Volume 39, Issue 1, January 2004

 

At Close Range
Blast Requirements are Increasing; Are you Prepared?

by Eric Facy

If the Lloyd D. George federal courthouse in Las Vegas is any indication of what is to come from the General Services Administration’s (GSA) ambitious courthouse program, a lot of glass will be used in the more than 100 projects planned over the next decade. Great. So what’s the big deal about a custom curtainwall built with high-performance glazing for maximum energy efficiency? Not much, unless you throw in blast mitigation as a requirement. Suddenly, the game changes and cascading effects ripple through the entire project life. In fact, this requirement alone could be the single greatest consequence to the owner, the glazing contractor, the manufacturer and the design team. 

Let’s put this into perspective. Blast mitigation is indeed a vast field, and different threats call for different responses. While quarter-inch, laminated glass may work in one case, another may require glass-clad polycarbonate. Some commonality, however, can be derived from these projects.

Occupant Protection
Most federal agencies now mandate blast mitigation for all new construction and most major retrofits. The two largest U.S. landlords, the Department of Defense and the GSA, have both issued standards governing the minimum blast-mitigation criteria for owned and soon-leased buildings. Military barracks, courthouses, federal buildings and airports are just some applications affected. The State Department also has stringent criteria for embassies and consular compounds. 

The main goal of these standards is to protect the occupants from the hazards of flying glass. You may have seen footage of a bomb explosion hitting a lite of annealed glass that breaks into shards, propelled into occupied space, injuring or killing those in its path. Clearly, strategies must be implemented to lessen this hazard, and glass can certainly be designed to mitigate or even resist the consequences of a blast. 

Four Elements
Laminated glass provides an excellent step in the right direction. Multiple and various interlayers, thicker glass and polycarbonates can also provide additional strength. Glass, however, is only the first in a four-variable equation. The frame must be designed to contain the glass; as it deflects and stretches, the glass will have a tendency to pull out of the frame. This can be solved through structural glazing or deep glass bites (more than 1 inch). Likewise, the frame must remain anchored to the wall solidly, and the wall itself must be designed to take the load transferred from the window. 

With these parameters in mind, how are project stakeholders affected by blast requirements? Owners, designers, installers and manufacturers are each affected by these products and requirements in different ways.

Owner
The owner is the one to worry initially about the building-threat assessment. In federal projects, usually an approach is mandated or suggested, but most often owners will require the services of a reputed blast consultant to quantify the potential threat and response.

Depending on the threat assessment, owners should choose a site that allows for maximum stand-off distance in the case of new construction. Building at a busy, urban intersection may incur a lot of additional building hardening cost.

Retrofit is more challenging, as the owner is constrained by existing conditions. Window replacement and wall hardening are often the only solutions for maximum blast 
mitigation.

Beyond adequate window design, the single most effective way for a building to resist a given blast load is to be as far away as possible from the bomb. Each additional foot encroached on the secure perimeter will quasi-exponentially increase the pressure and impulse that’s being protected against. For the owner and designer, this typically means surrounding a building with as much area as possible without vehicular access. Hand-carried devices tend to reduce the charge weight, thus the threat, dramatically. This could even be an opportunity to design a “green” building or obtain LEED credits by having parks and trees, water-retention ponds, access to public transportation etc., surrounding the building.

Definitions
Is safety and security glazing new ground for you? If you’re not sure of the differences in the two, or are unfamiliar with impact-resistance terminology we’ve compiled a glossary of some common words and definitions.

Blast-Resistant Laminates: Blast-resistant laminates can substantially reduce injury from flying glass resulting from direct blast shock waves (over-pressures). When properly designed, framed and anchored, blast-resistant laminates are capable of maintaining the integrity of the building envelope following an explosion and reducing interior damage. Large scale arena testing indicates that ¼-inch laminated glass installed in a standard frame withstands over-pressures up to 16 psi with phase durations of 10-20 msec and positive phase impulses of around 100 psi-msec. Quarter-inch laminated glass in a wet-glazed frame (structural silicone) withstands over-pressures up to 15 psi with positive phase durations of 15-20 msec and phase impulses of 130-150 psi-msec. (GANA Laminated Glazing Reference Manual, 2003 edition.)

Bullet-Resistant Laminates (multiple ply): Bullet-resistant laminates are designed to resist penetration from medium- to super-power small arms and high-power rifles. There are numerous types of bullet-resistant laminates, including all-glass laminates, glass-clad polycarbonate laminates and laminated polycarbonate (or other plastics). It is important for the designer/reader to understand these laminates must be tested to prove their performance to resist a certain ballistic threat; that a higher threat level than qualified may not offer similar performance attributes. It is therefore highly recommended the designer select the highest threat level deemed to be required. (GANA Laminated Glazing Reference Manual, 2003 edition.)

Burglar-Resistant Laminates: Burglar-resistant laminates are deterrents to smash-and-grab crime, and resist penetration from hand-held or hand-thrown objects such as hammers, crowbars, bats, knives, bricks and rocks. Typical applications include storefronts, displays, museums, hotels and motels, homes, offices and government buildings. (GANA Laminated Glazing Reference Manual, 2003 edition.)

Charge weight: Gross tare of a given explosive device, typically measured in pounds or kilograms. Usually, the device is expressed in TNT equivalent weight, using tables and calculations. 

Glass-Clad Polycarbonate and/or Laminated Polycarbonate: High and maximum security/institutional glass-clad polycarbonate laminates (multiple ply) are used in the same manner as institutional glazings and provide ballistic protection as well. The combination of materials will reduce the weight, thickness and frame requirements for a given level of security performance. (GANA Laminated Glazing Reference Manual, 2003 edition.)

