Bomb Blast Mitigating Glazing Design

Any attempt to design glazing that minimizes bodily injury and property dam­age during a bomb blast requires an understanding of the fracture behavior of architectural glazing subjected to blast loading. Traditional window glass de­sign methodology, which assumes that loads act quasi-statically with durations measured in seconds or longer periods, is not suitable for designing blast miti­gating windows.

It is important to emphasize the principal differences between static, dy­namic, and short-duration dynamic loads. Static loads, such as gravity loads, are assumed to act on a building structure for long periods of time and are not time dependent. Dynamic loads, such as induced by earthquakes or wind gusts, have strong time dependencies and their durations are typically measured in tenth of seconds up to several seconds. Short-duration dynamic loads, such as those induced by explosions or debris impact, are pulse loads with a duration that is about 1000 times shorter than the duration of a typical earthquake. In addition to their short-duration dynamic nature, air blast pressure loadings tend to have much larger magnitudes than wind and snow loadings that typi­cally govern window glass design.

The damage potential of explosive blasts lies in their ability to deliver ki­netic energy to the glass pane. Under static loading the glass pane reacts by bending, i. e., the deformation occurs while the load is acting on the glass pane. Dynamic wind or seismic loading increase relative slowly when compared to the structure’s natural frequency. When resisting the acceleration from such slow dynamic forces, the strain distribution is similar to the one observed under static loading; therefore, such situation can be treated in a quasi-static approach. However, if the positive phase duration of the blast pressure is shorter than the natural period of vibration of the glass pane, the response is described as impulsive. In this case, most of the deformation of the glass pane will occur after the blast loading has diminished. The dynamic air blast pres­sure loading associated with an explosion excites higher vibration modes in a window glass pane and causes much higher stresses and deflections than a quasi-static loading having the same magnitude of pressure [5].

Figure 3 shows qualitatively the relative amplitude-frequency distribution for different dynamic loads acting on a building. In this figure, for nonoscilla-

image206

FIG. 3—Qualitative amplitude-frequency distribution for different dynamic loads act­ing on a building (adapted from Ref [13] and modified by authors).

tory pulse loads, such as blast load, frequency is to be considered the inverse of pulse duration.

The best protection from a bomb is standoff distance since the loading associated with a given bomb decreases rapidly with increasing standoff dis­tance. Therefore, typically a perimeter wall is constructed at the property boundary to restrict access to the site and to achieve a certain minimum stand­off distance. However, the building’s facade is often its first and only defense against the effects of a bomb. The manner in which a facade responds to blast loading substantially affects the behavior of the building’s structure. Typically, the design philosophy is to focus on the post-damage behavior with the aim of having the building and its cladding components standing or attached long enough to evacuate every person and to protect occupants from injury or death resulting from flying debris.

Generally, the least hazardous post-damage behavior is achieved when the window units are composed of laminated or filmed glass. For a range of blast pressures and impulses, failed laminated or filmed glass panes retain the shards of glass, thereby limiting the extent of flying debris. Even when cracked by blast pressure, the outer glass layers of a laminated glass pane remain bonded to the inner plastic interlayer rather than forming free-flying shards, so long as the blast does not exceed the postulated maximum peak pressure and impulse used in dimensioning the blast mitigating glazing system [14-16]. For the laminated or filmed glazing system to be effective, the glass must remain attached to the suitably enhanced mullions or window frame. This is typically accomplished by means of a sufficiently strong structural silicone sealant. For laminated glass, retaining the glass pane may also be achieved by a sufficiently deep rebate. Standard glazing bites with gaskets will not restrain fractured laminated glass under air blast pressure loading and the entire glass pane may be pulled out from the rebates and dislodged from the frame as the glass deflects. On the other hand, the use of very deep bites with gaskets might restrain the blast – resistant glazing, but could lead to other problems such as thermal breakage in annealed laminated glass [5].

