Practical Experience in theDesign of Anchorages forCable Stayed Bridges

Juan B. RipollConsulting EngineerBarcelona, Spain

SynopsisThis paper traces the development

of a new cable stay anchorage, in-cluding the initial test program andsubsequent design solution. It alsodescribes the anchorage's first com-mercial application on Spain's longestcable stayed structure, the Barrios deLuna Bridge.

The practice of supporting bridges bycable stays has found renewed favor

during the past decade. Advantages of-fered by the recent free cantilever con-stnsction method are outweighed by es-callating costs when spans approach 650ft (200 m), thereby creating economicjustification for the single or twin towers

of a cable stay alternative.The principles underlying prestress-

ing can equally be extended to cablestayed bridges, rendering them into pre-stressed structures with external cableswhether the structure is of concreteor steel. Such bridges are now provingviable for spans exceeding 1500 ft (450m).

Several authoritative publications,such as the books by Leonhardt,'Podolny and Muller,' and Walther et a!,3and the recent PTI report,' have givenconsultants and contractors valuableguidelines for designing, specifying andconstructing cable stayed bridges.

At the recent FIP Congress in NewDelhi (February 1986) many noteworthypapers on cable stayed projects werepresented, indicating that progress inthis field is being attained worldwide.In these projects a wide variety of an-

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Fig. 1. Barrios de Luna Bridge — Aerial view of nearly completed structure.

chorages were specified for differingstay forms by various post-tensioningsystems.

The cable stay advantage was amplydemonstrated by a highway bridgewhich opened in 1983 in NorthernSpain. The Barrios de Luna's 1443 ft(440 m) center span is the longest con-crete deck yet constructed, with 324 ft(98.74 m) approach spans on either side.The bridge was designed by ProfessorJavier Manterola of the Madrid Instituteof Roads Engineering (see Figs. 1through 5).

A total of 110 pairs of stays support the74 ft (22.5 m) wide hollow-box pre-stressed deck from two 328 ft (100 m)towers. Coordinating erection of thestays with a well planned and tightlyscheduled concrete placement programenabled the stay erection to he com-pleted within one year, despite frost and

heavy snowfalls throughout the 1982-83winter.

This record construction rate was thedirect consequence of combining stayerection with their on-site assemblyfrom 0.6 in. (15.24 mm) diameterstrands; up to 70 in number, the longestbeing 722 ft (220 m).

The performance specification for thestays was very exacting, in keeping withsuch structures generally where light-ness of the deck exposes the stays towide-ranging loads of great amplitude.Design of the anchorage and positive in-stallation of the stays is, in consequence,fundamentally important to guaranteethe integrity of the bridge throughout itsprojected life.

The conflicting variables of achievingsafety in the presence of pronouncedstress reversals was the subject of closestudy. The following pages summarize

PCI JOURNAIJJuly-August 1986 95

on— , (C 7 n

EiO

001

-u 37'(II.25m)

c== E

LO

13' 24'(7.28m.)(3.97 m.1

TYPICAL CROSS SECTION

Fig. 2. Elevation and typical box girder cross section of Barrios de Luna Bridge, Spain.

developments achieved in order to finda suitable, reliable, and functional stayanchorage.

Selecting the StayIt seemed likely in this particular case

that any tangible saving over existingpractice would necessitate moving awayfrom prefabricated stays. At least thiswould engender some economy intransport, and probably save building anerection plant. The expense of repair-ing prefabricated cable sheaths, occa-sioned by unavoidable damage to suchbulky coils in transit, might also beeliminated.

Prestressing strand became the choiceof alternative steel that could be site as-sembled. Its fabrication into easilyerected stays imposed only a few extraproblems which were resolved withoutmuch difficulty.

