Volume 39, Issue 10,
Built to Perform
Challenges of Blast Window Design During Renovation and Retrofit
by Ken Sampson
The third anniversary of September 11, and the recent attack on the Australian Embassy in Indonesia underscores the continued need for blast protection in both target buildings and those in their proximity. A bomb does not discriminate and a blast wave will just as easily destroy the windows across the street as it will the windows of the target. Many of today’s buildings requiring blast protection must now undergo some degree of renovation. A critical component of securing the building envelope during renovation is blast window design and installation.
Prior to September 11, architects and building designers could never have anticipated the need for blast protection in an office building. Because of this, today’s architects, general contractors and glazing contractors are sometimes faced with enormous difficulties in providing blast windows that are compatible with existing conditions. These problems may include utilizing existing wall construction, providing ballistic and forced-entry protection in addition to blast protection or the need to maintain existing architectural appearance for historical preservation requirements.
Many companies manufacturing blast-resistant windows have been faced with such challenges and, when necessary, have designed completely new window systems for a particular application. Working with architects and contractors step by step to meet the challenge is a daily event. Architects should not settle for standard products on large projects and should seek out manufacturers willing to engage in the design phase of a project.
Today, not only do buildings owned and leased by the federal government require blast protection, but building owners in high-threat areas are also considering blast protection. Such is the case with the New York State Insurance Fund, located in downtown Manhattan, just blocks from the World Trade Center. The following is a case study of the building’s retrofit.
Building: New York State Insurance Fund
Location: Downtown Manhattan
Project Description: The New York State Insurance Fund decided to replace the existing windows on its 15-story building in lower Manhattan. Although not a target building, the building owner recognized the advantages of installing blast windows, given the building’s proximity to so many other potential targets. The owner desired operable windows and specified in-swing to allow for ease of cleaning.
• Provide 1,200 in-swing blast windows, capable of providing protection against a specified blast load (actual blast load value withheld to avoid disclosure). In-swing windows provide a unique challenge under load since the window is restrained only by a hinge mechanism and locking device, not the seating surface of a frame;
• Pass stringent water and air infiltration tests in accordance with ASTM X 283, ASTM X 332 and ASTM X 547; and
• Due to existing building construction and to avoid the increased cost of installing a separate sub-frame, only two bolts on each horizontal member into the existing structure were allowed, meaning the blast window could be held in place with only four bolts.
Solution: Our company took on this challenge and designed a new operable blast window specifically for this application, the USAW500. The first step was analysis of the existing wall structure to determine its suitability of accepting the blast load that the window would transfer to it. (In some cases this requires a separate analysis by a metallurgist or a materials engineer.)
With the wall composition clearly understood, a prototype window was tested at Baker Risk Engin-eering in San Antonio. Using only four bolts we designed for this application, the window passed stringent shock tube testing and achieved a General Ser-vices Administration (GSA) Performance Condition 3a.1 We attempted to meet Performance Condition 2, but a small fastener fell just inside the window, resulting in a 3a designation.
Blast Product Considerations
When purchasing a blast product, it is important to keep the following in mind:
• What is the threat needed to protect against?
• Will there be only one blast event?
• What degree of performance is required?
• Does the building have to return to or maintain operation after a blast event?
The first two are difficult questions to answer. If the answer to the last question is yes, then it is critically important to install windows that are built with a high enough safety factor to achieve a GSA Performance Condition 2 and allow a return to operations. (See box on page 82 for listing of GSA performance conditions.)
During a blast event, a blast window system should protect the occupants and allow for their safe escape. If the window is severely damaged and makes the building open to the environment, then it is no longer capable of protecting the property. Classified material may be compromised, theft may occur and severe weather may destroy electronic equipment and building components. These are all factors a building owner must consider when making the decision to purchase blast windows.
1See GSA Test Protocol: GSA-TS01-2003 available at www.oca.gsa.gov.
Ken Sampson is the founder and president of United States Bullet Proofing located in Hyattsville, Md.
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