Impulse: Area on the blast wave graph under the pressure-time waveform, i.e. compounded decaying pressure over time (milliseconds). Typically expressed in pounds per square inch (psi)*msec. (Applied Research Associates.)

Medium Security/Institutional Laminates: [Products that] offer improved detention security and provide unobstructed vision while eliminating the confined look of bars and metal screens. Institutional glazings offer prolonged physical impact resistance and extend the time required for penetration. Typical applications include penal institutions, detention centers, psychiatric hospitals and police stations. In addition, institutional laminated architectural glass provides increased protection in other high security applications such as embassies, computer centers and sensitive research centers. (GANA Laminated Glazing Reference Manual, 2003 edition.)

Pressure: [Dynamic peak overpressure] the highest pressure generated instantaneously by an explosion and recorded at a specific point (usually the target), typically measured in pounds per square inch (psi). (Applied Research Associates.)
Safety Glass: Flat (including bent) glass so constructed, treated or combined with other materials that, if broken by human contact, the likelihood and/or severity of cutting and piercing injuries that might result from such contact is reduced. (ASTM C 162-99 - Standard Terminology of Glass and Glass Products). 

Security Glass: [While very different from safety glass, security glass is more difficult to define empirically due to the breadth of areas covered. Security glass strives to protect against manmade or natural threats.] It is best understood through its specific categories, such as burglar resistant, bullet resistant, blast resistant, forced entry resistant, impact resistant. Additional references: ASTM F 1233, Standard Test Method for Security Glazing Materials and Systems; ASTM F 1641, Standard Test Method for Measuring Penetration Resistance of Security Glazing Using a Pendulum Impactor; ASTM F 1915, Standard Test Methods for Glazing Detention Facilities; UL 972, Standard for Burglary Resistant Glazing Materials.

Stand Off Distance: Straight line distance from an explosive device to a target, typically measured in feet or meters. 

Design Team
The architect needs to find appropriate building materials and structural elements. Again, in the case of new construction, wood windows in steel-stud walls may not be an optimal choice.

Glass should not be considered a weak design element in its own right. However, designing large, clerestory glass lites could become a window system problem. Using intermediate mullions, deeper glass bites or a catcher system (cable, muntin concept) might help break up the span.

Performance or reference specifications are fairly good options, where the blast load is either spelled out (pressure and impulse usually) or the standard or threat assessment is referenced. If sensitivity is a concern, government security clearances could be part of the bidding requirements.

As blast requirements become more stringent, the field of potential bidders and suppliers will likely dwindle. Having an industry representative and a blast consultant on the team as early as possible in the design phase may reduce lead times, product cost and installation issues dramatically.

Installer
The installer should be aware of potentially significant added weight of blast-fenestration products compared with other commercial products. Glass in particular could easily be double the square foot weight, by virtue of laminating several lites. This has implications for shipping and receiving, storing and installing products. Machinery and the need for a larger crew may impact the installation time and cost significantly.

Field glazing may be required to facilitate weight, shipping and schedules. However, most blast systems are structurally glazed, which can be an additional field challenge.
Installation from the outside is more prevalent with blast-resistant windows, mainly due to weight, wall access and occupancy.

Usual installation accessories such as trim and clip, receptor and subsill and strap anchors may not be appropriate or available for anchoring the majority of these systems. Instead, heavy-gauge, stainless steel bolts, countersunk or not, resin or masonry reinforcing anchors are used more commonly. This might deviate from established purchasing patterns and installation tools, and even require additional training. Quality assurance on each anchor might also be reinforced during commissioning, as adequate anchoring is vital to the entire system.

The surrounding conditions can also prove to be a challenge. For example, pre-drilled ½- to ¾-inch steel embedment plates can be grouted into the masonry opening to provide adequate support for the blast load. This adds to erection time, and quasi-perfection is necessary to match the holes with the window anchors. If drawings show a steel-stud wall, as the existing window is removed you might find your nearest stud and anchoring point 6 to 8 inches from the initial supposition. Surprises certainly do not disappear with blast window projects.

Manufacturer
So many systems, so few standards. Between punched openings, mullions, ribbon windows, curtainwall, storefront, doors, skylights and other window walls, manufacturers have almost endless possibilities and challenges in creating new blast-resistant products.

As most projects cater to specific threats, product applications are usually customized. Getting in the design process early on, and knowing product limitations and engineering solutions, can be the difference between success and failure for the entire project team. As such, always keep in mind manufacturability and installation methods.

As is the case for impact-resistant products, blast-resistant windows are a system, and the anchoring design must be sympathetic with the window. The manufacturer must provide recommendations and guidelines on how the anchor fits with the window. While wall design is not the responsibility of the manufacturer, it is helpful to know the surrounding conditions into which the anchor is going.

Thanks to decades of international test programs and research, the blast behavior of glass is fairly well known, and much data is available. This is not the case with frames and full systems. Very little data is available to manufacturers for reference as they design their products, and product-specific testing can seem like a risky, expensive task. Testing, however, can prove necessary to achieve optimal designs and cost/benefit ratios. Any window has some blast-mitigation capabilities, the issue is quantifying and equating it to the project’s specific need.

Increased Awareness
As we move forward and become more aware of the need for secure buildings, expect to see an increase in blast requirements, especially in federal projects. This will have huge implications for all involved. The learning curve is steep but rewards are sweet. We can all do our part in securing our future.


USG

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