Alternatives to Wood Poles and Crossarms

H. Robert Lash, Chief, and Norris Nicholson, Electrical

Engineer, Transmission Branch, RUS

1

ALTERNATIVES TO WOOD POLES AND CROSSARMS

For over twenty years the utility industry has been using tubular steel poles

for transmission structures. For the last three to four years, light duty tubular

steel poles began being used on distribution lines. This usage may be partly

due to the gradual increase in the cost of wood poles caused by harvesting

restrictions, supply and demand pressures and increases in preservative

costs. At the same time, technology has reduced the manufacturing costs for

light duty steel poles. Because steel poles are lighter in weight than

comparable wood poles, use of steel poles for distribution may have also

increased because they are perceived to be easier and faster to install. These

factors have led light duty steel poles to be competitive in cost compared to

wood poles for distribution lines when looking at the lifetime line cost.

The road to Steel Distribution Poles

For over sixty years wood poles construction has been the standard for

distribution lines at RUS (formerly REA). The use of steel poles to any

extent in REA transmission lines began in the early 70's. These poles were

designed using loading trees developed by the cooperatives or the consulting

engineer. The steel pole manufacturers realized that offering the industry a

standard pre-engineered pole would reduce the manufacturing costs and

would offer other benefits. In the mid 80's, the manufacturers preengineered their poles into standard classes and called them 'Wood

Equivalent poles'. The concept was to offer standard class poles that could

be interchangeable with comparable wood poles used in transmission lines.

Since most transmission lines are built to meet NESC Grade B construction,

the manufacturers developed their standard class (or pre-engineered) steel

poles based on the ratio of Grade B overload capacity factors (OLF). Thus,

using the ratio of overload factors for steel and wood for Grade B

construction (2.5/4.0) and using the load two feet from the top for wood

poles (ANSI O5.1), the manufactures developed their 'Wood Equivalent'

standard class poles and published them in their catalogs.
2

'Wood equivalency' and Grade C Construction

The concern of RUS is the topic of “Wood Pole Equivalency”. Each

material has it’s own properties, so it’s difficult to say that one material is

equivalent to another. The benefit of wood is its high strength to weight

ratio, great insulating properties, ease of field drilling, and low cost. Steel

poles can be manufactured to the required strength by varying the material

thickness or the pole diameter. There is no need to be concerned about knots

or sweep with steel poles.

RUS is concerned with making a direct substitution of wood poles with steel

poles of the same designation for distribution lines. The problem lies in the

differences in the OLF. The scariest story that has been relayed to RUS

involved a purchaser that used the following logic to determine what OLF to

use on the steel pole design. According to the NESC for distribution lines

(grade C construction), the OLF for wood poles is 2.0 at a crossing or 1.75

not at a crossing. If there is deterioration due to any cause such as decay or

mechanical damage, the pole is allowed to lose strength until the OLF

reaches1.33. This is a decrease of 1/3 its strength. Since steel is not

expected to lose any strength over time, the purchaser felt that it was safe to

buy a pole with an OLF of 1.33. This is 40 percent below the required OLF

required by the NESC for steel poles for new construction.

According to Table 253-2 of the NESC, for Grade C wood structures the

“when installed” OLF is 1.75 for transverse loads. Table 253-1 shows that

the OLF of Grade C steel poles is 2.2. This is where the problem arises.

The overload factor for steel is higher than the factor for wood for Grade C

construction.

As noted earlier, most manufacturers standardize on steel poles such that the

poles have a “wood pole equivalency” basis using Grade B requirements in

the NESC. Table 1 below summarizes the loads used to design poles:
3

Table 1

Wood Equivalent Steel Poles

Grade B Construction

Pole Class

Wood Pole

Tip Loads (lbs.)

Steel Pole

'Equivalent' Load (lbs.)

1 4500 2925

2 3700 2405

3 3000 1950

4 2400 1560

5 1900 1235

6 1500 975

Table 253-1 of the NESC requires that steel pole designs use an OLF of 2.5

for transverse wind load for Grade B construction. For Grade B construction

of wood pole the OLF of 4.0 is required on transverse loads. The required

steel pole strength to be equivalent to wood pole moment carrying capacity

for Grade B is a ratio of 2.5/4.0 or 62.5%.

Table 2 below equates the “wood equivalent” steel pole for Grade C

construction. The corresponding tip load for a class 4-40 (class 4, 40 foot)

steel pole for Grade C is 2565 lbs. From Table 1, the corresponding tip load

for a 4-40 for Grade B construction is 1560 lbs. When purchasing steel

poles, the grade of construction is the major determining factor.
4

Table 2

Wood Equivalent Steel Poles

Grade C Construction

Pole Class

Wood Pole

Tip Loads (lbs.)

