Detailing in Transition: Hybrid Walls and the Evolution of Terra Cotta Detailing Rachel L. Will and Edward A. Gerns Wiss, Janney, Elstner Associates 10 South LaSalle, Ste. 2600, Chicago, IL 60603 Phone: 312-372-0555 • E-mail: egerns@wje.com S y m p o s i u m o n B u i l d i n g E n v e l o p e T e c h n o l o g y • Oc t o be r 2 0 1 4 Wi l l a n d Ge r n s • 8 1 AB STRA CT The significant evolution in architectural terra cotta detailing relates to the development of hybrid wall systems, which combine characteristics of both load-bearing walls as well as true curtain walls. Between 1890 and 1940, hybrid wall systems were widely used throughout the United States, but were evolving as evidenced in the published standards and details. As the hybrid wall systems evolved and cladding was treated as a separate component of the wall system, lessons learned from early failures were subsequently incorporated into the industry literature. This presentation will discuss the changes that occurred during this time period. SPEA KER S Edward A. Gerns – Wiss, Janney, Elstner Associates Edward Gerns is a project manager and project architect/engineer experienced in the investigation and repair of deteriorated conditions in existing buildings. He performs evaluations of brick, terra cotta, and stone masonry; assesses causes of collapse or distress in existing cladding systems; and has inspected numerous structures damaged by wind, ice, snow, and fire. Gerns has overseen preparation of repair documents for contemporary and historic buildings and structures. His expertise includes exterior wall evaluation and restorations for buildings ranging from churches to high-rise offices. He also has extensive experience with all typical façade systems, including masonry, stone, concrete, exterior insulating finishing (EIFS), and metal and glass curtain walls. Rachel L. Will – Wiss, Janney, Elstner Associates Rachel Will performs building envelope evaluations and investigations of distressed and deteriorated conditions in existing buildings. She participates in various projects, including façade inspections, condition surveys, structural analyses, repair design, construction document preparation, and construction observation. Will’s expertise includes documentation and investigation of building façades, as well as preservation and repair of historic buildings. 8 2 • Wi l l a n d Ge r n s S y m p o s i u m o n B u i l d i n g E n v e l o p e T e c h n o l o g y • Oc t o be r 2 0 1 4 S y m p o s i u m o n B u i l d i n g E n v e l o p e T e c h n o l o g y • Oc t o be r 2 0 1 4 Wi l l a n d Ge r n s • 8 3 ABSTRACT Hybrid wall systems make up the majority of the masonry façades that were constructed shortly after the advent of the skeletal-frame structural system in the late 19th century. These façades represent the design progression from load-bearing mass masonry walls to the cavity-wall systems utilized today. While the introduction of the independent structural frame (steel or less commonly used reinforced concrete) was the driving force behind the shift in design theory regarding masonry façades acting as part of the structure to becoming an element of the building envelope, multiple other factors also played a role in this transition. Some of these elements included ventilation, efficient use of lighting energy, interest in maximizing leasable area and volume, the introduction and perfection of the plate-glass industry, and ultimately, the arrival of affordable and reliable mechanical systems. The technological achievements and advancements in civil engineering that occurred during this period significantly altered the built environment. Numerous unique wall systems—often proprietary in nature—were utilized for many building types throughout the United States during this period. As expected, building façades constructed in comparable periods were frequently designed and built with analogous systems; however, subtle nuances have been observed by the authors in later renditions of these systems, due to a better understanding of these wall systems gained through empirical or practical experience and numerous other external factors. This paper (by means of multiple case studies) focuses on the evolution of masonry façade design from the early load-bearing examples to the single-wythe, brick-clad curtain walls made famous following World War II, as well as the elements leading to such design alterations. BACKGROUND Prior to the 1870s, the exterior walls of a building functioned as both the building’s structural system and the enclosure for the interior space. The thickness of load-bearing mass walls was dependent on the thickness of the exterior walls and the load capacity of the underlying soil. Typically, the walls were constructed with an exterior wythe of character-defining masonry, such as brick or stone, and utilitarian brick masonry backup. The wythes of these walls were tied together by uniformly distributed individual units or headers, which are units turned perpendicular to the plane of