prestressing the cn tower journal/1976/may-1976/prestressing the cn...pects that governed the design...

Prestressing theCN TowerFranz Knoll, PE M. John Prosser, PEChief Engineer General ManagerNicolet, Carrier, Dressel VSL Canada Ltd.

and Associates, Ltd. Post-Tensioning SubcontractorsConsulting Engineers Stoney Creek, OntarioMontreal, Quebec

John Otter,* PEChief EngineerKilmer Van Nostrand

Co., Ltd.General ContractorsDownsview, Ontario

A major feature of this, the world's talleststructure, is that prestressing was usedextensively in both the foundation andsuperstructure of the Tower.This article describes the design andconstruction highlights in planning andbuilding the structure especially as relatedto the post-tensioning operations.

Franz Knoll M. John Prosser John Otter

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Toronto's CN Tower is at 1815 ftthe world's tallestt free-standing

structure (see Fig. 1).Built between 1973 and 1975, this

giant communications and observationTower is part of the Canadian NationalRailways Metro Centre redevelopmentscheme which envelops 190 acres ofrailway land between the central busi-ness district of Toronto and the water-front.

A major structural feature of theTower is that both its foundation andsuperstructure (for most of its height)were prestressed using in-place post-tensioning.

The 1450-ft concrete shaft of the su-perstructure has a three-legged crosssection which tapers upward. This cen-tral shaft was slipformed in about 8months from June 1973 to February1974.

In the summer of 1974 a skypodwith seven floors was added to the sup-erstructure to provide an observationrestaurant and service facilities forbroadcasters.

The basic supporting structure of theskypod is a set of twelve radially ar-ranged cantilevered concrete wallswhich were initially tied into the slip-formed shaft by means of stress bars.After the walls were completed, thecantilevered structure was stabilizedlaterally using a post-tensioned ringbeam.

In March and April 1975, the con-crete Tower was topped off by a 350-ft high steel mast. Erection was by heli-copter. The mast will support varioustransmission equipment for televisionand radio.

As a precaution, the mast was sub-sequently clad in a cylindrical skin ofglass-reinforced plastic to prevent ice

*Formerly, Project Engineer with VSL CanadaLtd., during construction of CN Tower.

tThe Sears Tower in Chicago is 1450 ft andOstankino Tower in Moscow is 1748 ft.

Fig. 1. Panoramic shot of CN Towernearing completion.

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build-up during the severe Toronto coldseason. There would be a danger oflarge chunks of ice breaking off andfalling onto buildings and streets.

This paper will review the designconsiderations that went into construct-ing the Tower and, in particular, theprestressing methods. The various as-pects that governed the design will bedetailed as well as a discussion of themethods and procedures used for thepost-tensioning operations. Especially,the prestressing of the foundation andshaft will be described including theauxiliary stressing of the anchorage zoneand stressing of the ring beam.

Design Considerations

Because of the sheer magnitude of theTower and the fact that fairly sophis-ticated construction methods were be-ing applied on a rather large scale,existing technology was very carefullyexamined to establish its range of val-idity and practicality to new conditions.

The following were some of the ma-jor technical areas of concern especiallythe design and construction considera-tions relating to the prestressing opera-tions. At this stage, no order of priorityis implied since it was impossible topredict with certainty where the diffi-culties would be concentrated.

The high magnitude of the forcesto be dealt with implied the use of alarge quantity of high strength steeltendons (a total in excess of 1000 tonswas eventually used). This figure maynot appear very sensational by today'sstandards; however, it should be notedthat the majority of the force was ap-plied basically to one single structuralmember, namely, the Tower shaft.

At an early design stage, it ap-peared logical to construct the Towershaft by the slipform technique. Sincethis method of construction is fairlysophisticated, it also meant that severe

restrictions would be imposed on suchitems as time schedules and space re-quirements relating to detailing, plac-ing, and stressing the tendons.1 Because of the record-breakingheight of the Tower, it was anticipatedthat construction would last through atleast one winter. Consequently, the con-creting and post-tensioning operationswould have to be planned to withstandvery severe weather conditions. (In ac-tuality, high winds and cold weatherconditions caused many difficulties andconstruction delays.)

The verticality and length of thetendons would be a significant depar-ture from previously-known practice,and would introduce a whole new set ofpractical considerations, relating to theplacing, securing, stressing, and grout-ing of the tendons. For instance, an ac-curate prediction of the post-tensioninglosses was very difficult because of in-sufficient experience with this type ofapplication.

Grouting of vertical ducts wouldcause substantial hydrostatic pressureinside the ducts, the consequences ofwhich were hard to predict.

The decision to use post-tensioningfor the foundation required extensivestudies on the behavior of the (frac-tured) rock base subject to tendonstressing.

Similarly, the decision to post-ten-sion the ring beam at the supportingstructure for the skypod posed intricateproblems of geometry, relating to re-stricted space and the requirement toproduce a uniform and symmetricalfinal stress in the twelve-sided polygon.

The fact that part of the tendonshad to be placed and stressed in thecold season, when the temperature ofthe concrete of the Tower walls wouldbe below freezing, made it impossibleto perform the grouting operation un-til many months later. Hence, the pos-sibility of corrosion had to be con-sidered.

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The above items were only the majorproblems anticipated which would af-fect the post-tensioning operations.There were many more design and con-struction problems of a more local andimmediate concern which came intoexistence during the construction per-iod. Some of these will be discussedlater on in the paper.

All the technical problems that couldbe anticipated had to be assessed andsolved at the time of the design pro-cess. Unavoidably, there were many un-certainities and possible hazards arisingfrom the unknown conditions and fromthe mode of construction.

This meant that many items had tobe estimated and compensated for bybuilt-in safety margins. This degree ofconservatism was justified because ofthe importance and unusual proportionsof the structure and the fact that evenrelatively minor construction problemsand delays could not be tolerated froman economic viewpoint.

The authors were relieved and satis-fied to find that the client basicallyconcurred with the conservative de-cisions. In retrospect, time has shownthat the built-in reserves were largelyjustified.

Because of the novelty of so manyaspects in the design and constructionof the CN Tower, all available tech-nical and management skills had to bemobilized. The usually distinct separa-tion of the responsibilities between thedesign team and the constructors had tobe abandoned. On this project it be-came paramount that there be the ut-most collaboration between the designengineers and the contractors.

