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Securing the weakest link

Advances in blast mitigation can save lives, if properly applied.


June 10, 2011
By Ron Phelps

Topics

As the trend continues to expand the use of glass in building envelopes
to create innovative architectural designs, the challenge of perimeter
security from a blast mitigation or crime perspective remains a daunting
task, to say the least.

As the trend continues to expand the use of glass in building envelopes to create innovative architectural designs, the challenge of perimeter security from a blast mitigation or crime perspective remains a daunting task, to say the least. Security consultants or specialists must take a number of factors into consideration that may include several restrictions to making changes to existing structures.

p18_gc-june11-innovations-gallery 
Blast mitigation technology has progressed to the point where even such buildings as the National Gallery of Canada, seen here, can be made safer. 3M upgraded the gallery’s windows in 2000. Building an understanding of blast mitigation standards and practices could land you some nice contracts in the ballooning institutional construction market.

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Once the threats have been revealed via TRA (Threat Risk Assessment), the design process of incorporating multiple layers of security begins. In terms of blast mitigation, increasing the stand-off distance is typically the first objective to minimize the blast pressure if at all possible.

The next step involves strengthening the building envelope to meet the required level of protection as per the TRA, and windows are generally considered to be the weakest link. While the interface between window frames and the structure of existing buildings can be strengthened, interior esthetics and costs typically direct the focus elsewhere. Securing the glass will be the focus of this article.

Since glass replacement to mitigate an explosive event (whether intentional or accidental) is expensive, retrofitting or upgrading the existing glass has been the most popular and proven method to improve life safety in existing buildings. This also meets sustainability objectives by reusing existing materials. Containing glass fragments during a blast is critical to protect people and property due to the high velocity of flying shards of glass.

The GSA test
In response to the heightened concern about terrorism, test methods were developed to mitigate hazards to people within buildings posed by glass shards propelled by the forces of an explosive event. The most recognized test method in North America remains the open-arena GSA (General Services Administration) test method that lists criteria of performance against a real blast event. This standard test method uses various blasts (using the equivalent of 200 to 500 pounds of TNT) and stand-off distances (ranging from 120 to 190 feet) to produce specific blast pressures on four-foot by 5 ½-foot windows equating to a four to five psi blast load.

Performance Condition 2 essentially means the glass breaks but is held in place. It is important to note that no security film on standard commercial IG units has met Performance Condition 1 in which the glass does not break. It is also important to note that the GSA is a minimum standard and four to five psi is a low blast load. In reality, a blast is likely to be bigger, closer and hitting larger windows than those specified in the GSA test. Therefore, higher blast load resistance (i.e., 14 psi) is desirable to provide maximum life safety protection.

The most popular method of upgrading existing glass is to retrofit applied, optically clear, security window film (also known as window laminate) since it holds the broken pieces together and greatly reduces glass fragmentation into the building. There are two types of security film that are polyester-based with a pressure-sensitive adhesive: basic layered (typically one to three layers from seven to 14 millimetres thick), and micro-layered (typically 28 and 42 layers from four to six millimetres thick).

Although basic polyester films can meet the lower blast loads, the specialized blast mitigation micro-layer films are more commonly selected on projects in Canada and throughout the world. This is due to the significant tear-resistance properties that 28- and 42-layered constructed films provide (more than four times the tear resistance of similar thicknesses of basic layered films).

It is well understood that tear resistance and elasticity of a security film are critical factors in creating a flexible membrane to help capture the energy from a blast regardless of attachment methods. Since micro-layer films are thinner than the basic polyester films, they are able to expand and absorb more energy without tearing, thus reducing stress on the frames. Once the micro-layer film has been penetrated by glass shards, it is able to withstand the positive/negative pressures of the blast combined with the sheer weight of the glass that can easily tear basic polyester, defeating the purpose.

Securing film is key
While attachment of the security film is optional on commercial IG units using micro-layered technology, it is best to attach the film to the frame to enhance the overall performance of life safety protection. Methods of attachment range from mechanical systems involving screws and metal materials to specialized wet glazing using a bead of structural adhesive. Typically the wet glaze method of attachment is favoured due to the elasticity of the bead and the lower installed cost.

Different levels of performance are achieved depending on which specialized wet glaze system is used. Although there are other methods to attach or capture glass fragments, such as catch-bar or cable systems and blast curtains, they are not normally used in Canada due to poor esthetics and higher costs.

Picking the right film
Performance-based specifications combined with installation requirements are becoming the most popular method to help ensure the best security film system is selected for the project. As with most quality manufactured products, the quality of workmanship required to install the system must be well understood before the tender is awarded to eliminate problems and project delays. This is especially true with optically clear security window film technologies that upgrade existing glass, since such installation imperfections as dust particles are noticeable and tend to be rejected.

There are also combined energy saving and security window films available that may be of value depending on the application and sun exposure of the existing glass. This can be determined through a site inspection by a specialist who is familiar with this form of technology and has the capability to conduct an energy audit.

Quality manufactured window security films also have a proprietary, scratch-resistant coating that allows the use of ammonia-based window cleaners with soft cloths, which will help to maintain your investment year after year. •

How to have a blast

Blast mitigation is rated according to how far glass fragments project into the space beyond the window when subjected to a standard explosive force. The American General Service Administration recognizes six levels (“Performance Conditions”) of blast protection.

Performance Condition

  1. Glass does not break.
  2. Glazing cracks but is retained by the frame. Dusting or very small fragments near sill or on floor acceptable.
  3. Glazing cracks. Fragments enter space and land on floor no further than 3.3 feet from the window.
  4. Glazing cracks. Fragments enter space and land on floor no further than 10 feet from the window.
  5. Glazing cracks. Fragments enter space and land on floor and impact a vertical witness panel at a distance of no more than 10 feet from the window at a height no greater than 2 feet above the floor.
  6. Glazing cracks and window system fails catastrophically. Fragments enter space impacting a vertical witness panel at a distance of no more than 10 feet from the window at a height greater than 2 feet above the floor.


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