Selecting the AnchorageIt seemed equally likely that job site

fabrication might offer opportunity forimproving the stay anchorage, and thisproved to be the case. By stressing thestay before filling the anchor socket withcompound, the structure's dead loadwill be borne by the wedges/compres-sion grip restraining each component

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Fig. 3. Box girder cantilever with moveable forms.

strand, leaving the socket filler com-pound when subsequently injected tosupport live load. This distribution offorce would be more satisfactory for therange and amplitude of stress reversalsthan occurs with a fully prefabricatedstay whose socket filler must absorbboth dead and live load as soon as it isbuilt into the stricture (Fig. 6).

This division of function could allowthe anchorage to be simplified and itssize and cost reduced.

Exacting requirements of the specifi-cation dictated the need for a distinctiveanchorage form which was selected inthe following way. The simplest choicewould be a conventional anchorage with

mechanical wedges in an anchor plateattached to a tubular transition elementand housing a ring deflector at the op-posing end (Fig. 7). All of these ele-ments are injected with cement grout.

Lack of cement bond at the interfaceA-B, between the plate and tubular ele-ment, is a fundamental weakness of suchan anchorage. If A P is live load, and Pdenotes dead load already supported bythe strands at the time cement grout isinjected, the length M-N of strand insidethe tubular element, including that ofM-Q within the anchor plate, creates atransfer length which elongates, pro-ducing a crack-like separation of theplate and grouted section along the

PCI JOURNAL/July-August 1966 97

Fig. 4, Soffit view of completed span.

plane A-B.With this configuration only bolts S

connect the plate and grouted section,and these could shear under cyclic load.For a fatigue load of 29 ksi (200 Nlmm2),interface separation might exceed 0.008in. (0.2 mm) allowing ingress of mois-ture into the anchorage's most sensitivepart. This would threaten the securityof the wedges by corroding thestressed strands and the anchoragecould be permanently damaged. Simi-larly, boles in the anchor plate offer apath for moisture to intrude into thewedges.

Vibration at the interface A-B, adja-cent to the wedges, raises doubts as totheir permanent seating. The possibility

of strands breaking under load causesfurther apprehension in that wedges re-straining cement grouted, post-tensionedtendons have been known to loosen dueto the shock generated. This fear wouldbe heightened where the anchored forceand the resultant wedging effect is lim-ited to 40 or 45 percent of the strand'sultimate strength. The use of keeperplates would not guarantee wedges re-seating if encrusted with grout.

These well-founded fears reinforcedthe belief that anchorage componentsand socket filler must be shielded frominterface movement, and be designed toensure that static load is supported byanchor elements and dynamic load ab-sorbed by the filler.

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Fig. 5. Stay anchorage beneath deck flange.

Performance of WedgesUnder Cyclic Load

Wedges used for conventional pre-stressed concrete are designed to sup-port loads ranging from 92 to 100 per-cent of ultimate tendon strength, as de-termined by national codes of practice.Such wedges usually have conic anglesgreater than the mating holes of anchor-age plates or barrels, and possess teethrelieved of incisive edges at their load-ing (narrow) ends, thereby ensuringhigh efficiency up to ultimate load (seeFig. 8a).

Numerous tests have been made inGermany to identify wedges capable ofsupporting cyclic loads of range,

amplitude and duration known to occurin cable stayed structures. From thestudies of Nurnberger,5 Rehm,s Pat-zack,' and Kohler," it can be demon-strated that a reversal of these matingangles and the resulting stress concen-trations occurring at M and N, will pro-duce a very high efficiency in cyclicterms provided a sharp tooth profile oc-cupies the full wedge length (see Fig.8b).

The sharp tooth profile which servesto prevent interface movement creates,in turn, a major disadvantage by bitingthe strand to a degree that drastically re-duces ultimate load efficiency from 100percent to approximately 70 percentwhen compared with conventional

PCI JOURNALJuly- August 1986 99

00ANCHORAGE

STAY

PREFABRICATED ANCHORAGEEPDXY SOCKET FILLINGCOMPOUND INJECTED BYTHE FACTORYACTOR

STAY LENGTHALL FORCES ACTING ON THE STAY ABSORBED BY THE SOCKETRILING COMPOUND. (MECHANICAL ANCHORING COMPONENTSHAVE NO PRACTICAL FUNCTION)

P+AP MAXIMUM FORCE WITHINTHE STAY.

