historical perspective on prestressed concrete journal/1976/septem… · historical perspective on...

Historical Perspective onPrestressed Concrete

David P. BillingtonProfessor of Civil EngineeringPrinceton UniversityPrinceton, New Jersey

The idea of prestressing, a prod-uct of the twentieth century, an-

nounced the single most significantnew direction in structural engineer-ing of any period in history.

It put into the hands of the design-er an ability to control structural be-havior at the same time as it enabledhim—or forced him—to think moredeeply about construction.

Moreover, the idea of prestressingopened up new possibilities for form.Ultimately, it is the new forms thatinfluence the general culture, andbecause these forms are visual wecan expect visual artists to be thefirst to sense a new direction.

Characteristically it was LeCor-busier, the most artistic of the greattwentieth century architects, whofirst announced the new idea dra-matically when, towards the end ofhis highly regarded Radiant City,written in 1933, he reported:'

I hadn't seen Freyssinet for years.Then he reappeared and told meall about the precise and very de-

manding research project in whichhe had been totally absorbed allthat time: the discovery of a newmaterial entirely different from anyother already in existence, five orsix times more resistant than thecements and steels now in use.LeCorbusier then quotes his friend

Eugene Freyssinet, speaking of hisdiscovery of prestressed concrete:

I reached my goal. So now I'mlooking around to see what I canuse this discovery of mine for. Andin my opinion, modern society needshousing, parks and highways.LeCorbusier responds to this pro-

gram by expressing his awe of theengineer:

What admirable powers of divina-tion in this man of science, of pre-cise and audacious calculations! Ata single glance—in three words—he summed up the whole programof the modern age. Into that oneshort sentence he has crammed avast wealth of poetry, of lyricism, ofsolidarity, of concern for mankindand the hearts of men.

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SynopsisThe author, who speaks fluent French andFlemish and spent some of the post World War IIyears in Europe studying engineering, presentsan essay on the origins and development ofprestressed concrete.Three men are singled out for having had themost profound influence on the development ofprestressed concrete—Freyssinet, Magnel,and Finsterwalder.Unquestionably, it was the painstaking pioneeringwork of Freyssinet that convinced theengineering world of the viability of prestressedconcrete.Throughout Freyssinet's life, there is one themethat keeps recurring time and again, namely,"a simplification of forms and an economy ofmeans."Magnel is noted as a great teacher and forcommunicating his ideas on prestressing to theEnglish-speaking world.Finsterwalder pioneered the development of thedouble cantilever method of bridge construction.In retrospect, the author regards the principleof prestressing as the single most importantconcept in engineering history.

The beginning of a new way ofbuilding does not usually bring forthsuch a florid outburst. Indeed Freys-sinet's own descriptions of hisachievement are entirely different,even though it too contains a passionand a vision.2

I decided to risk all that I had offortune, reputation and strength inmaking the idea of prestressing anindustrial reality. Foreseeing a longand hard struggle and a need forfinancial assistance, I took the pre-caution of taking out patents.

PCI JOURNAL/September-October 1976 49

The principle of prestressingis the single most importantconcept in the history of en-gineering.

He was at the time the co-managerof the large construction firm of En-treprises Limousin and M. Limousin,considering Freyssinet's ideas un-sound, refused to go along. As Freys-sinet later described it:

Convinced that my attemptswould soon ruin me, he consideredthat his friendship made it a dutyfor him to oppose at all cost whathe considered to be folly. For me,on the contrary, this folly, even if itwas to prove disastrous, was a mis-sion that I had to fulfill whateversacrifices might be required.

At the beginning, these sacrificeswere indeed considerable. I lost thebest of friends, a very good financialsituation, the joys given me by myprofession as an engineer and themany collaborators that I hadtrained and loved and worse, whoconsidered me as a deserter.

At the age of 50 I was abandon-ing a life that was already mappedout in order to throw myself intoone that was full of uncertaintiesand perils.To get some idea of the type of

person who would give up security toseek a new way of building, I shallgive a brief sketch of Freyssinet'spre-1933 background, along withsome assessment of his contribu-tions, and a discussion of how pre-stressing came to America afterWorld War II and flowered in thenineteen fifties.

For this last discussion, I shall fo-cus on the first two major Americanprestressing conferences, one at theMassachusetts Institute of Technol-ogy at Cambridge in 1951 and the

other at the University of California atBerkeley in 1957. Much as I wouldlike to explore in detail the wide de-velopments after 1957, I find theselast 20 years too broad for me tomake coherent in a short paper. In-stead, I shall end this discourse withseveral contemporary exampleswhose purpose is to show somethingof the continuing nature of the Euro-pean influence on American con-struction.

It is this last idea, often upsettingto the collective American ego, thatcontains a central cultural meaningof prestressing which springs fromthe fact that the structures of a localecharacterize the local culture per-haps better than any other set of arti-facts.

To focus on that fact and to narrowmy scope, I shall consider here onlybridges, even though we all knowthat prestressing has broad applica-tions to all kinds of buildings. Still theidea of prestressing arose out ofbridge design and its most impres-sive forms, from a purely engineeringviewpoint, appear in bridges.

Eugene Freyssinet(1879-1962)

Eugene Freyssinet was born in1879 in the provinces on the Correzeplateau east of Bordeau in a regionthat he later described:3

For many centuries, my ances-tors lived clinging to the flanks ofthe steep gorges through whichrush the torrents of the Correzeplateau. A land of forests and im-penetrable thickets with a harshclimate and a poor soil, it has,throughout the ages, been the re-fuge of the unsubdued and therebel.Seeing himself somewhat in that

light, Freyssinet went on to concludehow his heritage influenced building

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and went a long way toward explain-ing his willingness to risk all to workout his own unconventional ideas forprestressing.

Such conditions of backgroundand life have formed a tough, vio-lent and unsociable race, very poorand proud, little inclined to beg as-sistance •and which has wrenched,from its arid soil, all that it neededto live. Universal artisans, thesemen have created for themselves acivilization the main characteristicof which is an extreme concern forthe simplification of forms and econ-omy of means.Although his family moved to Paris

in the mid 1880's, he never liked thatcity, the "abominable Paris" hecalled it. It did not fit at all with theartisan world whose great love was"simplification of forms and economyof means."

