PRESTRESSINGWITH HIGH-TENSILESTEEL STRIPS

R. SaglioCommissariat a l'Energie AtomiqueParis, France

A. PuyoCoyne & BellierParis, France

J. PicautCoyne & BellierParis, France

High stress resistant steel can beobtained by quenching. The effi-ciency of the process depends onthe thickness of the steel. Thethinner the material, the more effi-cient the process becomes. As aconsequence, quenching is equallyefficient on a wire as on a somewhatthinner, but wider steel strip.

Such high-tensile strip exists com-mercially and is available from dif-ferent mills. The range of thick-nesses and widths varies with thesupplier, but 0.04 in. by 10 in. widestrip (0.1 x 25 cm.) is very common,sold in more than 1500-ft. (450 m.)coils. Ultimate stress is above 200,-000 psi (14,000 kg./sq.cm.) and ul-timate elongation is more than 2.5percent. Elastic limit is higher than160,000 psi (11,300 kg./sq.cm.).

The advantage of a steel strippost-tensioning system lies in sim-plified construction due to simpleanchoring devices and in easy ten-sioning techniques. The systempromises to offer advantages espe-cially for the construction of pres-

sure vessels where prestressingforces are very large.

CIRCUMFERENTIAL PRESTRESSING

General. The future of prestressedconcrete primary and secondary nu-clear containment vessels is closelylinked with the availability of highcapacity tendons, because conven-tional tendons are too weak for theoverall forces to which these struc-tures are subjected. Thus, an entirelynew approach to post-tensioning cir-cular vessels was developed.

It might be useful to quote a fewfigures concerning the prestress re-quired as an example. A pressurevessel designed to withstand a ser-vice pressure of 1400 psi (100 kg./sq.cm.), with internal and externaldiameters of 30 and 60 ft. (9 and18 m.) respectively, must withstanda prestressing force of the order of2000 tons per linear foot of cylinder;the cross section of each prestressingring must therefore exert a serviceforce of 10,000 tons, with rings at 5ft. (1.5 m.) on centers.

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This paper describes the potentials and features of a post-tensioningsystem which uses thin, high-strength steel strips as tendons.Its primary realm of application is in prestressed concretepressure vessels. The system can be used with radial jacking orwith a tensioning-by-winding technique. Tests to destruction ofan 800-ton capacity tendon are described in detail.

Preliminary project studies carriedout by the French Commissariat al'Energie Atomique and Coyne &Bellier, Consulting Engineers, Paris,France, have revealed the impor-tant advantages of moving all pre-stressing cable to the outer face ofthe vessel, in the form of indepen-dent high-tensile steel rings which,once tensioned, compress the con-crete wall.

The main advantages of externalrings are:

1. Greater ease of concreting2. Minimum volume of concrete

and reduced external diameter3. Homogeneity of stresses in the

concrete since there are nooverlapping cables

4. No bulky and costly steel bear-ing plates and anchorages

5. Ready access for visual inspec-tion

6. Savings in construction timeand labor

Design and prestressing methods forexternal rings. Tension rings areformed by winding high-tensile steel

strip around the vessel. If coil lengthis insufficient, a second coil is startedbefore the first one is completelywound around the vessel, so thatthe ends overlap, and winding con-tinues with the second coil. Thisoperation can be repeated indefi-nitely until the required ringstrength is reached. It is also pos-sible to wind two or more stripssimultaneously provided they beginand end on different turns. No weld-ing is needed.

Whatever the details of the pro-cess chosen to build the ring, therealways remains the outer end ofthe strip which has to be fixed toprevent it unrolling when tensioned.The best way consists of forcingthe last turn against the precedingones with an elastic force not ex-ceeding an average value of a fewtons per circumferential foot. Theresulting frictional resistance allowsthe extremity of the last turn towithstand the pull of the prestress.

Tensioning by radial jacking. One

January-February 1971ҟ 49

d eta,l . f iB.2

Fig. 1. Schematic view of post-tensioned vessel

method of tensioning the above de-scribed ring is to place jacks bear-ing on the walls of the vessel andto displace the ring radially out-wards. The head of the jack pusheson a bearing plate whose surfaceis curved to limit secondary stressesin the strip. All the jacks of onering act simultaneously. After ten-sioning, the ring has a curved poly-gonal shape. Once the ring is ten-sioned, its tension force is lockedin by wedges (Fig. 2). With thismethod losses of tension are negli-gible and it is easy to check orcorrect ring tension at any time.

Tensioning by winding. This pro-cess is similar to the Preload sys-tem, the difference being that theforce applied to a high-tensile stripis greater than the force applied toa wire. Furthermore, the strip isrolled on itself while wire has tobe wound around like thread on a

reel. In the tensioning machine thestrip is pressed without sliding be-tween pairs of rollers, these rollersbeing braked in order to tensionthe strip to its nominal value. Dueto the winding force each turn ispressed upon the underlying one.The inner end of the strip has tobe partly or totally anchored whenwinding begins. This can be doneby welding or with a friction an-choring device.

