Volume 36, Issue 12, December 2001

Consultants Corner

The Aftermath
        Interest in Blast-Resistant Products Intensifies 
by Mark Baker

In the wake of the September 11 terrorist attacks and the country’s increased awareness of the threat of terrorism, we have experienced a heightened interest in bomb blast-resistant glazing. The following are excerpts from a report, “Bomb-Resistant Cladding,” I prepared while working in London in 1993.
 

After the St. Mary’s Axe Bomb in Central London in 1992, commercial building developers and businessmen are becoming more interested in blast-resistant buildings. As a result, there are a growing number of window and curtainwall companies offering blast-resistant products.

Because this technology has been developed by the government and has not been made available to the public, designers and architects are often not in a position to evaluate what is offered.

The following are some basics of blast-resistance enhancements, which are useful in understanding risk assessment and damage limitation principals.

Blast-Resistant Basics
Bomb blasts are measured in terms of peak dynamic pressures and duration of blast, which are determined by the size of the device producing the explosion (in Kg of TNT) and the distance from the blast. Most damage caused by an explosion is the result of a primary blast wave (significant and rapid positive over pressure) moving outward from the detonation point and the negative under pressure that follows it.

Nick Johnson, principal engineer for the Explosion Protection Branch of the Special Services Group of the Department of the Environment in the United Kingdom, identified the following stages of designing a window 
or curtainwall with resistance enhancement:

1. Identification of Perceived Threat: Threats to a building or its occupants are dependent on whether a building or its neighbors are potential targets of terrorists, the vulnerability, accessibility and security of the area. It is also important to establish the origin of the threat, specifically internal versus external explosions.

2. Establishment of the Intended Level of Protection or Enhancement: The level of protection or conversely the level of damage that is permissible must be determined. Protection against loss of life or injury is a primary goal, followed by protection against damage to building contents and the structure itself. 

For external explosions flying glass causes the majority of property damage and human injury. It is generally agreed that if a curtainwall or window system can be designed so the glass remains in place, the risk to building occupants and pedestrians is reduced greatly. In addition, if the building envelope is not violated damage to the building contents due to blast energy is also limited.

3. Design Guidelines for Glass and Framing: Glass selection is a critical element of the design. Two options in the design of blast-resistant curtainwalls for external explosions are no-fail where the glass is designed to remain unbroken and fail-safe where the glass is designed to remain in the opening if broken. 

Unless detailed information about the potential blast is available, it is difficult to design a no-fail system economically. Unless the blast pressure is low, either method typically will involve laminated glass. Similar to hurricane-resistant design, deep glazing pockets and structural silicone sealants and flexible frames designed to absorb blast energy through deformation are utilized.

Conversely, internal bomb blasts are generally smaller; however, fail-safe or no-fail glazing systems prevents the venting of the blast pressure that is critical to damage limitation. As a result it is difficult to design against both an internal and external explosion.

Design Criteria
The design of bomb-resistant cladding systems is subject to the same difficulties as any other wall system:

1. Specification of Correct Design Criteria: The major hurdle for designing bomb-blast protection is establishing the size and location of the explosion. Will the bomb be inside or out? How close to the building? How big is the bomb? Unlike wind loads on a wall, the size of the explosion is variable. 

2. Proper Design and Material Selection: Once a design criteria is established, the design is rather straightforward. For example, for an external bomb, select the glass to resist the load, design a frame to retain the glass and design the anchorage to hold the frame to the structure.

3. Testing: Traditional mock-up tests, which are used regularly to test a curtainwall’s resistance to static structural loading, are not adequate to verify a system’s ability to resist blast pressures and transfer them to the structure. To confirm the adequacy of a design, analytical simulation or physical testing is required.

4. Proper Manufacture and Installation: As with all construction, a significant amount of error is made during the fabrication and installation of curtainwalls. For bomb-resistant construction, the effects of poor workmanship on the expected level of performance will be significant.

Going Forward
Following the 1995 bombing of the Alfred P. Murrah Federal Building in Oklahoma City, the GSA issued general guidelines for glazing protection of its buildings based on a formal assessment process. 

I’m sure we’ll be hearing more from the federal government regarding blast-resistant glazing in the months to come, in the wake of these recent terrorist attacks. 

m baker Mark Baker, P.E., serves as principal of IBA Consultant Office in Miami. If you have a question you would like Baker to answer in a future column, e-mail mbaker@glass.com.


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