the wall that span between adjacent wythes. These walls were intended to resist water infiltration by acting as a combination sponge and barrier, as well as unique design elements to help limit the water on the face of the wall, such as cornices, projecting water tables, etc. The majority of water and wind exposure was deflected by the outer surface of the walls. Moisture that bypassed the outer surface, through cracks and discontinuities in joints or masonry units, was absorbed by the wall system and eventually evaporated, presumably before reaching the interior surfaces. Wall thicknesses were somewhat empirical, based on practical structural considerations and prescribed codes. The mass of the wall, depending on climate, acted as both a water management system and thermal mass for passive heating and cooling. Addressing water infiltration and dampness in buildings has long been recognized as an important design and potential health issue. The notion of removing the moisture or minimizing the potential for it to reach the interior resulted in the development and evolution of cavity walls from mass walls. Vitruvius, in The Ten Books of Architecture, recognized that the need for removing moisture from walls by introducing ventilation into a wall system was one method of achieving this: “…if a wall is in a state of dampness all over, construct a second thin wall a little way from it on the inside, at a distance suited to circumstances; and in the space between these two walls, run a channel at a lower level than that of the apartment, with vents to the open air. Similarly, when the wall is brought up to the top, leave air holes there…” (Vitruvious, 1914 reprint) Palladio later stated: “It is very commendable in great fabricks [sic], to make some cavities in the thickness of the wall from the foundation to the roof, because they give vent to the windows and vapours, and cause them to do less damage to the building.” (Palladio, 1738) During the late 19th and early 20th centuries, various wall types were experimented with in building construction in an attempt to address moisture in wall systems and—to some extent—thermal performance. Detailing in Transition: Hybrid Walls and the Evolution of Terra Cotta Detailing Figure 1 – Examples of hollow wall systems. Reprinted from International Library of Technology, Common Brickwork, 1907. Hollow walls—sometimes referenced in period trade publications—essentially consisted of an inner wythe, outer wythe, and a “cavity” of sorts between the wythes, which was bisected by alternating header bricks creating voids between wythes (Figure 1). This wall type could save material costs, provide some thermal disruption, and minimize water infiltration. Similarly, a hollow-masonry wall was constructed of units that incorporated open spaces with the geometry of the units themselves (Figure 2). These types of units are still used today in the construction industry, with a similar notation of minimizing thermal bridges and taking advantage of the insulating properties of air spaces. Finally, the term cavity wall was defined as a wall consisting of an inner and outer wythe connected by metal anchors, resulting in an essentially clear cavity between the two wythes that ranged in width from 2 to 3 in. (Figure 3). These walls were thought to have improved thermal performance and reduced water infiltration. These developments in detailing masonry assemblies for improved thermal performance and reduced water infiltration— along with innovations and advances in structural and architectural design theory— resulted in seminal transitions in the detailing of the masonry façade. These detailing transitions and modifications are clearly visible in the continuum of masonry façades constructed throughout the late 19th and 20th centuries. SKELETON-FRAME STRUCTURAL SYSTEM A discussion of hybrid wall systems is not possible without including a brief discussion of the development of the skeletonframe structural system. The Industrial Revolution of the late 19th century led to greater use of steel as a building material. It was used in multiple capacities, including structural frames and the reinforcing for concrete used in structural frames. With advancements in mechanization, urban population growth, and the rise of the skyscraper, the development of the skeleton-frame building system quickly followed. Steel and reinforced- concrete-frame buildings became common practice in the building industry because of their economy, scale, and the speed of construction that could be achieved, in contrast to traditional loadbearing mass masonry buildings. HYBRID WALLS Wall systems that are classified as hybrid combine characteristics of both loadbearing wall systems, as well as true curtain walls (where the exterior cladding is completely independent from the structural system). For the purposes of this paper, hybrid walls are defined as having the following characteristics: • A three- to five-wythe exterior masonry wall system and a steel or reinforced concrete-frame