Comparison of the two tensioningmethods. Radial jacking allowschecking or readjusting tension atany time, whether by using the setof jacks used for tensioning or byplacing cast iron blocks supportingthe tensioned ring upon a dynamom-eter system. Tensioning by wind-ing under tension is usually moreeconomical since no supports areneeded.

The tensioning systems listed

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poion of the jacks during tensioning ring n° N(Axial cross sect ion X_X

ryҟrina n• N

(b)ҟ ...:

position oFThe jaclts before tensioning.( Axial Cross ' - Section)

(a)

Transverse Cross_ Section y Y_rx

N=N+2

--edg „gring Nil is to be tensioned

ringM42 is tensioned^ wedged.

(c) Lx

Fig. 2. Position of a radial jack under a ring to be tensionedJanuary-February 1971ҟ 51

above are not the only ones possible.The choice of a system dependson the design itself and on the de-sign criteria.

TEST OF A TENSION RING MODEL

General. A brief description of aring test run by the French AtomicEnergy Commission at the MarcouleCentre in France is given. The re-sults have been extensively reportedat the Euratom Conference held inBrussels in November 1969.

The test was primarily aimed atproving that high-tensile steel striphas similar tensile properties asstrand or wire. Other objectiveswere concerned with the ring sys-tem itself:

1. Problems which may arise

from the installation of thestrip

2. Friction anchorage of theouter end

3. Behavior when stress in thesteel exceeds nominal pre-stress

4. Uniformity of jack lift.Cost considerations led to 16 jacks

of 500 tons capacity being used fortensioning. The strip width of 10 in.(25 cm.) was greater than would beused on any prototype. It was as-sumed that any defects and theirpossible consequences would bemore visible with wide rather thannarrow strip of 4 to 6 in. (10 to 15cm.), while the installation and an-chorage of the outer turn wouldbe more difficult.

The test, thus being severe, was

Fig. 3. Model of concrete ring and high-tensile steel strip tension ring

52 PCI Journal

Fig. 4. Radial jack with bearing plate and friction bracket

supposed to reveal any possible diffi-culty that could be encountered withthe use of the high-tensile steel stripsystem.

Test arrangement. The concretering, as shown in Figs. 3 through5, is 2 ft. 2 in. (65 cm.) high. Itsinside diameter is 6 ft. 5 in. (2 m.),with a 0.2 in. (5 mm.) thick lineranchored to the concrete. The outerface is a 16-sided polygon inscribedin a circle of 12 ft. 10 in. (4 m.)diameter.

The sixteen jacks acting radially,fixed in position with bolts embed-ded in the concrete, are shown inFig. 3. Their nominal rating is 500tons, lift of 8 in. (20 cm.), and out-side diameter of 16 in. (40 cm.). Thepiston area is 93 sq. in. (600 sq. cm.).The ram pushes a bearing platewhose face in contact with the stripis a cylindrical surface of radius20 in. (50 cm.).

Fig. 4 shows the exterior bracketplate which is to develop the fric-tion needed for holding, and thereby

tensioning, the outer end of thestrip. This plate is fixed to the bear-ing plate by means of four boltswith spring washers so that a clamp-ing force of about 8 tons can beexerted.

The high-tensile strip material isa 0.75 percent carbon steel, hot andcold rolled and tempered:

Elastic limit = 190,000 psi (13,-500 kg. /sq.cm. )

Breaking strength = 225,000 psi(16,000 kg. /sq.cm. )

The strip is 0.04 in. (1 mm.) thickarid 10 in. (25 cm.) wide with slitedges, and is supplied in coils of14 in. (35 cm.) internal diameterand 28 in. (70 cm.) external diameter.Each coil weighs about 1350 lbs.(600 kg.) and is sufficient for about18 turns on the concrete ring.

The strip was simply installed bycarrying the coil around the con-crete ring, imposing a slight tensionon the strip during unwinding. Theprofile obtained was a regular cur-vilinear polygon.

January-February 1971ҟ 53

Fig. 5. Schematic view of tension ring tested to destruction

Execution and evaluation of the test.The test was divided into two partsbecause of the limited capacity ofthe jacks. In the first phase, the ringwas made up of two coils of stripas described above, making a totalof 37 turns of strip forming the ring.The maximum force applied to thering reached 925 tons, i.e. a meanstress of 142,000 psi (10,000 kg./sq/cm.), which is within the elasticrange of the steel used.

During the second phase, the ring

was made up of 18.5 turns, i.e. usingonly one coil. In this case it waspossible to carry the test to failurewith the jacks available. The forceapplied to the ring at failure was750 tons. Measurements were madeby strain gauges bonded to the innerand outer faces of the steel ring.Readings were compared withstresses deduced from the forces ap-plied by the jacks and from dis-placements.