As a student, he was only medi-ocre and thus rejected in 1898 forprestigious Ecole Polytechniquewhich did, however, accept him thefollowing year "with the not very bril-liant position of 161st.°' 4 Graduating19th, he succeeded in being accept-ed at the Ecole des Ponts et Chaus-sees where, for the first time, his arti-san love of building coincided withthat of his teachers, those "great ar-tisans with an enthusiasm for theirwork Resal, Sejourne, Rabut." It wasthere, in the lectures of CharlesRabut in 1903-04 that the idea ofprestressing first came to him:5

Freyssinet's artisan back-ground and love of buildingsinfluenced him to seek anengineering solution to hisstructures through "simpli-fication of forms and econ-omy of means."

Eugene Freyssinet—Morethan any other person, itwas the relentless pioneer-ing efforts of this coura-geous French engineer-builder who converted theconcept of prestressing in-

to a practical reality.

The idea of replacing the elasticforces that are created in the rein-forcements of concrete by deflexiondue to loads, by previously imposedand permanent stresses of sufficientvalue, came to my mind for thefirst time during a series of lecturesgiven by Charles Rabut at theEcole des Ponts et Chauss6es in1903-04. These lectures were de-voted, on the one hand, to rein-forced concrete and on the other,to the systematic study of spon-taneous orprovoked deflection instructures.

This idea never left him and servedas a guide as his early career fo-cussed on the building of bridges inthe wilderness of south centralFrance, where new ideas could flour-ish so long as they were based onthat artisan spirit of simplification offorms and economy of means. This


Fig. 1 Le Veurdre Bridge across the Allier River (1910-1911). Spans were67.5-72-67.5 m (22'3 .238-223 ft). This bridge incorporated the first use ofthrust by jacks at midspan for decentering and also compensating for

concrete creep and shrinkage.

was the same region in which GustavEiffel, 40 years before, had workedout new forms and economy in metalbridges.6

Two examples of Freyssinet's earlywork demonstrate both this spirit ofform and the guide of prestress, theBernard Arch of 1908 and the bridgeover the Allier at Le Veurdre (seeFig. 1) designed in 1907 and com-pleted in 1912.7

Towards 1906-07, the idea of ap-plying precompressions was firmenough in my mind to leac me todraw up a project for a 2500-toncapacity tie linking the two abut-ments of a 50-m span trial arch.

This tie and its arch were corn-pleted during the summer of 1903but a study of their deflexion andother observations taught me theexistence of creep in concrete, aphenomenon that was then unknownand even energetically denied byofficial science. In case of inducedpermanent stresses, this was a fear-some unknown. Immediately and ascarefully and completely as possi-ble, I began to study this problembut my efforts were rendered vainby my mobilisation in August 1914.At Le Veurdre, the situation was

more dramatic and the impact onFreyssinet's vision more lasting. Hehad volunteered to build three

bridges over the Allier River for aprice exactly one-third of that whichhad been bid. As a local engineer ofthe highway department, he had sug-gested that the bids be rejected andthat he be allowed to act as thebuilder for these bridges followinghis own designs.

As Freyssinet later described it:8Fifteen days later, an official let-

ter put me in charge of supervising,on behalf of the Public Authorities,the execution of these bridgeswhose designer I was, for whichwas to be the contractor and theplans of which had never been sub-mitted for anyone's approval. Mer-cier [Freyssinet's superior] then leftfor Portugal granting me unlimitedcredit out of his funds but withoutgiving me a single man, tool orpiece of advice. Never was a buildergiven such freedom. I was absolutemaster, receiving orders and advicefrom no-one.

This rather frightening responsibil-ity had an even more frightening con-clus'on when several months aftercompletion of the three-span bridgeat Le Veurdre, the 72m (238 ft) spanarches began to deflect downward atan accelerating rate.9

To halt this, all that was requiredwas to remove these joints [at thearch crown] after having, by using

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my decentering [sic] jacks in a newapplication, sufficiently raised thecrowns of the arches to do awaywith°the major part of the increasesof stress resulting from the defor-mation of the neutral axis of thearches. There could be no questionof informing the Head Engineer orthe Prefets in order to halt trafficfor they would have panicked andparalysed me and any day thatpassed might bring total collapsefor, at this moment, the strainswere increasing at a frighteningrate.These jacks, the so-called Freys-

sinet flat jacks, are still used in majorstructures today such as, for exam-ple, in the gigantic prestressed CNTower in Toronto; Freyssinet placedthem in the crown hinge and as hewent to describe it:1°

Returning to Moulins in the night,I jumped onto my bicycle and rodeto Veurdre to wake up Biguet andthree reliable men. The five of usthen re-inserted the decentering[sic] jacks—I had always kept thispossibility in reserve—and as soonas there was enough daylight touse the level and staffs; we beganto raise the three arches simul-taneously. It was market day andevery few minutes we had to inter-rupt the operation to allow a fewvehicles to pass. However, all endedwell and once more aligned, curedof the illness that had almostkilled it, the Veurdre bridge behavedperfectly until its destruction in thewar in 1940.Writing in 1949, about the com-

panion bridge at Bou'.iron, Freyssinetstated that:"

I have just seen it again andeven after [my larger and more re-cent arch bridges] I consider it.since the disappearance [sic] ofLe Veurdre, to be the finest of mybridges.In the process of creating these

wilderness works, Freyssinet laid theessential basis for prestressing

Had Freyssinet never pur-sued the idea of prestressing,he would still have been re-garded (along with RobertMaillart) as one of the twogreatest concrete structuralengineers in the first half ofthe twentieth century.

which, however, had to await almost20 years before it became more thanjust a special method of arch con-struction.

During the 1920's Freyssinet de-signed a series of arch structuresthat made him a world figure, notonly to engineers, but to architectsand artists as well. Had he neverpursued the idea of prestressing, hewould still have been regarded, alongwith Robert Maillart, as one of thetwo greatest concrete structural en-gineers in the first half of the twen-tieth century.12

The bridge over the Seine at Saint-Pierre du Vauvray, completed in1922, set the world's span record forconc°ete arches at 131 meters (432ft) and followed Freyssinet's methodof jacking the arch apart at the crownto compensate for rib shortening andto lift the structure off the scaffold.