ONLY CYCLIC LOAD( P)1S ABSORBED BY P+GP=MAXIMUM FORCE WITHINTYPE B THE SOCKET FILLING COMPOUND THE STAY.ANCHORAGEINJECTED ONHT E JOBSITE USI ::

.....PATENTED PROCESS

DEAD LOAD ( P) IS FULLY ABSORBED BY THE + P = DEAD LOADMECHANICAL WEDGES

TRANSFER LENGTH

EFFECTIVE STAY LENGTH AFTER CYCLIC LCAD USING TYPE B ANCHORAGES

LOAD ABSORBED BY THE SOCKET FILLING COMPOUND

Fig. 6. Diagramatic comparison between job site assembled and prefabricated stays.

d

WE

P+AP

Fig. 7. Conventional anchorages with mechanical wedges.

RELIEVEDTEETH

SHARP TEETHalong the wedge

Fig. 8. Alternative configurations of mating angles for welded anchorages.

PCI JOURNAL/July-August 1986 101

rT TJ 7 TT WedgeF765 I X15 I 2^ Q1 Q ^ Q ^ ^ — Q

1 2 15 50

# I I wire '♦

9F p

50 , Wedge bite 15. Wedge bite 2. Wedge bite

Fig. 9. Typical wedge bite and induced fretting pattern.(Courtesy of Prof. W. Kohler.)

TYPE B ADJUSTABLE ACTIVE ANCHDRM,E 4*ADJUSTABLE NUT

INO PLATE S

PLATE

ANCHOR SOCKET

CEMENT GRG

COMPROF

Fig. 10. Type B anchorages.

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Table 1. Results of fatigue tests with 270 K, 0.6 in. diameter strand.

Specimenmark

Cross-sec-tional area,

mm,

Minimmiistress,MPa

Maximumstress,MPa

Range ofstress,MPa

Fatiguelife,

cycles Remarks

1 143 540 780 240 2,341,0001 540 980 440 147,000 t2 143 540 840 300 3,135,0002 540 980 440 138,000 t3 143 540 890 350 259,000 t4 143 540 840 300 2,280,000 '4 540 980 440 161,000 t

*The specimen did not fail.tOne wire of the strand failed in the free span between the saws ofthe testing machine.Note: 1 in. = 25.4 mm; 1 in.' = 645 mm'; i psi = 0.006895 MPa.

wedges. Therefore, if the stay is to workwithin 45 percent of ultimate strength,whereas the wedge efficiency is limitedto 70 percent, its security is reduced toan unacceptable safety factor of 1.5.This, in itself, was a valid reason foravoiding the use of mechanical wedgesin the cable stay anchorages of Barriosde Luna Bridge where 100 percent effi-ciency was required.

The inability of wedges profiled forultimate efficiency to perform as capa-bly in cyclic terms, is due to interfacemovement at their leading ends wherethe teeth have been progressively re-lieved. Denied the restraint of sharpedges, they generate "fretting" of thestrand in the region of M in Fig. 8a,causing wires to break. "Fretting" pro-duced by wedge teeth can be clearlyseen in Fig. 9. A mechanical anchoragerelying only on wedges appears not tobe the best answer for cable stays since itfails to satisfy the conflicting criteria offatigue and efficiency response. Supple-mentary measures are, thus, necessary toguarantee structural integrity.

Type B Stay AnchorageIt was decided to design the anchor-

age to embody features that satisfiedthese performance parameters. In this,system, dynamic load AP is fully ab-

sorbed by the socket filling compoundwhich is comprised of epoxy resin, zincpowder and steel shot.

