Volume 41, Issue 11 - November 2006

Dynamic Glazing
Create New Opportunities for Your Company by Installing Blast-Resistant Systems
by Duf Hudson and Monte Monkress

For a growing number of glazing contractors, installing blast-resistant systems provides a significant source of revenue. Others shy away from these projects because of their complex nature. However, this market segment will continue to grow as terrorist activity across the globe escalates. Arm yourself with a better understanding of the design criteria and blast-mitigation requirements set forth by the government and create business opportunities that may have seemed out of reach. 

Who Can I Go to For Help?
Manufacturers and suppliers can be a great source of information and problem-solving when working on this type of project. While the post 9/11 world has made many of these manufacturers “specialists” when it comes to security systems, others offer systems that meet only the most minimum of blast-mitigation criteria. Be sure to communicate with your material suppliers clearly to make sure their systems meet your project’s requirements.

Keep in mind that hearing a supplier say, “Sure we have a blast system,” can mean many things and doesn’t define the level of protection provided very well. Protection against accidents, natural disasters or terrorist attacks can be achieved in several ways, and at differing levels of protection.

Understanding the Design Criteria
The General Services Administration (GSA) recommends that window systems be designed to mitigate the hazardous effects of flying glass during an explosion. The GSA has identified six glazing protection levels based on how far glass fragments enter a space and potentially injure its occupants (see sidebar on page 45).

The GSA blast protection criteria has been adopted by the multi-agency Interagency Security Committee (ISC) and is the most widely recognized classification of design levels for glazing hazard protection. The ASTM Hazard Rating is also included on the table for comparison purposes.

Selecting the Right System 
Often, the owner, designer or developer who is creating a blast-resistant building will consider a structural silicone glazing system, coupled with laminated glass construction at the inboard lite in the overall glass unit composition.

In this system, all edges of the glass are “glued” to the supporting frame by means of a high-performance structural silicone. Under blast conditions, the structural silicone retains the glass edges in place. While the glass may break, the PVB interlayer between the inboard lites of glass retains the glass in place to varying degrees, depending on the type of glass, size of the lite, thickness of the interlayer and extent of the blast.

While most of us are familiar with standard performance criteria listed in job specifications being in terms of pounds per square foot (psf), blast loads are measured in terms of pounds per square inch (psi; 1 psi equals 144 psf).

Consequently, a performance requirement number (referred to as peak blast pressure, or load) of 4 psi in a blast specification might appear disarmingly low, but when you translate that into the more familiar psf terms it becomes 576 psf.

Blast specifications will also include another element that must be considered when determining the type of glass and framing system to use: the duration, or impulse. This is measured in psi milliseconds (psi-msec). Both the pressure (load) and impulse (duration) are required to define the blast loading. 

An example of a specification requirement might read like this: “Systems must be capable of withstanding a peak blast pressure and impulse of 5.1 psi for a period of 42 psi-msec.”

Glass is usually the largest area of concern in a glazed frame, so some may think that to increase protection, just increase the glass strength, either by changing from annealed to heat-strengthened or tempered glass, increasing the thickness of the individual glass members within the insulating unit or by increasing the PVB laminate interlayer from .030 inches to .060 inches or even .090 inches.

Any of these approaches will change the way in which the frame supporting the glass reacts. The load on the glass is transferred to the surrounding frame, and then to the frame connections, which are typically not allowed to rotate more than 1 or 2 degrees. Making the glass stronger means that the framing connections must also be made stronger. There have been instances where merely changing the inboard lite of an insulating unit from annealed glass to heat-strengthened glass increased the number of fasteners at the framing connection from four to eight.

The Weakest Link
When designing window systems to resist blast forces it is also important that the glazing, framing and anchorage all be designed to withstand the required forces. Generally, the glazing should be the weak link. In other words, the window system should not prematurely fail and blow into occupied spaces due to failure of the frame or anchorage. This is a balanced design approach.

System framing design can be approached in several ways. Framing wall thicknesses can increase, system depth can increase, steel reinforcing can be added, fastener connections can increase in quantity and diameter (our company currently has a project using eight 7/16-inch fasteners at each mullion to sill and head connections) or all of the above.

Working toward the balanced approach should be the goal when selecting glass thickness, PVB interlayer thickness, framing type, framing depth, frame connections and anchor design, type and connections to the structure.

Examples
To illustrate, let’s look at two actual projects where the design team took different approaches with their specifications to meet the same end result.

The first blast project did not specify a load (peak blast pressure) or duration (positive phase impulse), only a required size of steel reinforcing, with no design flexibility. The second blast job, shortly thereafter, clearly spelled out the load and duration, allowing for value engineering of the design to meet the project’s requirements.

The first job, full of steel reinforcement, tested and achieved a peak blast pressure of 6.7 psi, with a positive phase impulse of 47 psi-msec, achieving a GSA condition 2 (ASTM minimal hazard) protection level. 

While the first system achieved very good numbers, the second job was designed and engineered to the load and duration specified. This allowed for the complete elimination of any steel reinforcement (thereby eliminating the cost of the steel to the customer and the additional labor required to install the steel loaded frames). Testing of this job yielded a peak blast pressure of 8.2 psi, with a positive phase impulse of 55 psi-msec, even better numbers than the frame loaded with steel, also achieving an ASTM minimal hazard rating.

Some window manufacturers achieve strength in their blast framing by anchoring the entire perimeter 6 inches or 12 inches on center, while others strive to maintain the historical method of anchoring window walls and curtainwalls at the head, sill and intermediate slab connections, and reducing the amount of steel reinforcing required.

There are several types of framing systems on the market that either are, or can be, designed to work with security glazing in providing a blast system. Investigate how these systems are designed to help you meet your project’s blast requirements, and look closely at the protection levels that each system gives you. 

With knowledge, confidence and a little help from a quality manufacturer you can blast into new business opportunities.

ISC Security Design Criteria Blast Protection Levels for Windows

Performance Conditon ASTM Hazard Rating GSA Description of Window Glazing Response 
1 No Break Glazing does not break. No visible damage to glazing or frame. 
2 No Hazard Glazing cracks but is retained by the frame. Dusting or very small fragments near sill or on floor acceptable. 
3a Minimal Hazard Glazing cracks. Fragments enter space and land on floor no further than 3.3 feet from the window. 
3b Very Low Hazard  Glazing cracks. Fragments enter space and land on floor no further than 10 feet from the window. 
4 Low Hazard 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. 
5 High Hazard Glazing cracks and window system fails catastrophically. 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 greater than 2 feet above the floor. 

USG
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