Structural Analysis of Historic Construction – D’Ayala & Fodde (eds)© 2008 Taylor & Francis Group, London, ISBN 978-0-415-46872-5

20th century curtain walls – loss of redundancy and increase in complexity

S.J. KelleyWiss, Janney, Elstner Associates, Inc. Chicago, Illinois, USA

ABSTRACT: Central to the development of the American skyscraper was the concept of the curtain wall. Thisdevelopment was rapid and was driven by the distinct cultural climate within American urban centers such asChicago and New York. Skyscrapers would not have been technically feasible without the lightweight curtainwall, and it was through the skyscraper that the curtain wall achieved its greatest realization. The principal andconfrontational factors of its development were economics and the need for fireproof buildings. The historicaldevelopment of the curtain wall is presented and a comparison of deterioration mechanisms between masonryand metal and glass curtain walls is discussed. Finally the issues of loss or redundancy and increase in complexityof contemporary curtain walls are discussed and strategies for diagnosis and conservation are presented.

1 INTRODUCTION

One of Oscar Wilde’s aphorisms regards the UnitedStates: “America is the only country that wentfrom barbarism to decadence without civilization inbetween.” The truth in these bon mots, at least with thetopic of curtain walls, was that curtain wall typologywould evolve as quickly as building codes would allowto accommodate ever-taller buildings in America’swild skyscraper boom.

Central to the development of the Americanskyscraper was the concept of the curtain wall, whichopened new avenues for architectural expression. Asdefined in the Kidder-Parker Architects’and Builders’Handbook in the first half of the 20th Century, a cur-tain wall is “an enclosing wall built and supportedbetween columns or piers, and on girders or other sup-port, and sustaining no weight other than its own.”Spanning between support points at floor levels, itsprimary functions are to provide weathertightness,provide a fire wall, and transfer laterally-induced loadsto the structural frame. The curtain wall does not havethe limitations of a bearing wall so that more wall areacan be opened up for glazing.

Numerous factors of change were involved withthe chief one being economic – buildings had to bebuilt ever taller, faster, and cheaper. These factorswere basic for a quickly growing and resource-richcountry. A reactive factor was the need for fireproofbuildings – this was not surprising in a country plaguedby urban fires throughout the 19th Century. From wallconstruction that consisted of heavy masonry to con-temporary metal and glass, curtain wall components

Figure 1. Installation of the prefabricated steel curtain wallat 860–880 Lake Shore Drive in Chicago, USA, one of thefrst residential skyscrapers to be clad entirely in glass andmetal (Mies van der Rohe, 1949–1951).

changed dramatically during the course of the 20thCentury. This curtain wall evolution resulted in shift-ing the functional burden onto thinner and differentmaterials.

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2 DUELING FACTORS IN AMERICANCONSTRUCTION: ECONOMY AND FIRESAFETY

Economy in construction of buildings is an axiomof modern construction worldwide and the excep-tions that one may cite would only prove the rule. InAmerica this axiom was very specific and tied to thedevelopment of the American West.

Consider the development of the balloon frame, adistinctly American framing technique that fulfilledthe requirements of a young and rapidly expandingcountry. In a balloon frame structure, studs run con-tinuously from the sill to the second floor top platewhich supports the roof structure. The second floorjoists are nailed to the studs and rest on ledger boardsthat are fit flush into the studs. This system relies onthe external sheathing rather than triangular bracingfor lateral strength.

It was characterized by the use of plentiful woodfrom America’s once ubiquitous forests, rapid con-struction, and the limited skill required by the workers.The advent of cheap machine-made nails, along withwater-powered sawmills in the early 19th century madeballoon framing highly attractive. Standard wood ele-ments such as studs were readily available. Althoughlumber was plentiful in 19th Century America, skilledlabor was not. Relatively unskilled workmen were ableto erect balloon frames quickly because the labor-intensive mortise and tenon connection of the bracedframe technique was supplanted by the use of cutnails. Without balloon framing, the western boom-towns of America and Canada certainly could not haveblossomed overnight.

