Poles & Supports

Without the Pole, There is nothing . . .

 

C. 1931 placed poles can show their age and tell an interesting story.

Hand drawn boiler flue tangent exchange structure, hand drawn by a Northwestern Bell Telephone Pioneer, 1983.

Doubtless, the wooden pole has had a starring role in the theatre of open wire as it began, expanded across the continent and served for the last three centuries.  Beyond open wire removal, it then continued its role in supporting aerial cable.

 

Let’s explore the myriad of ways the wooden pole has met the challenges of helping to make open wire the premier technology for so many years.  It goes without saying, that even with the demise of open wire outside plant, the pole stands steadfast supporting aerial cable, apparatus, power and CATV throughout the nation with an un-paralleled use over any other medium of structural support.  While there are competitors to the wooden pole as supports in the communications, power and CATV businesses, none have completely usurped its fundamental advantage completely nor monopolized the communications, power and lighting industries’ use of it over the last few centuries.

 

The pole is a versatile implement in not only supporting aerial wire and cable, but has found its post-open wire application, in conversions to buildings, corrals, fencing, decks, boat docks, railway timber short span bridges and other unique re-use capabilities.  Typically, after an open wire line is removed, many nearby property owners seek the former toll lead poles for local use and Bell System practices make it a point to indicate that donations to locals is permissible.

In our discussion today regarding the value of the wooden pole and its place in our utility culture, I am relying on information from the premier advocate of the wood pole: the North American Wood Pole Council.    Much information about the Council and its advocacy of the wooden pole can be found atwww.woodpoles.org.

 

The N.A.W.P.C. publishes numerous monographs on the wooden utility pole practice and its numerous other applications.  Here, for the purposes of expanding upon the topic of utility use for communications, we will dwelve into several topics to which any enthusiast of open wire is sure to find favor.

 

Since the dawn of the communications age, the wood pole has been the standard of telegraph, telephone and CATV companies in supporting conductors, hardware, crossarms, brackets, insulators, braces and apparatus.  Even today, the wood pole does not have a serious competitor in fulfilling its role to maintaining proper clearance height, the promotion of public safety, lineworker safety as well as equipment protection and the promotion of economical construction design. 

 

The wooden pole is a “green” material, offering significant advantage over competitors in both price, renewability and offering easy modifications in the field.

Because preservation techniques have rapidly kept pace with the preponderance of wood pole lines growing across the countryside, the wood pole can easily stand with little additional protection, for up to 90 years in many instances. 

 

Today, counting all the poles in the electric power industry, as well as the telecom and CATV industries, there are more than 130 million in use.  There is good reason beyond the three major advantages outlined previously to use the wooden pole: a) it is abundant in nature; b) handling the pole is comparatively easy; c) the fibrous nature of the heartwood is a natural electrical insulator; d) has an environmentally affable character during the life of its practical use; and finally, e) choices of wood species can be selected by the user for maximum length, strength and flexibility.  Later the intrusion of utility equipment into the urban and country landscape became an environmental appearance issue by the 1960s.  Happily, the wooden poles were found to blend into the natural surroundings with ease.

 

Common forest species of timber are offered for electric utility pole use:

 

Southern Yellow Pine

Douglas Fir

Western Red Cedar

Northern Red Pine

Common forest species of timber offered for Bell System and Independent telco use:

 

Western Red Cedar

Ponderosa Pine

Jack Pine

Lodgepole Pine

Red Pine

Douglas Fir

Southern Pine

Western Larch

It is important to note that all poles are treated throughout their entire length and impregnated accordingly through a pressure treatment.  The Bell, GT&E, Independents had choices as to the treatment selection in determining pole use:

 

Pentachlorophenol in petroleum

Cellon Process (Pentachlorophenol in LP Gas)

ACA (Ammonical Copper Arsenate [Chemonite]

CCA Type “A” (Chromated Copper Arsenate)

CCA Type “C” (Chromated Copper Arsenate)

 

As with other wood products, it is easy to fell these trees, machine them after harvesting, treat them with environmentally-friendly preservatives and classify them according to length butt sizes. 

 

Poles are available to the design engineer and construction forces through selection of pole “class.”  The “class” of a pole is determined by several factors.  First, what kinds of loads are expected to be impressed upon this structure?  What kinds of additional loads may be expected beyond the actual dead-weight of the conductors, crossarms, hardware, apparatus and equipment?  Is the line passing through a rugged terrain where diverse lengths and sizes will be required to maintain a “ruling span?”  Is this territory swampy?  Will a severe winter condition inflict high loads of ice and sleet on the conductors?  Does a line cross a deep valley or river?  Is it necessary to build “H-fixtures” or “A” frame structures to afford extra strength from a steep grade or tight angle?  These and many other questions require answers from staking engineers who study the line route.  From an analysis of the terrain, length of line and other factors, the quantity of poles is determined.  Also the specific classes for dead-ending, tangent, angle, transposition and special service requirement structures have to be included.

 

Both aerial open wire and suspended multi-pair copper cables put devious strain on each structure even in good outstanding weather conditions.  The real test of a pole’s integrity is in supporting these features in adverse weather conditions.  High winds, sleet, ice, high water and insect decay (in swampy areas).

 

Stress is a factor of tensile, compression and shear forces acting upon an object.  High torque loads–such as uneven twisting of crossarms by long insulators and pins can impress severe “shear” forces upon the pole tops.  “Longitudinal Load” is characterized by the uneven nature of spans and unbalanced conductors between poles, otherwise known as a form of “tensile” stress.  “Compression” stress can be a factor of a pole overloaded with equipment, conductors and even a lineman. Sometimes this is called a “vertical load.” Forces act to split and break the pole, typically at its weakest point: eight feet above the ground.