Steel Pole

'Equivalent' Load (lbs.)

1 4500 4810

2 3700 3960

3 3000 3210

4 2400 2565

5 1900 2030

6 1500 1605

An example will be used to demonstrate the confusion when using standard

class steel poles.

Example

A cooperative wants to build a distribution line meeting Grade C

construction. The design calls for 40-4 wood poles but the decision is made

that one section will use steel distribution poles. The transverse working

load (load without overload factors) is 2400/2.0= 1200 lbs. two feet from the

top of the wood pole. The common practice is to substitute a standard class

pole, designated as a 40-4, for the wood pole. From Table 1, the ultimate

load for the equivalent steel pole for Grade B would be 1560 lbs.

But, this is where the problem occurs. The “ultimate” load to which the

steel pole needs to be designed, should be 1200 lbs. (working load) x 2.2

(steel OLF)=2640 lbs.

In the Table 1 the 2640 lbs. ultimate load corresponds to a Class 1 steel pole.
5

If a 40-4 steel pole with a tip load of 1500 lbs. were selected, it would not

meet the NESC strength requirement

Electrical Effects

RUS strongly advocates a minimum lightning impulse withstand strength

(often incorrectly and simply referred to as a BIL level) of 300kV for

distribution pole top assemblies. A minimum of 300 kV withstand needs to

be maintained at dead-end assemblies. Withstand strengths of less than 300

kV will usually facilitate flashovers of lightning strikes to or in the

proximity of distribution lines. A recloser operation, which will cause lights

to flicker, is usually required to clear the resulting arc. Thus, a minimum of

300 kV withstand is required to maintain a reasonable quality of service.

Standard RUS pole top assemblies, with wood poles, have a minimum

withstand strength of 350 to 499 kV. If the steel pole design has a withstand

strength of less than 300kV, cooperatives should consider what additional

measures, such as the installation of surge arresters or the use of fiberglass

pole top pins to increase the withstand. The quality of service may be

negatively impacted if the withstand of less than 300 kV is maintained.

A steel pole may be used as a grounding conductor if the pole meets the

sufficient conductivity and low impedance requirements of the NESC and

RUS specifications. However, a directly embedded steel pole is not

recognized in the NESC as a grounding electrode. Thus NESC and RUS

requires that a separate driven electrode be used for all equipment, surge

arresters and other required system grounds, including grounding the poles

themselves, if needed.

Cooperatives should use stainless steel or galvanized steel ground rods and

non-copper ground wire in the soil on steel poles lines to mitigate the

corrosive effects of buried dissimilar metal in close proximity.

Raptor Protection

RUS advocates that distribution lines be designed and constructed in a way

that will minimize the electrocution of raptors. Distribution construction

with steel poles needs additional consideration because of the short distances

between the bare energized phase conductors and the grounded steel pole.
6

On single-phase lines, the installation of fiberglass-reinforced plastic poletop pins will minimize the number of electrocutions of small raptors. On

three-phase lines, some raptor protection can be achieved in an economical

manner by installing fiberglass-reinforced pole-top pins and perch guards on

the crossarms. Good raptor protection can be achieved on both single-phase

and three-phase structures by:

• Installing fiberglass-reinforced plastic pole-top pins;

• Using non-metallic crossarms and covering the pole, from the neutral up

to and including the top of the pole, with an insulating coating that has a

dielectric strength of at least 15,000 volts; and

• Using at least a 36-inch fiberglass-reinforced plastic guy strain insulators

and extension links for all connections to the pole above the neutral

position.

RUS is familiar enough with the use of light duty steel poles that their caseby-case approval has been streamlined. RUS needs the information

requested below to determine if the steel pole application would result in

safe reliable construction and meet all of RUS’ requirements:

1. Indicate the maximum number of steel poles to be used.

2. Indicate the name of the steel pole supplier.

3. Define the project or locations where steel poles will be installed.

4. Indicate reason for using steel poles.

5. Indicate that only RUS accepted materials are to be used.

6. Indicate that RUS standard construction is to be used.

7. If less than 300 kV withstand strength, briefly describe assemblies

and materials to be used and anticipated impact on reliability and

materials.

8. Describe raptor protection measures.

9. Indicate that the determination of the class of steel pole is based on

the proper engineering calculations.
7

Alternatives to Wood Crossarms

Along with the use of steel to construct light duty poles, there has been

considerable interest in alternative materials for crossarms. From the early

beginnings of the power industry wood has been the standard in overhead

construction due to its strength, durability, and electrical properties. The

relative abundance of wood made it an ideal raw material also. As

environmental restrictions become more stringent, along with economic

pressures, the need for an alternative material to wood has grown. Some

factors include the rising cost of production and environmental regulation of

wood preservatives. Recently the crossarm has been the subject of study in

the rural as well as urban utility companies. Co-ops are looking for products

with an increased useful life, as well as lower maintenance cost.