main structural system that supports floor loads • An exterior wythe supported by rolled steel or iron shapes attached to the main structural system and consisting of some combination of brick, terra cotta, and stone • Interior wythes header-bonded to each other and, to some extent, into 8 4 • Wi l l a n d Ge r n s S y m p o s i u m o n B u i l d i n g E n v e l o p e T e c h n o l o g y • Oc t o be r 2 0 1 4 Figure 2 – Hollow masonry wall system. Figure 3 – Cavity wall system. Figure 4 – Hybrid wall systems, reprinted from Good Practice in Construction, Philip Knobloch, 1923, Pencil Points Press. the outer wythe, consisting of brick masonry and/or extruded terra cotta blocks • An exterior wall system that is intended to function as a barrier system to manage water infiltration Between 1890 and 1940, this wall system was widely used throughout the United States. Numerous variations of the system still exist. Accepted industry standards and details were widely published, but individual architects and engineers often had their own approaches, as represented in Figure 4. HYBRID WALL SYSTEMS: BEHAVIOR At the most basic level, combining numerous materials with different properties, and ever-taller buildings with hybrid wall systems, introduced challenges that had not previously existed in monolithic mass walls. There was an understanding that differential movement of the curtain wall relative to the structural frame was an issue that should be addressed. As stated by Viollet-le-Duc: If, therefore, we undertake to encase an iron structure with a shell of masonry, that shell must be regarded only as an envelope, having no function other than supporting itself, without lending any support to the iron, or receiving any from it. Whenever an attempt has been made to mingle the two systems, mischief has resulted in the shape of dislocations and unequal settlements (Viollet-le-Duc, 1877). As the buildings became taller, the need to address lateral loads, largely ignored prior to this time, had to be considered. In 1894, lateral movement in curtain wall construction was actually being studied and analyzed. For example, lateral movements of steel-framed buildings in Chicago, including the 17-story Monadnock Building and the 14-story Pontiac Building, were documented under high-wind loads (Stebbing, 1894). ARCHITECTURAL TERRA COTTA Hybrid wall systems were the bridge between traditional mass walls and cavity walls. This transition was also accompanied by the evolution of the use of terra cotta cladding during the same period. Terra cotta has been used as a building material for thousands of years, yet its use as a cladding material generally coincided with the development of the skeleton-frame structural system and the skyscraper. Literally translated, terra cotta means “baked earth”—a mixture of clay and water that is fired to the point of sintering. Architectural terra cotta is defined as “clay products employed from structural decorative work which cannot be formed by machinery” (Heinrich, 1912). The term “architectural terra cotta” can include 19thcentury terra cotta, which was unglazed and buff or red in color; slip-glazed material coated with thin clay slurry for a matte finish; or glazed terra cotta, coated on the outer surface with a semi-vitreous or vitreous glaze created by adding fluxes and coloring agents to the clay slurry, which fuse into a layer of glass at the temperatures reached during firing (Stratton, 1996). Architectural terra cotta units are comprised of an outer shell and are braced with intermittent webs to prevent warping of the unit during firing, as well as adding strength to the units. Spacing of the webs was generally intended to create “cells,” or the area between webs (Figure 5). Bricks and mortar were used to fill the cells in the terra cotta units from behind to create a monolithic/hybrid wall system. Units were generally referred to as balanced units if they were installed within the plane of the wall, and unbalanced units if they projected from the wall and required additional anchorage support during installation. When compared to stone, the relatively “lighter” weight of terra cotta units allowed designers to more economically introduce cornices and other projecting building elements. The incorporation of these elements was utilized to help with water-shedding capabilities, which became an issue as the exterior wall mass decreased. Supplemental steel framing was anchored to the structural building frame to allow terra cotta units to be cantilevered from the plane of the exterior wall. Hung units, such as window heads, were supported by horizontal bars, which were inserted through holes in the webs and supported by J-hooks. The J-hooks were in turn anchored to struc- S y m p o s i u m o n B u i l d i n g E n v e l o p e T e c h n o l o g y • Oc t o be r 2 0 1 4 Wi l l a n d Ge r n s • 8 5 Figures 5A and 5B – Architectural terra cotta fabrication. tural steel elements such as shelf angles or spandrel beams. The structural elements could be located