 

Removed Burlington-Northern Santa Fe railroad communications poles removed from the main line Omaha-Denver route near Seward, Nebraska in 2003.

Pole Classes & Why They Are So Important

The National Electrical Safety Code, otherwise known by its initials, N.E.S.C., has developed a map of the United States conforming to the various “strata” of weather intensity upon the landscape.  The U. S. has three primary “loading” zones where it is possible to generalize design catagories of line design practice.  Design loading and weather conditions are both called to account.  These are:

 

Light – Ice loads of 0.00 inches @ 9 lbs. (60 mph) – Transverse

Medium – Ice loads of 0.25 inches @ 4 lbs. (40 mph) – Transverse

Heavy – Ice loads of 0.50 inches @ 4 lbs. (40 mph) – Transverse

 

Straight line spans (or tangent) are the dominating factor in designing most lines where mountainous areas do not impact the route of the system.  For example, in Iowa, South Dakota, Kansas and Texas areas–nearly all found in the “heavy” U. S. geography classification above, the nearest point from A to B is generally a straight, tangent parade of poles with a few exceptions, notably crossing railways, rivers, major freeways and abiding political boundaries.  The parameters deciding line design are the average mechanical strength and material load most poles carry.  

 

No one expects, however, that a highly engineered, milled and stoutly preserved pole can withstand a wind of a hurricane at 150 mph, nor a tornado ripping to shreds five miles of aerial cable and wire, nor an earthquake, rocking and tipping over miles of line.  There is a limit to even the defenders of the pole faith maintain.

But, in most instances, a properly “classed” pole line design can weather the elements through most of its lifetime without significant problem and with a few occasional replacements due to Acts of God and occasional traffic accidents, tree branch breaks and train derailments.

 

That being said, selecting a pole tangent class is dependent upon the number of conductors a pole is to carry.  For open wire bracket lines, this can be limited to six.  For crossarm lines, it can be from two to 16 aerial wires separated by air and pinned to insulators and pins.  To determine this need of “across” or “transverse” loading, one foot of a conductor is measured and weighed and then compared to the total span length.  Here’s the calculation:

(D=2R)W

Wc = __________________

 

 12

 

What does this mean?

 

Wc  =  Transverse wind load for 1′ of conductor in lbs./ft.

D  =  Diameter of the conductor in inches

R  =  Radial thickness of ice in inches

W  =   Magnitude of the wind force in lbs.

12  =   12 inches or 1′ of conductor

 

Once you have determined your pole stress and strength requirement, it is only necessary then to select the proper “class” of pole.

 

Class is determined by:

length of pole

top circumference

circumference measured six feet from the butt end

lengths vary in five foot steps

circumference from the top varies in two inch steps

 

Hence, 25′, 30′, 35′, 40′, 45′, 50′, 55′, and 15″, 17″, 19″, 21″, to 27″ minimum top circumference.  Here briefly is an example of pole strength according to class type:

 

Class                                   Lbs. Force Pole Must Be Able To Withstand With Load

_____________________________________________________________________

 

1                                         4,500

2                                         3,700

3                                         3,000

4                                         2,400

5                                         1,900

6                                         1,500

7                                         1,200

8                                         1,000 or less depending upon length

 

 

It is interesting to compare the strength of various species of poles.  Here are just a few comparisons of some early commonly used Bell System poles and those in use by Independent telcos for open wire construction.

 

 

Southern Yellow Pine, Douglas Fir & Western Hemlock

 

Pole

length

in feet:         Class:        Min. top circum.         Min. Circum.        Resist.

                                    in inches:                   6′ from butt       Moment

                                                                     in inches:           lbs./ft.

 

     25′             1                    27″                         34.5              83,020

     25′             3                    23″                         30.0              54,850

     25′             5                    19″                         26.0              35,920

 

     30′             2                    27″                         37.5             104,650

     30′             4                    21″                         30.0               53,800

     30′             6                    17″                         26.0               35,100

 

     35′             1                    27″                         40.0             125,000

     35′             2                    25″                         37.5             103,000

     35′             3                    23″                         35.0              83,750

     35′             4                    21″                         32.0              64,000

     35′             5                    19″                         30.0              52,700

     35′             6                    17″                         27.5              40,600

     35′             7                    15″                         25.5              32,380

 

     40′             1                    27″                         42.0            144,700

     40′             2                    25″                         39.5            120,400

     40′             3                    23″                         37.0              98,950

     40′             4                    21″                         34.0              76,700

     40′             5                    19″                         31.5              60,900

     40′             6                    17″                         29.0              47,600

     40′             7                    15″                         27.0              38,400

    

     45′             1                    27                          44.0             164,150

     45′             3                    23                          38.5             109,750

     45′             5                    19                          33.0               68,900

     45′             7                    15                          28.5               44,250

 

This is a partial table of strengths drawn from communications pole service statistics and the A.S.A. information.  While it is not complete, it does give the reader a good basic understanding of pole strengths according to “class” and bending movement strength.  No doubt about it, the wooden pole is composed of cellulose, a strong and flexible material–and if treated properly–is immune to most rot and insect decay over its usefull expected lifetime.

 

Lodgepole Pine, Jack Pine, Red Pine & Western Fir

 

Pole

length

in feet:         Class:        Min. top circum.         Min. Circum.        Resist.

