6.4.2 Window Design

Windows, once the sole responsibility of the architect, become a structural issue when explosive effects are taken into consideration. In designing windows to mitigate the effects of explosions they should first be designed to resist conventional loads and then be checked for explosive load effects and balanced design.

Balanced or capacity design philosophy means that the glass is designed to be no stronger than the weakest part of the overall window system, failing at pressure levels that do not exceed those of the frame, anchorage, and supporting wall system. If the glass is stronger than the supporting members, then the window is likely to fail with the whole panel entering into the building as a single unit, possibly with the frame, anchorage, and the wall attached. This failure mode is considered more hazardous than if the glass fragments enter the building, provided that the fragments are designed to minimize injuries. By using a damage-limiting approach, the damage sequence and extent of damage can be controlled.

Windows are typically the most vulnerable portion of any building. Though it may be impractical to design all the windows to resist a largescale explosive attack, it is desirable to limit the amount of hazardous glass breakage to reduce the injuries. Typical annealed glass windows break at low pressure and impulse levels and the shards created by broken windows are responsible for many of the injuries incurred during a large-scale explosive attack.

Designing windows to provide protection against the effects of explosions can be effective in reducing the glass laceration injuries in areas that are not directly across from the weapon. For a large-scale vehicle weapon, this pressure range is expected on the sides of surrounding buildings not facing the explosion or for smaller explosions in which pressures drop more rapidly with distance. Generally, it is not known on which side of the building the attack will occur, so all sides need to be protected. Window protection should be evaluated on a case-by-case basis by a qualified protective design consultant to develop a solution that meets established objectives. Several recommended solutions for the design of the window systems to reduce injuries to building occupants are provided in Figure 6-7.

Several approaches that can be taken to limit glass laceration injuries. One way is to reduce the number and size of windows. If blast-resistant walls are used, then fewer and/or smaller windows will allow less airblast to  enter the building, thus reducing the interior damage and injuries. Specific examples of how to incorporate these ideas into the design of a new building include (1) limiting the number of windows on the lower floors where the pressures would be higher during an external explosion; (2) using an internal atrium design with windows facing inward, not outward; (3) using clerestory windows, which are close to the ceiling, above the heads of the occupants; and (4) angling the windows away from the curb to reduce the pressure levels.

Glass curtain-wall, butt glazed, and Pilkington type systems have been found to perform surprisingly well in recent explosive tests with low explosive loads. In particular, glass curtain wall systems have been shown to accept larger deformations without the glass breaking hazardously, compared to rigidly supported punched window systems. Some design modifications to the connections, details, and member sizes may be required to optimize the performance.

Glass Design

Glass is often the weakest part of a building, breaking at low pressures compared to other components such as the floors, walls, or columns. Past incidents have shown that glass breakage and associated injuries may extend many thousands of feet in large external explosions. Highvelocity glass fragments have been shown to be a major contributor to injuries in such incidents. For incidents within downtown city areas, falling glass poses a major hazard to passersby and prolongs post-incident rescue and clean-up efforts by leaving tons of glass debris on the street. At this time, the issue of exterior debris is largely ignored by existing criteria. 

As part of the damage-limiting approach, glass failure is not quantified in terms of whether breakage occurs or not, but rather by the hazard it causes to the occupants. Two failure modes that reduce the hazard posed by window glass are

• glass that breaks but is retained by the frame and
• glass fragments exit the frame and fall within three to ten feet of the window.

The glass performance conditions are defined based on empirical data from explosive tests performed in a cubical space with a 10- foot dimension (Table 6-1). The performance condition ranges from 1, which corresponds to not breaking, to 5, which corresponds to hazardous flying debris at a distance of 10 feet from the window (see Figure 6-8). Generally a performance condition 3 or 4 is considered acceptable for buildings that are not at high risk of attack. At this level, the window breaks and fragments fly into the building but land harmlessly within 10 feet of the window or impact a witness panel 10 feet away, no more than 2 feet above the floor level. The design goal is to achieve a performance level less than 4 for 90 percent of the windows.