These important economic factors of construction –namely rapid construction and limited skill required bythe worker – set a trend for American and then Worldconstruction of large buildings in 20th century.

Another important but reactive factor was the needfor fireproof buildings as can be seen in Chicago.Chicago’s building history recommenced after 1871,when the Great Chicago fire devastated the City.Despite this catastrophe – and due to a strong localeconomy, the high cost of land, and rapidly evolvingbuilding technologies – Chicago quickly rose like thePhoenix and afforded an opportunity for the realizationof innovations in architecture, engineering, and con-struction which established it as the birthplace of theskyscraper.The answer of who invented the skyscraperis debated to this day and depends entirely upon whatare considered to be the defining characteristics. Com-parative characteristics are the separation of the wallfrom the frame, the first use of the iron frame, appear-ance of the beam-column moment connection, heightlimits, use of the passenger elevator, and theories offrame stiffness. For this paper we use the wall andframe separation definition.

These structures needed to be fireproofed andthe urban fires revealed that iron work, thoughinflammable would yield and fail beneath the flames.George H. Johnson, an English-educated archi-tect working with the Architectural Iron Worksof New York founded a business to manufacture thefireproof terra cotta tile that he patented in 1871. Ontrips Johnson made to Chicago to promote the sale ofhis fireproof tile may have become a precedent for theWilliam leBaron Jenney’s 11-story Home InsuranceBuilding (Chicago, 1885). The use of masonry, andlots of it, in skyscraper construction would prove to bedifficult to move beyond in curtain wall developmentdue to its fire resistant qualities.

Following these 19th Century building trends camethe development of Modern Movement technologiescharacterized by experimentation and innovation inconstruction materials and techniques, many of whichhad neither the resilience nor the longevity of tradi-tional construction. Materials such as cinder concrete,plywood, and particleboard; and systems includingpanelized construction, and thin curtain walls with lit-tle redundancy came into use. This experimentationwith materials and systems remains unabated today.

3 THE DEVELOPMENT OF THESKYSCRAPER TYPOLOGY IN AMERICA

The curtain wall dichotomy can be traced back tonumerous 19th and early 20th Century antecedents.In the United States curtain wall development becameintertwined with that of the skeleton frame. Skyscrap-ers would not have been technically feasible withoutthe lightweight curtain wall. And it was through theskyscraper that the curtain wall achieved its greatestrealization.

3.1 The skeleton frame

In the nineteenth century, engineers first utilized metalframe construction in bridges, factories, and ware-houses, and cast and wrought iron were the majormetals used in construction. The invention of theBessemer process in England in 1856 made it pos-sible to produce large quantities of steel affordably.In the United States, steel production on a large scalewas realized in the 1870s. The transition from ironto steel was gradual, with iron still used in buildingconstruction as late as the 1890s.

Arguably, the first metal skeleton-framed skyscraperwas the Home Insurance Building designed by Jenneyin Chicago. In this structure, column loads were trans-ferred to stone pier footings via the metal framewithout load-bearing masonry walls. Each level ofthe exterior wall was supported on a shelf angle fixedto the spandrel girder. At that time skyscrapers weredesigned without lateral bracing under the assumption

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Figure 2. The 20 story Masonic Temple in Chicago with alaterally braced steel frame clad in a masonry curtain wallskyscraper (Burnham and Root, 1892).

that the heavy masonry cladding provided sufficientrigidity for the whole structure.

As skeleton framing came into common use,masonry bearing wall construction reached its prac-tical limit. Burnham and Root’s 16-story MonadnockBlock (Chicago, 1891) utilized traditional load-bearing masonry walls which at grade were almost 2meters thick. This building also utilized the first rigidframe for lateral stiffness. At the same time, Burn-ham and Root developed a complete steel frame forthe Rand McNally Building (Chicago, 1890). Theyalso developed a steel frame laterally stiffened witha diagonal bracing system in the 20-story MasonicTemple (Figure 2). Within a three year period thesame firm had erected a bearing wall; steel frame; anda diagonal-braced metal frame skyscraper revealingthe experimental and quickly evolving nature of thetypology.