                                    in inches:                   6′ from butt       Moment

                                                                     in inches:           lbs./ft.

 

     25′             1                 27″                          36.0              84,700

     25′             3                 23                           31.0               53,920

     25′             5                 19                           27.0               35,800

 

     30′             2                 25 ”                        36.5                86,100

     30′             4                 21                          31.5                55,450

     30′             6                 17″                         27.0                35,030

 

     35′             1                  27″                        41.5              124,500

     35′             2                  25″                        38.5                99,400

     35′             3                  23″                        36.0                81,250

     35′             4                  21″                        33.5                65,450

     35′             5                  19″                        31.0                51,850

     35′             6                  17″                        28.5                40,300

     35′             7                  15″                        26.5                32,400

 

     40′             1                   27″                       44.0              148,400

     40′             2                   25″                       41.0              120,000

     40′             3                   23″                       38.0                95,500

     40′             4                   21″                       35.5                77,900

     40′             5                   19″                       33.0                62,600

     40′             6                   17″                       30.5                49,400

     40′             7                   15″                       28.0                38,200

   

     45′             1                    27                       46.0               167,300

     45′             3                    23                       40.0               109,800

     45′             5                    19                       34.5                 70,300

     45′             7                    15                       29.5                 43,800

 

Western Red Cedar

 

Pole

length

in feet:         Class:        Min. top circum.         Min. Circum.        Resist.

                                    in inches:                   6′ from butt       Moment

                                                                     in inches:           lbs./ft.

 

     25′             1                 27″                         38.0             85,000

     25′             3                 23                          33.0              55,500

     25′             5                 19                          28.5              36,050

 

     30′             2                 25 ”                        38.5               86,350

     30′             4                 21                          33.0               54,580

     30′             6                 17″                         28.5               34,920

 

     35′             1                  27″                        43.5              121,650

     35′             2                  25″                        41.0              101,850

     35′             3                  23″                        38.0                81,100

     35′             4                  21″                        35.5                66,120

     35′             5                  19″                        32.5                50,720

     35′             6                  17″                        30.5                41,920

     35′             7                  15″                        28.0                32,420

 

     40′             1                   27″                       46.0              143,900

     40′             2                   25″                       43.5              121,700

     40′             3                   23″                       40.5                98,200

     40′             4                   21″                       37.5                77,950

     40′             5                   19″                       34.5                60,700

     40′             6                   17″                       32.0                48,420

     40′             7                   15″                       00.0                00,000

  

     45′             1                    27                       48.5               165,500

     45′             3                    23                       42.5               111,850

     45′             5                    19                       36.5                 70,700

     45′             7                    15                       00.0                 00,000

 

Northern White Cedar

 

Pole

length

in feet:         Class:        Min. top circum.         Min. Circum.        Resist.

                                    in inches:                   6′ from butt       Moment

                                                                     in inches:           lbs./ft.

 

     25′             1                 27″                           43.5               83,150

     25′             3                 23                            38.0               55,500

     25′             5                 19                            32.5               34,750

 

     30′             2                 25 ”                          44.5                86,000

     30′             4                 21                            38.5                55,920

     30′             6                 17″                           33.0                35,070

 

     35′             1                  27″                          50.5              122,400

     35′             2                  25″                          47.5             101,850

     35′             3                  23″                          44.0                80,950

     35′             4                  21″                          41.0                65,500

     35′             5                  19″                          38.0                52,130

     35′             6                  17″                          35.0                40,720

     35′             7                  15″                          32.5                32,600

 

     40′             1                   27″                         53.5              145,500

     40′             2                   25″                         50.0              118,800

     40′             3                   23″                         46.5                95,500

     40′             4                   21″                         43.5                78,200

     40′             5                   19″                         40.0                70,800

     40′             6                   17″                         37.0                48,120

     40′             7                   15″                         00.0                00,000

 

     45′             1                    27                         56.0               163,350

     45′             3                    23                         49.0               109,750

     45′             5                    19                         42.0                 68,900

     45′             7                    15                         00.0                 00,000

 

 

A typical class four pole carrying both telecom and alternating current power lateral with common neutral.

A Tree . . . Is a Heavy Object

I don’t know about you . . . but I had the pleasure of moving a Class 3 40′ pole complete with steps, temporary step hardware, bridling runs, terminals and hardware (the ten crossarms had been removed) over three hundred feet by myself, following the unloading of this beast from transport from Tyndall, South Dakota.  Left in the driveway, it was necessary for me to move it to a safe location over numerous obstacles and onto its secure perch–above the ground by three feet–to avoid rot and other issues.

 

. . . without a doubt . . . moving a pole such as this . . . is not something I would suggest for those of you at home to do . . . as trees are heavy.  And . . . that is essentially what a pole is; with branches, a lengthy top and roots removed.  To give you an idea of how this can be done, consider my project: to move this object, I had the laws of gravity (physics) and a fence stretcher.  That’s it!  From sunup at 7:00 am in the morning until I breathed my last gasp of moving effort at 10:30 pm in the darkness the same day, completed this effort.

 

The pole in my grip was a Douglas Fir tree, cut and treated in the year 1968 . . . a comparatively new pole.  For a Class 3 40′ pole the weight was (not including the hardware): 1,040 lbs!

 

So, while were at it, let’s examine some of the typical weights of Douglas Fir poles in open wire use. 

 

A relatively small bracket lead to farms might utilize a 20′ Class 5 pole.  It’s weight: 290 lbs.

 

An exchange line carrying a ten-pin arm only without an underslung aerial cable might be 25′ high and a Class 3.  It’s weight: 480 lbs.