Alternatives, such as fiber composites and recycled materials such as rubber,

are being investigated as possible foundations for new crossarms. These

materials are expected to uphold the same standards or exceed those of

existing wood crossarms. The utility industry has taken a special interest

lately in fiberglass as an alternative material for wood crossarms.

Fiber Composites

Composites consist of thermoset resin and fiber reinforcement. The resin, in

its liquid form, is combined with the fiber and then cured into a solid

laminate. There are numerous composite resins and reinforcements, each

with a unique property to impart to the finished product. However, most

composite fabrication utilizes one of at least six major family groups of resin

systems1:

 

1 Lacovara, B., What Every Engineer Should Know About Composites, Composites Fabricators

Association, http://www.cfa-hq.org, 1999.

• Polyester

• Vinylester

• Modified acrylic

• Epoxy

• Phenolic

• Urethane

The type of resin system used is selected based on the functionality of the

product. Production cost is also a major factor in the system selection. Just

as there are numerous composite resins, there are a number of reinforcement
8

fibers available. The reinforcement fiber, just as the resin selection, depends

on the functionality of the end product. Some reinforcement fibers include:

• Kevlar;

• Carbon;

• Glass.

Glass fiber is the most commonly used of the on the market today. Most

manufacturers utilize glass fibers due to its low cost with exceptional

strength characteristics. Glass fiber is the optimum choice in crossarm

production because the material is easier to work with in the manufacturing

process.

RUS Requirements

Electric Staff Division has recently developed an Items Required Sheet for

listing fiberglass crossarms in Informational Publication 200-1, RUS List of

Materials. In order to gain acceptance there are some basic requirements

that must be shared with wood crossarms:

• Meet the same load capacity as the standard wood crossarms (unbraced);

• Meet the same cross section as the wood crossarm;

• Meet the same environmental requirements as the wood crossarm.

These are some basic requirements that RUS feels will provide Co-ops and

their customers with quality service comparable to that of wood. RUS also

requests that certain design and test requirements be met, all of which are

outlined in Items Required in an Application for RUS Acceptance of

Fiberglass Crossarms.

Testing

A vertical and longitudinal test should meet or exceed the ultimate moment

capacity and deflection characteristics of an equivalent wood crossarm.

ANSI 05.3 - 1995 Annex B outlines the standard procedure in this test. A

moment of rupture (MOR) equivalent to 7400 psi and a moment of elasticity

(MOE) equivalent to 1.8 x 106

 psi are thresholds RUS recommends for this

test. These guidelines are based on each major axis of and equivalent wood

crossarms.
9

The transverse pin test requires that a load be applied to 1-3/8" thread pin

with a 2-1/4" washer mounted on the fiberglass crossarm. This pin is item f

in the RUS List of Materials. The crossarm is to be loaded to 1650 lbs. or

until the ultimate transverse load is achieved. The fiberglass crossarm must

withstand a 750 lb. transverse load without any crushing of the arm.

An electrical test as well as an aging and weathering test should be

performed also. For the electrical test, dry flashover between the mounting

hole and the farthest end pin hole should be measured. ASTM G53 specifies

how the accelerated weathering and ultraviolet tests should be fashioned. At

least 2500 hours of ultraviolet aging should be conducted without any

deterioration of the arm.

Design

RUS requests a number of design features for fiberglass crossarms that

should maintain the standard of wood arms and smooth the transition to

fiberglass crossarms. For ease of installation, RUS requests that the

attachment method of the crossarm to the pole be consistent or the same as

that of the wood arm. It is also requested that the arm be equivalent in size

to the standard RUS wood arm along with the ability to be field drilled

through the centerline of each major axis.

For safety and durability, crossarms should be designed for a minimum of 30

years exterior exposure and for a minimum crushing load 500 psi under the

washer without any permanent deformation or damage.
ITEMS REQUIRED IN AN APPLICATION FOR RUS ACCEPTANCE

OF FIBERGLASS CROSSARMS

1. Test Requirements:

♦ Vertical and longitudinal crossarms tests - The fiberglass crossarms should meet or exceed ultimate

moment capacity and deflection characteristics of equivalent wood arms for of each major axis (wood

crossarm based on an MOR of 7400 psi and MOE of 1.8x106 psi). Test crossarms in accordance with

ANSI 05.3 – 1995 Annex B.