directly above the hung units, embedded within the brick masonry, or placed behind additional terra cotta units within the system. The increasing popularity of using terra cotta as a cladding material was attributable to three distinct factors: fire-resistant characteristics, the innovation of building technology and steel or concrete construction, and flexibility of design at a low cost (Kurutz, 1989) as explained below: • Fireproof: Following the great 19thcentury fires of Chicago and New York City, fireproof construction became essential. Architectural terra cotta provided protection for structural steel and wrought- or cast-iron elements (Tunick, 1997). In addition to architectural terra cotta used as a facing material, structural clay block was another terra cotta product that became a common fireproof, nonstructural, lightweight replacement for masonry fill material. • Lightweight: Terra cotta, being lighter than their stone counterparts, required less labor to maneuver and set the individual units. • Economical: Terra cotta was first used as a substitute for traditional cladding materials, often mimicking the appearance of brownstone or limestone. With the advance of ceramic glazes, terra cotta would also evolve into an aesthetic expression of its own. Terra cotta was cheaper and faster to produce than cutting and carving individual stone units, while easily adapted to the same traditional details. Installation Installation methods for architectural terra cotta evolved along with innovation in the construction industry. Terra cotta was first used as a masonry component integrated into load-bearing walls. As defined above, these systems are consistent with hybrid walls in that open terra cotta units were keyed into brick masonry, fitting brick and mortar tightly between the structural walls and webs to secure each unit with a minimal use of (metal) fasteners or ties (National Terra Cotta Society, 1914). As construction methods changed and terra cotta was used more as a cladding material, the installation detailing for terra cotta would change as well (National Terra Cotta Society, 1927). The early practice of filling units with brick and mortar shifted to rely heavily on ferrous fasteners or attachments to secure terra cotta cladding to the larger steel-frame superstructure. The use of copper alloys for fasteners was typically limited to areas such as cornices, water tables, or parapets, which were more likely to have exposure to moisture and resulting corrosion (Figure 6). Filling Terra Cotta One design issue that is still debated is the filling of terra cotta units to improve performance and durability. In 1927, the National Terra Cotta Society stated: Exposed free-standing construction, subject to the absorption of water through mortar joints and liable to injury from subsequent freezing or the expansion of improper filling material, should generally be left unfilled (National Terra Cotta Society, 1927). Additionally: [The filling of voids in terra cotta] is a subject that has been very thoroughly discussed by [the National Terra Cotta Society] and yet it remains a debated question. In fact, one portion of this organization believes in complete filling, while there seems to be another group who are doubtful about its value (Johnson, 1926). In 1927, the National Terra Cotta Society listed the following as advantages to filling or partially filling terra cotta units: 8 6 • Wi l l a n d Ge r n s S y m p o s i u m o n B u i l d i n g E n v e l o p e T e c h n o l o g y • Oc t o be r 2 0 1 4 Figures 6A and 6B – Representative terra cotta installation details. 1. It makes a rigid construction. 2. It offers some protection to corrosion of anchors. 3. By the mechanical bond, it may tend to hold pieces in place, perhaps even when cracked. 4. It tends to carry the terra cotta block weight immediately to the back wall (thus minimizing the load on individual blocks). 5. It may reduce the accumulation of large water pockets. 6. It acts as a sound absorbent, getting away from the resonating box effect of unfilled sections. 7. It furnishes a tie system in many conditions, where the metallic tie anchor system is hard to place. 8. It tends toward a complete unit construction, which possibly is an advantage in localities subject to heavy vibrations or earthquake shocks. 9. By some it is claimed that filling strengthens the terra cotta block. The Society also provided the following disadvantages to filling or partly filling terra cotta voids: 1. Concrete is known to expand under certain conditions with age, especially if it becomes water-soaked; and such expansion may rupture the terra cotta. 2. The absorption of water and subsequent freezing may cause damage. 3. A system of filled blocks keyed tightly and quite individually to the backing wall will not permit full play to the block in the adjustment necessary for temperature changes and the deflections from various causes. 