 

A toll lead carrying 40 wires on four ten pin arms perhaps festooned with a lead cable as well underneath the fourth arm might require a 35′ pole, Class 2.  It’s weight: 960 lbs.

 

A pole supporting six arms, non-self supporting cables, constructed to make a 45-degree angle with four guys attached might demand a 50′ pole, Class 1 . . . it’s weight (without the arms and hardware)?  1,970 lbs.!

 

You see . . . a lowly wooden pole . . . deserves respect!

Four technologies are seen in this view: open wire, aerial multi-pair copper cable (remote subscriber terminal), where fiber is brought up from the ground as well as a buried multi-pair copper cable.

A Concrete Investment . . .

. . . And you thought the concrete pole was a recent development?

Today, concrete street lighting standards and electric utility “poles”–perhaps “structures” would be a better term–have replaced many wooden, steel and aluminum supports throughout the country.  In southeastern Colorado, the finality of a major sleet storm ten years ago brought about the gradual replacement of a large portion of the REC’s distribution system’s shattered wooden poles with galvanized steel . . . along with their attendant costs.  

 

Here in north central Kansas, a rural cooperative has introduced a number of concrete poles into the electric distribution system, mainly because their strength obviates guying (if placed and selected properly) and avoids short dip distribution lateral lines from a deadend, which cannot be guyed owing to the highway easemental obstruction problem.

 

Yes, concrete is a viable alternative to electric power, area outdoor lighting, CATV and telecommunication pole needs.  And . . . yes . . . they areexpensive. However, the lifetime of a concrete “pole” can be double that of a wooden pole and these can be re-used if necessary, where it is more likely a wooden pole could not after 40 years or so.

 

But you might think that the concrete pole is a recent . . . development.  If you think so . . . you are . . . dead wrong!  The earliest known and documented use of a concrete pole in America were those made by Col. G. M. Totten, Chief Engineer of the Panama Railroad, in 1856.  These were moreposts than poles, yet they could be festooned with brackets, insulators and wires carrying them about twelve feet above the ground and were both six inch and eight inch topped circumferences with a 15-inch square base bolted to an anchor.  They lasted for a short time, owing to some significant stresses and lack of internal reinforcement.  They cracked, external surfaces spalled and then collapsed. There was enough in their failure–despite their early failure–to catagorize them as a “partial success” and the idea proceeded to its second incarnation: the square wooden center style.  

 

Unfortunately, due to the terrific amount of rain, vegetation, insect problems and humidity, the moisture permeated the wooden internal structure and caused it to expand and swell.  Again, the concrete buckled and these were deemed an abject failure.

 

While the vast number of the earlier 1856 poles failed, there were some which significantly succeeded beyond expectations and stood providing telegraph service until the United States took over the Panama Railroad.  Which, was about 30 years or so.

 

The French, whose work in the Central American area to build a canal earlier than Americans, took notice of these remarkably resiliant concrete “survivors.”  Europe was experimenting with concrete poles for railway systems and postal service telegraph and telephone during the late 1800s.  In 1896, a French engineer, M. Henebique, designed trolley car catenary poles for the Le Mans Tramway Company.

 

Recognizing that concrete in bridges and other buildings was being placed with internal reinforcement, he incorporated into the interior of these castings iron rods and twisted cables.  M. Porcheddu, in Bologne, Italy, another eager engineer-entrepreneur, tried a similar system test with concrete poles and these experienced a good service life without significant incident.  It was apparent that the concrete was of such quality that novel structures could be designed and incorporated into utility service quite unremarkably.

 

Wood reared its grainy face once more in the tale of concrete poles, when in the year 1900, the use of concrete poles planted around the decayed butts of older power and communications poles, could therefore increase the lifetime of a formerly all wooden pole by 50%.

 

Enter the Germans.  At Meissen, located on the Elbe River, a company–Otto & Schlosser– devised a new direction for concrete pole design.    They plucked a wooden form, around which steel reinforcing was wrapped and wet cement being forced within the form revolving horizontally.  ‘Kinda like the aluminum horizontally spun pole for street lighting–except a lot more heavy–but strong!

 

A Swiss company, Siegiwart devised a steel core like the Schlosser company–but built so that the internal steel core would slowly collapse with the entrance of cement and lock the reinforcement within the pole.  Many of these poles ended up on the streets of Zurich and may be seen today. 

Erection of a concrete pole early 1900s.

Bob Cummings, a civil engineer–not the “Love That Bob” character of Hitchcockian fame, built at his little foundry, some experimental concrete poles. His little factory located in Hampton, Virginia, turned out a number of these triangular poles.  They were an equallateral shape of one foot to each side in width implanted with 3/4″ iron rods planted on each corner to maintain rigidity. No real evidence has been provided as to their success, but the idea was taking on life and breathing new life into an old idea here in the United States.

 

By the year 1904, the United Traction Company in Albany, New York looked deeply into the use of square reinforced poles with chamfered corners and installed many on their electric catanary trolley system.  These remained in use until the system was dismantled in the 1950s.