♦ Transverse Pin Test

a. Transverse load to be applied to a 1-3/8" thread pin (item f) with a 2 1/4" washer mounted on

the fiberglass crossarm.

b. No crushing of the fiberglass member is permitted for a transverse load up to

750 lbs transverse load.

c. Transverse load to be gradually increased to 1650 lbs or ultimate, whichever comes first.

Report results.

♦ Longitudinal Pin Test - Apply 700 lbs to a 1 3/8" thread pin (item f) with a 2 1/4" washer in the

longitudinal direction.

♦ Electrical Test - Perform a dry flashover test between the crossarm mounting hole and the farthest

end pin hole. Report the results.

♦ Weathering and Aging Tests - Crossarms shall be tested for accelerated weathering and ultraviolet

aging for 2500 hours without any deterioration in accordance with ASTM G53, Practice for operating

light-and water-exposure apparatus (fluorescence UV - condensation type) for exposure of nonmetallic materials;

2. Design Requirements:

♦ Arms shall be equivalent in size to RUS standard size wood arms;

♦ Crossarm must be able to be field drilled through the center line of each major axis;

♦ Attachment method of the crossarm to the pole must be consistent or same as wood the crossarm;

♦ Crossarms shall be designed for a minimum of 30 years of exterior exposure.

♦ Crossarms shall be foam-filled to eliminate water ingress. Filler shall be closed cell and

completely fill the crossarm. End caps shall be permanently affixed;

♦ Each crossarm shall be permanently marked with the manufacturer’s name or logo and date of

manufacture.

♦ Crossarm shall be designed for a minimum crushing load of 500 psi under washer without any

permanent deformation or damage.

Refer to “General Requirements in an Application for RUS Acceptance of a Product” for additional

information.
1

Version 5

Jan 13, 2000

RUS Guidelines and Approval for the Use of Distribution Steel Poles

The Rural Utilities Service (RUS) will consider a borrower’s written request to use distribution

steel poles for site specific projects on a case-by-case trial basis to gain experience. Before

granting approval, RUS needs sufficient information to assure that the application of steel poles

will result in safe and reliable construction and meet RUS requirements.

Borrowers requesting RUS approval to use distribution steel poles are asked to read the following

guidelines and design information and to furnish RUS with the information requested in Part II.

Part I: RUS Guidelines and Design Information for Using Distribution Steel Poles

A: MATERIALS

Except for various miscellaneous material items, RUS regulations require that borrowers use

materials that RUS has fully, conditionally or technically accepted. A compilation of fully and

conditionally accepted materials may be found in Informational Publication 202-1, “List of

Materials Acceptable for Use on Systems of RUS Electrification Borrowers” (List of Materials).

This List of Materials can be accessed through the internet at http://www.usda.gov/rus/listof.htm.

For information on technically accepted items and other questions regarding materials, please

contact:

Mr. Harvey Bowles, Chair

Technical Standards Committee “A” (Electric)

Rural Utilities Service, Stop 1569

1400 Independence Avenue SW

Washington DC 20250-1569

Phone: (202) 720-0980

Fax: (202) 720-7491

Email: hbowles@rus.usda.gov

Borrowers requesting RUS approval of materials not presently accepted, for use with steel poles

or any other application, are asked to provide: a description of the material, catalog sheets, test

results, and the name and address of the manufacturer. Such requests should be sent to the

appropriate regional Engineering Branch Chief. (See Section G)

B: LIGHTNING IMPULSE WITHSTAND STRENGTH and SURGE PROTECTION

A lightning impulse withstand strength, often called Basic Impulse Insulation Level or BIL, of

less than 300 kV on distribution pole top assemblies will usually facilitate flashovers of lightning

strikes to or near distribution lines. A recloser operation, which will cause lights to flicker, is

usually required to clear the resulting arc. RUS advocates a minimum of 300 kV withstand

strength (dry flashover, phase-to-phase and phase-to-ground) to minimize recloser operations and
2

thus improve the quality of service. This level is especially important on deadends where voltage

doubling can occur.

A withstand strength of 300 kV (dry flashover) can be achieved on steel poles by using many of

the standard RUS pole-top assemblies and installing a fiberglass-reinforced plastic pole-top pin

(item “b (2)” in the List of Materials) on the phase conductor attached to the very top of the pole.

A 300 kV lightning impulse withstand strength (dry flashover) can be attained on a steel pole

deadend structure by installing a 24 inch (minimum length) insulated extension link (item “eu” in

the List of Materials) between the primary deadend suspension insulators and the steel pole.

Borrowers do not need additional RUS approval to use the above two material items or the

resulting modified standard pole top assemblies.

