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BY J. FRED TRIGGS, D.SC., P.E.*

It is literally impossible to open a contemporary mag-azine devoted to the building arts without cominga c ross some re f e rence to shells and shell technology.Twenty years ago, shells were almost unheard of. Wheredid they come from? Why their popularity, and now that

we have them, what do they mean to the building arts?According to modern building term in olo gy, the shell

is any laminar membrane capable of enclosing spaceand supporting loads through the exertion of dire c ts t resses re s o l ved in the boundaries of the stru c t u re.

Therefore, domes and vaults, acting in direct compres-sion, are shells. These are synclastic shells, either singlyas in the vault, or doubly curved as with the dome. Butt h e re are other shells, some of which are anticlastic ordoubly curved in opposing dire c t i o n s. Ce rtainly themost convenient of these and perhaps the most dramat-ic are the hyperbolic paraboloids.

The history of the origin of shells begins about fivethousand years ago at Sa q q a ra in Egypt. It was at thistime and place that the first architect of record, Imhotep,

constructed the stepped pyramid of Zozer which is today

the oldest existing building in the world. There is no di-rect connection between the py ramid of Zozer andshells, but at the time of its construction, a commemo-ra t i ve gesture on the part of the builder consisted ofe recting prototypes re p re sen t a t i ve of the stru c t u res ofthe time in a grouping around the tomb. These pro t o-type buildings have long ceased to exist as stru c t u re s,but archeologists, reconstructing shapes from fragmentss t rewn about the ground, have theori zed that some ofthese prototype structures had barrel vaulted roofs.

Little or nothing remains now of any shells construct-ed by the Ancient Egyptians of the Old Kingdom, but thebuilders of the Middle Kingdom, some thousand ye ars

l a t e r, roofed their cave tombs with flat compre s s i o na rched vau l ts, and a few hundred years later, Mi no an sand Cretans built their beehive domes. As far as weknow, their knowledge of descript ive geometry was in-adequate to reveal that they were building conoids.

The Romans did not invent the round or segmentalarch (its origin is vague but must have been Asiatic) butthey brought the art of arch, dome and vault building toa peak of perfection. They built domes, half domes, bar-rel vaults and intercepting round vaul t s. They built the

The Airpor t Te rminal Bui ld ing at Lambert Fie ld, St.Louis, is a superb example of shell concrete. Three setsof intersecting barrel shells 4 12 inches thick make upthe roof, which provides an interior over 400 feet longwithout supporting columns.

THE MEANING OF SHELLS

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Baths of Ca racalla and they built the Pantheon. Theirwork with domes and vaults laid the foundation for ane m p irical system of ru dime n tary shell technology. TheRomans possessed natural cement and had they applied

just a li ttle imagination, they would undoubtedly havebeen able to construct reinforced concrete shells. How-e ve r, two thousand years was to elapse before this wasdone.

The decline and fall of the Roman Em p i re was un-doubtedly hastened by the removal of the world capitalby Constantine to Byzantium. However the shift of cul-tu ral emphasis to the East brought to bear on the con-struction arts certain new ideas and fresh personalities.These new ideas, bred of an Eastern culture, resulted ina revival of shell technology and a far wider use of shellsthan had existed to this time. One of the noblest build-ings of this or any other time is the Hagia Sophia or

Church of the Divine Wisdom which is sometimes nowcalled St. Sophia. Anthemius and Isod o rous, arch ite c tsfor this church, solved the problem of fitting a ro u n ddome on a square base by contriving a sort of five-sideds p h erical triangle called the pendentive. This would bedone today simply by squaring the plan of the domewith vertical intersecting arches which, all in all, seemsto be a far simpler solution but which would have beenquite impossible with materials available in 537 A.D.

Fi ve to seven hundred years passed after the con-s t ruction of Hagia Sophia with l ittle or n o pro g ress inshell technology except the repetition of the oniondome. Then the Cathedral Builders began the construc-

tion of the pointed and groined vaults of the Gothic Pe-riod. Notre Dame, Chartres, Saint Denis and many oth-er cathedrals we re built and added to the empiri c a ltechniques of construction of the vault and shell. Veryoften this pro g ress was fraught with much tra va i l .Months or years of work as well as many lives were lostwhen some too daring master builder miscalculated hisvault thrust or over-extended his ratio of length to rise.But these shells we re built a nd some of the successfulones still stand today. The beginning of the Renaissancebrought with it a revival of dome construction and Bra-m a n t es and Michael Angelos Dome of Saint Peter is asplendid example of the shell work of that time.

The Re n a iss a n ce, with its emphasis on the art s, gaveway to the Age of Reason with its emphasis on creati vethinking and finally to the materialism of the IndustrialAge. Galileo, Kepler, Bacon and Newton had pointed theway to a culture based upon scientific reason. Then with

The barrel or cylindrical shell is the simplest andcommonest form of the single curvature shell. It is notquite economical, but adapts very well to a wi de range ofarchit ect ural needs. Some common variations are shownin the sketc hes.