 

Now, railroads became interested and not passive observers of the growth of concrete pole applications for their systems–particularly for the electrified railways and signal system facilities use.   The Pittsburgh, Cincinnati, Chicago & St. Louis Railroad selected a dozen or so concrete poles in 1906.  Their performance was outstanding, so the company in 1908 ordered fifty class 3 35′ poles to which they affixed one ten pin ten-foot crossarm [this was limited within the city limits by ordinance of the town of New Brighton, PA].  The railroad then countered this ordinance–having designated “crossarms” as the wire support limiting factor–and defied the city by using steel brackets supporting ten more conductors below the existing arm.  City ordinance said nothing about a “cable” attachment, so a 50-pair paper/lead cable was also attached to the bottom of the existing 10-pin top arm.  As this installation was an outstanding success, with the line having more than equalled those more stout wooden “A” poles stress onslaught of weight and pull.  At the time, these innovative concrete poles cost a whopping $15.00 a piece and weighed 2,300 lbs.!

 

The only negative instance of concrete problem was that where the 5/8″ bolt entered the pole and exited the arm, concrete cracked and flaked at the gain.

 

So, despite the minor degredation, and the expense, 40′ concrete poles were similarly employed by the same railroad at special locations in 1911: Union City, Indiana, Crestline, PA–where 50′ pole lengths were erected and in Youngstown, OH, where the Western Union Telegraph Company eagerly became involved when 50′ lengths were required over existing power and trolley lines.

AT&T engineers were also studying the solution to the wooden pole deterioration problem and also the answer to special structure considerations, where it was necessary to have significantly higher strength for river, canyon, highway and clearing long spans over physical obstacles.

 

The Ontario Power Company employed the use of concrete square poles for their distribution system and found that they were very economical–if they built them in-house–which the utility did.  These poles were 60′ high and carried 4.0 copper conductors in arrangements of six to eight lines.  These were used to cross portions of Niagara Falls on the Canadian side.

 

Hence AT&T took a serious look at concrete poles and purchased them to be employed on a transcontinental cable line in Pennsylvania–of which the old open wire line being replaced was previously mounted on stout wooden poles.

Completed concrete toll lead depiction, around early 1900s. Unknown location.

 

Here’s what made the railroads, Western Union, and shortly thereafter, AT&T, pick up their heads and take notice about the use of concrete for aerial wire and cable supports:

 

While the further “next step” application of concrete to constructing crossarms was dismissed, the design, further production, and use of concrete poles by utilities and transportation companies was shown to greatly exceeded the comparative longevity of conventional wooden poles.

On the question of “square” vs. “round” poles: the Pennsylvania Railroad found a rectangular hollow design weighed more than the round type with a hollow center–in fact, about 1/6th more–proving that when shape is concerned, mechanical gains can be used for crossarm attachment and economics greatly predominate in this variance of design.

Experimental tests.  Concrete poles failed in tests when they were subjected to bending motions 50% higher than the calculated strength of the poles. Similarly constructed poles of 30′ design–as compared to the longer lengths–could withstand 1500 lbs. pull applied two feet from the top with the but five feet six inches in the ground or otherwise anchored. Again, the structures only met failure when the calculated load was exceeded by fully 50%!

Large safety factors are inherent in the concrete pole.  The best concrete poles are a) round; b) hollow and c) properly reinforced internally.  The hollow nature of these poles allows some weight loss, while maintaining structural integrity.

Guy wires are avoided in the use of a properly designed and placed concrete pole.

Concrete poles are justified when they are properly designed at the shop for their specific application; field changes cannot be easily made by linemen or installation forces.  Measure twice and order once!

 

It is quite remarkable to note that the Ancient and highly innovative Romans, devised cement and cured concrete taking it from zero (the invention itself!) to 10,000 lbs. per square inch stress.  Today, in our age such poles are designed for 60,000 lbs. per square inch stress.  One must salute . . . Roman . . . ingenuity!

Hayday Of The Crossarm

Rural & Exchange Voice Frequency Crossarm Design

Before we embark on a discussion of crossarm design, it would be well to note pin numbering systems for aerial open wire.  I’ve often been asked, “If cables have multi-pair combinations of colors to keep them separate from the central office end to the subscriber terminal, how in the world would you keep track of row upon row of similar-appearing open wire pairs?”

 

The answer is simple: pin numbering schemes.  These schemes came about from the earliest days of open wire, because early transmission telephone engineers faced and surmounted this arguably monumental challenge, before you ever raised the question!  Maintaining pair identity was indeed essential to a well operated phone system.  In the days when even early cables were made up of lead-cladded paper-insulated pairs, their location within the cable was one method to keep each pair’s separate location located from one end of the cable to the terminal on the other end.  However, when you look back at early photos of New York in the pre-Blizzard of ’88 photographs, 90′ poles carrying 50 ten-pin arms, this dilemma was even more substantial to the outside plant engineer.

 

Below, is a sketch of typical crossarm numbering schemes.  I’ve included the typical four-pin rural subscriber’s farm crossarm type construction, the exchange-type six-pin and the basic exchange and carrier ten-pin arms.

 

Note that when pins are removed from either end of the ten-pin arm, the numbering scheme remains the same for the smaller arms.  If it is decided to run two open wire pairs to a farm, for example, but there is some potential for a third pair, then a six-pin arm would be used.  The two “used” pairs would take up pins No. 3, No. 4, No. 5 and No. 6.  If a third pair were used, it would take the void left by empty pins No. 7 and No. 8.  So two pins on the far end of the six pin arm would be used as well as the pole pair.  This is traditional design technique for not only Bell Companies’ lines but GT&E and the vast networks of Independent Companies, too.

How open wire line wires are numbered. Top 4-pin arm; middle: 6-pin arm; bottom 10A or 10B arms. Sketch by D. Schema

On large toll leads between cities or transcontinental toll facilities, the most important circuits were on the top arms.  Primary D.C. telegraph circuits, major program circuits, high quality voice channel and carrier, were carried on these circuits.  There might also be alarm circuits, FAA communications links and AM and FM radio network programming carried on these top arms. 