This greatly increases safety by limit-ing the wedges to supporting dead loadP. Both P + A P are then transferred tothe bridge through the socket. Crackscannot appear at the anchor plate andgrouted socket interface. Fig. 10 showsthe major features of active and passiveanchorages in the longitudinal section.

Interface Between Stay, Groutand Socket Filler

The interface between stay grout (ce-ment or otherwise) and epoxy-basedsocket filler will always gravitate to ahorizontal ellipse as shown in Fig. 10.Any shear developing along this planewill be contained if the interface ismainly confined within the socket andits tubular extension.

Test Results on Type BAnchorages

Anchorages for the Barrios de LunaBridge were tested at the Otto Graf In-stitute in West Germany and will bedetailed later in the paper. Other testsaffording similar results were made atMunich University in the same country,and at the EMPA in Zurich, Switzer-

PCI JOURNAL/July-August 1986 103

Table 2. Test results of 0.6 in. diameter wires.

Yield Upper Tensile Modulus ofSection point load strength elasticity Yields

Specimen S Rm FR, R,,, F, A,number torn, N/mrn kN N?mm2 kNlmm2 percent

1 143 1692 267.4 1870 193 4.62 143 1692 266.8 1866 192 4.43 143 1689 267.9 1873 193 3.7

Mean 143 1691 267.4 1870 193 4.2

Note: 1 in. = 25.4 min; 1 ft = 0.305 m.

Table 3. Prestressing cable elongation during continuous cyclic testing.

Lengthvariations'

Elongationsfrom F. to F.

Elongationincreaset

Prestressingcable effective

lengthF,, F.Number All ale at = at; - Al i , aAl dof cycles inm mm min mm rn

0 0 3.06 3.06 - 3.36418,000 0.06 3.20 3.14 0.08 3.44868,000 0.07 3.20 3.13 0.07 3.43

1,565,000 0.05 3.20 3.15 0.09 3.452,000,000 0.02 3.20 3.18 0.12 3.49

'Length variations with Fa lower load in the beginning of the cyclic continuous teat (N -0).tTaken as elongation in the beginning of the test (N=0).Note: 1 in? = 645 nun'; 1 ksi = 6.895 Nimm'; 1 kip - 4.45 kN.

land. Strands to provide high perfor-mance cyclic loadings of ASTM 270 Kgrade, were used for test specimens.

A fatigue performance of 26 ksi (180N/mmi ) in the anchorage called for astrand of 35 ksi (240 N/mm 2 ) guaranteedminimum fatigue performance to allowfor the reduced anchorage efficiencyanticipated. Table 1 gives test results onthe strand to verify its suitability. Table2 exhibits other pretest strand data.

An 1810.6 in. sample stay typifying ajob site specimen was assembled, thenexposed to continuous cyclic loadingwithin the specified parameters:

Lower load F.= 540 x 18 x 140 x 10-s= 1361 kN (306 kips)

Upper load F.

=720x18x140x10 -3= 1814 kN (408 kips)

Amplitude 2 = 180 Nlmma (26 ksi)which was obtained from the nominalsection of the strands.Frequency n = 2.67 HzMaximum number ofcyclesN = 2x106

The test rig with mounted specimen isshown in Fig. 11.

Elongation increased during the test.After the first load cycle it was 0.12 in.(3.06 rnm), which, applying Hooke'sLaw, gives an effective length of:

E(AL)SL = (Fo - F„) 25.4

193 x 3.06 x 18 x 143(1814 - 1361) 25.4

= 132.28 in. (3.36 m)

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The elongation after completing 2 x10 13 cycles had increased to 0.125 in.(3.18 mm), corresponding to a correctedeffective length of 137.40 in. (3.49 m).The overall increase compared with theunstressed strand was 5.12 in. (130 mm)(see Table 3).

Since the measured distance betweenstay anchor plates was 155.91 in. (3.96m), this afforded evidence that thesocket filler prevented cyclic load fromreaching the wedges.