4. The thermal expansion of concrete is different and greater than that of terra cotta, and it is in general a better conductor of heat; besides, the terra cotta would be subjected directly to the sun’s rays, while the filling is protected, so that possibly the combination may set up differential strains that would prove undesirable, especially in some climates. 5. The increased rigidity of filled construction may throw heavy stresses on the terra cotta in buildings subjected to heavy wind pressure. 6. The dead weight of the fill, if not properly keyed and supported by the backing wall, may add just so much more load to the lower block courses and thus cause crushing of the terra cotta. 7. Filling adds extra weight to the structure, which in modern buildings means heavier structural sections with resultant increased cost, to which, of course, must be added the extra cost of the fill itself. To illustrate that the issue remains debated in the industry, the European approach to filling terra cotta differs from that of the U.S. In John Fidler’s article titled “Fragile Remains: An International Review of Problems in the Decay and Treatment of Architectural Terracotta and Faience” (Fiddler, 1994), he states: The voids in terra cotta block were often filled with a concrete packing of cement: crushed ballast (maximum diameter 20 mm) in a ratio of 1:7 or 8. During the 1920s, “breeze” aggregate was used in order to save weight, and it was soon discovered that could lead to problems. Apparently, the breeze became swollen when wet, and the expansion is thought to have caused stress cracking in the terra cotta. DISTRESS MECHANISMS Many of the design and anchorage detailing changes were based on in-service performance of terra cotta cladding. As distress began to manifest, the industry recognized the need to evolve. One of the unique characteristics of clay-based masonry units is that they are at their smallest dimensions when leaving the kiln after the firing process, due to the virtual absence of moisture in the units. As the units are exposed to the normal in-service environment, they S y m p o s i u m o n B u i l d i n g E n v e l o p e T e c h n o l o g y • Oc t o be r 2 0 1 4 Wi l l a n d Ge r n s • 8 7 Figures 7A and 7B – Examples of frame shrinkage distress. absorb atmospheric moisture and expand in volume. Once the masonry unit’s moisture content reaches equilibrium with the environment, the volume stabilizes, with only minor increases over time and some cyclic changes in volume due to seasonal fluctuations in environmental moisture. The irreversible expansion due to initial and progressive moisture absorption, as well as the cyclical volume changes due to seasonal effects, can impart significant stresses on brick masonry. Newer masonry buildings incorporate expansion joints that limit the accumulation of these stresses and provide a location where the cyclical and permanent volume changes can be accommodated— but older curtain walls typically did not effectively accommodate these stresses. Another condition unique to some masonry-clad structures is the impact of irreversible shrinkage and creep over time. The amount of shrinkage is in proportion to the height of the structural frame. This type of volume change results in a vertical shortening of the structural frame, which is in conflict with most masonry wall materials that remain constant or increase in volume over time. When load is applied to a structure and sustained, it will initially deflect and continue to deform over time. This long-term change in volume due to the application of load is referred to as “creep.” Creep typically results in a continual vertical shortening of the structural frame and becomes greater as the load increases (Figures 7A and 7B). Similar to shrinkage, the majority of creep will impact a structure shortly after loading, but can continue to have a modest effect throughout the life of the structure. As terra cotta cladding systems age, the passivity of the mortar decreases and discontinuities in the enclosure develop that result in increased water infiltration and corrosion of the underlying steel. The accumulation of corrosive scale on the shelf angles used to support the exterior cladding material further exacerbates the locked-in stresses due to differential movement of the structural frame and terra cotta cladding elements (Figures 8A and 8B). The corrosion of exposed steel has long been recognized as a potential problem. Numerous methods of limiting corrosion have been employed throughout history, including boiling the iron in tallow, cover- 8 8 • Wi l l a n d Ge r n s S y m p o s i u m o n B u i l d i n g E n v e l o p e T e c h n o l o g y • Oc t o be r 2 0 1 4 Figures 8A and 8B – Corrosion-related distress. Figure 9 – 1924 Atlantic terra cotta details. Figure 10 – Northwestern terra cotta detailing, circa 1911. ing it with pitch or varnish, or coating it in molten tin or zinc—otherwise known as galvanizing. Generally, the corrosion process of metal components within a masonry wall system can be divided into three phases. The first phase includes the initial 30 years of service