The friction force exerted on the

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CAC

-nCD

c'DCK

V

15

B

M N

FIRST TURNҟ 2nd!TUQN

.^- 710T ( 18.5turnsIҟ i.3^

ҟ

^J^– –t 5 ( r` 7

N umber of jacks 16radial force applied on

i the bracketsw

iҟ I

/ --5ymbollzed curvelplastic range (ring force 710 T } 4^

' —Symbolized curve elastic range (ring force • 92

ҟ

^y Symboled theoretical curve for f.0.1—^2Theoretical curve for f = O1--1r I i i

Friction ibracbe#s Numbers2 3 4ҟ5ҟ6ҟ7ҟ8 9 10 11ҟ12 13 14 15 16 17 18 19ҟ 1

1 2 3 4 5 6 7

Fig. 6. Number of friction brackets vs. ring tension stress

Fig. 7. Stress-strain curve for test of ring to failure

Fai/u^m MY IɁ- Chap 67n".

3\ \\\\\\\\\\\\\\1\\\ F ,I.. n* to o,, n- 17 yap 3 //.

Fig. 8. Failure mode of steel strip ring

outer turn by the clamping brackets(Fig. 4) can be calculated by count-ing the number of brackets on whichsliding occurs and by knowing thetensile stress prevailing in the ring.This relation is illustrated in Fig. 6for a coefficient of friction, f = 0.1.The curves show theoretical and ex-

perimental results.The strain gauges showed that

tension in each cross section of thering between adjacent jacks was uni-form in the elastic range. Due toplastic deformation near failuresome irregular stress distribution be-tween jacks developed. In general,

January-February 1971ҟ 57

however, it can be stated that withinthe test elastic range (37 turns), cor-relations between the forces appliedby the jacks and those deduced fromcalibrated strain gauges were verygood.

Concerning the test to failure(18.5 turns), the results are sum-marized in Fig. 7. This graph showsthe stress-strain curve derived fromring force and displacement mea-surements, and the curve obtainedfrom the calibrated strain gauges.The first curve lies approximately5 percent above the latter, whichcan be explained by the frictionbetween jack rams and cylindersinduced by uneven lateral displace-ment. Jack friction is negligible onlywhen the jacks act perfectly radial.

The ultimate elongation was 2.8percent whereas 5.5 percent was ob-tained on samples. This discrepancyis similar to results obtained withother post-tensioning material.

Failure was progressive. Fig. 8shows the two points where the stripfailed. No failure occurred else-where.

STRIP TENDONS

The use of high-tensile steel stripis not restricted to tension rings.The following outline deals withtheir application to tendons. A ten-don differs from a ring by the factthat it has two ends. The majorproblem, therefore, is how to an-chor the individual strips. Theoryand experience prove that if a cableis clamped externally, the cable'sultimate load decreases because ofthe clamping force. It appears,therefore, advantageous to use afriction anchorage device which hasno notching effect on the strip ma-terial.

One possibility of making a ten-don out of high-tensile strip is shown

Fig. 9. Strip tendon with jacking heads

in Fig. 9. The semi-circles act asanchoring heads and can be jacked.This arrangement might be advan-tageous when the linear portion canbe located outside the concrete.

A U-shaped cable can be developedfrom unequal lengths of strip asshown in Figs. 10 (a) and (b) . Thetwo free ends of the cable are inter-locked around a half-cylinder likeplaying cards and anchorage is en-sured by interface friction, which canbe increased by applying a forceradially to the cable.

Another way of anchoring thestrip ends is shown in Figs. 11(a)through (d). The principle consistsin forcing each layer of strip, A,against an adjacent layer or a fric-tion plate, B, so that their load istransferred by friction. The frictionplates can be stacked in the sameway as the layers of strip and asingle normal compressive force, N,can anchor as many strip layers asfeasible.

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(a) (b)

The friction plates can be splitup longitudinally into several ele-ments to give an anchorage ofsmaller transverse area but greaterlongitudinal length, Fig. 11 (d). Inthis case, the friction, plates are sup-ported on the shoulders of the mildsteel box. The box walls havethreaded holes for bolting on a thicksteel cover plate forcing the wholeassembly together by means of anelastic device precompressed witha given pressure

In general, it can be stated thatstrip tendons differ from other ten-dons in that safe and compact an-chorages can be provided for high-

capacity tendons of several thou-sand tons. Such friction anchorages,in addition to their compact size,allow applying a corrosion protec-tive coating to the strip steel beforeit leaves the mill, which will elimi-nate the need for additional corro-sion protection of the tendon at thesite.

CONCLUSION

It has been shown how high-tensile steel strips can be used inpost-tensioning, either as tensionrings or as tendons. Their assetslie primarily in simple anchorage

Fig. 10. Strip tendon with lapped spliceJanuary- February 1971ҟ 59

a

INA g

B

(a)

Fig. 11. Friction-type anchorage devices

devices due to the large area avail-able for developing friction withoutnotching.

In continuous strips, as used fortension rings of post-tensioned pres-sure vessels, the anchorages are

(d)

eliminated entirely. This and thepossibility of tensioning by radialjacking or by winding make high-strength steel strip tendons an effi-cient system for post-tensioningpressure vessels.

Discussion of this paper is invited. Please forward your discussion to PCI Headquartersby May I to permit publication in the May-June 1971 issue of the PCI JOURNAL

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