 

As the arms descend, the circuits convey fewer high capacity circuits and greater priority programming after the 40th pin number.  Arms might be carrying regional, or exchange circuits locally on the same pole from pin numbers 41 to whatever last numbered pair is located on the structure.

Story of the Crossarm

The eight-foot long “N” Arm with ten pins.

The communications-style crossarm (sometimes erroneously termed the “crossbar”) is a fundamental architectural component of the well-dressed toll lead.  While in Europe and Asia, where the use of steel, iron or aluminum insulator supports were directly attached into a vertical array along the pole were used, North American communications companies gravitated towards the crossarm.  This was probably because of two factors: first, wood was plentiful in the numerous forests of the west and southeast; secondly, in Europe, property laws dictated use of aerial space differently than American easement case law. 

 

Furthermore, the large amount of timber and forestry products available during the westward U. S. and Canadian territorial railway expansion, could easily be made into arms and poles.  It didn’t take long for the railway companies to adorn their aerial signal circuit poles with horizontally applied multiple circuit pairs because it eliminated a really tall pole–as in European areas–and the arms could accomodate as many as 10 wires–sometimes more–on a single pole structure.  Western Union in the early days used the wooden side bracket.  However, railways added their expansive circuits for track control, signals and communications.  The idea of “dark fiber” for fiber optic transmission today–meaning a company like AT&T, or Metropolitan Fiber Systems–might put in extra fibers which would not be used, except as “leased” fiber for other companies’ use.  We might use the the term “dark  wire.”  Railroads had placed this “dark” open wire, where they leased “arm bandwidth” for a particular crossarm pin positions to Western Union, Postal Telegraph and Overland Telegraph for their links.

 

Wood products were easily ordered, could be later chemically treated and field changes in the arms could be easily made without special tooling or pre-order instructions.

 

Crossarms used by the civilian telecommunications companies were not all that different from the transportation companies (e.g. railways); so we will blend our discussion of arm specifications between the two concurrently.

 

The 10A and 10B Arms

 

While, it would seem that the smaller numbered pin crossarms might be considered first–especially in regards to AT&T and the Independents, the classic arm of the “Spirit of Service” and the Ice Storm of 1888, will be considered first as it was the first official arm spec adopted by most companies in the early years. 

 

The Bell and Indendent telephone companies settled on the specifications of the typical communications arm relatively quickly.  AT&T constructed some of their first major toll leads using the traditional 10′ 10-pin, 12-inches between pins and 16-inches between the pole pairs.  This was an 1881 Bell specification.  To this day, it has not changed. 

 

For clearance justification for lineworkers and some tight right-of-way easements, the “10B” arm came into use by the turn of the last century.  It was quite popular with many railways as a standard.  Bell and its Independent telephony counterparts used it only when it was required by engineers’ design plans for special circumstances.  These arms were typically ones with rounded roofs on them to allow rain water to run off and prevent decay.

 

The railways in the late 1880s and early 1890s came up with the 6-pin arm and 8-pin arm–the former not having much viability beyond the late 1910 era.  However, the six-pin arm was used commonly on exchange and rural subscriber circuits of the Independents and Bell companies.  REA devised the “6B” arm in the 1950s.  To my knowledge, there was no “6B” arm in use by the Bell companies, as it does not appear in the Bell System Practices–the bible of AT&T design, installation and maintenance guidelines.

 

The 10N Arm

 

The “10N” arm was a specification in use by the Bell and Independent companies.  It was eight foot, six inches long, but used ten pins.  The 10N was incorporated into linework where a short 10-pin arm was desired in reducing overhang on private property or where it was needed to eliminate a significant amount of tree trimming.  The strength of the arm allowed it to be used on spans where they did not exceed 300 feet between poles.  The date of first use of the 10N arm is uncertain; it is possible it entered the Bell and Independent’s inventory around post-World War II.

 

The 10N arm required the use of significantly smaller steel brace drop.  The braces were 20-inch style, unlike the 10A or 10B with 30-inch steel drop braces.  Some Indendents in Iowa–such as ConTel–used the 10N significantly throughout their construction and design in western Iowa.  I recall many ConTel open wire lines which were composed exclusively of 10N arm construction.  Most of these bit the dust after 1974.  These were used primary as “exchange” arms, and saw little use by Bell or Independents as “toll” inventory features of their lines.  These also had rounded roofs of a concave surface to allow rain drainage.

 

Special Service Requirements: 16A and 16B Arms

 

Let’s discuss the more unusual arms for special service in the exchange circuit construction venue.  These were the “16A” and “16B” arms.   These were used by Bell as supports for rural and exchange lines exclusively.  They used 30-inch span braces of steel.

 

The 16A crossarm was used where the operating company expected traffic to require more than five, but not eight pairs of conductors.  The other factor was pole height.  It would be a major setback to have to add an extra conventional 6A, 10A, 10N arm where the pole was already too short.  

 

The 16A arm was also called upon to support conductors on a “joint-use” pole.  Here the ultimate required circuits would exceed five, but not seven, pairs of wires and clearance requirements would, again as in our prior example, not support the placement of an extra arm above or below the existing one.  

 

The 16A’s limitation on span was significant; it could not be used on lines where the span lengths exceeded 200 feet between poles.  Below we compare the specifications of these arms through illustration.

 

Let’s discuss the variation: the “16B” arm.  This also, like the 16A, was used primarily on rural and exchange lines only.  The tight and crowded conditions where pins were specified on this arm necessitated its use for spans of only 150 feet between poles.  Both 16A and 16B used 30-inch span steel braces.

