Production and Erection ofPrestressed Concrete Poles

for a RailroadElectrification Project

Leonard G. McSaveneyChief EngineerFirth Stresscrete DivisionFirth Industries LimitedAuckland, New Zealand

During the oil crisis of the 1970s theNew Zealand Government com-

mitted itself to doing all it could to re-duce the effect on the country of futurerises in oil prices. One of the options atthat time was to convert the most heav-ily trafficked section of the New Zea-land Railways Corporation system fromdiesel powered to electric locomotives.

In 1981 the decision was made toelectrify a major part of the North Islandmain trunk line. While in this time oflowering world oil prices and risingelectricity costs the initial economicsmay he hard to justify, the long term ad-vantages of the scheme will benefit boththe Railways Corporation and the coun-try in the future.

The section to be electrified, throughthe center of the North Island, contained

many steep grades through difficult ter-rain. In addition to the savings in im-ported fuel, electric locomotives offeredthe additional advantages of better trac-tive power on the steep sections of track,resulting in longer trains hauling greaterloads, increased reliability and im-proved operating efficiency. Electriclocomotives also have the advantagethat regenerative braking on the down-hill runs can be used to power other Io-comotives on the system.

In 1983 bids were called worldwidefor the electrification of a 172 km (108mile) section of track using a 25,000 voltalternating current system. This projectwas split into several contracts: Loco-motives, Signals and Communications,and Traction Overhead (catenary cablesand support structures). Local contracts

42

were also let for track realignment, tun-nel alterations and general upgrading ofthe line.

Included in the pole supply bid con-ditions was the option for the New Zea-land Railways Corporation to extend theStage I contract to Stage 11, a further 230km (144 miles) of track.

The contract for the Traction Over-head portion of the project, valued atNZ$35,000,000 (US$20,000,000), waswon by a joint venture company: Mc-Connell Dowell Constructors Ltd fromNew Zealand and Multi ConstructionEngineering Ltd from Australia. TheMcConnell Dowell — MCE Joint Ven-ture awarded a subcontract to FirthStresscrete, a division of Firth Indus-tries Ltd, for the design and supply ofthe prestressed concrete poles.

Electrical SystemThe prestressed concrete poles sup-

port a combination of conductors sus-pended from a steel framework. The25kV system comprises five conductors:the earth wire connected to the back ofthe pole, the protection wire separatedfrom the pole by two 3kV porcelain in-sulators, the auto-transformer feederwire supported from a 25kV insulatorand the contact and the catenary wiressuspended over the middle of the trackfrom a steel tube frame with 25kV insu-Iators at its end.

All the conductors, with the exceptionof the earth wire, are insulated from thepole's "earth potential" by the 3kVstand-off insulator between the pole andthe rectangular hollow steel stand-offtube. The stand-off tube allows forheight adjustments to the cantilever armwhen registering the equipment.

Pole OptionsPreliminary discussions with the New

Zealand Railways Corporation had indi-cated that approximately 4000 poleswould be required for Stage I and a fur-

PCI JOURNAL/September-October 1987

SynopsisA long line production method for

producing prestressed concrete polesto support overhead catenary wiresfor a railroad electrification project hasbeen developed in New Zealand-

The method is ideally suited to pro-ducing poles economically from aconventional multiproduct preten-sioned precast concrete factory usingsemiskilled labor.

This paper describes the evolutionof the design concept, optimum poleshapes, quality assurance, productionand installation methods.

they 6000 poles for Stage II. Since thereseemed little likelihood of further sec-tions of track being electrified, a pro-duction process that could produce10,000 poles over a 4-year period was allthat was required. Because of the rela-tively small number of poles, FirthStresscrete felt that a sophisticated spe-cial purpose factory to produce the spunhollow circular poles that are used inother electric rail systems around theworld could not be justified for thiscontract.

The company chose, therefore, to baseits bid on a pole that could be manufac-hired in any of its existing prestressingfactories along the route of the electri-fied track. The method chosen had to fiton existing long line pretensioningbeds, be able to use concrete from aready mixed concrete truck, and not re-quire any production plant or laborskills that are not normally associatedwith the production of structural precastpretensioned concrete flooring or bridgeunits.

The bid specification provided anumber of material options for the poles:steel, timber and concrete. All alterna-tives were investigated by the contractor

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A VARIES

80

3.73'

A

—s5 I

170 ( 8.63"1

SECTION A-A

All Holes 25 mm Die.!T'1

---I I - 250 1 9.6 .7 1 -- - - 455 1 181

NO TES AND DESIGN LOADINGS

Pole Length 10.000m. (32 = 9-7)

Transverse W.L. 6-80 MN (1530 Ibs.)

Down Line W1. (.70 NN (380 IL's.)

Load Applied from top at 305 mm (?-0)

Safety Factor at W.L. r2Ground Line 2000 mm (6"-7)

Weight of Pole 1100 kg (2424 IL's)

Fig- 1. Front and side elevations, cross-sectional details, design loadings and otherparticulars of Pole Type 510C 10 m (32 ft 9.70 in.).