The designated maximum transverse load on fiberglass-reinforced plastic pole-top pins is 500

pounds. The maximum line angles for this loading limitation can be found in Table I of RUS

Bulletin 1728F-803, “Specifications and Drawings for 24.9/14.4 kV Line Construction.”

RUS recommends the installation of surge arresters at 800 foot to 1,200 foot intervals and at

deadends on all distribution lines, which are exposed, to frequent lightning strikes. This

recommendation is especially applicable to distribution lines built with steel poles because of

their generally lower lightning impulse withstand strengths. An adequate number of installed

surge arresters minimizes the number of lightning flashovers and the resulting momentary

outages and damaged insulators.

C. GROUNDS, GROUNDING

The National Electrical Safety Code (NESC) requires that all non current-carrying metallic

members on a line support structure be effectively grounded. Thus, each steel pole needs to be

effectively bonded to all primary and secondary neutrals, down guys, messengers, and all other

metallic attachments to the pole. Other NESC grounding requirements may also apply.

A steel pole may be used as a grounding conductor if the pole meets the sufficient conductivity

and low impedance requirements of the NESC.

Since a directly embedded steel pole is not recognized in the NESC as a grounding electrode,

separate driven ground rods or grounding electrodes need to be used for all equipment, surge

arresters and other required system grounds. The use of stainless steel or galvanized steel ground

rods and non-copper ground wires in the soil near steel pole distribution lines will help to mitigate

the corrosive effects of dissimilar metals buried in close proximity.

D: COSTS AND ECONOMIC STUDIES

RUS does not require borrowers to provide any economic studies or cost comparisons to justify

the use of steel distribution poles instead of wood poles. However, borrowers are encouraged to

compare the initial and long-term estimated installed cost of equivalent distribution structures or

lines constructed with steel poles versus wood poles. Borrowers may, at their discretion, furnish

the results of their cost estimates to RUS.
3

Questions or comments regarding Sections B through D above are welcomed by and should be

sent to:

Jim Bohlk, Electrical Engineer

Rural Utilities Service, Stop 1569

1400 Independence Avenue SW

Washington DC 20250-1569

Phone: (202) 720-1967

Fax: (202) 720-7491

Email: jbohlk@rus.usda.gov

E: RAPTOR PROTECTION USING STEEL POLES

RUS advocates that distribution lines be designed and constructed in a way that will minimize the

electrocution of raptors. Distribution construction with steel poles need extraordinary

consideration because of the short distances between the bare energized phase conductors and the

grounded steel pole.

On single-phase lines, the installation of 24-inch long fiberglass-reinforced plastic pole-top pins

(“item b (2)” in the List of Materials) will minimize the electrocution of small raptors. On threephase lines, some raptor protection can be achieved in an economical manner by installing

fiberglass-reinforced pole-top pins and perch guards on the crossarms as shown on assembly

VP3.3G in Bulletin 1728F-803.

Good raptor protection can be achieved on both single-phase and three-phase structures by:

♦ Installing 24-inch long fiberglass-reinforced plastic pole-top pins;

♦ Using non-metallic crossarms and covering the pole, from the neutral up to and including the

top of the pole, with an insulating coating that has a dielectric strength of at least 15,000

volts; and,

♦ Using 36 inch (minimum length) fiberglass-reinforced plastic guy strain insulators (item “w”)

and extension links (item “eu”) for all connections to the pole above the neutral position.

(See Bulletin 1728F-803, assemblies VA5.4 and E5.1G)

Any questions or comments regarding raptor protection can be directed to:

Dennis Rankin

Rural Utilities Service, Stop 1571

1400 Independence Avenue SW

Washington DC 20250-1569

Phone: (202) 720-1953

Fax: (202) 720-1820

Email: drankin@rus.usda.gov
4

F: SELECTION OF STEEL DISTRIBUTION POLES

Generally, a wood pole cannot be replaced with a steel distribution pole of the same class

because of NESC strength requirements. After the selection of the NESC grade of construction,

certain “design load” calculations are required to determine the minimum class of a distribution

steel pole that can be used in lieu of a wood pole for standard RUS pole-top assemblies. The

calculations involve the overload factors and strength factors, for both wood and steel poles, as

found in Tables 253-1 and 261-1A of the 1997 edition of the NESC. (Note that some of these

values will probably be changed in the next edition of the NESC.) RUS has performed the

calculations for steel pole “design loads” for various poles under 60 ft in height and not at

crossings. The results are shown in the tables, which follow.

RUS regulations require a minimum of NESC Grade C construction in the design and

construction of distribution lines and structures. Section 24, Grades of Construction, of the

NESC, and thus RUS, may require higher grades of construction for certain conditions.