Several years later, the structural innovations of theChicago school were taken further by D.H. Burnhamin the Reliance Building (Chicago, 1895). The exte-rior bays were designed as rigid steel frames, andtwo-story columns erected with staggered joints fur-ther increased frame rigidity. The construction ofthe Reliance Building also illustrated all of the

Figure 3. The New York Daily News building with amasonry curtain wall that has been completely stripped ofdetail as a cost measure (Hood and Howells, 1930).

familiar construction techniques which had come intowidespread use at the end of the 19th century to facil-itate rapid construction. The construction site was litso work could continue into the night. Work spaceswere enclosed and heated so the project could proceedduring the winter months. The structural frame, whichbegan erection in mid July of 1894, was topped off onAugust 1 and required only 15 days.

Steel and then concrete skeleton framing soonbecame universally accepted for skyscrapers. There-after, improvements of known design methods encour-aged the construction of increasingly taller buildings.

4 THE MASONRY CURTAIN WALL AND ITSDETERIORATION MECHANISMS

A good place to begin discussion of the early curtainwall is with the Reliance Building in Chicago, the firstskyscraper to fully utilize terra cotta as a cladding. Theterra cotta units of the curtain wall are connected to agridwork of cast-iron mullions, lintels, and sills whichspan between levels. Unlike the Home Insurance andother similar buildings, the Reliance frame did not relyupon the masonry curtain wall for lateral support.

In New York City the steel frame and masonry cur-tain wall became established with theAmerican Surety

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Building in New York City (Price, 1894), and, onceadopted, skyscraper heights increased dramatically inthat city. The once impressive twenty-story buildingsof Chicago were overshadowed by buildings 100, 200,and finally 238 meters with the Woolworth Building(Cass Gilbert, 1913). The Woolworth building utilizedthe latest developments in steel frame construction, butits curtain wall had not abandoned the use of masonryconstruction and ornament.

The First World War administered the final blowto the arts and crafts movement in Europe, and themachine became the basis of a new architecture. Mod-ern European architecture required that the labor ofproducing the parts be performed in the factory ratherthan by craftsmen on site. German intellectuals werein awe of the example of the American skyscraper,a strong symbol of the new world for which theyendeavored. Ludwig Mies van der Rohe, prior to relo-cating in Chicago, prepared a series of unrealizedprojects in which the most famous came to be knownas the “Glass Skyscraper,” a highrise enveloped totallyin glass.

Contemporary with the European modernists,America had entered a second skyscraper era. The postWorld War I period brought a demand for increasedspeed in design and erection. Curtain wall con-struction, however, continued to utilize the masonrytechniques that had been developed by the turn-of-the-century. Though mass production and standardizationhad begun to impact the details of architecture, theuse of prefabrication and the module were not yetextensively used.

H. R. Dowsell, one of the architects of the EmpireState Building (New York City, 1931) wrote of themasonry curtain wall, “Tradition has clung to the her-itage of thick masonry walls. We inherited masonrywalls and seem unable to outgrow our inheritance.The idea that masonry is the only form of permanentconstruction was so deeply rooted that practically allbuilding codes made masonry walls mandatory . . .”(Engineering News Record, 19 February 1931).

The masonry trend continued into the Great Depres-sion while stripping away any costly ornamenta-tion. Examples would include the New York DailyNews (Figure 3), McGraw-Hill building (Hood andFouilhoux, 1932), and the PSFS Building (Howe andLescase, 1931). However, due to the Great Depressionand the Second World War, construction grinded to ahalt for almost 20 years.

