9/6/2017· Failure Analysis
By: Dr. Thomas Read
As "water hammer" (i.e. liquid impact) has the potential to cause glass bottle failure, Read Consulting LLC created two such failures to study the resulting fracture surface details.
For hundreds of years, building walls were constructed of stone or brick and were sometimes several feet thick. Such massive walls served well not only to sup port the floors and roof but also to keep out the weather.
In the early twentieth century, economic pressures and the need for taller buildings led to the design of a structural building frame and thinner walls. Such walls required a radical change in water leakage resistance design.
Early traditional masonry barrier walls control water leakage by absorbing water that penetrates the exposed surface and slowly dispersing it as water vapor (see figure). This outward flow of moisture is driven by vapor pressure related to the thermal gradient across the wall. (The interior wall surface is usually warmer than the exposed wall surface.)
In the 1950s, cavity walls came into existence. Cavity walls control water leakage via a continuous air space in the wall that channels water to where it can be diverted outside by internal flashings and weep holes. Since both barrier and cavity wall systems are designed to control water that enters the wall, a limited amount of imperfections in the wall's exposed surface is tolerable.
The skin barrier wall system has evolved over me put 20 years or so This new wall system must deflect all water at the surface to avoid leakage. Any breach in the wall's surface allows uncontrolled water entry to the wall system and building interior. Continuous watertight integrity of both the skin material and elastomeric joint sealants is essential.
Variations of Skin Barrier Walls
Several different types of wall systems fit the skin barrier category. One common type is a thin composite wall panel constructed with a cementitious backer board and faced with thin stone or ceramic tiles. The skin, which is usually about 25 mm (1 in.) thick, is prefabricated or field applied to a light-gage steel stud framing system (figure omitted).
Another popular skin barrier wall system involves a thin (about 1.5 mm[1/16 in .] thick) polymer-based skin material that is trowel-applied to a rigid insulation board and reinforced with a woven glass fiber mesh. Other skin barrier wall systems have skins of metal or glass panels, ceramic plates, or fiber-reinforced concrete.
The consequences of water penetration through the surface of properly constructed traditional barrier and cavity masonry walls is seldom disastrous. Sealant failure or minor cracks in cavity walls result in more water entry to the cavity but not always leakage to the interior. Sustained water penetration through skin barrier walls, however, almost always leads to water-damaged wall framing, interior finishes, wet insulation, and even mold growth inside the walls.
Design and Maintenance Considerations
Many wall elements require maintenance. Mortar joints in conventional masonry walls require tuckpointing . In any wall system, sealant in joints around windows, expansion joints, and other moving wall joints must be replaced periodically. Since the skin barrier wall's surface is the only line of defense against leaks, maintaining the sealant joints and controlling water entry from other sources (copings, roof flashings, etc.) is critical.
Since skin barrier walls are economical and popular, they are likely here to stay. And with special design attention and proper maintenance, they may perform reasonably well. However, to avoid costly failures, designers and owners must fully understand the lack of leakage control redundancy and heightened maintenance requirements with this wall system.
For a copy of this article and others, please contact WJE.
Kimball J. Beasley is a structural engineer and senior consultant with the Princeton office of Wiss, Janney, Elstner Associates, Inc. Over the past 25 years he has
investigated failures and problems of almost 1,000 buildings and structures.
See Wiss Janney's Profile on Experts.com.
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