Then several years later, he won acompetition for a far larger project,the spanning of the Elhorn river atPlougastel, a project which occupiedhim until 1930. Here, Freyssinet de-signed three hollow-box arches eachof 186 meters (614 ft) in span (seeFig. 2) and again the arches werejacked apart at their crowns by acontrolled prestress.

It was in the course of studies forthis impressive project, that he tookup the study of creep and shrinkagein concrete:13


Fig. 2. Plougastel Bridge across the Elorn River (1930). Three spans of 180m (584 ft). This bridge had the largest span in reinforced concrete at that

time.

. . . to know whether one couldcreate permanent prestresses inconcrete in spite of its slow strains

Here was a statement of the prob-lem somewhat more general than thespecific question of designingarches. Thus, in 1926 he organized aset of experiments and began re-search which was published posthu-mously as "the Relations Betweenthe Strains and Constitution of Ce-ments and Colloidal Structured Ma-terials" (1926-1929).14

Freyssinet's motivation was pri-marily to understand structures madeof concrete rather than the structureof concrete. Indeed he ends thistreatise with the conclusion that"arches with spans in excess of 1000meters" can be built "at a far lowercost than a suspension bridge of thesame span." 15 His major work didnot, however, lie in that direction.

By 1928, with Plougastel well under

way, Freyssinet had recognized themore general significance of pre-stressing, patenting his ideas inFrance, Britain, and the UnitedStates. 6 For the next 4 years, he de-voted his full attention to the poten-tials of prestressing.

In November of 1932, Freyssinetsat down and wrote out his progressat the request of the editor of a newjournal Science et Industrie. In oneof its early issues, dated January1933, Freyssinet's article "New Ideasand Methods" appeared."

Beginning with his ideas on the"thermodynamic theory of binders,"he proceeded to analyze the behav-ior of cement, of concrete, and ofreinforced concrete all from the per-spective of a scientist. He describedtests and their results and further ex-plained how stresses over a crosssection arise from shrinkage, fromaxial compression and from bending.Finally, in the fourth of his six chap-

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ters, he outlined the "conditions forthe practical use 'of prestressing. "18

1. Using metals with a very highelastic limit.

2. Submitting them to very stronginitial tensions, much greater than50 kg/mm 2 (70,000 psi).

3. Associating them with con-cretes of a very low, constant andwell-known rate of deformability,which offer the additional advantageof very high and regular strengths ofresistance.

In present-day terms, Freyssinethad established the need for highstrength steel, for tensioning it to ahigh initial stress, and for highstrength concrete to reduce to a`minimum the loss of initial prestress.

Although many engineers had pro-posed the idea of prestress even asfar back as 1886, no one had basedhis idea on a clear understanding ofthe properties of the concrete. Thus,all previous ideas had failed to pro-duce what is now called prestressedconcrete.' 9

Freyssinet saw, in general, thewide potential for his idea, whichused what he called treated con-crete, but in particular, he had greatdifficulty in establishing any commer-cial value for it. Partly of course, 1933was the depression, but partly too itwas a genuinely radical idea. Seen asa means for improving arch design,his system of crown jacking was ac-cepted both in Europe and the UnitedStates and used as early as 1930 inOregon;2 ° but seen as a new mate-rial, prestressing found little applica-tion in its early years.

Freyssinet himself developed afactory at Montargis where he manu-factured prestressed concrete poles(see Fig. 3) for electric lines, but hecould not make it succeed. The fac-tory closed not long after his 1933 ar-ticle appeared and as Freyssinet lat-er put it "our factory was without

Fig. 3. Production of prestressedconcrete poles (1933).

customers and was only good forscrap; my wife and I were ruined. "21

But not for long, because in 1935he had the opportunity to prove themerits of prestressing by saving theMartime Terminal at Le Havre, partsof which had been settling into theharbor at the alarming rate of about1 in. (2.54 cm) per month.

Freyssinet proposed to consolidatethe foundations by prestressing andhis success so convinced the Frenchauthorities that they then supportednumerous large-scale projects be-tween 1935 and 1939 where pre-stressing proved its practical merit.Freyssinet's retrospective attitude on


Fig. 4. Armet Bridge across the Marne River (1946). This elegant bridge(built by Freyssinet) was the first major bridge built of precast prestressed

segmental construction.

the Marine Terminal restorationsintriguing:22

Would I have had the courage totake responsibility had this not con-stituted for me too, the only chanceof rescuing from oblivion the tech-niques that had cost me my fortuneand five years of the hardest work• . . it was perhaps a chance ofsaving my confidence in myself andin the worth of my effort.

Here Freyssinet intimates the rolethat chance plays in providing oppor-tunities even though eventual suc-cess surely depended upon thoselong years between 1903 and 1933of direct field experience in struc-tures. But Freyssinet's ability totransform his ideas into new struc-tures lay less in chance or even inlong experience. As he himself saidof those experienced engineers be-fore him who had had the notion ofprestressing:23

When by chance, they ap-proached this domain, the absenceof a directing idea prevented thedrawing of conclusions that were

I loved this art of building whichI conceived in the same way as myartisan ancestors, as a means ofreducing to the extreme, the hu-man toil necessary to attain a use-ful goal . . . from the bridges ofSeptfonds and LeVeurdre to thoseof the Marne (see Fig. 4) and Ca-racas . . . (two of his best-knownpost World War II bridge projects)

More specifically this love went todefine prestressing as an entirelynew material with the widest possibleapplication. For Freyssinet "the fieldsof prestressed and reinforced con-crete have no common frontier;"either a structure is fully prestressedor it is not to be called prestressedconcrete.

We do not need to accept that

is of any practical consequence.This directing idea, for Freyssinet,

was in general that simplification offorms and economy of means socharacteristic of his artisan heritage;or as he said once to young engi-neers:24

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rigid definition today to recognizehow essential it apparently was toFreyssinet to have this idea of pre-stressing as a new material in orderto direct his energies into practicalapplications. It was to be crack free;a structure in which the elongationof the high strength steel was to beindependent of the strain in the con-crete.