LONG BARRELS

SHORT BARRELS

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no thicker than 5/8 of an inch. Solution of this problemwas obviously impossible using conventional methodsof construction. Candela solved the problem with thehyperbolic paraboloid shell. The cosmic ray laboratorywas built in 1951.

Since 1951, Felix Candela has designed and built liter-ally thousands of shells, but more important to the ben-efit of mankind, he has published his discove ries anddisseminated his broad knowledge of shell technology.

Paralleling the work of Candela, in a somewhat differ-ent medium, has been the work of Pier Luigi Ne rv i .Working mostly in It a ly, Nervi is motivated by substan-tially the same ideals, namely the desire for economics t ru c t u re without sacrifice of delicacy of pro p o rt i o n .Nervis work in shells somewhat antidated that of Can-dela and has been different in technique and applica-tion. Whereas Candela has emphasized the curve gener-ating straight line and cast-in-place concrete, Nervi hasutilized a precast technique relying on post-stressing forcontinuity. It is perhaps significant that Nervi jointly withCandela occupies an Architectural Chair at Harvard Uni-versity.

The techniques developed by Ne rvi in the precastingof shell elements have been adopted and expanded inmany other parts of the world. A report on the visit of an

A m e rican delegation to observe concrete and pre-s t ressed concrete engineering in the U.S.S.R. cites atsome length the work being done in Soviet Russia onprecast concrete techniques in general and shell castingin particular. From this report it is apparent that the Rus-sians are precasting roof shells, hyperbolic para b o l o i dthin shells and corrugated thin shell units.

Subsequent to the pioneer work of the thin shell inno-vators, many other practitioners of the building arts havetaken up the challenge. Considerable work in thin shells

is being done in Eu ro p e, Mexico and South Ameri c awhile, in this country, we have seen the beginnings of theuse of this technique.

The author has devoted considerable attention to thisnew technique of building science and, in addition toan extensive use of shells for foundations*, has designedseveral hyperbolic paraboloid shell roof buildings, a hy-perbolic paraboloid bandstand for an amusement parkand even a barrel shell revetment to contain a stream.

We have glanced rather casually at the origin of shellsbut we have not answe red the vital questionwhat dothey mean to the building arts? As one who has been us-ing shells and particularly the hyperbolic para b o l o i dsince 1956, the writer may be allowed to express somepersonal opinions on the valid use of shells and their fu-t u re in the building industry. It seems that the logicaluse of the shell as a building technique needs to be basi-cally for economic purposes. With certain reservations, it

Economy in the use of mat erial and low forming cost aresignificant advant ages of hyperbolic paraboloid surfaces.Just a few of the many variations are shown here. In each ofthese shells the edges terminate along st raight lines formedby the surface.

*See Shell Foundations, Concrete Construction, Jan-uary, 1961, page 6.

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is most difficult to be in sympathy with the use of shellsfor startling effect like an appliqued embroidery. Howev-er most shells are of such a form as to create stimulatingvisual appeal and consequently may well be used to en-close space for worship or exhibition. But genera l l yspeaking t hough their pri m a ry application may proveto be in the industrial field for warehouses and factories

where they may most economically provide safe fire re-sistant and flexibly arranged shelter.

Early experience with the construction of shell-roofedbuildings re veals that such construction may be ob-tained for approximately one-half to two-thirds conven-tional building cost. This is true in spite of two basic de-ficiencies which still exist. The first of these is that fewconstructors are completely familiar with methods andtechniques of forming and placing concrete in shells.The second is that the precasting of shells on a commer-cial basis has not yet been developed.

It is the writers belief that shell construction in the lat-ter part of the Twentieth Ce n t u ry presents one of thechallenges of this generation; howe ve r, it seems ex-t remely unlikely that the full worth of the shell will be

realized in our generation. The reasons for this outlooka re highly complex and have to do with both the tem-p e rament of the designers who must initiate the shellproject and the entrenchment of certain prejudices re-g a rding the use of competitive materials and pro c e-dures.

Despite the very obvious economies of shell construc-tion, very few projects of this nature are being construct-ed. Our experience has taught us that the know-how re-q u i red for successful execution of this type ofc o n s t ruction can be acquired on a s ingle job and thatthis execution need not require construction specialists.

What then is the basic cause of the potential designersapathy tow a rd this technique? Pe rhaps this hesitancymay be due to the philosophy expressed in Alexander

Popes advice, Be not the first by whom the new are triednor yet the last to lay the old aside. Or it may be thatthe owner resists t he idea of a change in constru c t i o nmethods with which he is not entirely familiar. Howevercomments gleaned from conversation with some de-signers tend to indicate that professional fee structuresmay have a bearing on the problem. It may be that thedesigner is fearful that pre vailing fees may fail to com-pensate for the additional effort involved in the applica-tion of a somewhat daring innovation.

If this is a consideration, it seems quite in order toquote from a lecture given by Felix Candela at theCarnegie Institute of Technology, April 24, 1961, When

people ask me why I build so many hyperbolic parabo-loid shells, I tell them simply that I have a large familyand need the money.

*The author is a consulting engineer with offices inPittsburgh, Pennsylvania.

PUBLICATION #C620193Copyright 1962, The Aberdeen Group

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