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and the New Zealand Railways Corpo-ration, taking full account of both theelectrical and structural properties ofthe materials.

The final decision in favor of pre-stressed concrete was based on thismaterial offering the most economicalsolution, together with an aestheticallypleasing appearance when combinedwith the overhead equipment.

Pole prices in Firth Stresscrete'sNZ$3,000,000 (US$1,700,000) pole sup-ply contract ranged from NZ$150.00(USS84.00) per pole to NZ$190.00(US$106.00) per pole for the typicalpoles, and up to NZ$350.00 (US$196.00)per pole for the special poles.

Design and form costs were coveredby a separate lump sum payment.

Design and EngineeringFirth Stresscrete is New Zealand's

largest manufacturer of precast pre-stressed concrete. The company hasbeen designing and manufacturing pre-stressed concrete power poles in NewZealand since the early 1950s.

Computer aided designs for a range ofdifferent poles have been well proven inservice and by load tests.

It was decided to adapt a standard ta-pered I-section pole to give a series ofpoles to suit the varying load and serviceconditions. Fig. 1 shows front and sideelevations, cross section, design load-ings and other details of a typical pole.

Each pole location is designed anddetailed for its particular loads. Thepoles, cantilever arms and foundationsare selected to provide the most eco-nomical solution for each location. Polesare manufactured to various lengths andstrengths to cater for varying groundlev-els and bending moments.

Twelve types of poles were even-tually required. These were producedfrom five different molds. The fivemolds produced:

1. Long and short poles for straighttrack.

2. Long and short poles for curvedtrack.

3. Extra long poles for steep foun-dation sites.

4. Crossing loop poles with canti-lever arms on each side.

5. Poles to bolt on the sides ofbridges.

6. Portal poles to support steel crossbeams over several tracks in sta-tion areas.

7. Headspan poles to support multi-ple conductors off a suspendedcatenary wire for multiple tracksin marshalling yards.

S. Bolted base poles for use on padfootings.

9. Substation poles and overheadfeeder poles.

10. Clearance poles to raise otherpower distribution lines abovethe main power feed wire.

All poles were designed to have com-patible strand patterns using 12.5 mm(V in.) diameter seven wire strand, Thisenabled them to be cast end to end onlong line stressing beds.

The poles were designed and manu-factured to New Zealand Standard NZS3115 (1980) "Concrete Poles for Electri-cal Transmission and Distribution."This is a performance standard allowingany recognized concrete design methodto be used but specifying cracking mo-ments and ultimate capacity in terms ofdesign working loads.

The standard specifies test loadingprocedures to destruction to prove thedesign and also load tests to workingloads as part of the quality assuranceprocedure.

The design parameters set out in thestandard are:

1. No cracks under working loads.2. A factor of safety against collapse of

2.0.3. A down line strength of 25 percent

of the transverse strength.4. Tip deflection under working load

of 1150 of the height above ground.5. Minimum concrete cover to any

PCI JOURNAUSeptember-October 1987 45

steel reinforcement of 20 mm (0.8in.).

The only amendment to this standardwas to reduce the allowable tip deflec-tion to limit the deflection at conductorlevel to 50 mm (2 in.), This was done toensure that the conductor wire wouldnot be blown off the locomotive panto-graph contact under maximum operatingwind speeds.

Form Design and PoleProduction

An essential item in reducing produc-tion costs is an efficient and durableform. Hinged steel forms were used thathave required very little maintenanceafter the 700 casting days that they havebeen in production to date.

The poles are cured by circulating hotwater through tubes built into the forms.Insulated fabric covers are used to min-imize heat losses. The design 28-daycylinder strength of the poles is 45 MPa(6500 psi) and transfer strengths in ex-

cess of the 28 MPa (4000 psi) minimumare achieved after 16 hours of curing.

The factory producing the poles isalso manufacturing poles for local powersupply authorities, flooring products forbuildings, and bridge beams. No specialskills are called for in the production ofthe railroad poles and no additional me-chanized equipment was required. Theminimum equipment is one overheadcrane to strip the poles from the moldsand a forklift to stockpile the poles andto load the rail wagons. Production laboraverages 3 to 4 man hours per pole andthe poles are produced at a rate of 90poles per week.

With each pole made for a specifictrack location, schedules of poles aregiven as soon as the pole location anddesign parameters are determined bythe main contractor and the New Zea-land Railways. A wagon Ioading se-quence is also given for each wagon loadof poles. This ensures that poles can beplaced from the train in the correct se-quence.

Fig. 2. Hi-Rail crane with auger attachment prepares to excavate a pole foundation.

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Quality AssurancePoles are manufactured to a rigid

quality assurance program. This startswith the design of the product and themolds; includes quality control checkson the component materials, the pro-duction methods, the finished product,and transport and handling.