Masonry curtain walls that we care for today con-sist of facing material backed by several wythes ofbrick or clay tile, for a total wall thickness of as muchas 30 cm or more. Metal shelf angles to support themasonry infill are anchored to the structure at the floorlevels. Flashing was not commonly used and corrosionof the shelf angles and other metal embedments is asignificant cause of distress.

Another issue is the differential movement of thecurtain wall relative to the structural frame. By 1894,lateral movement in curtain wall construction wasactually being studied and analyzed. The masonrycladding is also exposed to temperature-related move-ments, while the embedded frame is protected. Struc-tural frames shorten under dead load and materialcreep. Conversely, fired-clay masonry curtain wallsexpand due to the intake of moisture. Early curtainwalls were not built to accommodate these differentialmovements resulting in the introduction of unantic-ipated stresses into the curtain wall and frame. Thisproblem came to be understood as evidenced by dis-placement measurements that were performed duringconstruction of the Empire State building in 1931. Thehorizontal deflection of the top of the Empire StateBuilding was monitored by the American Institute ofSteel Construction. Measurements were also made todetermine exactly how much lower the various floorswere from their theoretical position. These measure-ments showed that the 85th floor was 16 cm. belowits theoretical elevation. (H.G. Balcom, “New York’sTallest Skyscraper,” Civil Engineering 1 no 6, March1931).

To address this differential movement, a pressure-relieving joint or “cowing” was developed by the1930s. Cowing composed of corrugated lead was typ-ically laid horizontally at mid-span in selected levelsof the skyscraper facade. This form of pressure reliefwas utilized in masonry envelopes until the 1960s.

Besides serving the function of transmitting windloads to the structural frame, the curtain wall alsomust resist moisture infiltration. To enter the interior,the moisture must pass through the mass of the wall;however it may take a relatively long period of timethis infiltration to become apparent. The mass of thewall may become saturated with slowly developingdetrimental results.

5 THE METAL AND GLASS CURTAIN WALLAND ITS DETERIORATION MECHANISMS

New technologies resulting from World War II had agreat influence on the acceptance of the machine-mademetal and glass curtain wall. Given the abundant post-war supply, aluminum was reasonably priced. Therewas experimentation with mild steel, stainless steel,and bronze as well. Extruded components were suit-able for standardization and could be prefabricated fordelivery to the site. This was important because laborhad become a significant part of construction costs.The new curtain wall technology further decreasedbuilding weight and construction cost, and increasedusable floor area. Prefabricated construction was lesslimited by cold temperatures which prohibited erec-tion of “wet” walls of brick and mortar. The invention

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Figure 4. The Lever House in NewYork where the prefabri-cated metal and glass curtain wall realized its potential (SOM,1952).

and development of float glass by Alistair Pilkingtonin the 1950s would make large panes of glass afford-ably available. American architectural philosophers ofthe day lauded the fact that craftsmanship had beentransplanted from the site to the factory.

One of the first post-war buildings to be constructedwith a glass curtain wall was the Equitable Building(Pietro Belluschi, 1948) in Portland, Oregon. Belluschiwas able to take advantage of leftover aluminum stock-piled for World War II by smelters and to utilizeassembly techniques derived from West Coast airplaneplants. The 860–880 Lake Shore Drive buildings inChicago (Figure 1) were among the first residentialbuildings in the United States to be sheathed entirelyin glass, and were the realization of Mies’ 1920 pro-posal for a glass skyscraper. The steel, aluminum, andglass skin was assembled on the buildings’ roofs intwo story high units, and then lowered into place onthe facade.

At the Lever House (Figure 4), the curtain wall hasan interior frame of mild steel clad with stainless steel.At the United Nations Secretariat Building (Harrisonand Abramovitz, 1950), curtain walls were conceivedas an assembly of aluminum windows held in placewith a grid of reinforced mullions. At both buildings,the lower portion of the curtain wall at each level wasbacked up by a concrete masonry wall to provide the

fire rating that code officials felt was not providedby the curtain wall. The masonry wall could not beimmediately abandoned though it became hidden fromview.