For example, initial specificationsand design plans had to be continuouslyadjusted and adapted to the conditionsand difficulties encountered as con-struction progressed. The eventual suc-cess of this collaboration is manifestedby the fact that this paper is a jointlyauthored report.

Prestressing theFoundation

The foundation of the CN Tower isbased on the Ordovician rocks of theDundas formation. The soils consist ofhorizontally layered shales and lime-stones, with occasional planes of clay-filled seams. Borings showed that ran-dom vertical cracks and separations alsoexist.

The footing slab, as it was even-tually constructed, was the result of alengthy design study. The decision touse prestressing, including the propor-tioning and layout of the post-tension-ing tendons, was only one of the majorfeatures of the structure.

The geometry of the Y-shaped slabwas largely determined by the shape ofthe Tower shaft, and by the fact thatthe foundation had to be situated at arelatively shallow depth below the fu-ture Tower base. Essentially, the foun-dation slab performs the same functionas a conventional flat footing which sup-ports the dead weight of the superstruc-ture plus its own weight as well asoverturning moments due to horizontalloads.

There exists, however, one significantdifference from the usual type of flatfooting besides the enormous scale,i.e., the spreading of the vertical loadsto the interface of the rock base is, atleast partially, performed by the Towershaft itself, due to the tapering of thewalls.

Thus, the function of the Tower foot-ing proper is essentially reduced to thatof the lower part of a flat footing, name-ly, that of resisting tensile stresses.

Obviously, in typical flat footings allthese tensile stresses would be resistedby reinforcing steel or, perhaps, bypost-tensioning in some cases. However,because of the unusual proportions ofthis structure, serious consideration wasgiven to having the horizontal spread-

PCI JOURNAL/May-June 1976 87

RJIIIIttIWATERTABLE

FRICTIONAT BASE

-TENSIONING

OI TENSILE STRESSES IN CONCRETE

FRICTION FORCES BETWEEN ROCK AND CONCRETE

O3 PASSIVE RESISTANCE OF ROCK BANK

Fig. 2. Loading pattern of foundation slab.

ing forces resisted in another mannerthan by building in steel elements. Thealternate principle would be analogousto the way an arched structure is sup-ported, i.e., the rock base would pro-vide the lateral resistance against move-ment of the springings. This method,of course, would provide a substantialsavings in cost because the tension ele-ments could be omitted.

In the case of the CN Tower, threemechanisms can be distinguished inwhich the spreading forces and result-ing stresses would be dissipated (seeFig. 2):

1. By tensile stresses in the concreteof the foundation slab.

2. By friction activated at the inter-face of the rock and concrete.

3. By passive resistance of the rockwall around the perimeter.

The following reasoning was thenused:

Because of the nature of the rock(thin layers with embedments of clay,and high pore water pressures in re-sponse to sudden dynamic loading), themobilization of friction forces appeared

doubtful and would occur only aftersome movement had taken place.

In turn, this mechanism might causecracking in the foundation. This crack-ing would be most undesirable becausethe water table is located about 30 ftabove the foundation underside.

Similar conclusions can be drawn foran edge resistance of the non-excavatedrock. Reactive forces would be mobil-ized only after some elongation of thefoundation slab because of the macro-scopic compressibility of the rock (ver-tical cracks). Again, the foundationwould be initially exposed to high ten-sile stresses that would result in unde-sirable cracks.

Thus, in order to safeguard againstthe entry of water, as well as to preventlarge-scale cracking, it was decided toprestress the concrete footing. In thismanner, the entire slab would be de-signed to be crack-free and carry notensile stresses in the concrete.

During the stressing of the tendons itwas confirmed that the foundation slabdid indeed behave in the anticipatedmanner and that friction forces were

88

Fig. 3. Section through foundation.

not activated despite the fact that hori-zontal movements occurred.

The layout of the tendons followeda symmetrical triangular pattern. Fur-thermore, the hollow core of the footingslab had to be avoided because thisspace (see Fig. 3) was reserved for thestressing operation of the Tower shaftpost-tensioning. The pattern eventuallydecided upon is shown in Fig. 4.

During construction, and particularlyduring the stressing sequence of thetendons, the effect of movements in therock base had to be considered.

It was found that one of the criticalfailure mechanisms of the entire Towerstructure would be governed by theconcentration of shear stresses aroundthe extreme edges on the leeward sideof the footing. The relatively thin layersof the rock and its vertical separationsindicated that indeed, this had to be amajor consideration, since the risk ofpremature shearing of consecutive lay-ers of rock had to be minimized.

Thus, any disturbance of the rock,because of the foundation moving ontop of it during prestressing, had to be

avoided in these zones. In particular,existing vertical separations in the rockcould not be allowed to expand or formnew fissures around the edges of theslab. This, in turn, meant that all move-ment of the slab at its extremities hadto be eliminated and the point of zeromotion, due to prestressing had to benear the end if each leg.

The above problem was solved byintroducing open joints (see Fig. 4)near the center of the Tower, whereflat jacks were applied to counteractshortening of the slab due to prestress-ing.

The stressing of the 48 cable tendonswas performed by alternating with thepressurization of the movement con-trolling the flat jacks. The tendons werestressed from both ends to achieve asymmetrical pattern of stresses.

Each cable unit was initially stressedto 528 kips and then released to 400kips. The average two-dimensionalstress thus introduced in the slab isabout 100 psi.

The movements that were observedduring the stressing operation con-


CONTROL JOINT 1 - 1

Fig. 4. Plan of foundation showing layout of prestressing tendons and section of control joint locating flat jacks.

firmed the anticipated behavior of the concrete slab. The points of zero movement were actually found to be located near the ends of the three legs.

An interesting question was whether or not the compressive stresses introduced by the post-tensioning would in- volve a portion of the rock base through friction. From the measured movement of the slabs it appeared that this was not the case. It was found that the actual displacements corresponded closely with the theoretical elastic de- formations of the concrete. Therefore, it was concluded that all compressive stresses must have remained within the footing. Consequently, it was assumed that frictional engagement of the rock layers was largely nonexistent.

Fig. 4 is a foundation plan showing the tendon layout. Also shown is a sec-

tion of the control joints and location of flat jacks. Fig. 5 shows the actual installation of tendons and mild steel reinforcement prior to concreting.

Problems and solutions

Some practical difficulties arose during the construction of the foundation.

