Battling explosives: Department of Defense criteria for blast-resistant glazing
December 1, 2006
COMMERCIAL, RETAIL, FABRICATION : SAFETY GLAZING
Glass hazards often represent a significant component of the risk to personnel during a terrorist explosives attack. Whether from a vehicle-borne improvised explosive device or hand-placed explosive charges, blast pressures can overwhelm conventional glazing and result in hazardous glass fragments in occupied areas.
Injuries from glass include lacerations and serious injuries from less obvious glass dust that can get embedded in eyes. A number of solutions for glass-hazard mitigation fortunately are ready to be applied to new and existing facilities.
While commercial buildings in selected metropolitan areas are subject to attack, military facilities represent the greater risk in terms of number of people exposed. Following the attack on Khobar Towers in Saudi Arabia in 1996, it became apparent that additional protection for personnel was needed, regardless of where they were stationed. In this incident, 19 soldiers living in conventional housing were killed and more than 200 were injured.
To address this problem, the Department of Defense developed a set of minimum standards to be incorporated into all military construction, or MILCON, funded projects. These standards included glazing protection. The first set of standards was published as Interim Antiterrorism/Force Protection Standards in December 1999. This document applied to new construction and to major modernization projects, beginning with fiscal year 2002. The standards focused on occupied buildings. The standards recognized that it is much cheaper to incorporate blast-resistant features into new structures than to retrofit an existing building. The criteria were based on minimal threats and were not intended to totally prevent blast damage against the target facililty but rather to minimize the collateral effects of a bomb blast in nearby occupied buildings. For the target facility, the criteria were specifically developed to prevent mass casualties.
The standards applied to facilities with occupancy greater than one per 400 square feet to exclude industrial and storage facilities. The focus was on troop billeting and “primary gathering structures,” which have the highest occupancies.
The initial standard contained requirements for a number of configurations, siting and building construction issues involving protection from terrorist attack. Three levels of protection were described along with the required glazing performance following a blast. Section AP 2.4.3 of that document laid out the only requirement for protective glazing, which was 1⁄4-inch laminated annealed glass.
This interim standard also provided for levels of protection beyond that provided by the minimum standards. These levels of protection were to be set on a project specific basis by the planning team. At the Low level of protection, required for primary gathering buildings, glazing fragments propelled a few feet into the structure were permitted. At the Medium level, glazing was permitted to break but must remain in the frame. High level protection required that the glass did not break.
UFC arrives, unofficially
In July 2002, a major revision of the antiterrorism standards was prepared as part of the Unified Facilities Criteria. Designated 04-010-01, the document was in draft form and was never made official, although the wording was often used in project specifications. The criteria were applicable to “buildings, or parts of buildings, owned, leased, privatized or otherwise occupied, managed, or controlled by or for DoD.” Consistent with the Interim Standards, they were intended to provide a level of protection for buildings even with no specific threat so all DoD facilities would provide some meaningful degree of protection. The criteria specifically recognized that resources for protection are limited, and the required measures had to be cost effective.
Criteria in the UFC was applicable, for 2004 and beyond, to new buildings and buildings undergoing major modernization, defined as projects that exceed 50 percent of the replacement cost of the building. Facilities undergoing conversion of use for different functions also were required to meet the provisions of the standards. Leased facilities were covered by the criteria when more than 25 percent of the building was occupied by DoD personnel and applied to new or renewed leases for 2005 and beyond. Window-replacement projects also invoked the window-protection portions of the criteria. The criteria addressed a set of specific building functions that were exempt from some or all of the provisions.
The draft 2002 document focused attention on standoff distance as an important measure for protection of personnel. Blast loads decay rapidly with distance, and any increase in standoff distance reduces the potential for building damage and thus occupant vulnerability.
Required levels of protection were established based on occupancy types. Specific structural damage and glazing hazards were associated with these defined levels of protection and potential injury levels.