Deadend structures and line angle structures involve additional calculations (such as loading

trees) to determine the required minimum steel pole strength and pole class. Thus, RUS

advocates that these types of structures (and steel pole selection) be designed (1) under the

direction of a registered professional engineer, and (2) meet NESC Grade B strength

requirements.

The design of unguyed angle and dead-end steel pole structures should consider pole deflection

and greater embedment depths. Extreme ice conditions and appropriate high winds should be

considered in the design loads.

Questions or comments regarding proper selection and installation of steel poles should be sent

to:

Robert Lash, Chief

Transmission Branch

Rural Utilities Service, Stop 1569

1400 Independence Avenue SW

Washington DC 20250-1569

Phone: (202) 720-0486

Fax: (202) 720-7491

Email: blash@rus.usda.gov

or

Donald Heald, Engineer

Rural Utilities Service, Stop 1569

1400 Independence Avenue SW

Washington DC 20250-1569

Phone: (202) 720-9102

Fax: (202) 720-7491

Email: dheald@rus.usda.gov
5

Required Steel Pole Design Loads

(Columns 1 and 2 from American National Standards Institute (ANSI 05.1)

(Design loads 2 feet from top of pole)

TABLE 1 - NESC GRADE C STRUCTURES

(RUS Tangent and Small Angle Assemblies) (Not at a Crossing)

(For New and Replaced Grade C Structures)

ANSI 0.51

Wood Pole

Class

Wood Pole

Design Load

(lbs.)

Steel Pole

Design Load

(lbs.)

H1 5400 5770

1 4500 4810

2 3700 3960

3 3000 3210

4 2400 2570

5 1900 2030

6 1500 1600

7 1200 1290

TABLE 2 - NESC GRADE B STRUCTURES

(RUS Tangent and Small Angle Assemblies) (Not at a Crossing)

(For New and Replaced Grade B Structures)

ANSI 0.51

Wood Pole

Class

Wood Pole

Design Load

(lbs.)

Steel Pole

Design Load

(lbs.)

H1 5400 3510

1 4500 2930

2 3700 2410

3 3000 1950

4 2400 1560

5 1900 1240

G: REQUEST FOR RUS APPROVAL TO USE STEEL DISTRIBUTION POLES

Borrowers requesting RUS approval to use steel distribution poles should send their written

request and supporting information to the appropriate regional Engineering Branch Chief at the

address given below.

Northern Region Southern Region

Charles M. Philpott, Chief

Northern Engineering Branch

Rural Utilities Service, Stop 1566

1400 Independence Avenue SW

Washington DC 20250-1566

Phone: (202) 720-1432

Fax: (202) 720-1411

Email: cphilpot@rus.usda.gov

Louis Riggs, Acting Chief

Southern Engineering Branch

Rural Utilities Service, Stop 1567

1400 Independence Avenue SW

Washington DC 20250-1567

Phone: (202) 720-0848

Fax: (202) 720-0097

Email: lriggs@rdmail.rural.usda.gov
6

Part II: Information Needed by RUS for Case-by-Case Approval of Steel

Distribution Poles

Before granting approval, RUS needs all of the information requested below to determine if the

steel pole application will result in safe and reliable construction and meets all of RUS’s

requirements.

1. Indicate the maximum number of steel poles to be used.

2. Indicate the name of the steel pole manufacturer.

3. Define the project or location(s) where the steel poles will be installed.

4. In addition to “experimental purposes to obtain experience”, furnish sound reason(s) for

using steel poles.

5. Indicate that only RUS accepted materials are to be used. (Otherwise, see Section A of

steel pole guidelines.)

6. Indicate that only RUS standard construction is to be used. (Otherwise, see Sections A

and B of steel pole guidelines. Please furnish sufficient dimensioned drawings and other

technical information for RUS’ evaluation of the design.)

7. (If, and only if, the design has less than a 300 kV withstand strength [see guidelines,

Section B], then briefly describe assemblies and materials to be used and anticipated

impact [if any] on reliability and materials.)

8. Describe raptor protection measures, if any, that are to be incorporated into the design.

(See guidelines, Section D.) (Note that RUS recommends that raptor protection be

considered in distribution line designs, especially lines using steel poles, even though

neither all lines nor all areas may require raptor protection.)

9. Indicate that the determination of the class of the steel poles for each application is based

on the proper engineering calculations performed by a competent person. (See

guidelines, Section F.)
BIOGRAPHICAL SKETCH

H. ROBERT LASH

H.Robert Lash is presently Chief of the Transmission Branch, Electric Staff Division. In

this position he supervises the review of transmission line designs, substation designs,

contract and policy review and revision, and other technical areas of support for the area

offices. Bob is a member of IEEE, and American Wood Preservers’ Association and sits

on several ANSI subcommittees.