One has only to read his writingsto realize that Freyssinet was morean advocate than a teacher; more anoriginator of ideas than one who ex-plains them to others. In his writings,he can even now communicate clear-ly to us his passion but not so wellhis technical concepts.

It took another sort of person tomake clear the simplicity of pre-stressing and especially to bring it tothe United States. Probably the mostinfluential engineer to do this wasGustave Magnel.

Gustave Magnel(1889-1955)

After having graduated from theUniversity of Ghent in Belgium, Mag-nel spent the years of World War I inEngland where he helped train Brit-ish engineers in reinforced concrete.Aside from establishing his teachingtalent, this experience gave him afull command of the English lan-guage .25

In 1922, Magnel was appointed alecturer at Ghent to teach reinforcedconcrete, in 1927 named docent, andin 1937 made professor and directorof the Laboratory for Reinforced Con-crete. 26 Although French was hismother tongue, he switched histeaching to Flemish (Dutch) when theUniversity at Ghent changed lan-guages in the late 1920's. He couldthus teach fluently in at least threelanguages.

In addition to teaching, he was a

Gustave Magnel—This mul-ti-talented Belgian profes-sor combined his design,research, teaching, andwriting skills to communi-cate his knowledge of pre-stressed concrete to the

English-speaking world.

prolific writer, an experienced de-signer, and an able researcher bythe time the second World War iso-lated him in Belgium. During thoseyears then he began to exploreFreyssinet's ideas and to carry outsome research on his own.

Thus, when the war ended andbuilding in Europe began again at anaccelerating rate, Magnel was one ofthe few engineers with long experi-ence in reinforced concrete, who atthe same time had mastered theideas of prestressing, and what iseven more important, who was ideallysuited to communicate those ideasto the English-speaking world.

He had already written at leastnine books, some of which had gonethrough three editions when, in 1948,he wrote Le Beton Precontraintwhich was immediately published in


The single most significantcharacteristic of Magnel washis ability to communicate asexemplified by his teachings,and prolific writings—andmore importantly to translatethose ideas to the Englishspeaking world.

Freyssinet and Magnelin Contrast

Magnel's books and writings(English editions) clearly ex-plain the idea of prestressingin terms of structural me-chanics, whereas Freyssinetin his 1949 article gives amore descriptive, but lesspractical, discussion.2'Freyssinet's writing is morestimulating to an experi-enced engineer, Magnel'smore useful to one unac-quainted with prestressing.Whereas Freyssinet sawprestressed concrete as acompletely new material, es-sentially different from rein-forced concrete, Magnel em-phasized rather the simplicityof design as it related tothe, by then accepted, ideasabout reinforced concrete.Freyssinet exhorts the de-signer to rethink concretestructures from a totally newperspective while Magneldemonstrates how proce-dures already known by apracticing engineer in 1948can be used to design mem-bers of prestressed concrete.

English, went through three Britisheditions and was also later publishedin the United States.27

But the single most significantcharacteristic of Magnet was his abil-ity to teach. As one of the few Ameri-cans who followed a complete se-quence of his courses at Ghent,can state that he was the best teach-er I ever had. His efforts in teaching,writing and research were to simpli-fy. As he wrote in his book on pre-stressing:29

In the writer's opinion this prob-lem (of computing the ultimatestrength of prestressed beams)should be solved with the leastpossible calculations, as calcula-tions are based on assumptionswhich may lead to wrong results.His suspicions of complex calcula-

tions was balanced by his confidencein tests and full-scale observations.

It is therefore proposed to useknown experimental results to pro-duce a reasonable formula, avoid-ing the temptations to confuse theproblem with pseudo-scientific frills.It was this drive for simple, prac-

tical formulas and explanationswhich, combined with his long ex-perience, lent credibility to Magnel'senthusiasm of prestressing. Thus,when the opportunity arose in 1948to explore the possibility of building amajor public structure of prestressedconcrete, it was not surprising thatthe American engineers involvedwould turn to the Belgian, Magnet,for a design.

The Walnut Lane Bridge

In a speech given at the FirstUnited States Conference on Pre-stressed Concrete, Samuel S. Baxter,later to become president of theASCE, stated that had the originalarch design for the new Walnut LaneBridge been bid below the engi-neers estimate:3o

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It is also quite possible that thisFirst Conference on PrestressedConcrete might not now be in ses-sion. . .His claim was probably correct,

even though prestressing was al-ready being tried out by 1951 andsome conference would soon havebeen arranged thereafter. Still thisPhiladelphia bridge served to charac-terize the potential for prestressedconcrete because of its large-scale,160-ft main spans, because of itsconstruction economy, and becauseof its acceptance, not only by city en-gineers, but also by a powerful cityArt Jury, two types of people normal-ly associated with traditional atti-tudes.

As Baxter explained it, the stone-faced arch design of 1974 obtaineda low bid of $1,047.790 compared tothe engineers estimate of $900,000.By law, if the low bid exceeds theestimate, it is rejected. Thus, thecity engineers began to search foranother solution, of which two arose.

The first was a plan to remove thestone facing which in the low bidamounted to the astounding sum of$486,490! Here the Art Jury objectedto the mass of an unfaced arch. Thesecond solution suggested itself al-most by accident.31

The Bureau of Engineering, Sur-veys and Zoning at that time wasconstructing large circular sludgetanks at its new Northeast Treat-ment Works. These were being builtby the Preload Corporation of NewYork (sub-contractors for VirginiaEngineering Company of NewportNews, Virginia), using the prestress-ing technique of winding wiresaround a thin core. The chance re-mark of Mr. E. R. Schofield, whowas at that time Chief of the De-sign Division of the Bureau of Engi-neering, Surveys and Zoning, to arepresentative of the Preload Cor-poration, led to a decision to ex-

plore the use of prestressed con-crete for this bridge. Among thosewith whom Mr. Schofield talkedwere Mr. L. Coff, Consulting Engi-neer of New York, and representa-tives of the Preload Corporation.Contracts were also made with Pro-fessor Gustave Magnel in Belgium.The city decided to follow Mag-

nel's ideas for a prestressed con-crete girder design but they still hadto convince the Art Jury. Baxter re-cords their response, surely one ofthe most historcally significant eventsin the relationship between structureand aesthetics.32

