Volume 38, Issue 6, June 2003
The Big Bang
Understanding the Composition and Elements of Blast-Resistant Systems
by Ken Hays
he demand for and marketing of blast-resistant windows and curtainwall represents a tremendous opportunity for the glass and glazing industry. This demand poses two unique challenges and risks to potential product providers and installers who must understand the fundamental ways in which blast resistance differs from traditional glazing industry knowledge.
This dilemma is really twofold: first, the fundamental differences that make blast-resistant products radically different from standard or custom windows and curtainwall must be identified and understood. Second, providers must be educated on the technology necessary to design, manufacture and/or install blast-resistant products.
Blast Resistance Evolution
Until three or four years ago, blast-resistant window products were manufactured and sold primarily by a handful of firms that, for the most part, evolved from the prison window industry. The windows came in standard shapes, sizes and profiles and, owing to the needs of the clients, were designed to resist a very big bomb. The architectural and aesthetic design of embassy exteriors was driven by what was offered in vendors’ catalogs, because size and appearance variation were severely limited. Accordingly, most embassy exteriors were designed to appear as box-like, concrete structures with rectangular, punched openings to accommodate windows. The market was simply not sufficient enough to support expenditures on variability in size, appearance, anchoring or new technology to reduce the cost of windows.
Changes began at the Department of State level with construction of the new U.S. Embassy in Moscow and the GSA standard for blast mitigation in new and retrofitted buildings. The Moscow Station was the first truly commercial-type, high-threat-level, blast-resistant office building with a high-concept architectural appearance built by or for the United States.
Blast engineers designing exterior building claddings are focused on threat assessments, standoff, glazing makeup, capture methods and anchorage. There are precious few capable of this and even fewer who have actually participated in a building exterior design. The traditional challenges of wind-load performance and considerations for air and water infiltration and thermal characteristics are not typically within the purview of blast-design engineers. Blast designers are concerned with either strength or flexibility to withstand the specified blast load. They generally look to others, such as window and curtainwall designers, to address the architectural performance considerations.
Understanding The Unique Nature
Windows and curtainwall that are designed, tested and certified to meet hurricane requirements can vary in size by minus 10 percent from the size of the tested and certified unit. Standard or custom windows and curtainwall can vary over a wide range of parameters with test data and calculations remaining valid. This design-use latitude seldom applies in the design and installation of blast-resistant products. Relatively minor changes in threat levels, floor spans or spans between anchor points, anchor spacings, mullion spacings and face dimensions or size of the window in any dimension will likely require a re-analysis of the window’s performance characteristics.
Variability in these parameters is more forgiving at low threat levels. However, as the blast-load requirement increases, this variability without follow-on analysis diminishes to zero for any component. Moreover, varying field conditions for the product installation create special engineering demands regardless of whether the blast-load criteria are low or high. When embed or anchor points are misallocated, concrete is high/low, in or out, opening sizes vary or distances from the window frame to anchor points as designed vary, an engineering review of each specific condition that deviates from the design in the field is required. These deviations must be identified, analyzed and documented carefully for both specific situations, with records maintained that verify the fix was implemented at that certain location. While on the surface this may appear radical, remember that these systems are designed to protect people’s lives—not just to address comfort.
This field-fix situation is further complicated by the fact that straightforward field-engineering fixes do not apply. Field corrections traditionally accepted because they are simply stronger are not necessarily the solution for accommodating blast design. In fact, they may be detrimental to performance, since the field corrections designer must understand the “design intent” of the entity that designed the window and its connections. Therein lies the methodology of receiving the blast load and developing the transfer path of that load into the structure. Modifications affecting that path and the non-linear dynamics of blast loading on the various window, glass and connection components are of equal importance.
Accordingly, the liability of the window and curtainwall manufacturer extends to the quality and exactitude of the field installation in a manner never contemplated previously by glass manufacturers. The manufacturer’s liability is tied inextricably from the initial design to the completed installation in the most severe terms.
Getting in the Know
The community of owners and architects is sorely lacking in specific, or even general knowledge, of these challenges. Owing to this lack of knowledge, a real peril to building occupants exists when the uninformed or the unscrupulous vender promotes and installs a “standardized” blast-resistant product and suggests that it is variable across a wide range of architectural considerations and anchor conditions. Taking an analyzed and tested window product to market, even one designed to address low threat levels, poses another risk to the vendor. Tremendous development expenditures may be earmarked for a product that, while appearing highly marketable, is useful only in a very narrow set of applications.
For instance, reducing the size of the window may require reducing the strength of the glazing makeup. Reduction in the total surface area upon which the loading is gathered may transfer more load into the capture mechanism and mullion. Should a blast occur, it is important that the glass fail first. If the glass resists the loading for an unanticipated duration, the entire window unit might become a missile.
Before employing a blast designer, find out how many commercial building applications he has engineered. Before hiring a window or curtainwall designer, make sure he understands the ramifications of blast with reference to his design.
Ken Hays is the executive vice president, physical security division, of Masonry Arts Inc. in Bessemer, Ala.
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