Stronger Towers With10% of the Weight of Steeltowers that yields lightweight structures with less wind loading that canbe stacked much faster than comparable steel towers.

By ttlifte Khu ttich Wilson, Ph.lf.;PatfiGl Burfie; and Leslie llerig

Carbon fiber offers an alternative in the manufacfure of telecommunications

rs are arduous to constructto manufacture, and they use

e resources that manv sav are iny. An alternative to using

anufacture towers is to use. It is noncoffosive and has

ical conductivity. It weighsl0 percent of the weight of

comparable steel towers. And the lightweight of carbon fiber reduces the timerequired to erect towers to a matterof hours versus the days or weeks re-quired for steel. In addition, stackingthe sections does not require the use ofa heavy lift crane. (See Figure 1)

6 10,000oz3ogoor s,ooo

0 0.005 0.01 0.015 0.02 0.025 0.03 0.035 0.04 0.045 0.05WEIGHT DENSITY {POUNDS/INGH)

Figure 1. GeoStrut carbon-fiber tower member weight characteristics.

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American Consulting Technology &Research (ACTR) is the designer andowner of the core technologies in-volved and is co-developing, alongwith Globel ink Telecom, the f i rstheavy-load structure made from car-bon flber. This new support member isconfigured for tower applications as-sembled in three- or four-leg self-sup-porting, guyed, monopole, telescopingand transportable models. The struc-ture is manufactured in a round latticematrix of carbon-fiber strands layeredin a geometric pattern named GeoStrut.(See Figure 2)

The strut lattice configuration hashalf of the wind load of a solid surface,yet it maintains a high antenna load ca-pacity. In some cases, the reduced windload permits up to a 40 percent reduc-tion in the foundation requiremerrt, amajor cost item in tower erection. Car-bon fiber excels in abrasion resistance,disaster survivability (fire, hurricaneand earthquake) and maintenance be-cause it doesn't corrode.

In addition to the economic con-siderations, the carbon-fiber tower isenvironmentally friendly because ituses readily available raw material andeliminates hot-dip galvanizing, a pro-cess that is more costly and compara-tively not as environmentally friendly.The carbon-fiber raw material is ex-tracted and processed in all areas of theUnited States with no negative impacton the environment.

The carbon-fiber composition sur-vives dramatic temperature extremeswhile maintaining extremely high ten-sile strength, as evidenced by its useon the NASA space shuttle, the F-35fighter and the Boeing Dreamliner.

Table 1 compares the tensile strength,elasticity and density of several typesof construction materials.

As a high-end material, carbon fiberhas perfonnance characteristics wellbeyond those of standard materials.The tensile strength of the carbon fiberis more than nine times the strength ofGrade 304 stainless steel. The elasticity

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Round lattice matrix Two views of a monopole tower configurationwith section lugs

Th ree-leg ged self-s u pporti ngtower configuration

Figure 2. The structure is manufactured in a round lattice matrix of carbon fiber strands layered in a geometric pat-torn. Section lugs male diameter changes.

of the carbon fiber is another way ofviewing material stiffness. Carbon fiberis the stiffest material listed in Table 1.

These material properties are com-bined with light weight in the finalproduct. Grade 304 stainless steel ismore than five times as dense as carbonfiber. Carbon fiber's lower density andhigher tensile strength allow the de-signer to create new structures to carrythe same loads as steel.

The combination of lower densityand higher tensile strength of the car-bon-fiber members allows the desien

of structures to carry higher loads ona support system that has been reduced90 percent in overall dead weight.

Table 2 rs a direct comparison of prop-erties of two 1O0-foottall, 36-inch-di-ameter wind-turbine monopoles made ofdifferent materials. The table shows thatthe structures exhibit the same projectedarea and are subjected to the same windload (40 psf : 100 mph wind speed).The lattice design of the carbon-fibermembers allows air to pass through thestructure, as indicated by the lower dragcoefficient. The result of this 50 percent

reduction in drag coefficient withoutcables is a 50 percent reduction in wind-load forces. This reduced load reducesthe foundation required to resist the loadreactions. The actual foundation reduc-tion varies as a function of the amountof cabling required because cables havetheir own drag coefficient, which is un-accounted for in this analysis.

The steel tower in this examplewould be made with 1/8-inch-thick self-supporting sheet metal, which is compa-rable to a tower with a steel frame coatedwith ll32-inch-thick sheet metal. The

GeoStrut Technology Advantages Garbon-fiber Applicationso Lightest mass-production-capable carbon o Telecommunications antennas

fiber structure in the world o Wind turbineso Comparable unit pricing o Util i ty and telephone pole (environmentally safe foro Highest weight-to-strength ratio direct-bury applications)o Highest antenna load capacity o Building framingo Lowest shipping and handling cost o Pier and platform supportso Lowest installation labor cost o Highway barrier (yields w/strength-absorbing impacto Shortest installation time forces, saving lives)o Lowest maintenance o Fence post or f lagpoleo Longest lifespan

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carbon-fiber tower would weigh 275pounds. The steel tower would weigh4,873 pounds, 17.72 times as much.