The Art Jury, however, on seeingthe preliminary sketches for the newbridge agreed that the comparative-ly slim lines of the new bridgewould not require stone facing.Thus, a major structure in one of

Philadelphia's most elegant naturalsettings became possible because itsappearance was pleasing enough topermit it to be economical. The lowbid in 1949 was $597,600 for thebridge and $100,783 for the ap-proaches; Baxter estimated that thisamounted to a "net minimum saving(of) approximately $76,000" over anarch without stone facing in 1949.33

Moreover, the Art Jury would prob-ably have required a rubbed finish onthe bare concrete arch, adding atleast $40,000 and making the pre-stressed solution a minimum of $116,-000 less than the arch. The saving ofover 16 percent clearly made thislarge-scale work possible and influ-enced the way prestressing enteredAmerican practice. Of the thirty pa-pers presented at MIT in August of1951, five were by people directlyconnected to the Walnut Lane Bridge.

Another feature of this bridge wasthe full-scale test to destruction ofone of its 160-ft (48.5 m) long girders.Although, perhaps, unnecessary inprinciple, this test did serve dramat-ically to demonstrate, in practice,


Fig. 5. Artist's painting of Walnut Lane Bridge after completion (1951).

Particulars ofWalnut Lane Bridge

The first prestressed concretebridge to be constructed in theUnited States was Philadelphia'sWalnut Lane Bridge. This bridge,which was conceived and designedby Gustave Magnel, was completedii 1950. It contains three simplysupported girders with a centerspan of 155 ft and two end spansof 74 ft each. The girders are (-shaped, 79 in. deep with 52-in.flanges.The flanges in the center span arebutted, but in the end span thebeams are placed 8 ft 8 in. on cen-ters and the slab cast-in-place. Thegirders were tensioned by the Mag-nel method, i.e., using two strandsat a time.The Preload Co. of New Yorkerected this structure. The girderswere cast on falswork at the bridgesite and moved horizontally intoposition on the foundation.

and in front of at least 500 engineers,the high overload capacity of thebridge built along these new lines.34

F'.q. 5 is an artist's picture of theWalnut Lane Bridge soon after com-pletion in 1951. Fig. 6 is a shot ofthe same bridge taken in 1976.

Thus, the Walnut Lane Bridge putbefore the American structural engi-neer the image of new possibilitiesfor safe, economical, and elegantstructures. Yet these obvious advan-tages came together with a set ofquestions even doubts, that all cen-tered on a suspicion of Europeanideas that has existed in America atleast since the time of Emerson'sAmerican Scholar speech of 1837.In Emerson's terms, the doubts fo-cussed on the need to think deeplyabout the local American environ-ment and to create works of art,political structures and scholarshipthat would be distinct and originalrather than merely copying Euro-pean taste.

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Fig. 6. Walnut Lane Bridge as it appears today (1976).

In modern engineering terms,these doubts center on the differencebetween labor and materials. Laborbeing cheaper in Europe means thatmaterial savings dominate designideas, whereas materials beingcheaper in America, labor savingsare supposed to dominate designideas in the United States.

We need to look critically at thesecliches today. They reflect in part aquestionably conservative attitudetoward design and a justifiably cauti-ous attitude toward building.

The Walnut Lane Bridge raisedagain this question of labor and ma-terials and it was criticized for beingtoo much a European design. Magnelhad made the design through thePreload Corporation whose vicepresident Cruzon Dobell reportedthat "it took 152 man hours to assem-ble and install one ton of prestress-ing wire" for the bridge.35

Magnel, himself, was astounded at

the problems associated with gettingAmerican industry to manufacturespecial fittings. He used to lamentthat all would have been well, if in-stead of 20 end cable fittings, hecould have ordered one million!

Admiral Jelley, Chief of the Bureauof Yards and Docks, perhaps sum-marized best this viewpoint in his

In the immediate post WorldWar II years, there was awidely held view that laborbeing cheaper in Europemeant that material savingsdominated design ideas,whereas materials beingcheaper in America, laborsavings were supposed todominate design ideas in theUnited States.


"Closing Summary" to the MIT Con-ference:

We have seen American adapta-tions of European practice in bridgeconstruction. The Walnut LaneBridge in particular was a directapplication of Dr. Magnel's system.

However, the Arroyo Seco footbridge (California's first prestressedbridge) had an interesting departurefrom European precedents—a but-ton type of anchorage was used.think that this is significant becausef consider that European ideasshould not be copied blindly. Con-struction conditions in this country,particularly trade practices, pre-clude this. American engineersmust find and develop their ownsolutions.This was the situation at the end

of 1951. The idea of prestressing waswell accepted. Its safety and econ-omy seemed possible and its visualpotential a reality. Now began thelong process, even now unfinished,for American engineers to find anddevelop their own solutions.

Developments 1951-1957

In his 1951 discussion at MIT,W. E. Dean noted that in following aprestressed design through to cal-culations and plans:36

We encounter a number of factorsthat are puzzling to say the least.All of these are capable of solutionand it is evident that our Europeancounterparts have solved them totheir satisfaction, but whether wecan adapt our practice and con-cepts of safety to European thinkingremain to be seen.This skeptical view, common

among structural engineers in 1951,had by 1957 changed radically asDean himself expressed in the open-ing speech of the World Conferenceon Prestressed Concrete held inBerkeley in July 1957:37

Those who have been associated

FORMATION OF FIPThe Federation Internation-ale de la Precontrainte (FIP)was officially inaugurated ata meeting held at the Uni-versity Engineering Depart-ment, Cambridge, England,on August 29, 1952.

This meeting represented theculmination of the efforts ofseveral eminent internationalengineers and researchworkers who had held meet-ings and discussions over a2-year period, and in whichEugene Freyssinet and Gus-tave Magnel played promi-nent roles.

