struts and nylon tendons for his models, such as "Fair Leda early study" (1960).

Bamboo

Snelson used black bamboo and nylon in his tensegrity sculpture, "For George" (1970).

Metal

Metal is the material of choice in deployed tensegrity structures. Steel and aluminum are preferred.

Such metallic struts may be unfinished, black (lacquer) steel, carbon steel, stainless steel or galvanized steel, brass, and ductile iron. Aluminum pipe or tubing is popular as it weighs less than iron or steel. Inconel, chrome moly, and titanium steel alloys are less common, as their benefit is in high temperature and pressure systems, and these stresses are not common in tensegrity structures.

Copper

Copper is a softer metal and is not normally used for tensegrity structures. The Copper Institute, however, published a procedure for using copper struts in a coffee table. See How To Build A 3 Strut Copper Base for a Table.

Aluminum

Snelson used aluminum struts in his tensegrity sculpture, "Rainbow Arch" (2001), "Study for Able Charlie" (1978), and others. His most common choice for deployments is aluminum struts with stainless steel tendons. The original maquette for "Black E. C. Tower" (1969) used black anodized aluminum.

Bruce Hamilton uses Easton 9075 T - 9 annodized aluminum fitted with 8 - 32 threaded inserts. For more information, see his website.

Brass

Snelson used brass struts and stainless steel tendons in his tensegrity sculpture, "Fat Rador" (1975-1978).

Steel

Stainless steel. When steel is not stainless steel, it needs to be painted.

Snelson used painted steel in his tensegrity sculpture, "Northwood I" (1969). He typically uses stainless steel struts such as in "Northwood III" (1970), "V-X" (1968), "Vortex III" (2002), and others.

Metal Shafts

de Jong, Gerald reported in August 2010, "I happen to be on vacation in Toronto and I just came back from my favorite store in the whole city called Active Surplus. I spent a few hundred dollars because I found the most fabulous metal shafts, surely meant for another purpose but perfect for building rock-solid tensegrities. Nice stainless steel, perfectly uniform, and with ridges and notches that will eventually lend themselves for some high-tension connections as soon as I figure out how to do it. I'm wondering what kind of tension line I should be looking for if I want these things to be strung like guitars. It usually takes me a few months of gestation before I figure out the best way to put these things together, but this was such a goldmine of uniform bars I couldn't let it pass me by."

Plastic

Plastic tubing is widely used for its light weight, chemical resistance, non-corrosive properties, and ease of making connections. Plastic materials include polyvinyl chloride (PVC), chlorinated polyvinyl chloride (CPVC), fibre reinforced plastic (FRP), reinforced polymer mortar (RPMP)[2], polypropylene (PP), polyethylene (PE),cross-linked high-density polyethylene (PEX), polybutylene (PB), and acrylonitrile butadiene styrene (ABS), for example.

PVC Pipe

PVC pipes are popular in low cost tensegrity models. Make.com published a procedure for a PVC-pipe strut-based 3 strut tensegrity table.

Drinking Straws

Straws are convenient for fast models. See How To Build A 30 Strut Soda Straw Dodecahedron.

Concrete

No known tensegrity has been made with concrete struts.

Ceramic

No known tensegrity has been made with ceramic struts.

Light Fixtures

Light fixtures can fulfill the role of struts, particularly Fluorescent Light Tubes, as they are straight. See Lighting Fixture.