A carbon-fiber monopole can betransported and delivered with a pickuptruck or easily flown by small helicopterto remote locations. At the end of the firstday, the tower crew can have the carbon-fiber monopole assembled and erected

in place. The standard steel monopolerequires hear,y equipment for transport,delivery and erection. Additionally, thesteel monopole foundation is twice thesize of its carbon-fiber counterpart.

Designs are available to use carbon fi-ber forthree- or four-leg stand-alone tow-ers extending as much as 400 feet high.Using a helicopter, the lightweight tower

can be set into place with snug-fittingcollars. A 400-foot tower would weighabout 2,000 pounds, and its load-bearingcapacity compared with steel would beabout two to one, allowing more anten-nas to be affixed to the structure.

Designs are available to use car-bon fiber for guyed towers as tall as300 feet with a base diameter as wideas 24 inches, weighing less than 1,000pounds. Generally, the higher the tower,the larger the diameter for guy cablerequired to resist the forces. The para-dox is that as the guy-cable diameterincreases, so does the dead weight.Thus. the tower size must increase tosupport the compression load from thedead weight of the guy cables.

The carbon-fiber tower, with half thewind load and twice the load-bearingstrength, allows the size of the guy ca-bles to be dramatically reduced. Takentogether, all these factors relate directlyto material cost and the cost of labor

18.000 1 .53

1018 Mi ld Stee l 30,000 7.87

Wood (Douglas Fir) 1,936 0 .55

Table 1. Material comparison.Tensile strength: The higher the number, the greater the force required tocause structural failure.Modulus: Elasticity is an indicator of a material's ability to deform andreturn to original size and shape. The lower the number, the greater its elasticity.

I characteristic and a measure of mass per unit of volume.

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Wind,Force-s : So.lid-steel Monopole GeoStrut Monopole

oD in (m) 36 (0 .914 36 (0 .914)

Height ft (m) 100 (30.4s) 100 (30.48)Protected Area ft'(m') 300 (27.87) 300 (27.87)Dra$"'coefficient cp "'.:' t.oeg 0.536Total Weight Lbs (kg) 4873 (2215)- 275 (125)

Wipd,'Load,,gi$f'{MP6).,,:1.,r.1,.1 401,1(39,71) ,,,,,, ,,,, 4a $g.71)Total Wind Force kips (kn) 108.6 (483.1) 54.8 (243.8)

Table 2. A comparison of wind forces on a 1O0-foot wind turbine tower made as asol id cyl inder and of GeoStrut tube.* The tower weight is based on an equivalent 1O0-foot steel cyl inder of 1/8-inchwal l th ickness.

to stack the tower. They allow heavterantenna loads with the potential forgreater profit for the tower owner.

The rigidity of the material of whichtowers are made has an effect on con-struction cost. When the carbon-fiberstrand layers are increased, the structurebecomes considerably more rigid thansteel. Increased rigidity allows a reduc-tion in the number of guy-cable eleva-tion points that affect twist and sway.

A telescoping carbon-fiber towerthat collapses within itself for transportor storage and that can be quickly ex-tended for set up has been designed forheights as great as 100 f-eet. SLrcli a towerwould weigh 150 poLrnds and wor-rld beeasy to transport in the back of a pick-up truck. A 60-foot telescoping towermade of steel with conrparable strengthwould require an 8-fbot-square base,whereas a 60-foot carbon-fiber tower's

base would be only 18 inches, and thetower would weigh 128 pounds.

Carbon fiber is known throughoutthe world as a sLlperior rlaterial. Previ-ously, high cost and a low prodr-rctionrate harnpered its potential for wideruse. T l re Ia t t i ce s t ruc tu re n ra i r r ta inscarbon fiber's strength while redr-rcingthe amount of rraterial used to keep thecost under control . The resul t is a prod-r-rct that outperfbrrls other carbon-fiberstructures and that can be bLrilt anderected at a fraction of the price.

Mike Kipp is chief technology officerof American Consul t ing Technology &Research, Provo, Utah. His emai l ad-dress is mkipp@actr.com. Erich Wilson,Ph.D. , i s an eng ineer w i th ACTR. Pat -rick Burke is chief technology officer atGlobeLin k Telecom, Anchorage, Alaska ;pb u rke@g lobel i n ktel. com. Lesl ie Derrigis Globel ink 's director of business de-velopment; ld e rrig@g lobel i n ktel.com.

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Your towers are built to nuithstand tough conditions,Your Enuironmental Gompliance and Revieuus shouldwithstand tough scrutiny.

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Mav 2009