Most fittingly, the first presi-dent of FIP was Freyssinetand the first deputy-generalvice-president was Magnel.

Today, FIP has MemberGroups in 44 countries andFIP observers in some 25other countries. The currentpresident of FIP is Ben C.Gerwick, Jr. of the Universityof California at Berkeley.

One of FIP's principal activi-ties has been to organizeinternational congresses andspecial symposia throughoutthe world. It has held Con-gresses in London (1953),Amsterdam (1955), Berlin(1958), Rome and Naples(1962), Paris (1966), Prague(1970), and New York (1974).The eighth Congress willbe held in London in May1978.

62

with the prestressed concrete fieldfor the past several years have rea-son to be proud. Their past effortshave been spectacularly satisfying,present development is stimulating,the future appears to be not onlypromising but almost fantastic inits potential for the use and maturityof prestressed concrete design. Forin the utilization of this economical,versatile and highly adaptable ma-terial we are barely coming of age.The intervening years brought pre-

stressed concrete into the main-stream of American constructionpractice; it moved from the provinceof the pioneers such as Freyssinetand Magnel into the practice of allstructural engineers.

In the United States, those 6 yearssaw the organization of the Pre-stressed Concrete Institute at Tampa,Florida, in July of 1954, the publica-tion of the first specification for pre-tensioned prestressed concrete onOctober 7, 1954, by the PCI and inthe same year the "Criteria for Pre-stressed Concrete Bridges" by theBureau of Public Roads; and the ap-pearance of American textbooks onprestressed concrete structures, themost widely used being that by T. Y.Lin, written largely during his one-year Fulbrig'ht Fellowship at Ghentwith Gustave Magnel.

These events were the evidencesof the rapid growth of prestressedconcrete throughout the UnitedStates and the World; and it was thisgrowth that the Berkeley Conferencesummarized in the summer of 1957.

Five reports on American bridgesappeared and they characterizedwell the developments since 1951:

First, a discussion by Arthur L. El-liott on construction experience withprestressed concrete in Californiawhere the state bridge departmenthad already contracted for over 60projects. 38 Elliott focussed on theirproblems, especially with inexperi-

FORMATION OF PCIThe Prestressed ConcreteInstitute (PCI) was organizedat a special meeting in Tam-pa, Florida, July 1954. Twoyears later the first issue ofthe PCI JOURNAL was pub-lished.In November of 1959, the PCImoved its headquarters toChicago.Today, the PCI is an umbrellaorganization made up of over2000 producer and suppliercompanies, affiliated stateassociations, professionalengineers and architects,and students.The current president of PCIis Chicago-based consultingengineer Eugene P. Holland.

enced contractors, but clearlyshowed that his bridge division hadmade major progress in using thenew ideas.

The second report by E. L. Erick-son, chief, Bridge Division Bureau ofPublic Roads, described the newlypublished "Criteria for PrestressedConcrete Bridges," which had al-ready played a central role in en-couraging bridge designers to tryprestressing and which was begin-ning to open up its design for bridgesof the interstate highway system en-acted into law by the Congress in1956.3

The third report by Wayne F. Palm-er, described "The 24-Mile LakePontchartrain Prestressed Bridge"recently completed near New Or-leans.40 This immense project sig-nalled the practicality of preten-sioned precast elements and proved


Major InternationalPrestressingConferences

In North America

First United States Confer-ence on Prestressed Con-crete, Massachusetts In-stitute of Technology,Cambridge, Mass., August14-16, 1951.

Canadian Conference onPrestressed Concrete,Toronto, Ontario, January28-29, 1954.

World Conference on Pre-stressed Concrete, Univer-sity of California at Ber-keley, Calif., July 29-August 3, 1957.

FIP/PCI Congress, NewYork, N.Y., May 26-June 1,1974.

ACI-CEB-PCI-FIP Symposi-um, ACI Annual Conven-tion, Philadelphia, Pa.,March 31-April 1, 1976.

T. Y. Lin Symposium on Pre-stressed Concrete—Past-Present—Future, l'niver-sity of California at Ber-keley, Calif., June 5, 1976.

by competitive bidding to be sub-stantially cheaper than the competi-tive bid in steel. As Palmer put it:41

When bids for both the steel andthe concrete designs were opened,it was clear that on a project of thiskind steel was no longer a seriouscompetitor.The fourth report dealt with the

bridges for the Illinois toll highway onwhich the decision had been made tostandardize precast factory-made

prestressed elements for 224 of the289 bridges. 42 Again comparisonswith steel designs indicated theeconomy of prestressed concrete.For the prestressed concrete bridges,the unit costs were $13.10 per sq ftwhereas for the steel $16.50 per sq ft,a very substantial difference.

The fifth and final American reporton bridges described a full-scale loadtest of one bridge for the Illinois tollhighway.43

What these reports show is a ma-jor shift in focus, compared to thebridge reports at the MIT Conference6 years earlier, a shift away from in-dividual custom-made projects likethe Walnut Lane Bridge and towardsmass produced fabrication on im-mense projects or, as in the case ofCalifornia, the wide use of prestress-ing by a single public agency. Thereis no mention of the Walnut LaneBridge in any of these articles andvery little reference to European ex-perience. As Dean stated in his open-ing remarks:44

It appears that in the field of rela-tively small standardized, mass pro-duced parts, United States construc-tion is presently outstanding. In longspans, continuous structures andthe more daring structural applica-tions, foreign technology leads.As if to pick up Dean's challenge

about Europe's lead in daring struc-tures, T. Y. Lin, the ConferenceChairman, closed the proceedings bypresenting a set of drawings by anarchitect for daring structures of pre-stressed concrete. Lin's own careersince 1957 bears out clearly the posi-tive results of such futuristic stimu-lus.

What I wish to add here is theparallel stimulus of looking back atsome equally dramatic design ideaswhich arose during those early yearsbefore the Berkeley Conference. Es-pecially important are the ideas of

64

Ulrich Finsterwalder, who duringthat same period in the 1950's tookprestressing and made it a construc-tion technique as well as a designidea.