life of the building and represents the period of time when the underlying steel is protected by the alkalinity of the environment and various coatings that may have been applied to the steel. During the second phase, while the protective systems deteriorate, the steel begins to corrode as it becomes exposed to water and oxygen. This results in the third phase, where significant distress will manifest as the cladding system attempts to accommodate the accumulated scale, which occupies four to ten times the volume of the uncorroded steel. TERRA COTTA DETAILING MODIFICATIONS: 1914 TO 1927 Many important lessons learned— attributable to installation issues and observed distress in hybrid terra cotta cladding systems—were incorporated into accepted industry standards of the time. These changes are evident when reviewing articles and product literature, but are most clearly illustrated in terra cotta industry publications. The National Terra Cotta Society was formed around 1910, primarily to encourage designers to use terra cotta in building construction. The society produced guide specifications and details in a folio that was first published in 1914. Prior to this time, many books and articles had been written by architects and engineers on the use of terra cotta as a construction and architectural material. Terra cotta manufacturers of the time also provided reference details to enable designers to understand the detailing and to promote its use (Figures 9 and 10). Within the industry, there were many misconceptions and significant variations in the quality of the terra cotta being produced. Much like many other materials manufactured during this time period, no standards existed for the material. As architectural terra cotta became more widely used, and building heights continued to increase, the early terra-cotta-clad buildings began to age and the first generation of issues emerged. The industry had to address these problems in order to remain viable. Improper maintenance of these early buildings, in combination with significant variations in the material properties of the terra cotta, resulted in the recognition that guidelines should be established. The body of knowledge represented in the 1914 edition of the National Terra Cotta Society’s folio was generally limited to approximately 20 years of building construction. By the 1927 edition, however, almost 35 years of construction, life cycle maintenance, and costs had occurred. The 13 additional years of experience pushed many of the early buildings through their critical initial maintenance cycles. Failures from improper applications, poor detailing, the inability to manage water infiltration appropriately, and a general misunderstanding of material properties resulted in significant modifications to the folio. A comparison of some of the details and specifications provide insight into the lessons learned. Clearly listed in the introduction of the 1927 edition, S y m p o s i u m o n B u i l d i n g E n v e l o p e T e c h n o l o g y • Oc t o be r 2 0 1 4 Wi l l a n d Ge r n s • 8 9 Figures 12A and 12B – 1925 patent for horizontal expansion joint. Figures 11A and 11B – Representative details from the 1914 (left) and 1927 edition (right). the following modifications are identified: • The need for continuous support of the terra cotta cladding at each floor line • The need to incorporate vertical and horizontal expansion provisions into the cladding systems • Modifications required to the detailing of concrete frame buildings to accommodate the frame shrinkage and creep of the underlying frame. • The need to protect embedded steel anchorages and support components from corrosion • No filling of freestanding elements • The need to incorporate flashings and drips to minimize water infiltration and manage water movement over the face of the façade. Figure 11 illustrates a comparison of representative details from the 1914 edition (on the left) to the 1927 edition (on the right). While the modifications were subtle, they nonetheless depicted a change in the general approach of detailing the cladding system. Fundamentally, the 1927 edition began to differentiate the terra cotta as a separate component of the wall system (moving towards the current-day cavity wall) rather than completely integral to the backup wall system (a clear example of a hybrid wall). OTHER DEVELOPMENTS While the National Terra Cotta Society was certainly the most widely recognized authority in the terra cotta industry during this period, it is also worth looking at some of the other developments that were occurring in the industry at this time. The 1925 patent (Figure 12) clearly illustrates the notion of terra cotta as a cladding element. Shelf angle support is shown as a system that isolates each floor rather than allowing for some load sharing and shedding between floors. The shelf angles extend more than half the thickness of the terra cotta, in contrast to the minimal support shown in the National Terra Cotta Society’s 1914 details. In