Billeting and primary gathering buildings were assigned a Low level of protection. Most other inhabited buildings, with 11 or more personnel, were assigned a Very Low level. The higher level recognized the increased risk with a greater concentration of personnel and the attractiveness of the target.
Medium and High levels of protection also were delineated but were not specifically assigned to a particular occupancy or building function.
In terms of glazing hazards, the High protection level required that glazing not be broken. At Medium level, glass could be broken but must remain in the frame. Low protection required glazing to fall within one meter of the opening corresponding to a Very Low Hazard in ASTM F 1642.
Hazard levels definition for testing
At Very Low level of protection, glazing could be propelled into the interior of a building but with some mitigation of the hazard, such as laminated glass or safety film for existing buildings. This hazard level is designated Low in ASTM F 1642. Buildings with a protection level of Below AT Standards—AT or anti-terrorist—would be expected to sustain high hazard levels fragments.
The UFC-04-010-01 draft of 2002 established 23 standards addressing a wide range of facility layout and construction parameters that directly impact the vulnerability of occupants. In addition, 16 recommendations were defined to improve protection on a non-mandatory basis. Standard 11 dealt specifically with windows and was applicable to windows and frames. Laminated glass, 1⁄4-inch minimum, was required for exterior windows and glazed doors. For insulating units, the inner lite was required to be laminated. Recommendation 16 encouraged planners and designers to minimize the number and size of windows.
Framing and connection details
One of the more significant changes in the draft UFC from the Interim Standards was a defined static load level for frames, mullions and hardware of 1 pound per square inch applied to the glazing. Frame and mullion deflections were limited to L/160, where “L” was the unsupported member length. The intent of the minimum uniform load was to ensure window framing had flexural strength to develop the capacity of the laminated glass. This approach made design of these frames more accessible to suppliers than a specified dynamic load. Most fame manufacturers are not versed in dynamic analysis but are competent with static loads.
A minimum bite also was specified to prevent premature ejection of the lite from the frame before the laminated glazing capacity was reached. This minimum bite was 3⁄8-inch for structurally glazed lites, installed with a structural silicone, and 1 inch for windows that were not structurally glazed, installed with glazing tape or ethylene propylene diene monomer gasket.
Additionally, frame connections to the surrounding walls had to be designed to resist a tension load of 200 pounds per inch and a shear load of 75 ppi acting simultaneously. This load was specified to reflect the reactions imparted by laminated glazing with a 0.030-inch poly vinyl butyral interlayer responding in tensile membrane with an assumed deformation of about 20 degrees out of plane. The full strength of the PVB interlayer was used in determining these loads. These loads were significant and required heavy frames to provide adequate response, even using ultimate load capacity.
The first official UFC 4-010-01 was promulgated in October 2003, making it the second official criteria dealing with antiterrorism across DoD. Window requirements contained in Standard 10 in this document are similar to the 2002 draft version. The most significant change is the specification of two uniform loads to be used for design of connections of the frame to the structure, depending on vision area of the glazing. For windows with a vision area smaller than 10.8 square feet, the static design load is 10.8 psi. The uniform design load for larger windows that are smaller than 32 square feet is 4.4 psi. In addition to the two shear/ connection loads, the 1-psi load for design of the frames in flexure re-mained but the allowable deformation limit was increased to L/60. Aluminum and steel were the only framing allowed unless otherwise demonstrated to provide acceptable response. Ultimate strength analysis is permitted for steel frames and a stress equal to a 0.2 percent offset yield strength was specified for aluminum frames.
The guidance to minimize number and sizes of windows was moved to Recommendation 17. Language for the recommendation remained the same as the 2002 draft standards.
Revisions to the October 2003 UFC 4-010-01 were initially prepared in draft form in October 2005 with a final revision submitted for staffing in March 2006. The current draft version of July 2006 incorporated staffing comments and is currently up for signature by the DoD engineer board.