The use of a semi-rigid sheath for the cable ducts proved to be troublesome in combination with the mass casting of a large concrete body. For example, many duct supports appeared to have been necessary. These were apparently not provided in sufficient number and the resulting displacements of cable ducts impaired the eventual sleeving-in of the strands. This operation was scheduled to take place after the concreting.

Fig. 5. Layout of prestressing tendons and mild steel remrorcement prior wconcreting the foundation.

Unfortunately, the duct displace-ments resulted in delays and in theeventual loss of some of the tendonsoriginally provided because of blockedor kinked ducts that would not allowsleeving-in of cables.

The alternating stressing from bothends also resulted in the loss of onecable, due to grip failure on strands inthe far anchor head. This cable was

later replaced but the energy containedin the tendon under stress proved to beso great as to balistically lift thestressing jack off its seat.

Fortunately, no injuries resulted fromthe incident. In retrospect, it is felt thata safety grip on the far end of theanchor might have prevented such ac-

cidents.

Prestressing the Tower Shaft

Design criteriaThe basic design philosophy for theTower shaft was established quite earlyin the study period. It was decided thatthe Tower shaft would be fully pre-stressed (post-tensioned). Hence, notensile stresses under expected extremeloads were to occur in the concrete.

In this regard, extreme loads weredefined to be maximum wind effectswith a return period of 50 years. Thisdescription relates closely to the defini-tion of wind loads in building codes

which calls for a statistically based re-turn period of 10 to 100 years.

For the extreme fiber in compression,a stress of 2500 psi was allowed as anupper limit on the gross area of crosssection. This allowable stress includesall effects due to gravity loads, wind,and post-tensioning.

The walls were built with a specifiedcompressive strength of concrete of5000 psi. The percentage of mild steelreinforcement used in the vertical direc-tion was between 0.5 and 1.0 percent.


The actual compressive strength ofthe concrete (from cyclinder tests)greatly exceeded the minimum specifiedconcrete strength. However, at the timeof design it was felt imprudent to spec-ify very high strength concrete (andhence higher allowable stresses) becauseof the many uncertainties such as elasticshortening, shrinkage and creep, heatof hydration, and so forth.

In the event of extreme wind condi-tions (exceeding the values definedabove), the Tower would actually be-come a "partially" prestressed structurewith possible formation of cracks. How-ever, this possibility was not judged suf-ficiently serious to warrant additionalexpenditures of steel and concrete, con-sidering the rarity of this event.

Ultimate strength (limit state) analy-sis also showed that with the amountof post-tensioning and reinforcing steelfurnished, a sufficient safety factor

ING

IDS INNDATION

Fig. 6. Anchorage of Tower tendons.

_7UPPER BOUND

EFFECTIVEPOST-TENSIONING

LOWER BOUND

Fig. 7. Upper and lower bounds ofTower prestressing force with values

effectively achieved.

against collapse was provided. (Notethat the effective load factor for windforces over the 50-year extreme periodis approximately 2.3 for the limit statewhich includes all dynamic and second-order effects.)

The post-tensioning of the Tower:shaft consists of 144 tendons which inturn are composed of 16 to 31 %-in.diameter strands of seven wires each.The cable units are uniformly spreadover much of the Tower walls, with theexception of the side faces of the wingsections.

All the Tower walls extend upwardswith approximately constant thicknessfrom the base to their top termination.

The location of tendons was govern-ed by considerations of uniform stressand some practical problems which willbe discussed below.

A concentration of all tendons at theextreme ends of the Tower wings wouldhave slightly increased the ultimatestrength of the shaft. However, this ar-rangement would have resulted in con-gestion with accompanying difficult ge-ometrical and structural problemsaround the anchorage areas. Hence, amore uniform pattern of tendons wasadopted.

All the cables were anchored in thebase, with the anchor head accessible-from the hollow core of the footing.The cables then extend to the ton of therespective Tower walls with the topanchor head at the termination of thewalls (see Fig. 6).

Stressing was successfully executedfrom the base with the exception of onecable group, the reasons for which willbe discussed below. Stressing at thelower end was planned in order tominimize disruptions and delays to theslipform operation.

Note that the sheltered areas in thefoundation area (see Fig. 6) allowedthe intricate stressing operations to pro-ceed while work overhead was continu-ing. However, when stressing was con-

ducted from the top, as it had to bedone for one group of cables, the opera-tion caused a 2-week delay.

Fig. 7 shows the constraints placedupon the prestressing design in theTower walls together with the effectiveamount of post-tensioning eventuallyachieved. (The actual distribution oftendon units throughout the Towershaft is summarized in Fig. 8).

It may be noted from Fig. 7 that themost economical force diagram shouldfollow closely the lower bound curve.In fact, the effective prestressing forcedoes so only in a very approximatemanner.

Apart from some considerations re-lating to construction stages, this ap-proximation arises from the very prac-tical problem of detail design, and ofconstruction sequence. For example,

Fig. 8. Distribution of tendon unitsthrough Tower shaft.

)S)

)S)

DS)

NDS)

STRANDS)

ITS'RANDS)

93

Fig. 9. Slipforming of Tower walls(August 1973).

the top termination of the tendons ne-cessitates a halt in the continuous slip-form operation. Rather bulky and deli-cate equipment had to be hauled in forplacing the cables and anchors. Theconcrete casting, curing, and finishingoperations as well as the reinforcingsteel setting cannot proceed while thepost-tensioning tendons are placed.That is, both operations cannot be con-ducted simultaneously.

Hence, the number of times the slip-forming could be disrupted, includingall preparations for stop and go, had tobe kept to an absolute minimum.

It was therefore decided to sacrificesome materials in order to allow for asmoother slipform operation. To furtherexpedite matters, the post-tensioningtendons were arranged in large groupsand were conveniently placed so that alltop anchorages of cables were at thetop terminations of individual Towerwalls.

This feature also avoided the difficul-ties that would have originated from acable termination detail in the face of awall.

Figs. 9 and 10 show various stages ofslipforming the Tower shaft. This slip-forming operation took about 8 monthsto complete.

Contrary to an earlier intention, notendons were provided in the side facesof the Tower supporting wings. Theseare triangular in elevation and no tophead of these walls exists. The change-over of cables from these walls intoothers would have posed too many diffi-culties for placing and, thus, had to beabandoned.