Prior to gaining RUS in 1983, Bob was employed by Burns & McDonnell Consultants

and Joslyn Manufacturing.

He graduated from Kent State University in 1980 with a MBA and SUNY College of

Environmental Science and Forestry in 1974 with a BS in Wood Products Engineering.

NORRIS NICHOLSON

Norris Nicholson is an electrical engineer employed in the Electric Staff Division of the

Rural Utilities Service. Mr. Nicholson began his career with the Rural Electrification

Administration in 1993 as an intern student with the Automated Information Systems

Division. He has been working in the Transmission Branch of the Electric Staff Division

over the past year in developing agency guidelines and standards for use by RUS

engineers, borrowers, and their consulting engineers. He is active in the Electric

Engineering Committee of RUS. Mr. Nicholson obtained his BSEE from Southern A & M

College and University, Baton Rouge, Louisiana.
ALTERNATIVES

TO

WOOD POLES

AND CROSSARMS

(MARCH 14, 2000)

Hot Topic

Light Duty Steel Poles and

Fiberglass Crossarms
Light Duty Steel Poles

nNot promoting wide spread

substitution

nAre advantageous under

certain conditions
Wood Pole Costs

nHarvesting Restrictions

nSupply & Demand

nPreservative Costs
Steel Pole Designs

nOriginally designed using

loading trees

nOffered in pre-engineered

classes

nPublished transmission class

pole values

2 feet

TIP LOAD
Wood Pole Equivalency

nDifficult because of inherent

differences in materials

 Steel Distribution Poles

 Pros Cons

n Long Life

n Lighter Weight

n Lower

Construction Cost

n No Defects

n Low Maintenance

n No Long-term

Experience

n Lack of Linecrew

Experience

n Harder to Field

Drill

n Difficult to Repair

n Conductivity
 Wood Poles

 Pros Cons

n 70 Years of

Experience

n Field Framing

n Repair

n Conductivity

n Decay

n Disposal

n Weight

n Preservatives

Comparison of Materials

Many intangibles to

consider
RUS Concerns

Direct Substitution Using

Pre-Engineered Classes

Grade “B” Construction

Transverse Loads

 Overload Factor

 Wood - 4.0

 Steel - 2.5
Wood Equivalent Steel

Poles

Grade “B” Construction

Pole Class Wood Steel

 Tip Load Tip Load

1 4500 2925

2 3777 2405

3 3000 1950

4 2400 1560

5 1900 1235

Grade “C” Construction

Transverse Loads

 Overload Factor

 Wood - 2.67 at crossings

 2.0 not at crossings

 Steel - 2.20
Wood Equivalent Steel

Poles

Grade “C” Construction

Pole Class Wood Steel

 Tip Load Tip Load

1 4500 4810

2 3777 3960

3 3000 3210

4 2400 2565

5 1900 2030

Example:

Wood Pole Design

Base Pole - 40ft. Class 4

Grade “C” Construction
Horizontal Tip Loads

n Wood Tip Load - 2400 lbs.

n Steel Tip Load - 2565 lbs.

Pole Selection from

Transmission Tables

2565 lbs.- Class 1

Class 4 from same table 40%

undersized
Distribution Steel Poles

Electrical

Effects

Wood Poles

RUS standard pole top

assemblies with wood

poles - lightning impulse

withstand of 350-499 kV
When using steel poles

RUS advocates minimum

lightning Impulse withstand

around 300 kV

When using steel poles

Withstands strengths

around 300 kV could

facilitate flashovers of

lightning strokes
When using steel poles

Recloser operation needed

to clear resulting arcLights flickers

When using steel poles

Need approximately

300 kV withstand for

acceptable quality of

service
When using steel poles

RUS recommends surge

arresters or fiberglass pole

top pins to increase

withstand.

When using steel poles

NESC does not recognize

direct embedded steel pole

as a grounding electrode
When using steel poles

RUS and NESC require a

separate driven electrode

for all equipment, surge

arresters and required

system grounds

Believe it or not!!!!!!