Ulrich Finsterwalder

Finsterwalder, like Freyssinet, is abuilder whose designs have frequent-ly been constructed only becausethey were bid below other competingdesigns. His major bridge idea, de-veloped after World War II, is thedouble cantilever, built entirely with-out scaffolding. Figs. 7 through 12show several of his major works.

Like Freyssinet and Magnel, Fin-sterwalder came to prestressing witha long experience in reinforced con-crete and especially like Freyssinet,in arch and thin shell structures. Fin-sterwalder learned mathematicswhile in a French prison camp duringWorld War I and after the war he putthat to use in shell theory whichserved as a basis for the many thinshell concrete structures designedand built by Dyckerhoff and WidmannA. G. starting in the mid 1920's.

In 1937 he began work developinga prestressing system and designedand built his first bridge. 45 Then afterWorld War II, his major prestressingwork began, which he reported on inAmerica in 1952.46

His work since that time is sobroad and varied that it defies simplecharacterization, except to say thatmore than anyone else, perhaps, Fin-sterwalder has shown that pre-stressed concrete can be a safe,economical, and elegant solution toalmost any major structural problemthat exists in the modern world.

In a 1970 interview, Finsterwaldermentioned that his favorite bridgewas the Mangfall Bridge, under de-sign just at the time of the BerkeleyConference. With this bridge, as in

Ulrich Finsterwalder—Thisimaginative German engi-neer-constructor has playeda very significant role in ad-vancing the state-of-the-artof prestressed concrete es-pecially his development ofthe double cantilever meth-od of erection in bridge

construction.

other cases, Finsterwalder sought toshow that prestressed concrete couldcompete directly with steel, not onlyin cost, but also in the reduction inits depth.

In the Mangfall, built by his canti-lever method but made with opentruss-like walls, his idea was to dup-licate the girder depth of the steelbridge built in the late 1930's anddestroyed in World War II. Not onlydid he succeed technically, but indesigning a two-level bridge (Fig. 9),he provided the pedestrian with oneof the most spectacular crossingssince the Brooklyn Bridge.

Finsterwalder has the idea of pro-viding a prestressed concrete alter-native to every steel bridge designincluding those with very long spanswhich have previously been the sole


Fig. 7. Elztal Bridge (1965).

Fig. 8. Mosel Bridge (1965) at Schweich-Longuich.

Fig. 9. Mangfall Bridge (1947) at Darching.

Fig. 10. Bendorf Bridge over Rhine (1962).

Fig. 11. Second Main Bridge (Frankfurt-Sindlingen).

Fig. 12. Rhein-Main-Flughafen Airport Terminal(1968) near Frankfurt.

Fig. 13. Rio Colorado Bridge.

68

Fig. 14. Chillon Viaduct.

province of suspension bridges. Hisstress ribbon bridge, conceived atabout the same time as the Mangfalldesign, carried prestressed concretefar beyond its previous limits. Hemade a design for the BosporusBridge which would have had a freespan of 454 m (1500 ft).47

The Recent Past

Appropriately, the first work will bethe 1972 highway bridge over the RioColorado (see Fig. 13) in Costa Ricadesigned by T. Y. Lin Internationaland spanning 145 m (479 ft) betweensupports over a 91 m (300 ft) deepvalley. 48 This new form shows itsstructural logic clearly in the almostpolygonal lower chord, its delicateverticals and its straight light hori-zontal roadway.

The same sense of form appears inthe recent Chillon Viaduct (see Fig.14) along the northeast shore of LakeGeneva and designed by Prof. Piguetof Lausanne. Here the design waschosen after a competition in whichthe criterion of aesthetics played amajor role. The double cantilevermethod used precast elements post-tensioned together and the total finalcost of the 2 1h-kilometer (1 1/2 miles)viaduct was only $14 per sq ft.49

Finally, and departing slightly fromthe historical focus, is the newestbridge of Switzerland's most talentedcontemporary bridge designer, Chris-tian Menn. It will be on the road go-

ing over the Simplon Pass and it re-flects the continual search for formin prestressed concrete.

In a way, each of these three re-cent works are by mature designerswho have worked with prestressingsince its early days of the 1950's.Thus, they are very like the earlierpioneers whose major contributionscame after a long contemplation ofstructures.

Whether they would admit it or not,the crucial factor was the study ofhistory. Not a study of names anddates, but rather of forms and of full-scale behavior. Ideas in structurecome from understanding clearly theworks of the recent past; but theyalso come from abroad as well asfrom home.

In understanding more clearly theworks of Freyssinet, Magnel, and Fin-sterwalder, American engineers havebegun adapting prestressed concreteto American conditions. In so doing,designers like T. Y. Lin, engineers ofthe state of California, and othershave shown how new American formscan become a central part of the re-cent past that engineers everywherewill need to study.

Freyssinet's difficult years from1928 to 1935 have led to new formsthat have become symbols of how thestructural environment of the late20th century can be built, not onlyto save materials and money but alsoto add elegance and dignity equalto any period in mankind's history.


Notes and References

1. Le Corbusier, The Radiant City, Paris1933, (English edition 1967), P. 340. Iam indebted to Mary McLeod, a grad-uate student in architecture, who toldme of this quote.

2. Freyssinet, E., "Eugene Freyssinet byHimself," Travaux, April-May 1966.(Extracts from a lecture given byFreyssinet on May 21, 1954, and pub-lished in Travaux, June, 1954), p. 10.

3. Ibid, p. 3.4. Ibid, p. 5.5. Freyssinet, E., "A General Introduc-

tion to the Idea of Prestressing,"Travaux, April-May, 1966. (Originallywritten in 1949), p. 19.

6. Insolera, I., "I Grandi Viadotti di Eif-fel Net Massif Central," Zodiac Xlll,1964, pp. 61-111.

7. Freyssinet, op. cit., 1949, p. 19.8. Freyssinet, op. cit., 1954, p. 8.9. Ibid, p. 9.

10. Ibid, p. 9.11. Ibid, p. 9.12. Freyssinet and Maillart were the only

engineers elected in 1937 as honor-ary corresponding members of theRoyal Institute of British Architectsalong with eminent architects includ-ing Le Corbusier. See Collins, G.,"The Discovery of Maillart as Artist,"Maillart Papers, Princeton, 1973, P.38. Freyssinet was discovered beforeMaillart and written about in the1920's. Le Corbusier used Freys-sinet's work to illustrate his mostwidely read work Towards a NewArchitecture, Paris, 1923, pp. 263-265.