This change caused some ratherlarge stress concentrations at the top ofthe cross walls. However, these stressconcentrations were resisted using re-inforcing steel and auxiliary stress barsin one instance (see p. 107).

All the cables were introduced in thewalls in one piece, thus avoiding coup-ling devices and intermediate stressingstages. This produced rather largestress elongations (see Table 1).

It was also necessary to introducesome reinforcing steel for temporaryconstruction conditions, with part ofthe Tower shaft unstressed and the slip-form moving ahead. Some of this re-inforcing steel would not have beenneeded in the final structure but itscost was considered a fair expense inexchange for greater ease of construc-tion and continuity.

Table 1. Tendon elongations forvarious cable groups.

Cablegroup

Cable length,ft

Total elongation,in.

Al 1487 110A2 1378 103B 1262 98D 562 44E 180 15FG 1125 89

94

At the time of design neither theexact sequence of construction nor therelationship of the construction pro-cess to the seasons were known. Wea-ther conditions were anticipated tocause delays and difficulties but to whatmeasure this would occur was impos-sible to assess during the planningstage.

Other anticipated difficulties in-cluded the blockage of cable ducts dueto denting or chunks of wet concretefalling into the sheath. The possibilityof cleaning the ducts before insertingthe tendons was very doubtful.

In general, the placing and stressingof vertical post-tensioning units of thislength could meet with many unfor-seen difficulties. It was quite probable,therefore, that individual strands, or en-tire units, would be lost. Recovery orreplacement of these would cause ex-cessive delays and costs.

In addition, much uncertainty existedduring the design stage as to the pre-stress that would effectively beachieved at the critical points of theshaft. This was due to lack of experi-ence in estimating the amount of pre-stress losses and due to practical diffi-culties which could not be predicted.

Therefore, a somewhat larger amountof prestressing than initially called forwas introduced to compensate for thisuncertainty. This excess of post-tension-ing was continually reassessed and ad-justed as the operation proceeded in re-lation to the hazards and difficultiesanticipated for each group of stressingand the experience gained during con-struction.

Compensatory measures included theprovision of additional ducts which, ifnot used, would be left empty andgrouted simultaneously with the cables.Also the choice of diameter for thesheath used was made in such a wayas to allow for additional strands to beplaced over and above the projectednumber. This flexibility allowed the

Fig. 10. Slipforming of Tower walls(November 1973).


LEGEND• CABLE COMPLETELY PLACED B STRESSEDr CABLE IN COMPLETELY PLACED ANDNR

STRANDS BROKEN OR LOST• CABLE COMPLETELY PLACED B STRESSED

AFTER CLEARING WITH DIAMOND DRILL®DUCT BLOCKEDo UNUSED SPARE DUCT

\••**t 0••••

Fig. 11. Historical review of post-tensioning results.

possibility of replacing tendons thatwere partially or entirely lost or im-possible to place or stress.

Fig. 11 illustrates the arrangement ofthese precautions and their final usagewhich largely justified their implemen-tation.

The practical instances that led tothis will be briefly reviewed below:

1. All ducts were built-in with diamet-ers of % in. or somewhat larger than re-quired for the projected amount of strandsto be inserted, This precaution would haveallowed an additional 20 percent of strandsabove the projected number.

2. For one group of cables, two spareducts were inserted for every ten cables tobe placed. In actuality, these ducts wereused eventually since it proved impossiblein some instances to remove blockages, orto insert all the strands required.

3. An overdesign of about 5 percent wasprovided in the number of strands to com-

pensate for the uncertainties due to pre-stress losses and unforseen constructionproblems.

4. As a back-up measure, the top an-chor head was chosen to be of the stressanchor type. This precaution allowedstressing to be conducted from the top endeither as a replacement to base stressingor as a supplementary measure for frictionand other losses in excess of that anticipat-ed.

Fig. 11 summarizes pictorially thefollowing items where things wentwrong and could be compensated bythe precautions provided:

1. Ducts blocked.2. Loss of strands due to problems

with tendon threading.3. Strands lost due to loss of grip at

the brake and at the stressing jack.4. Strands snapped during stressing

operation.In some instances the built-in re-

96

serves were more than sufficient where-as in other cases they were used upalmost completely.

Prestress lossesIt is fair to say that an accurate pre-diction of prestress losses due tocreep, shrinkage, elastic shortening, andsteel relaxation is difficult to determine.It might also be said that very greataccuracy is not really necessary. Thisproved to be the case with the CNTower. However, it was necessary toinvestigate the losses problem to someextent before moving on with con-fidence.

Recent revisions to the AASHTOSpecifications and PCI recommenda-tions suggest that prestress losses havesometimes been grossly underestimatedin the past. Coincidentally, the servicerecord also suggests that these under-estimations have not been critical. Fur-thermore, an overestimation of prestresslosses does not necessarily produce aconservative design.

The evidence for underestimationwas deduced from tests which appearto have accounted for all the significantparameters quite rigorously.

The parameters and their variationsin relation to the CN Tower are de-scribed below. The basic approach tak-en was to compare the conditions in theTower, item for item, with those exist-ing in standard bridge construction forwhich experience is extensive.

1. Concrete shrinkage—The concrete inthe Tower did not exhibit shrinkage char-acteristics that differ markedly from thenorm. In addition, much of the concretewas bound to be many months old at thetime of tensioning.

2. Elastic shortening—This would bequite variable from cable group to cablegroup and from level to level. Since muchof the effective prestress is derived fromthe dead weight of the structure and be-cause most of the weight would be actingbefore tensioning of the major cable groups

(i.e., longer cables), it was judged that thecondition in the Tower was superior to astandard application.

3. Concrete creep—The concrete in theTower did not differ from "normal" con-crete in its creep characteristics. Since thesummation of prestress from post-tension-ing and from the structure's dead weightwas within the normal range, no particu-lar departures were anticipated. Actually,the situation would be more favorable forthe longer cables since much of the creepdue to dead weight prestress would havedissipated before stressing.

4. Strand relaxation—Recent findings re-garding relaxation losses were accepted asbeing qualitatively correct, i.e., that lossesincrease as the initial strand stress increas-es. It was, therefore, established that thelosses to be expected in the Tower shouldnot exceed those in normal practice. Infact, there was every expectation that theywould be less. Furthermore, since much ofthe prestress effect was due to weight ofthe structure, variations in prestress losswould have an even smaller effect on per-formance than is normally the case.