The approach to curtain wall design that quicklyevolved was to make the joints as weathertight as pos-sible, then provide positive means for conducting anywater leakage out of the wall.Thus an interior drainagesystem was provided to collect water that leaks throughthe cladding and direct it back to the exterior.

The development of new curtain wall materialsoccurred in the post-war years: thin stone veneers,precast concrete, brick veneers, and structural siliconeglazed facades.To bring things full circle, the aestheticdevelopment of “Post-Modernism” led to a return toearlier architectonic forms, but not a return to earliermethods of construction.

Metal and glass curtain walls that we care for todayconsist of factory-fabricated and preassembled metalunits that are connected to the structural frame. Glasshas gone through radical technical developments andis typically no longer a monolithic material. In a cur-tain wall it can appear as an insulated glass unit ora sandwich of materials developed to strengthen it. Itmay have clear, colored, or reflective coatings installedon one or more of its surfaces or may have transpar-ent or translucent colorants integral with the glass.The assemblies may also include panels of aluminum,ceramics, precast concrete, or stone.

Though not the vision of 1950s designers, metal andglass curtain walls are wholly reliant on sealants to per-form adequately. High performance sealants includenewly developed families of elastomeric sealants suchas polysulfides, solvent acrylics, urethanes, and sili-cones. Numerous sealant products are now availableand have been developed to be used in either curtainwall glazing, installation, or repair.

Modern structural frames are more flexible becausethey are designed to tighter limits with less mate-rial and are more exposed to temperature extremesthan the frames of a masonry-clad building. Prefab-ricated curtain wall units are detailed to accommodatethese increased movements. However, lateral move-ments of the frame and differential movement betweenthe frame and the cladding can lead to distress in theglass and metal curtain wall.

6 LOSS OF REDUNDANCY AND INCREASEIN COMPLEXITY

In a general sense, redundancy in design can preventfailures of a building system. Structural redundancy,allows for loads to follow an alternate path if theprimary supports fail. Redundancy of design for cur-tain walls can apply not only to structural but toperformance issues such as weathertightness.

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The mass of the masonry curtain wall provided aredundant system with empirically developed strengthcapacities that extend far beyond that required. Whilelack of maintenance could lead to water infiltrationproblems or deterioration of the cladding system, theseproblems do not become immediately apparent butbecome manifest only over a period of time.

In the metal and glass curtain wall strength capac-ities are engineered to meet specific standards andredundancy is reduced. An engineered system of per-formance redundancy has been introduced that pro-vides a weathertight barrier while providing internaldrainage as well. However, a breakdown of these pro-tective systems in the metal-and-glass curtain wallwould lead to immediate water leakage on the interior.

High-tech materials have been adopted for use onthese curtain walls that offer unique maintenance chal-lenges. Tempered glass may fail due to nickel-sulfideinclusions. Thin-stone veneers may become distressedfrom material weakness, loss of strength hysteresis, ordeterioration due to anchorage. Incompatible materialssuch as certain sealants and building stone or dissimi-lar metals can cause staining which can pose long-termmaintenance problems. Little is known regarding thelong term performance of sealants that are now beingused to hold glass panels in place thus eliminating themetal grid.

7 CONCLUSIONS

Due to the sophisticated engineered systems that havebeen introduced, and the decreased redundancy, metaland glass curtain wall systems pose unique and com-plex problems. Diagnosis of this unique typology hasbeen made much easier by following the guidelinesprovided in the ICOMOS Charter: Principles for theAnalysis, Conservation and Structural Restoration ofArchitectural Heritage (ISCARSAH Principles) thatwas ratified by the ICOMOS 14th General Assemblyin Zimbabwe in 2003.The ISCARSAH Principles pro-vide a comprehensive and well considered philosophyfor building diagnostics which proves just as valu-able (if not more so) for engineered systems as theywould for empirically designed medieval structurescomposed of masonry and wood.