13. Freyssinet, op. cit., 1954, p. 10.14. Freyssinet, E., "The Relations Be-

tween the Strains and Constitution ofCements and Colloidal StructuredMaterials," Travaux, April-May, 1966,p. 570 (originally written between1926-1929).

15. Ibid, p. 605.16. Billig, Kurt, Prestressed Concrete,

London, 1952, pp. 28 and 43.17. Freyssinet, E., "New Ideas and Meth-

ods," Science et Industrie, January

1933, published also in English inTrauvaux, April-May, 1966, pp. 607-622.

18. Ibid, P. 617.19. Apparently the only exception was

R. E. Dill who in 1925 did recognizethe importance of prestressing lossesdue to creep and shrinkage of theconcrete. See Billig, op. cit., pp. 32-34. P. H. Jackson 'had, among hisnumerous patents, one that in retro-spect can be called prestressing, butas far as is known had no influenceon others and was never used prac-tically.

20. McCullough, C. B., and Thayer, E. S.,Elastic Arch Bridges, New York, 1931,pp. 325-358.

21. Freyssinet, op. cit., 1954, p. 12.22. Ibid, p. 13.23. Freyssinet, op. cit., 1949, p. 25.24. Freyssinet, op. cit. 1954, p. 18.25. Evans, R. H., "Speech," In Memoriam

Gustave Magnel, October 1956, p. 51.26. Anseele, E., "Speech," Ibid, p. 31.27. Magnel, Gustave, Prestressed Con-

crete, Third Edition, London, 1954.28. Freyssinet, Eugene, "A General In-

troduction to the Idea of Prestress-ing," Travaux, April-May, 1966, pp.19-49, originally published in 1949.

29. Magnel, op. cit., p. 84.30. Baxter, S. S., and Barofsky, M., "Con-

struction of the Walnut Lane Bridge,"Proceedings, First United States Con-ference on Prestressed Concrete,Massachusetts Institute of Technol-ogy, Cambridge, August 1951, P. 47.

31. Ibid, pp. 47-48. According to M. Forn-erod, then Chief Engineer for Preload,it was one of his engineers, CharlesZollmai, later one of the organizersof the Berkeley Conference, who sug-gested Magnel. Zollman had been astudent of Magnet's at Ghent andhad helped him with the English lan-guage version of his book.

32. Ibid, p. 48.33. Ibid, p. 48. The low bid in 1947 for the

arch without stone facing $561,300which when corrected from the rise

70

39.

40.

in costs according to the EngineeringNews-Record cost index would haverisen to $673,560 in 1949.

34. Anderson, A. 'R., "Field Testing ofPrestressed Concrete Structures,"Proceedings, First United States Con-ference on Prestressed Concrete,Massachusetts Institute of Technol-ogy, Cambridge, August 1951, pp.215-217. The test was described byG. Magnel in "Prototype PrestressedBeam Justifies Walnut Lane Bridge,"AC! Journal, Proceedings V. 47, De-cember 1950, pp. 301-316; and byM. Fornerod in the Bulletin of theIABSE, Zurich, 1950; it also appearsin detail in Magnel's book, op. cit.,pp. 188-200.

35 Dobell. C., "Prestressed ConcreteTanks," Proceedings, First UnitedStates Conference on PrestressedConcrete. Massachusetts Institute ofTechnology, Cambridge, August 1951,p. 9.

36. Dean, W. E.. "General Design andEconomic Co-siderations in thePlanning of Prestressed Concrete

Structures—Discussion," Proceedings,First United States Conference onPrestressed Concrete, MassachusettsInstitute of Technology, Cambridge,August 19 1. p. 184.

37 Dean, W. E., "Prestressed Concrete—A Youth Comes of Age," Proceed-ings, World Conference on Pre-stressed Concrete. Universit y of Cal-ifornia, Berkeley, July 29-August 3,1957. p. Al-3.

38. Elliott. A. L., "Construction Experi-ence with Prestressed Concrete inCalifornia," Proceedings, World Con-ference on Prestressed Concrete,University of California, Berkeley,July 29-August 3, 1957, p. A8-1.

Erickson, E. L., "The Bureau of Pub-lic Roads, Criteria for PrestressedConcrete Bridges," Proceedings,World Conference on PrestressedConcrete, University of California,Berkeley, July 29-August 3, 1957, p.A9-1.Palmer, Wayne F., "The 24-Mile LakePontchartrain Prestressed Bridge,"P, oceedings, World Conference onPrestressed Concrete, University ofCalifornia, Berkeley, July 29-August3, 1957, p. A10-1.

41, lb'd, p. A10-2.42. Bender, M. E., "Prestressed Concrete

Bridges for the Illinois Toll Highway,"Proceedings, World Conference onPrestressed Concrete, University ofCalifornia, Berkeley, July 29-August3, 1957, p. All-1.

43, Janney, J., and Eney, W. J., "FullScale Test of Bridges on NorthernIllinois Toll Highways," Proceedings,World Conference on PrestressedConcrete, University of California,Berkeley, July 29-August 3, 1957, p.Al2-1.

44. Dean. op. cit., p. Al-l.45. Finsterwalder, U., "Eisenbetontrager

met selbsttatiger Vorspannunq." DerBauingenieur, Heft 35/36, 1938.

46. Finsterwalder, U., "Free-Span Pre-stressed Concrete Bridge," AC! Jour-nal, Proceedings V. 49. No. 3, No-vember 1952, pp. 225-232.

47. Finsterwalder. U., "Prestressed Con-crete Bridge Construction," AC!Journal, Proceedings V. 62, No. 9,September 1965, pp. 1037-1046.

48. "Inverted Suspension Span is Simpleand Cheap," Engineering News Rec-ord, May 11, 1972, pp. 27-31.

49. Menn, C., "New Bridges," Mail/artPapers, Princeton, 1973, p. 115.


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