Part of the requirements of NZS 3115requires one pole in every 100 to be testloaded up to the design working load. Atthis load it must have no cracks and itsdeflection must be within ±15 percentof the average deflection for all poles ofthat type tested.

The aim of the quality assurance pro-gram is to ensure that all poles leavingthe factory are satisfactory. Remedialwork at remote sites, often without roadaccess, is very expensive.

Pole InstallationInstallation of these poles through

particularly rugged sections of the cen-tral North Island of New Zealand ishampered by lack of access. Often theonly access is from the rail track. This isa single track and must be cleared forthe passage of trains at regular times.

This problem has stretched the in-genuity of the McConnell Dowell -MCE joint venture and has led them todevelop a novel series of dual road-railand rail mounted machines. These ma-chines are able to perform all the majorpole installation functions and arequickly able to lift themselves clear ofthe track to enable trains to pass.

The installation starts with the pas-sage of a supply train hauling wagonloads of poles. The poles are laid along-side the track by a hydraulic cranemounted on a rail wagon. Workmen thendress the poles with the insulators andstand-off tube, and fit any other specialhardware. When the poles have beenlaid out, the foundation holes are au-gered by two rail mounted hydrauliccranes equipped with power swivels andpendulum augers (Fig. 2). A third rail

Fig. 3. Pole installation using rail mountedcrane.

mounted Palfinger crane (Fig. 3) followsthe auger cranes and lifts the poles intoposition where they are temporarilybraced in the correct alignment.

Concrete is delivered to the founda-tion holes by a hydraulically driven railmounted truck transporter (Fig. 4). Thistransporter is capable of carrying a fullyloaded ready-mixed concrete truck atspeeds of up to 25 km per hour (15 milesper hour). This machine is also able tolift itself clear of the track onto trans-portable stands similar to those used bynormal track maintenance machines.The concrete transporter has proven tohe so ideal for the job that a further fivemachines have been sold for use onAustralian railroads.

Fig. 5 shows the equipment used forrunning out the contact wire.

The conductors were installed using arail mounted truck with a hydraulic lift

PCI JOURNALSeptember-October 1987 47

Fig. 4. Rail mounted concrete transporter for casting in place pole foundations.

Fig. 5. Equipment used for running out the contact wire.

platform, as shown in Fig. 6.Fig. 7 shows the final adjustment of

contact wire and the finished pole con-figuration.

Using the above specialized equip-ment, the joint venture's small highlymotivated crew have been able to installpoles at a rate of tip to 150 poles per

5-day week.The erection of the overhead con-

ductors is split into three separate oper-ations:

1. Running out and sagging of thefixed termination conductors.

2. Running out and tensioning of thecounterweight tensioned contact

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Fig. 6. Hi-Rail truck with hydraulic lift platform installing conductors.

and catenary system.3. Registration to line and level of the

contact and catenary wires andfinal adjustment to the cantilevers,

For each of these operations equip-ment has been developed using bothroad-rail vehicles and modified NewZealand Railways rolling stock.

Foundations

An economical method of overcomingvarying ground levels and soil bearingcapacities without the need to produce awide range of different pole lengths wasessential to reduce both the pole pro-duction costs and the cost of foundations.

PCf JOURNAL'September-October 1967 49

Fig. 7. Final adjustment of contact wire showing finished pole configuration.

The solution arrived at, after exten-sive site testing, was able to accommo-date the typical range of ground levels,and soils varying from poorly compactedembankment fill to well compacted soilsand soft rock, using only two lengths ofpoles.

Concluding RemarksThe choice of an I-shaped section re-

stilts in a pole that ideally matches theservice loads. The pole is designed for

the load of wind on the wires in the di-rection transverse to the track. In theopposite down line direction the loadcapacity of 25 percent of the transversestrength provides an adequate marginfor handling loads, wire tensioningloads and accidental overloads.

By choosing a production method thatis compatible with the normal range ofproducts in a typical precast prestressedconcrete factory, the system is economicalfor small production runs in factories thatwish to diversify their product range.

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The prestressed concrete poles forthis railroad electrification project havewon the New Zealand Concrete Soci-ety's 1986 Prestressed Concrete Award.

In making this award, the judges wereimpressed by both the structural effi-ciency of the design and the aestheticallypleasing appearance of the slenderfluted poles. They were also impressedby the on-going commercial implica-tions of the project. The design andmanufacturing systems have been sosuccessful that they have been licensedfor use in other countries.

The use of these slender prestressedconcrete poles has minimized the in-trusion of the overhead electrificationon the predominantly rural environmentthat the railroad passes through.

Stage I of this railroad electrificationhas been completed. Stage II is due tohe completed by February 1988. Thesuccessful manufacture and installationover more than 8000 poles without aproblem is a tribute to Firth Stress-crete's production personnel and toMcConnell Dowell — MCE's fieldcrews.

NOTE: Discussion of this article is invited. Please submityour comments to PCI Headquarters by June 1, '1988.

PCI JOURNAL/September-October 1987 51