The breakdown of anticipated lossesfor the longer cables was:

Shrinkage ............... 2.5 ksiElastic shortening ......... 1.8 ksiConcrete creep ........... 11.0 ksiSteel relaxation ........... 13.0 ksi

Total ................... 28.3 ksi

The total loss was, in fact, so close tothe traditional value of 25 ksi that itwas decided to abandon the sophistica-tion inherent in the extra 3.3 ksi and toadopt the standard value.

Friction and wobble lossesThere were several factors in the post-tensioning of the Tower that were out-side the realm of traditional experience.These included:

1. Cables that were extremely long (upto 1500 ft).

2. Ducts that were essentially straightand vertical for their full length. The onlycurves of significance in most of the ductswere those of the bottom anchorages.


3. The use of rigid sheathing. This wassuggested by the prestresser, essentially asa means for increasing the reliability of theducts but would also have a significant ef-fect on friction.

4. The use of galvanized sheathing. Thismade the ducts entirely unrusted at thetime of stressing.

5. The use of templates to set the ductswith minimum deviations.

6. The use of "oversized" ducts for pro-vision of space capacity.

These conditions, if cumulated, pro-duced uncertainties far in excess of theones concerning other losses. Thus, acareful procedure had to be used, in-volving continuous application of ex-perience gathered during the construc-tion, and constant updating of proce-dures from newly gathered information.

It should be noted that the manufac-turer's guaranteed cable coefficients ofwobble and friction were K = 0.0005and tk = 0.2, respectively. This meantthat 920 tons of vertical post-tensioningwould be needed involving the stress-ing of 208 cable ends.

However, for design purposes the co-efficients used were K = 0.0001 andpt = 0.15. Actually then, only 840 tonsof prestressing had to be installed ne-cessitating only 144 cable ends to bestressed. Thus, the trial and projectionmethod eventually saved 8 percent inmaterials and 30 percent in stressingoperations.

InstallationRigid helically corrugated sheath madefrom 24-gage corrugated steel sheetwere installed, with diameters of 4 and4½ in. Splicing was achieved by sleev-ing adjacent pieces with an overlappingsection of slightly larger diameter.

The length of pieces varied from 5to 20 ft, varying upon location of indi-vidual cables and the position of par-ticular obstructions. The sheaths wereplaced from the main (middle) deck ofthe moving stage which also containedthe main slipform and served as a plat-

form for placing reinforcing steel andinserts, and depositing and vibratingthe concrete. However, the workingspace was extremely constricted be-cause the lifting equipment, as well asone upper deck, was located above thisplatform. Because of these obstructions,ducts could not be easily removed orsimilar measures taken. Hence, this wasone reason for using relatively shortlengths of sheath units.

Various alternate materials for theducts and installation methods wereconsidered. For example, corrugatedplastic tubes, semi-rigid ducts, and theforming of a hole in the slipformedwalls by means of a mandrel, were con-sidered initially. However, rigid sheathswere selected because they were strong,were easily handled and installed, andwere relatively cheap.

Despite rough handling, the ductswere not damaged and the blockagescreated by denting, punching or burn-ing and other mistreatment were few.

Each cable duct was provided witha cap after every new unit was installedto prevent foreign material from enter-ing and blocking the ducts. In retro-spect, this measure proved quite suc-cessful except for four ducts which be-came clogged during construction.Three of these ducts were subsequentlycleared by using a flexible shaft dia-mond drill. This instrument was veryeffective in removing concrete chunks(some 400 ft deep) from the ducts.Nevertheless, this drilling operation wasvery expensive and caused many de-lays. Therefore, not all the ducts werecleaned out in this manner.

Fig. 12 shows a series of cappedsheaths while Fig. 13 depicts the in-stallation of duct sheaths from the "mo-ving deck."

The joints of the sheath were allwrapped with tape immediately afterplacing. This technique prevented ce-ment paste from entering.

Grout vents were installed in pairs at

98

regular intervals of 100 ft, with a dis-tance of 5 ft between the two vents ofeach pair. The lower vents of each pairwould then serve as a control outlet forthe grout pumped in from below, whilethe upper one formed the inlet for thesubsequent grout lift above.

Fig. 14 shows the installation ofgrout vents.

A very important task was to uncov-er the grout vent openings immediate-ly after the slipform operation beforethe concrete hardened over the pipeheads.

The installation of cables started im-mediately on completion of the respec-tive walls. First the anchor heads andbase plates were placed.

The strands were sleeved-in from thetop, one at a time. First, they were un-wound from the reels and then sleevedthrough a specially designed rig intothe top anchor head. The speed of the

Fig. 12. Series of capped sheathsat base of Tower shaft.

Fig. 13. Installation oftendon duct sheath from

"moving deck."

Fig. 14. Installation ofgrout vents. They weredone in pairs at regular

intervals.


Fig. 15. Feeding stranddirectly from reel.

strand when descending into the holdwas mainly governed by its own weightand controlled by a manual brake ap-plied at the reel (see Fig. 15).

This operation required considerableskill on the part of the prestresser. De-pending on the speed and other circum-stances, the strands have a tendency tojam in the duct before reaching thelower end. These tie-ups had to be un-done which at times caused consider-able delays.

Although the sleeving-in of strandsby this method was generally successfulin that strands could be placed at a highrate, the process can be dangerous attimes. For example, the strand had atendency to coil up at the lower end ofthe duct when brake control at the up-per end failed and the speed of thestrand was mainly governed by its ownweight.

Various devices and speeds of feed-ing were used for this operation. Themost successful procedure turned outalso to be the simplest. In this methodthe strands were led directly from thereel to the anchor head. Control wasmaintained by a man-operated brake atthe revolving reel.

Some stressing difficulties were re-lated to the placement method. For ex-ample, the consecutive insertion ofstrands at high speed can lead to coil-ing up of strands around each other in-side the duct to form a knot in zones ofsharp curvature.

In one cable group, the location ofthis knot was sufficiently close to thestress anchor head (at the base) to pre-vent stressing from the bottom end. Theknot would have been pulled towardsthe stress anchor head, the entangledstrands not being able to free them-selves for their final position in the in-dividually-sleeved anchor block.Strands then became kinked andsnapped due to concentrated overstress.