 

The ultimate number of circuits for the 16B could exceed five, but not eight pairs of wires.  Furthermore, these arms were crowned with a rounded roof to allow rain drainage from the surface to prevent decay.

 

The DE or “Dead End” Arm

 

The “DE” crossarm for exchange use came into being when dead-ending open wire advanced beyond the application of standard double arms with standard wooden or steel pin termination styles.  When the “C” type deadend–that is a cotter key type clevis bracket equipped with a wet or dry process porcelain spool came into being in the late 1920s, the “DE” arm was devised.

 

The arm is configured in length to several sizes.  Very commonly they were sizes to fit the standard 4-pin, 6-pin, 10-pin arms of their standard sizes.  Later, construction standards were liberalized to use shorter length DE arms for ten pin tangent line termination.  All used the standard or variations of, the C-type deadends or oval eyes inserted with smaller guy strain deadend insulators used (as by many railways and independents telcos).

 

Using a standard 5/8″ crossarm bolt of the proper length to assemble double arms, or with special hardware to fasten a single arm, the unit also used 30-inch steel braces to prevent deflection torque.

 

Typically, the DE arm deadended three to five pairs of rural or exchange wires on non-joint or jointly used REA, Bell or Independent pole lines.  Sometimes, it was necessary on toll leads to mount filters, termination equipment, protectors and other related materials on the common arm where the C-type deadends were fastened.  This required a standard longer arm.  30-inch steel braces were traditionally applied to this construction and the arms were typically not “rounded” on their roofs, but rather a 1/2-inch 45-degree cut was made along the top sides to lessen rain pooling on its surface; otherwise it had a “square” top.

 

There were also other concerns: where toll facilities required a special wire spacing, as in the case of carrier systems of various classes, the arms might be required to be longer, even with fewer conductors.  DE arms were very common at junction and termination poles, where the special situation demanded extra climbing space or strength at angles.  The Mitchell-Tyndall-Wagner toll junction/termination pole which is featured in another topic of this site, is a prime example. 

 

The Two, Four and Six Pin Crossarms

 

The Independent telcos and Bell System designs were roughly the same.  Both adopted the two pin, four and six pin arms.  The railways had been using some of the smaller pin numbered arms for some time, although each railway had adopted their own independent bearing on what they desired for wire spacing requirements.

 

Regarding the Independents and AT&T, the two pin (2A) crossarm was a simple affair, using a wooden arm only one foot in length, equipped with two wooden locust pins and a 5/8″ diameter bolt hole in the middle.  A single 20-inch steel span brace was fastened by a single carriage bolt and then a lag screw tightened the unit to the pole.

 

The four-pin (4A) crossarm also used one 20-inch span steel brace and carriage bolt through the left side of the face of the arm.  It was 4′ long and equipped with four wooden pins to carry four wires (or two pairs).  When a four-pin arm was placed below REA electric distribution construction or any other electric utility’s facilities on joint-use poles, Bell required two (2) twenty-inch braces to be used.

 

The good ole 6′ six-pin arm (6A) was quite common in the hayday of the crossarm.  Typically it would be seen on county and state highways paralleling them singularly or below a 10-pin arm.  This 6-foot wooden arm was required to be erected with two 20″ crossarm braces and drilled in the middle with a 5/8″ bolt hole.  There was a second variation of this unit: the “6B” arm.  There was just a little more climbing space between the pole pairs and the three pins on either side of the pole were shoved to the right and left so that clearances could be established for linework or easements which were restrictive.

 

Interestingly enough, the H-Fixture we so commonly associate with the 10A arms, also had a variation with the REA specifications.  They had an H-Fixture using the 6A arm.  Rare little critter–never saw any–however, this specification does exist in the 1948 borrower’s specifications guide.

 

The Guard Arm for Exchange and Rural Circuits

 

The “Guard Arm” came into use early in urban areas as applied to lead cable lines in rear alleys and early suburban city areas to mount drops and protect the underslung cable from errant tree branches and falling debris.  The guard arm would be drilled at each end of its four-foot length so that guard arm hooks (sometimes insulated, sometimes not) could be installed.  Additionally, drive rings could be inserted into the arm so connections to the cable terminal could be made.

 

The Guard Arm was typically not designed with a curved, concave surface “roof” area; it was squared off and was 4 1/4″ high by 3 1/4″ wide.  No other dimensional differences were seen in this exclusively 4-foot arm.

 

Poles Apart: The H-Fixture Configuration

 

The “H-Fixture,” which was a major architectural identifier of some of the nation’s longest and most important long distance toll leads, required a crossarm equipped with two sets of braces: one set of 20-inch steel span types and another set of two 30-inch steel span types.  If the H-Fixture was double armed, then an additional duplication of the above was required.

 

The “H-Fixture” was truly a hybrid design constructed and placed at varying intervals along a toll lead’s route to reduce the amount of damage caused by “cascading” or a crippling failure due to large scale wind, sleet or ice damage, upon a long series of tangent open wire toll lines.  The two pole structure was a complicated affair requiring two 5/8″ diameter bolt holes 40″ from crossarm center on either sides of the arm.  This would afix the arm to the two support poles on either side.

 

Additionally, there was a need to re-bore holes for pins carrying the insulators and wires because the poles took up valuable space between wires and pairs.  Typically, on voice frequency lines, pin numbers: 2 and 3 were affected; and conversely, on the opposing side of the arm, pin numbers: 8 and 9 were potentially blocked by the pole circumference.  These changes were typically “field” drilled, as it was not always certain what the actual changes might be.  The structure was usually assembled on the ground and lifted to the two augered holes in the soil, dropped and then the pole tamped with surrounding soil or gravel.