Principal changes in the document that dealt with glazing hazards were revisions to the protection level descriptions, making them more qualitative rather than quantitative, something they were never meant to be.
Standard 10 was revised extensively to incorporate other design procedures. The procedure in Standard 10 is limited to laminated glass.
The procedures provide for design of glazing and framing based on specific design charge weights and standoff distances. The procedure will cover response of the laminated glass for Medium, Low and Very Low protection levels. However, the standoff distance used cannot be greater than the minimum conventional construction standoff associated with the Low level of protection, even if more standoff is available. ASTM F 2248 is used to develop a uniform load, which is then used in ASTM E 1300 to determine the glazing section requirement. In order to design to the Low and Very Low levels of protection, certain modifications to ASTM F 2248 and ASTM E 1300 are provided in Standard 10 since the procedures when unmodified will result in response to a Medium protection level. Two tables are included in the standard to provide specific guidance for laminated single pane and insulated glass units with an inboard laminated pane. The criteria specify an overriding minimum thickness of 1⁄4-inch laminated glass section with an 0.030-inch polyvinyl butyral interlayer.
The standard references the DoD Security Engineering Facilities Design Manual for Medium and Higher protection levels, for charge weights and standoffs outside the range of ASTM F 2248 and for alternatives to laminated glass.
Glazing bite is determined in accordance with ASTM F 2248. Standard 10 requires silicone application on both sides of a single-pane laminated glass panel for structural glazed systems but only requires application on the inboard side for IGUs.
The design load developed in ASTM F 2248 is also used for frame design to allowable stress levels. Deflections are limited to L/160, where “ L” is the length of the supported edge of the glazing. For punched windows, “L” is the span of the glazing without regard to intermediate supports. ASTM F 2248 also is to be used for development of connection design loads. Allowable loads, as determined by the connection manufacturer, are to be used. This simplifies the design procedure for glazing contractors who are used to working with allowable loads rather than ultimate loads.
Connections must prevent the frame from becoming separated from the wall in which the window is installed. This connection design can be shown by calculation or by testing. Calculations must include all elements in the load path and all potential modes of response including flexure, bearing, shear, compression and tension.
Structural supports and the structural support connections, which transfer the loads from the windows, are to be designed using nominal strength for the loads developed with ASTM F 2248. For laminated glass, the design load is equal to eight times the resistance of the glazing determined from ASTM E 1300. Structural element connection loads do not have to be distributed through the rest of the structure because it is assumed that the loads will be dissipated into the structure.
Alternate glazing and methods of analysis can be used if designed for the specified protection levels. This allows the use of HazL, or Window Fragment Hazard Level Analysis software program, and other codes for glazing design with the provision that the standoff distance cannot be greater than the minimum standoff distance in the criteria.
The standard does allow testing to demonstrate protection levels. This is most applicable to systems that are not easily simulated numerically such as energy absorbing systems. The testing must include the entire window system, including connections, and be conducted in accordance with ASTM F 1642. Testing samples must include connection type, number and configuration to be installed in the subject project. Any deviations have to be addressed by calculation.
Retrofits using safety film or blast shades are not permitted for new construction, major modernization or window replacement projects. Film and shades are permitted for leased facilities and for windows in existing buildings that are not subject to the triggers for replacement but are being retrofit solely to reduce risk. Protection levels for retrofit should be shown through calculation or testing.
Criteria applies; be aware
UFC 04-010-01 was years in the making and is still undergoing revision but it has brought some standardization to the glazing industry for blast applications in much the same way the U.S. General Services Administration test and specification criteria did for civilian agencies. The UFC has the potential for greater impact on the industry than the GSA criteria due to the mandatory nature of the standards and the large number of military structures involved. As standard designs are developed and tested, the cost of individual designs will come down and the impact project budgets will be reduced. Contractors and suppliers need to be well versed in the requirements because of the potential impact to construction cost. Missing these provisions at the bid stage can put a tight squeeze on profit margins during construction.