The stressing operation then had tobe moved to the top end for this groupof tendons where it was successfullyexecuted though with considerable de-lay. The lost strands were replaced inadjacent ducts (see Fig. 11).

Stressing procedureThe stressing method used dependedon whether the tendon was stressed atthe top or the bottom. For bottom-stressed cables, there was a region ofsharp curvature just beyond the anchor-age that caused rapid stress change inthe tendon as it rounded the bend.

This had the effect of absorbingmuch of the anchorage "set effect" thatis typical of individually wedged multi-strand systems. There was, therefore,little to be gained by "overstressing" oroverpulling the cable and then rebasingit.

Stressing was, consequently, verystraight forward; the cable was pulledto a specified pressure at which pointthe wedges were easily set.

100

Fig. 16. Strands arrive at basethrough 1500-ft long duct.

Fig. 17. Closeup of stressing inprotective cavern.

Fig. 16 shows newly arrived strandat base of Tower while Fig. 17 showsthe stressing operation in the protectivecavern.

For the tendons that were stressed atthe top end, the situation was different,since there was no adjacent zone ofsharp curvature. Overpulling was ad-vantageous here in getting the point ofmaximum stress to move a considerabledistance down the cable.

Tendons were pulled to a specifiedpressure at which point the wedgeswere set. The complete anchor headwas then pulled a specified distance andthen released back through this dis-tance.

Fig. 18 shows the stressing operationfrom the top anchorage.

The long stress elongations made itnecessary to reset the jacks many times.For cables over 1000 ft long, the effec-

Fig. 18. Stressing tendons from topapplying back-up procedure.


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Testing—The first experiment consist-ed of a series of tests using 24-gagerigid ducts (same duct material intend-ed for use in the Tower). The ductswere 100 and 200 ft long and weremounted on a 200-ft guyed scaffoldingtower.

Figs. 20 and 21 show the full-scaleexperimental setup for testing the grou-ting system.

At first, a water-cement ratio for thegrout of about 0.44 was specified. Un-fortunately, this ratio, when combinedwith the type of mixing equipmentused (a colcrete colloidal mixer), pro-duced a grout that was excessively vis-cous. Pumping pressures became exces-sively high and there was too little timebetween mixing and the time when thegrout was no longer pumpable.

Hence, an increase of the water-ce-ment ratio to about 0.50 became nec-essary. This mix resulted in a flow conetime of approximately 12 sec., allowingfor efficient pumping. An averagestrength of just over 5000 psi wasachieved. This strength was consideredsatisfactory since the main function ofthe grout was to be for corrosion pro-tection.

The water-reducing agent (in liquidform) was found to perform satisfactori-ly.

Expansive agents were also added insome of the initial tests. The intent wasto counteract shrinkage and separationfrom the steel sheath. However, due tothe hydrostatic pressure of the 100-fthigh grout lifts, a very undesirable phe-nomenon occurred. The gas producedby the expansive agent -(in this casealuminium powder) was forced out ofthe mix and pressured to the top of thegrout lift. There it collected in large-sized bubbles which, when hydration ofthe grout had occurred, resulted in asection of grout that was of an extreme-ly porous and loose composition.

This portion extended for about 5 ftin height and, though its strength was

Fig. 20. Full-scale testingof grout.

Fig. 21. Closeup of grout testin fills at base.


POSSIBLE CRACKS0M

PROPAGATING 0

-4— .4— P -p.—,, in

DUCT, 4" TO 4 1'2" o24-GA. SHEATH

JQ

EXTRA TIE' \,CONCRETE OF*4 AT 18" VARIOUS AGES

Fig. 22. Hydrostatic grout pressure.

sufficient to sustain the hydrostatic pres-sure of the subsequent grout lift above(about 100 psi), it was completely in-sufficient for structural purposes andcorrosion protection. Consequently, theuse of gas-producing expansive agentswere abandoned and tests without thechemical additive proved satisfactory.

The tests also established that sub-stantial amounts of bleed water ac-cumulates at the top of a lift. This phe-nomenon, however, was much less pro-nounced in the actual grouting of theTower, although the precise reasons forthis are unknown.

Subsequent to the testing, variousportions of the grouted ducts wereopened and inspected. It was found

Table 2. Summary of averagestrengths of two mixes

Water- 7-daycement Flow test, averageratio sec. strength, psi Remarks

0.45 13 5500 Insufficientfilling and wirecoverage;pumpingdifficult

0.53 10 4700 Satisfactory

Note: The average 28-day strength for grout in-jected into the Tower cable ducts was over5000 psi.

that with the relatively viscous groutresulting from a water-cement ratio of0.45, incomplete filling and wire cover-age resulted. In some cases cavities inexcess of 1 in. were present. This wasjudged to be unacceptable and so awater-cement ratio slightly more than0.50 was used and proved satisfactory.The strength obtained from the twomixes was also measured, from 2-in.cubes (see Table 2). It was, as expect-ed, considerably lower for the morefluid mix which was, however, consid-ered a fair exchange for better corro-sion.

A second series of tests was per-formed to investigate possible prob-lems, especially the resistance of theconcrete Tower walls against splitting.With fresh grout inside the ducts, con-siderable hydrostatic pressure would ex-ist.

The behavior of a brittle body underhydrostatic pressure in an enclosed cy-lindrical surface is very complex. It issuffice to say that an analytical methodwhich could predict crack propagationin time and space was judged most un-reliable. Consequently, it was decidedto conduct a series of tests to simulate

104

8'-O'

'B - L I-- °_TEE____---s—__—___—__—___—_—__-- _--__

A

A.

PLANINJECTION

TUBES

A'-4 112"I. D. DUCT17 STRANDS

'B'- 4" I.D. DUCT7 STRANDS

-HANDPRESSURE

PUMP

PLYWOODDIVIDER

SECTION A-A

Fig. 23. Horizontal grout test panel.

the actual conditions in the Tower wallsas closely as possible (see Fig. 22).

A test panel (see Fig. 23) was con-structed and subjected to the longitudi-nal compressive stress that would existin the Tower walls at the time of grout-ing (an average value of 1000 psi waschosen). Reinforcement was providedreflecting conditions in the critical(thinnest) portion of the wall that wouldbe subject to grouting pressures.

The test panel was divided into twoseparate strips by building paper. Oneof the strips was transversely reinforcedto verify the effectiveness of this instopping the propagation of crackswhile the other did not have any tiesacross the probable splitting region.

