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Walls Still Growing Up Age of Steel The prevalence of solid masonry began to wane in Western societies in the late 19th century, with the introduction of steel frame construction. While effective for low-rise construction, solid masonry buildings were inadequate to meet the needs of industrialized cities, which began to grow vertically. The Monadnock Building (1891) in Chicago exemplifies the difficulties of building tall structures with load-bearing walls. Its 16-story north half is the tallest conventional building in the United States that is supported solely by solid masonry walls. Six-foot-thick walls at the base of the Monadnock Building support the great weight of the masonry above. The building's tremendous weight caused it to settle nearly 20 inches (50 centimeters) into the ground before stabilizing, making it necessary for occupants to step down from the sidewalk to the ground floor. Solid masonry construction was also expensive and labor intensive. In contrast, steel frames were cheaper and could be erected in a fraction of the time. Remarkably, construction of the Empire State Building (1931) took almost a full year less than that of the Monadnock Building, despite rising 86 stories higher. Other factors, such as the development of safe passenger elevators and advances in the mass production of steel, helped to seal the fate of solid masonry construction in the United States. Although structurally different, early steel frame buildings still had thick masonry walls. While the steel frame provided the primary structural support, the exterior ("infill") walls of early steel frame buildings were composed of several wythes of masonry. The masonry surrounded the steel frame, or was connected to the frame using rigid components, creating composite wall systems. This combination of old and new building techniques became known as "transitional masonry." Like their load-bearing predecessors, these buildings relied on the storage capacity of the exterior masonry to control the elements. This construction style posed several problems. Because the steel frame was rigidly connected to the exterior walls, without provision for differential expansion, contraction, and deflection between the disparate materials, movement of the building frame could lead to cracking and buckling of the masonry. In addition, corrosion of the steel frame due to exposure to water in the damp masonry could also cause masonry displacement. Unfortunately, such problems took many decades to become apparent. New York's first facade inspection ordinance, prompted by a fatality from falling masonry, was not written into law until 1980. Age of Curtains Transitional masonry construction gradually evolved into a new building style, that of lightweight "curtain wall" construction installed over internal structural framing. This construction was not only lighter than masonry, but slip connections to the structure allowed independent movement of the cladding and the framing. While this greatly reduced the risk of cladding failure due to differential movement, curtain wall construction introduced a new set of problems. Unlike masonry, lightweight claddings typically have little or no storage capacity for heat and moisture. They rely on waterproofing and thermal insulation to control the elements. Early curtain wall systems were built without redundancy; that is, they relied on the exterior surface of the wall being waterproof to prevent leakage into the building. As designers began to understand the differences between masonry and curtain wall construction, new designs developed that included multiple planes of protection. The most common of these is the drainage-plane wall system in which the primary waterproofing is concealed within the wall construction. The cladding is not expected to be completely waterproof but acts as a rainscreen, preventing most rainwater from reaching the internal waterproofing. An air space behind the cladding promotes drainage of penetrating water, with flashing at floor lines and wall openings to collect and direct water back to the exterior. Controlling water penetration is only one of several challenges that designers faced in the development of contemporary wall construction. The trend toward airtight buildings led to a variety of potential moisture problems unrelated to exterior water leakage. This is worsened by the fact that many modern materials such as wood fiberboard and paper-faced gypsum products are, unlike traditional masonry materials, easily damaged by water. The introduction of insulation in curtain wall assemblies created cold planes within the wall system that are subject to condensation due to air leakage and water vapor diffusion. Similarly, the design of air-tight enclosures without adequate mechanical ventilation can lead to excessively high interior moisture levels due to occupant respiration, artificial humidification, and so on. Condensation on windows, doors, and other interior surfaces, combined with hidden condensation within the walls, can produce damage equal to or greater than that from exterior water leakage. Understanding the dynamic performance of these wall systems, the proper location of components such as insulation and vapor retarders, and the design of appropriate HVAC systems are required to prevent such problems. Walls Today Over the course of building history, solid masonry construction became a well understood and finely tuned art. Many of these buildings are still in use hundreds, even thousands, of years later, standing as monuments to the skill and experience of their creators. While contemporary building designers have skills that are technically more sophisticated than those of their predecessors, they are, by comparison, still lacking in experience. This is partially because the fast pace of technical change in the construction industry does not allow much time for new technologies to be evaluated. Rather, many new technologies are incorporated into buildings before their behavior is fully understood. Transitional masonry, an example of technology outpacing experience, demonstrated that problems can go undiscovered for decades. While solid masonry construction techniques evolved over thousands of years, today's designers have just over 100 years of experience in applying cladding over a structural steel frame. As designers, we need to be aware of this and understand that we are, indeed, still learning. Cooperation in sharing knowledge and experience between architects, engineers, and manufacturers, is essential. Discuss this article in the Architecture Forum... Sean M. O'Brien is a staff engineer with the New York City office of Simpson Gumpertz & Heger Inc., a design and consulting engineering firm that designs, investigates, and retrofits buildings and structures. He specializes in building science and building envelope performance. Resources
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