The preferred solution for new construction is to use laminated annealed (i.e., float) glass with structural sealant around the inside perimeter. For insulated units, only the inner pane needs to be laminated. The lamination holds the shards of glass together in explosive events, reducing its ability to cause laceration injuries. The structural sealant helps to hold the pane in the frame for higher loads. Annealed glass is used because it has a breaking strength that is about one-half that of heat-strengthened glass and about one-fourth as strong as tempered glass. Using annealed glass becomes particularly important for buildings with lightweight exterior walls using for instance, metal studs, dry wall, and brick façade. Use the thinnest overall glass thickness that is acceptable based on conventional load requirements. Also, it is important to use an interlayer thickness that is 60 mil thick rather than 30 mil thick, as is used in conventional applications. This layup has been shown to perform well in low-pressure regions (i.e., under about 5 psi). If a 60 mil polyvinyl butaryl (PVB) layer is used, the tension membrane forces into the framing members need to be considered in design.

To make sure that the components supporting the glass are stronger than the glass itself, specify a window breakage strength that is high compared to what is used in conventional design. The breakage strength in window design may be specified as a function of the number of windows expected to break at that load. For instance, in conventional design, it is typical to use a breakage pressure corresponding to eight breaks out of 1000. When a lot of glass breakage is expected, such as for an explosive incident, use a pressure corresponding to 750 breaks out of 1000 to increase confidence that the frame does not fail, too. Glass breakage strength values may be obtained from window manufacturers. 

Mullion Design

The frame members connecting adjoining windows are referred to as mullions. These members may be designed in two ways. Using a static approach, the breaking strength of the window glass is applied to the mullion; alternatively, a dynamic load can be applied using the peak pressure and impulse values. The static approach may lead to a design that is not practical, because the mullion can become very deep and heavy, driving up the weight and cost of the window system. It may also not be consistent with the overall architectural objectives for the project.

As with frames, it is good engineering practice to limit the number of interlocking parts used for the mullion.

Frame and Anchorage Design

Window frames need to retain the glass so that the entire pane does not become a single large unit of flying debris. It also needs to be designed to resist the breaking stress of the window glass.

To retain the glass in the frame, a minimum of a ¼-inch bead of structural sealant (e.g., silicone) should be used around the inner perimeter of the window. The allowable tensile strength should be at least 20 psi. Also, the window bite (i.e., the depth of window captured by the frame) needs to be at least ½ inch. The structural sealant recommendations should be determined on a case-by-case basis. In some applications, the structural sealant may govern the overall design of the window system.

Frame and anchorage design is performed by applying the breaking strength of the window to the frame and the fasteners. In most  conventionally designed buildings, the frames will be aluminum. In some applications, steel frames are used. Also, in lobby areas where large panes of glass are used, a larger bite with more structural sealant may be needed.

Inoperable windows are generally recommended for air-blast mitigating designs. However, some operable window designs are conceptually viable. For instance, designs in which the window rotates about a horizontal hinge at the head or sill and opens in the outward direction may perform adequately. In these designs, the window will slam shut in an explosion event. If this type of design is used, the governing design parameter may be the capacity of the hinges and/or hardware.

Wall Design

The supporting wall response should be checked using approaches similar to those for frames and mullions. It does not make sense, and is potentially highly hazardous, to have a wall system that is weaker than windows. Remember that the maximum strength of any wall system needs to be at least equal to the window strength. If the walls are unable to accept the loads transmitted by the mullions, the mullions may need to be anchored into the structural slabs or spandrel beams. Anchoring into columns is generally discouraged, because it increases the tributary area of lateral load that is transferred into the columns and may cause instability.

The balanced-design approach is particularly challenging in the design of ballistic-resistant and forced-entry-resistant windows, which consist of one or more inches of glass and polycarbonate. These windows can easily become stronger than the supporting wall. In these cases, the windows may need to be designed for the design threat air-blast pressure levels under the implicit assumption that balanced-design conditions will not be met for larger loads.

Multi-hazard Considerations

Under normal operating conditions, windows perform several functions listed below.

• They permit light to enter building.
• They save energy by reducing thermal transmission.
• They make the building quieter by reducing acoustic transmission.

Explosions are one of a number of abnormal loading conditions that the building may undergo. Some of the others are

• fire,
• earthquake,
• hurricane,
• gun fire, and
• forced entry.

When developing a protection strategy for windows to mitigate the effects of a particular explosion threat scenario, it is important to consider how this protection may interfere with some of these other functions or other explosion threat scenarios. Some questions that may be worthwhile to consider are listed below.

• If an internal explosion occurs, will the upgraded windows increase smoke inhalation injuries by preventing the smoke to vent through windows that would normally break in an explosion event?
• If a fire occurs, will it be more difficult to break the protected windows to vent the building and gain access to the injured?
• Will a window upgrade that is intended to protect the occupants worsen the hazard to passersby?