Volume 36, Issue 2, February 2001
Why Bad Things Happen
to Good Buildings
by Thomas A. Schwartz
This article originally appeared
in the Winter 2000 issue of Architecture Boston.
Most people in the construction industry have had this experience: someone (a client, a
guy at a cocktail party, your Aunt Ethel) asks, Why cant you people build
things the way you used to? Why do new buildings fall apart? Depending upon the
circumstances, the question is posed in tones ranging from outrage to jocularity to
sincere curiosity. It could be easily dismissed as an unsophisticated query from someone
who doesnt appreciate the complex art and science of construction today. But it is
based on a valid observation. We dont make buildings the way we used to, and
sometimes we suffer the consequences.
In the first half of the 20th century, building facades underwent a transition. Walls consisted of layers of masonry (wythes), which formed part of the load-bearing structure of the building. Floors were supported by the exterior walls and moved with the walls. The masonry also provided resistance to water penetration, aided by design features such as drip edges that reduced water flow over the façade and protected the more vulnerable joints. Any moisture that did migrate to the inside evaporated readily because their materials had inherent durability and because the thermal mass of these walls mitigated temperature swings. But, by todays standards, these walls were expensive, energy-inefficient, and limited in design flexibility. Something else was neededsomething that offered improved thermal performance, greater design freedom and lower cost.
Curtainwalls proved to be the answer. The transition to curtainwalls began in the late 1800s and continued until the middle of the 1900s. In the early part of the transition, walls were still massive, but they no longer supported the structures floor loads. In the latter part of the transition, lightweight walls were hung on the structural frame. It soon became apparent that the lack of mass and lack of inherent resistance to the effects of water exposure required new ways of managing water penetration and protecting vulnerable materials such as cavity drainage systems, internal waterproofing elements and durable flashing materials. But they havent evolved without a few bumps in the road.
Why is it that water problems are a bigger issue now for buildings than they have ever been? The weather hasnt changed significantly. What has changed, however, is demonstrated by three recent trends: over-reliance on sealants to do the job of waterproofing; the push to make buildings air-tight to reduce energy costs; and the widespread use of moisture-sensitive materials in wall construction.
Instead of providing redundancies to serve as fail-safe protection against water penetration, designers and contractors began to rely solely on surface-sealed barrier walls. Metal flashings that once were soldered are now lapped and sealed. We ask more of sealant performance than we have had reason to expect. The result has been too many walls that leak immediately after construction.
Even small amounts of water penetration can have serious consequences. Improvement in air tightness can paradoxically create problems in moisture retention, because the lack of airflow slows drying. Water that might have penetrated and then evaporated within a few days may now require weeks to dry during which time a building might be exposed to additional rainstorms. The net result is accumulation of moisture, prolonged high humidity, and even saturation within the wall cavities, creating an environment ripe for rapid deterioration and mold growth.
Despite the relatively high moisture absorption of the transitional wall system of the early-20th century, many of these walls survived well with minimal attention for many decades. The transitional walls were primarily constructed of stone, brick and mortar that could remain wet without rapid deterioration. Compare this with walls assembled today from soluble gypsum sheathing boards, corrosion-prone light-gauge metal studs and insulating glass with water-degradable edge seals.
The transition from masonry load-bearing walls in many cases meant a transition to glass, now one of the most common cladding materials. The quintessentially brittle material, glass has introduced its own set of challenges. Glass is usually installed in metal framing systems, which means that differential thermal movement is inevitable: aluminum frames subjected to a change in temperature can move about 2.5 times more than glass subjected to the same change. Breakage can result if the design does not accommodate this differential movement.
Bad things happen to some good, even great, buildings. Sometimes good buildings fail because their designers have pushed the limits of technology in order to create something newFrank Lloyd Wrights Falling Water is now undergoing structural repair to its famous cantilevers. Sometimes they fail because of a defect in a common material or component12 years after its construction, the building at 303 Congress Street in Boston suddenly settled 6-inches due to errors in the production of the concrete mix that was used in its precast piles; the building was eventually demolished. And sometimes buildings fail through a combination of these scenarioswhen familiar materials and technologies are used in new ways. Bostonians are familiar with one of the most famous example: the John Hancock Tower, which was clad in more than an acre of plywood after its reflective glass began to fracture in 1972 and 1973.
A gag order imposed on the parties to the resulting legal dispute prevented the release of the facts regarding the cause of the breakagegiving rise to many theories and myths, some of which exist to this day. Initially, many design professionals thought the reason for the breakage lay in the fact that the tower swayed excessively in the wind. Although it was indeed swaying substantially, this was not the reason for the glass breakage. Another hypothesis was that wind forces at hot spots, which resulted from the rhomboid shape of the tower, caused overstressing of the glass. Substantial hot spots did exist, but only a small percentage of the glass was subject to anything near the load for which it had been designed. Still another myth was that the window broke because of the stress they endured from the settlement of the towers foundation.
But in fact, extraordinary external forces and the buildings structural design were not the primary cause of the failure. The problem actually lay in the insulating glass.
The insulating glass units that made up the façade were fabricated with a thin lead tape spacer to separate the two panes of glass. The tape was soldered to the glass after the edge of the glass was coated with a film of copper to make it more receptive to the solder. This created a tenacious bond between the spacer and the glass, which constituted the products greater strength as well as the source of it demise.
The lead-tape seal insulating unit was the premier product of the time. It was expensive, and it performed very well with relatively small-size clear glassthe typicial application in the 1940s through the 1960s. By the late 1960s, however, large-size glass with tints and reflective coatings became popular. The large sizes and increased thermal loads associated with the tints and coatings caused substantial differential movement and increased stress along the glass-to-tape bond, and eventually, the bond began to separate. The bond, however, was so strong in some areas that the tape ripped microscopically small pieces of glass from the glass surface. These sites concentrated stress from wind loads and ultimately proved catastrophic.
Not everything we did in years past was good. Not everything we do now is bad. But innovation isand should berelentless. And with innovation comes reduced predictability and increased risk. To meet the challenge that innovation presents, we must use the lessons of our history coupled with sound technical fundamentals and a healthy dose of common sense.
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