However there is a wide chasm that presently existsbetween professionals in the preservation communityand environmental professionals. The obvious debateis whether it is a sustainable strategy to maintainan aesthetic and keep original material or whetherit is more important to achieve better water shed-ding and thermal performance by compromising theoriginal material. Windows are sources of heat lossand gain and curtain walls even more so. This wasnot a great concern to designers of large structuresenveloped in glass prior to the Energy Crisis of 1973.However energy use is of primary importance today

and regulated by the US Government. Consequentlythere are numerous instances where curtain walls arediscarded and replaced rather than conserved.

Another issue is obsolescence. Curtain walls are theembodiment of pure hard economics on the construc-tion industry. Early metal and glass curtain walls, beingavant garde, were not designed with a discernible lifes-pan.The aluminum was cheap and easy to fabricate andnot finished in a way that would preserve its luster.Some glass products were manufactured using tech-nically sophisticated and obsolete processes, and itis no longer feasible to authentically recreate them.Original sealants or sealants used for repair are notalways compatible with sealants that are presentlyavailable.

A third issue is defining significance of the metaland glass curtain wall. Is significance defined by thematerial itself or by the transparency that it provides?Perhaps a key to all of the issues cited can be foundin the Nara Document on Authenticity, “authenticityjudgments may be linked to the worth of a great vari-ety of sources of information. Aspects of the sourcesmay include form and design, materials and substance,use and function, traditions and techniques, locationand setting, and spirit and feeling, and other externalfactors.” This statement infers that the historic fabricdoes not necessarily need to be conserved to maintainintegrity.

Perhaps our approaches to energy inefficient build-ing claddings will need to be reassessed as we enterinto a new energy paradigm. Be that as it may, asour post-World War II buildings become historic land-marks, the challenges that we face will be complexas well and will require greater degrees of scientificinquiry and the active participation of the engineer.

REFERENCES

Kelley, S., “The Glass and Metal Curtain Wall: History, Diag-nostics, and Treatment,” Preserving Post-War Heritage,Edited by Susan MacDonald, London: Donhead, 2001.

Kelley, S., “Conflicts and Challenges in Preserving CurtainWalls,” APT Bulletin, Volume 32, Number 1, 2001.

Kelley, S., “Office Buildings of the Chicago School: TheRestoration of the Reliance Building,” KonservierungDer Moderne, ICOMOS, Hefte des Deutschen NationalKomitees XXIV, Munich 1998.

Kelley, S., and Dennis Johnson, “The Glass and Metal Cur-tainwall: The History and Diagnostics,” Modern Move-ment Heritage, London: E & FN Spon, 1998.

Kelley, S., and B. Kaskel. “Curtain Walls in the USA: Fail-ures, Investigation, and Repair,” Curtain Wall Refurbish-ment, A Challenge to Manage, Eindhoven, Netherlands:DOCOMOMO International, 1997.

Kelley, S., “The History of the Glass and Metal Curtain Wall:From the Reliance Building to the Lever House,” WindowRehabilitation Guide for Historic Buildings, WashingtonDC: Historic Preservation Education Foundation, 1997.

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Kelley, S., “Aluminum: History and Conservation.”TwentiethCentury Building Materials.Thomas C. Jester, editor. NewYork, New York: McGraw-Hill, 1995.

Kelley, S., “The History of the Curtain Wall: From Crafts-manship to Machine Made.” Preserving the Recent Past.D. Slaton and R. Shiffer, editors. Washington, D.C.:Historic Preservation Education Foundation, 1995.

Hunderman, H., J. Koerber, and S. Kelley. “Curtain WallDevelopment: The Loss of Redundancy.” D. Slaton and

R. Shiffer, editors. Preserving the Recent Past. Washing-ton, D.C.: Historic Preservation Education Foundation,1995.

Kelley, S., C. Paulson, and D. Slaton. “Assessment Tech-niques Utilized with Historic American Highrises,” inStructural Repair and Maintenance of Historical Build-ings. Southampton, England: Computational MechanicsPublications, 1989.

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