Ultimate Load Testingand Results

The specimen stay was loaded to fail-ure after completing continuous cyclictesting. Effective free length of strandsgrew with load increase, measuredelongations being computed utilizingHooke's Law. Using relative values ofdiameter and elastic modulus, it wasevident that effective free length wassignificantly lower than the distancebetween the shims and sockets for val-ues less than 0.8 of the nominal breakingload. (Since the 0.8 nominal breakingload is approximately equal to the strandyield strength, R, = 0.01, it may be con-cluded that Hooke's Law is correctlyapplied to this load.)

Strand displacement in the socketproved to be insignificant, owing to ad-equate resistance by the epoxy resinfilling. No slipping of the strands wasdiscernable up to breaking.

The 4728 kN (1063 kips) ultimate loadwas fully applied. All 18 strands brokealmost simultaneously with all rupturesoccurring in their free lengths, Nocracking, breaking or yielding occurredin the anchorages.

Anchorage efficiency obtained bycomparing maximum load applied to thespecimen stay cable with that of eachstrand's maximum load was:

472818x267.4 =0.08

The total strand elongation at rupture

PCI JOURNALIJuly-August 1986

9A

111k

Fig. 11. Stay specimen in testing rig atOtto Graf Institute in Stuttgart.

was 5.71 in. (145 mm). This was only 15percent below the mean yield value forsingle strands established in the Iabo-ratory before running the test (see Table2). Even at rupture the strand lengthembedded into the epoxy filling wassmaller than the stay free length. Thetotal yield in the free length of strandsmust, therefore, be considered asslightly higher. The ratio betweenupper and nominal cable stress was:

F,,, _ 4728 = 1.06FN4660

frilly satisfying the specifications.

Range of Use ofType B Anchorage

In general, it can be stated that theType B anchorage is suitable for a stressamplitude range of about two-thirds thestrand maximum over 2 x 10 8 cycles.

105

REFERENCES

1. Leonhardt, Fritz, Bridges, Aesthetics andDesign, Deutsche Verlags-Anstalt GmbH,Stuttgart, West Germany, 1982.

2. Podolny, Walter Jr., and Muller, Jean M.,Construction and Design of PrestressedConcrete Segmental Bridges, John Wiley& Sons, Inc., Somerset, New Jersey, 1982.

3. Walther, Rene, Houriet, Bernard, Isler,Walmar, and Moia, Pierre, PontsHaubanes (Cable Stayed Bridges), PressesPolytechniques Romandes, Lausanne,Switzerland, 1985, 202 pp.

4. PT! Ad Hoc Committee on Cable-StayedBridges, "Recommendations for StayCable Design and Testing," Post-Tensioning Institute, Phoenix, Arizona,January 1986, 22 pp.

5. Nurnherger, U., "Behavior of StressedSteel in Wedged Anchorages Under StaticLoading" (Verhalten von Spannstahlen inKielverankerungen unter Statischer Be-lastung), Bauingenieur, V. 56, 1981, pp.

25-32.6. Rehm, G., "Wedge and Stay Anchorages

for Dynamically Stressed Tension Mem-bers of High-Tensile Wires" (Kiel andKemmverankerunger fur dynamischbeanspruchte Augglieder aus hochfestenDrahtern), Bauingenieur, V. 52, 1978, pp.287-298.

7. Patzack, M., "Basic Investigations IntoStatic Dynamic Loading Capacity ofMetallic Cable Groutings (Filled Anchor-ages)," IGrundlagenuntersttchungen zurstatischen and dynamischen Belasbarkeitvon metallischen Drahseilvergiissen(Vergussverankerungen)], Mitteilungen,4511978, SFB 64, Universitat Stuttgart,West Germany.

8. Kohler, W., "Fatigue Behavior of Steeland Concrete Construction," Ermu-dungsverhalten von Stahl and Beton-bauten), IVBH, Lausanne, Switzerland,1982.

NOTE: Discussion of this paper is invited. Please submit yourcomments to PCI Headquarters by March 1, 1987.

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