 

It is interesting to note linguistically, that the electric utility used a similar structure from its early years to the present, but gave them the technical moniker: “H-Frame,” instead of “H-Fixture,” unlike the communications companies.

 

Open Wire Carrier Crossarm Designs

 

The advance of open wire electronic carrier systems necessitated the modification of crossarm and pin spacing design.  This first came about in the 1920s with the application of 3-channel carrier systems on a single pair of wires.  The service could be highly effective and efficient, only if certain design and construction qualifications were taken into effect.

 

The 1929 Transcontinental AT&T toll lead from Colorado to western Nevada, is a case in point.  This line was a heavy-haul transmission link supplanting the original 1913-1914 four-wire aerial lead.  Quickly, Bell Labs engineers decided the best possible service with this new carrier system might be improved with crossarm pin spacing and new transposition arrangements.  While the length of the standard wooden arms for the Bell and Indendents did not change; the pin positions and type of pin material did. 

 

The REA and RUS telephone borrowers used the 2-pin arm–typically in a “side arm” design” because it was quickly determined that for joint use REA power distribution lines, it was much easier to erect an arm and modify it than to place side wooden brackets.  That was the logic there.

 

When REA and RUS added extra pairs, it was not a problem to remove the little two-pin side brackets and place a four, six or ten pin arm in its place–rather than to tear down side bracket construction.

Crossarms and Carrier Technology

Open wire lines were originally designed to carry voice-frequency (low) channels.  When electronics began to emerge and mature in the early 1920s, technology presented itself in the form of carrier for outside transmission communications plant.  The first carrier was that of Bell System Type “C”, which was a three-channel carrier on one pair of wires.  A significant leap beyond existing phantomed circuit design and construction. 

 

The re-construction of the original 1913-1914 two pair aerial toll lead in the Utah and Nevada deserts commenced with a grand rebirth–a fourty-wire toll lead using carrier on all but its pole pairs–which continued the voice frequency phantomed construction technique.

 

Up until 1937, open wire had carried frequencies up to 140-kC per second as in the development of the 12-channel Type “J” system.  While construction requirements in physical outside plant had been demanding with the three-channel system, the improvements in capacity with Type “J” came with a cost: the most stringent demands were placed upon design, construction and configuration of the new technology.  Clearances between crossarms, spacing between pairs–and downright elimination of pole phantomed pairs–called for new crossarm designs.

 

When in the post-World War II era, traffic demands were put upon existing exchange circuits, a new carrier–Type “O” was configured to make the most use of these open wire leads until buried cable could take their place.  Economically, it made sense and retirement of all aerial wire in some mountainous and plains area states, was not an easy option.  There, the use of existing arms could be retrofitted with new technology and hardware so that hugely expensive conversions would not have to be made immediately.  Open wire in those remote areas was yet the best alternative to expensive buried cable.

 

The major point between crossarm architectural changes revolved around transmission efficiencies.  The problem of mutual interference and coupling made changes a necessity–especially in the world of carrier’s high frequency technology.

 

With these changes in the arm designs, came better transposition techniques, insulator designs, hybrid pin choices and high strength conductor with excellent transmission characteristics.  Ties for insulator attachment also changed.  Bonding and grounding of the pins below each pair on the crossarm were sometimes advanced to eliminate leakage current problems.  Sag limits on wire spans were confined to very exacting specifications–sometimes to as little as one-quarter inch!

The Carrier Crossarms: S6 and W6 Styles

The “S”teel 6-inch pair separation arm and the “W”ooden pin 6-inch separation arms maintained the standard 10′ length and treatment.   The use of these arms was confined to open wire high frequency carrier systems where it was possible to either combine phantom pole pairs with the multi-channel carrier systems on the same arms.  There were four types of pin arrangements as shown below.  Sketch by D. Schema.

 

Sometimes it was necessary to use existing crossarms, such as the 10A, and retrofit them for the use of carrier.  The conversion was simple.  The wooden “B” bushing could be inserted in existing 1 1/4″ diameter pin holes where it a new CS or steel pin with wooden cob could be used; no change there.  However, on the spacings, it was then necessary to make field adjustments to the existing arms.  If the arms were not too deteriorated, the existing arm could be bored to place new steel pins beside existing holes. 

S-8 and W-8 Carrier Crossarm Configurations

S-8 ten foot crossarm with steel CS pins and CSA Pyrex glassware. Photo credit: Tom Hagen.

W-8 Crossarm crossarm configuration with wooden pins. Pole pairs absent. 16-channel carrier. Note use of Hemingray No. 45 DP clear glass insulators.

These crossarms were used on carrier toll leads where eight-inch separation between the wires of a pair were used.  Again 30-inch steel span braces were used as in conventional 10A and 10B arms.

 

The arm would carry four pairs and could be a new configuration, assembled and erected to creat a new lead, or could be the modification of an existing lead from voice frequency toll to high frequency carrier.  The transmission engineer would specify in his plans, with notes, as to which configuration would be used per the type and classification of carrier system.

 

There were seven types of crossarm pin configurations for the S8 and W8 arms.  In all these forms, the design allowed for 16-inches between the pole pairs (on voice frequency phantomed construction) except on one design, where the difference was 32-inches between the carrier pairs–and phantomed voice frequency circuits were dropped entirely.