The inside of the ducts was pres-surized with water and tested to de-struction (splitting of the concrete).The splitting pressure varied from 300to 509 psi. It was found that transversereinforcement effectively stoppedcracks from extending over longer dis-tances.

Based on these results the following

provisions were introduced into the de-sign and construction procedure:

1. Specify a maximum height of groutlifts not exceeding 100 ft. (This height canbe pumped with a maximum pressure atthe pump of little over 100 psi.)

2. Provide a pressure relief valve at theinlet point which was to be set about 100psi to prevent accidental overpressuriza-tion. (This relief valve was, however elim-inated when grouting was executed afterabout 1 year when the concrete of thewalls had reached its final strength.)

3. Furnish transverse reinforcing steel ofone No. 4 bar at 18 in. spacing for everycable.

As a result of these procedures nosplitting of the wall concrete was ob-served during construction.

Procedure and observations—Thegrouting operation was based on theprinciple of hatching the grout near theinlet elevation for each lift. This en-tailed the use of a vertically movableplatform to support the grout mixer,cements, water, and other materials.

The vertical grout test (describedabove) had shown that there was a pos-


sibility of substantial problems from ex-cess bleed water. Accordingly, a doublevent system with a 5-ft vertical separa-tion between outlet and inlet vents wasadopted. The idea was that up to 5 ftof bleed water could be drained backout after each lift. The total quantity offree water in the ducts would thus staywithin reason.

In practice, however, the bleed waterwas relatively small and was not cumu-lative with respect to successive lifts,i.e., they did not vary markedly withthe length of cable.

The top of each cable was normallyprovided with tall, large diameter,standpipes to collect the bleed waterand to allow the accumulation of goodgrout under the bleed water.

The shortest cable group (180 ftlong) showed bleed quantities equiva-lent to between 5 ft (in one case) and14 ft of water in the ducts. The longestcable group showed similar quantitiesof bleed water. (Note that this observa-tion appears to contradict experiencereported in the literature by MorrisSchupack.)

A variety of investigations were madeduring the grouting to confirm this pe-culiar occurrence. The conclusion wasthat the large amount of segregationexpected was not, in fact, occurring. Noproven explanation of this was actuallyfound. However, the speculations arebased on the following:

1. The extraordinarily dry condition ofthe ducts and strands prior to grouting wasnot foreseen or replicated in the initialtests. The dryness resulted from the appli-cation of a "dry air" corrosion preventiontechnique throughout the winter preceed-ing grouting.

2. The water transport mechanism as-sumption presumes that free water at thebase of a grout column can infiltrate be-tween the outer wires of the strand. It canthen rise upwards in the space betweenthe outer wires and the core wire driven bya hydrostatic head differential due to the

difference in specific gravity between waterand grout. It is assumed that the suspendedcement particles are too large to get intothis space.

To prevent this mechanism from takingplace would require that the space be-tween the outer wires is either reducedto the point that it clogs quickly with ce-ment particles or increased to the pointwhere it is fully infiltrated. Since the spacebetween wires does, in fact, become smal-ler when strand is stressed and since pre-viously reported tests seem to have beendone with unstressed strand, the formerexplanation seems more likely. Further-more, the constant quantity of bleed waterirrespective of cable lengths would seem toindicate that, after a certain amount ofwater escapes, the space becomes cloggedand no more water can be transportedaway.

It is, therefore, concluded that thewater transport mechanism, that hasbeen found by others and that was al-so confirmed in our own tests, probablyhas a limited capacity to remove waterfrom a grout column. Since the mix re-maining is comparable to that used ingrouting most structures, it is felt thatthe lack of apparent bleed water hasnot been detrimental to the structure.

Corrosion protectionGrout was to be used as the main agentto protect the strands against corrosion.However, since most of the cableswould have to be placed during thewinter, cold temperatures would haveprecluded proper setting of the grout.This left the tendons unprotected andthus a means had to be found to pro-tect the strands against corrosion.

The use of alcohol to fill the ductvoids and various types of coatingswere investigated but were found to beimpractical or insufficient.

The method chosen was a dry-air sys-tem that eliminated moisture until thegrouting could be done in the summerof 1974.

106

The specification required that theequipment supply 400 cu ft per min, ofoil-free dry air at ambient temperature,but with a dew point temperature of— 100 F (— 55 C). Standard componentswere used, which included two com-pressors coupled to two heatless dryingsystems and filters. All this equipmentwas housed in a 40-ft insulated traileradjacent to the job site.

The system operated as follows: theair is compressed and then passedthrough a filter to remove oil and otherparticles whereupon the air is againcompressed and refiltered. It then pass-es through the final dryer where the re-mainder of the moisture is removed. Ac-tivated alumina is used as the dryingagent.

A large-diameter polyethylene pipeconnected the air-drying system to thebase of the Tower, where smaller pipes

connected the main pipe to the lowestgrout vent in each duct. All intermedi-ate grout vents were closed off, an op-eration taking 2 weeks, since all workhad to be performed from swing stages.At the top of each duct, a fiberglass ormetal cover was placed over the ex-posed anchor head.

The extremely dry air was thenpumped into the bottom end of eachduct. A measured orifice at the top ofeach duct ensured up to three completeair changes per hour. When eventuallythe fiberglass covers were removedfrom the anchor heads and strand, thesteel was found to be uncorroded. Onthis basis, it was concluded that the en-tire length of cable was likewise un-corroded because the top end is wheremost moisture would gather, thus in-ducing corrosion.

Auxiliary Stressing of Anchorage Zone

At the top end of the wing sections (co-inciding with the base of the upper ac-commodation levels), three conditionsarose:

1. The top anchorage of the largestgroup of post-tensioning units [3 x 10cables at 700 kips (318 tons) nominal stres-ing force] was to be anchored at the ex-terior edge of the wing sections at a dis-tance of about 10 ft from the centralhexagonal part of the Tower. However, atthe time of stressing, the central Towerwall would not carry substantial loads.Hence, a situation existed where highstresses would be introduced at the outsideedges of a multicellular truncated tower(see Fig. 24). This constituted a large-scaleanchorage zone with high local tensilestresses.

2. Later, during construction the stresscondition would reverse itself. Higher loadswould be concentrated in the central por-tion due to the weight of the upper part ofthe Tower walls in addition to the stagepost-tensioning (see Fig. 25).