RUS has streamlined

Case-By-Case

Approval
Information Needed by RUS

nNumber of Steel Poles

nLocation of Installation

Information Needed by RUS

nSteel Pole Manufacturer

nJustification-

 usually to “gain experience”
Information Needed by RUS

nStatement if RUS standard pole

top assembly is bring used

nStatement concerning the use

of RUS accepted materials

Information Needed by RUS

nStatement on method used to

achieve acceptable lightning

withstand strength

nAssurance that steel pole was

selected using proper

engineering calculations
Information Needed by RUS

nIf non-standard pole top

assembly is to be used:

l Dimensional Drawings

lTechnical information to

evaluate 

Information Needed by RUS

nIf “unlisted” material are to be

used:

l Detailed description of 

material for evaluation
Information Needed by RUS

nStatement that Raptor

protection was considered and

what measures are being taken
ALTERNATIVES

TO

WOOD CROSSARMS

Norris W. Nicholson

Electric Staff Division-Transmission Branch

Physical Makeup

LIQUID RESIN REINFORCEMENT FIBER

COMPOSITE STRUCTURE
Major Resin Groups

u Polyester

u Vinylester

u Epoxy

u Acrylic

u Urethane

u Phenolic

Resin System Costs

Polyester

Vinyl

Epoxy

$$$$$ 

Low

High
Reinforcement Fibers

ËKevlar

ËCarbon

ËGlass

Transmission & Distribution

115 kV

Transmission Assembly

15 kV

Distribution Line
Advantages & Disadvantages

ËWeight

ËApplication

ËInsect/Bird Resistance

ËDeflection

ËBlooming

ËCost

Basic Guidelines

ËSame load capacity as the standard wood

crossarm

ËSame cross section as the wood crossarm

ËSame environmental exposure requirements

as the wood crossarm
ITEMS REQUIRED IN AN APPLICATION FOR RUS ACCEPTANCE

OF FIBERGLASS CROSSARMS

1. Test Requirements:

♦ Vertical and longitudinal crossarms tests - The fiberglass crossarms should meet or

exceed ultimate moment capacity and deflection characteristics of equivalent wood arms

for of each major axis (wood crossarm based on an MOR of 7400 psi and MOE of

1.8x106 psi) . Test crossarms in accordance with ANSI 05.3 – 1995 Annex B.

♦ Transverse Pin Test

a. Transverse load to be applied to a 1-3/8" thread pin (item f) with a 2 1/4" washer

mounted on the fiberglass crossarm.

b. No crushing of the fiberglass member is permitted for a transverse load up to

750 lbs transverse load.

c. Transverse load to be gradually increased to 1650 lbs or ultimate, whichever

comes first. Report results.

♦ Longitudinal Pin Test - Apply 700 lbs to a 1 3/8" thread pin (item f) with a 2 1/4"

washer in the longitudinal direction.

♦ Electrical Test - Perform a dry flashover test between the crossarm mounting hole and

the farthest end pin hole. Report the results.

♦ Weathering and Aging Tests - Crossarms shall be tested for accelerated weathering

and ultraviolet aging for 2500 hours without any deterioration in accordance with ASTM

G53, Practice for operating light-and water-exposure apparatus (fluorescence UV -

condensation type) for exposure of non-metallic materials;

2. Design Requirements:

♦ Arms shall be equivalent in size to RUS standard size wood arms;

♦ Crossarm must be able to be field drilled through the center line of each major axis;

♦ Attachment method of the crossarm to the pole must be consistent or same as wood

the crossarm;

♦ Crossarms shall be designed for a minimum of 30 years of exterior exposure.

♦ Crossarms shall be foam-filled to eliminate water ingress. Filler shall be closed cell

and completely fill the crossarm. End caps shall be permanently affixed;

♦ Each crossarm shall be permanently marked with the manufacturer’s name or logo and

date of manufacture.

♦ Crossarm shall be designed for a minimum crushing load of 500 psi under washer

without any permanent deformation or damage.

Refer to “General Requirements in an Application for RUS Acceptance of a Product”

for additional information.

Design

Items Required

ËEquivalent to standard size RUS crossarms

ËConsistent pole attachment

ËField drillable

ËFoam filled

ËMinimum 500 psi washer crushing load

ËMinimum 30 yrs. exterior exposure

ËPermanently marked
Test

Items Required

ËVertical & Longitudinal

ËTransverse Pin

ËLongitudinal Pin

ËElectrical

ËWeathering & Aging

Vertical & Longitudinal Test

ËModulus of Rupture (MOR)

‰7400 psi

ËModulus of Elasticity (MOE)

‰1.8 x 106 psi

(ANSI 05.3 - 1995 Annex B)
Transverse Pin Test

Ë1650 lbs. Or ultimate transverse moment

Ë750 lbs. Crushing withstand

Longitudinal Pin Test

700 lbs. Load to 1 3/8” thread pin with 2 1/4” washer

Electrical Test

ËDry flashover

Weathering & Ultraviolet Test

2500 hours ultraviolet aging & accelerated

weathering

 (ASTM G53)
Other

Applications

Substations
Decking

Walkways Bridges