addition, a gap is clearly shown below the horizontal leg of the shelf angle—presumably to allow for deflection of the horizontal leg, expansion of the masonry, and frame shrinkage. The system seems to have been copied almost verbatim in the 1935 patent (Figure 13), but a compressible corrugated lead sheet, known as lead cowing, was introduced where the joint had previously been left open. These systems had existed prior to the 1920s, but became commonplace in many of the skyscrapers of the late 1920s. The 1927 cornice support system (Figure 14) reflects the generally understood notion of allowing cornice and water table elements to remain as light as possible. In this instance, the wall system itself is shown as a solid element, but the projecting portion of the façade is treated essentially as an independent element. As illustrated in another 1927 patent (Figure 15), some details still conceptualized a monolithic wall system. The impetus of the patent is an anchorage system for terra cotta on a concrete frame substrate, but the units are depicted to be filled solid. 9 0 • Wi l l a n d Ge r n s S y m p o s i u m o n B u i l d i n g E n v e l o p e T e c h n o l o g y • Oc t o be r 2 0 1 4 Figure 13 – 1935 patent for lead cowing for horizontal expansion joint. Figure 14 – 1927 patent for cornice anchorage. Figure 15 – 1927 patent for terra cotta cladding anchorage system. This example illustrates that even as late as 1927, the material was not fully understood throughout the industry, since the units were shown to be filled and conceptualized as a solid mass wall rather than a cladding system. By this point, the Terra Cotta Society had modified its details related to filling units and was recommending against the practice. Finally, the joint treatment shown in the 1932 patent (Figure 16) illustrates an attempt to mitigate water infiltration through the joint, or perhaps weeping incidental water out of the system. This method installs a noncorroding metal such as zinc or copper into pre-cut grooves at the joint between two units, so that it is “positively spanning and sealing the mortar joints and directing the seepage, if any, to the place where it will do the least damage.” The configuration of the components indicates an understanding of the importance of the joints in the water management system, but also seems to be an attempt at the introduction of a flashing system of sorts into the wall assembly. CONCLUSION At the most basic level, masonry façade deterioration is the result of numerous factors, including differential material properties, movements, moisture, temperature fluctuation, and gravity. Terra-cottaclad hybrid wall systems constructed between 1890 and 1940 have a unique set of characteristics that clearly show how designers of the time were gaining knowledge of these wall systems through the experience of initial decades of their performance. Lessons learned in the industry from early failures related to water infiltration lead to incorporation of weeps and flashing to minimize corrosion and improve water management. Provisions for movement- and load-path-related distress lead to modification of unit and anchorage geometry, as well as incorporation of movementrelated detailing. The manufacturers’ understanding of these factors resulted in relatively rapid changes in the industry standards for terra cotta detailing. REFERENCES John Fiddler (September 1994). “Architectural Ceramics: Their History, Manufacture and Conservation.” Proceedings of a Joint Symposium of English Heritage and the United Kingdom Institute for Conservation. English Heritage. pp. 22-25. Robert Johnson (1926). “Principles of Terra Cotta Construction.” Directory of Engineering. pp. 22-23. Gary Kurutz (1989). Architectural Terra Cotta of Gladding McBean. Wingate Press. p. 8. National Terra Cotta Society (1927). Terra Cotta – Standard Construction. (Original work published 1914.) Andrea Palladio (1965). The Four Books of Architecture. New York: Dover Publications, Inc. p. 7. (Original work published 1738.) Marcus Vitruvious Pollio (1914). Vitruvious, The Ten Books on Architecture (Morris Morgan, Trans.). London: Harvard University Press. p. 209. Heinrich Reis (1912). Building Stones and Clay Products: A Handbook for Architects. New York: John Wiley & Sons. p. 320. William Stebbing (March 1, 1894). Engineering News. p. 471. Michael Stratton (1996). “Nature of Terra Cotta and Faience.” In Jeanne Marie Teutonico (Ed.), Architectural Ceramics: Their History Manufacture and Conservation. London: English Heritage. pp. 50-52. Susan Tunick 1997. Terra-Cotta Skyline: New York’s Architectural Ornament. New York: Princeton Architectural Press. p. 7. Eugene-Emmanuel Viollet-le-Duc (1987). Lectures on Architecture, Vol. 2, Lecture 11). New York: Dover Publications Inc., p. 2. (Original work published 1877.) S y m p o s i u m o n B u i l d i n g E n v e l o p e T e c h n o l o g y • Oc t o be r 2 0 1 4 Wi l l a n d Ge r n s • 9 1 Figure 16 – 1932 patent for joint treatment in head joints between units.