3. The erection and concreting of thesupporting bracket walls for the upper ac-commodation levels resulted in substantialhorizontal loads on the same portion of theTower shaft (see Fig. 26), before the sub-porting ring beam would be post-tensionedand act as the final lateral support of thecantilever walls. This, in effect, again re-versed the stress condition.

All three conditions produced highlocal radial tensile stresses. The transferof these stresses to the central hexagon-al cell had to occur within the boundsof the slipformed Tower walls. Hence,the need for prestressing was indicated.

A system of stress bars was selectedthat allowed the placing and stressingof the tendons in stages. In the firststage, the stress bars were installedwithin the slipformed walls to counter-act the tensile stresses due to anchorageof the Tower post-tensioning cables. Inthe second stage, coupling was addedas part of the scaffolding for the upper


Fig. 24. Stage 1 loads concentrated at Fig. 25. Stage 2 loads concentrated in outside edges of truncated Tower. Note central portion of truncated Tower. Note that shaded areas denote tensile stress. that shaded areas denote tensile stress.

Fig. 26. Stage 3 horizontal loads

from bracket walls. Note that shaded

areas denote tensile stress.

Fig. 27. Arrangement of stress bars for lateral support.

accommodation. This provided the re-quired temporary lateral support for theweight of the concrete walls (see Fig.27).

Stressing was carried out for eachstage as soon as the concrete reached asufficient strength. Since completion,no cracking in the concrete has beenobserved.

Ring Beam (UpperAccommodation)

The main lateral containment of the up-per accommodation structure is formedby a post-tensioned concrete ring beamat the top of the bracket walls (see Fig.28).

The ring beam acts within the floorof the second level (outdoor observa-tion) and was constructed as the lastelement of the concrete base structurefor the "pod," in the 1974-1975 winter.

fhe main function of the ring beamis to resist direct tension with only min-or bending due to the weight of thefloor it supports. Consequently, thepost-tensioning was arranged in a con-centric and symmetrical manner.

The ring beam forms a twelve-sidedpolygon, reflecting the radial loads itwill withstand. Its final post-tensioningforce, after all losses, was estimated tobe 2400 kips (1090 tons) and the ulti-mate tensile strength of the cables wasspecified as 4200 kips (1900 tons). Thisforce is arranged in twelve cable units,consisting of 49 wires each, with an ul-timate tensile strength of 241 ksi (170kg/mm2), so that in every section ofthe beam six units are present.

The most delicate design problemwas to arrange the tendon units in asymmetrical pattern all around, includ-ing all stress transfers due to friction atstressing. All anchors had to be locatedtowards the inside of the beam. This re-

Fig. 28. Layout of tendons for ring beam.


suited in a rather intricate placing pat-tern which is shown schematically inFig. 28.

Each cable is placed in identical posi-tion, reaching around 180 deg, andstaggered every 30 deg. All 24 anchorswere stressed sequentially to ensure theclosest approximation of symmetry atall times. Stressing elongations werefound to be very uniform, with a maxi-mum variation of about 5 percent on anaverage elongation of 10 1/z in.

Closing Remarks

The CN Tower, with the exception ofsome portions designed in structuralsteel, is a fully-prestressed concretestructure. Essentially, all prestressing(or post-tensioning) is arranged con-centrically and designed to disallowtensile stresses.

Thus, full prestressing has been pro-

APEX OFTANGENTIAL

CONE 4RESULTANT OFLATERAL LOADS

II , It ABOVE SECTION

DIRECTION1 IX-X

OF STRESSES I I

(V=O) I

x--II--- --- x

Fig. 29. Idealized Tower stresses.

vided within working load levels (asdefined by estimates of dead and liveloads and statistical 50-year windloads). For loadings in excess of theselevels, the structure will become partial-ly prestressed and tensile stresses willoccur in certain portions of the con-crete.

All prestressing is essentially uniaxial.Therefore, the above statements mustbe qualified since tensile stresses willalso occur at working load, transverseto the main stress direction. Most ofthese transverse stresses are, however,quite small.

For example, the shear stresses dueto wind are greatly reduced because ofthe tapering shape of the Tower. Theshape approaches the idealized shellstructure shown in Fig. 29, which fora lateral load resultant coinciding withthe apex of the tangential cone, doesnot produce shear stresses.

Equilibrium to the exterior lateralload is then held by the horizontal com-ponents of the wall stresses which aredirect (normal) stresses but have an in-clination equal to that of the wall.

Other sources of tensile stresses exist,and reinforcing steel was providedthroughout for the effects of local bend-ing, shrinkage, temperature, torsion,and other stress conditions.

Fig. 30 shows the CN Tower in Mayof 1975.

Credits

Owners: CN Tower Ltd., Toronto, On-tario.

Architect: John Andrews International,Roger du Toit; the Webb ZerafaMenkes Housden Partnership, Toron-to, Ontario.

Structural Consultants: Nicolet, Carri-er, Dressel and Associates, Limited,Montreal, Quebec.

Manager Contractor: Foundation Co.of Canada, Toronto, Ontario.

110

Foundation Post-Tensioning: ConencoCanada Ltd., Toronto, Ontario..

Tower Shaft Post-Tensioning: VSLCanada Ltd., Stoney Creek, Ontario.

Auxiliary Stressing of Anchorage Zone:Supply: Dywidag Canada Ltd.,Toronto, Ontario.Construction: VSL Canada Ltd.,Stoney Creek, Ontario.

Ring Beam, Upper Accommodation:BBR Canada Ltd., Toronto, Ontario.

Acknowledgment

The authors wish to express their sin-cere gratitude to all participants in theCN Tower project for their help andcollaboration in preparing this report.Particular thanks must go to FrankTam, resident structural engineer, fordocumenting the pertinent records dur-ing the construction period.

A project of this magnitude can onlybe built when a very congenial and un-derstanding atmosphere exists betweenall participants. This inspiring collabor-ation will long be remembered by theauthors. Lastly, one important ingredi-ent for this smooth teamwork was theowner's generous cooperation, withoutwhich monumental structures of thistype cannot be successfully built.

A heartfelt thank you to all! Fig. 30. CN Tower from water-front (May 1975).

Discussion of this paper is invited.Please forward your comments toPCI Headquarters by October 1, 1976.


prestressing the cn tower journal/1976/may-1976/prestressing the cn...pects that governed the design...

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