Maximum protective performance is achieved by securely bonding the glass pane to its frame or mullions by means of a structural silicone sealant, thus enabling the cracked laminated or filmed glass to behave as a ductile membrane that bulges inwards, assuming an external explosion event. If the impact-absorbing interlayer or film remains intact and the fixation of the glass pane is maintained, the glazing prevents blast pressures from entering the building and at most a fine glass dust is detached from the surface of the inner laminated glass pane. Most of the causes of injury are thereby removed.

Stretching the laminated glass polyvinyl butyral (PVB) interlayer beyond the limit of its ductility (ca. 200 % at room temperature) causes it to tear. Even after tearing has commenced, the laminated glass pane will continue to offer some blast resistance until the opening is substantial. This is because as it deforms and ruptures, the applied blast pressure will diminish over time so that the residual blast pulse, which does eventually enter the interior of the build­ing, will be less than the full pulse [17].

The ability of laminated or filmed glass to absorb and dissipate blast energy is well proven, both in tests and actual terrorist bomb explosions [5,7,8]. How­ever, in order to effectively utilize the membrane capacity of laminated or filmed glass and to transfer the blast load to the window frame or mullions, the glazing must be securely adhered to its supporting structure. Silicone sealants provide unique benefits to these window designs due to their inherent strength properties, resulting in their ability to anchor the glazing in the framing during a bomb blast event. However, the silicone sealant is only one component of a glazing system. A glazing system that meets the testing and code requirements for bomb-blast resistant glazing must successfully integrate the frame or mul – lion and its anchorage, glass or other glazing materials, protective film or in­terlayer, and silicone sealant into a system capable of transferring the sustained forces to the structural building slabs. Various recommendations and standards dealing with the testing, classification, and guidance for installation have been developed (see, for example, Refs. [18-26], and the discussion of International and European Standards in Ref [27]).

The most commonly used performance specifications for bomb blast miti­gation in the United States are the GSA (Government Services Administration) Levels C and D [18]. The GSA levels are defined in terms of (1) an overpressure, and (2) an impulse (integrated pressure duration product). Level C specifies a peak pressure of 27.6 kPa and an impulse loading of 193 kPa■ ms, while Level D specifies a peak pressure of 69.0 kPa and an impulse loading of 614 kPa■ ms. The behavior of the glazing under the given pressure impulse conditions is then classified in terms of the breakage mode with the classification ranging from Category 1 (no break) to Category 5 (high hazard). The intent of these criteria is to reduce, but not necessarily eliminate, the potential hazards, recognizing that not all windows will survive a bomb attack.

However, it is important to note that terrorist bombing is a very low prob­ability event. Most of the time during their service life, bomb-blast resistant windows and facades must provide their regular function, which is to protect the building and its inhabitants from the exterior environment while allowing

image207

FIG. 4—Schematic model of window behavior upon impact of blast load.

light to enter the building and its occupants to visually observe the outside world. Furthermore, since one cannot predict if and when a building will be exposed to a blast event, all materials used in the building facade must have durability characteristics that will allow them to withstand the blast regardless whether the building just has been completed or already has been in use for the past 10 or 20 years. Silicone sealants excel in their long-term elasticity, adhe­sion and durability; properties that are important for the proper functioning of a facade, both during regular weather exposure as well as during a bomb-blast event [28].

Behavior of Bonded Architectural Glazing Subjected to Bomb Blast Loading

When a window glazed with a laminated or filmed glass that is bonded to the frame successfully passes air blast pressure loading, the following stages can be observed (as shown schematically in Fig. 4):

1. The glass pane deforms elastically and stores some of the impacting blast energy.

2. The glass pane(s) fracture(s), and the fracture dissipate(s) the stored energy.

3. The film or interlayer remains bonded to the glass shards as it deforms (stretches) in an elastoplastic manner and the deformation dissipates most of the impacting blast energy, causing the filmed or laminated glazing to deflect considerably (300 to 400 mm is not uncommon for larger glazing sizes).

image208

4. The frame and support system (mullions) deform and dissipate the re­maining blast energy.

The overall design philosophy for bomb blast mitigating windows requires the laminated or filmed glass to break as this consequently allows the interlayer or film to stretch, causing the large-scale dissipation of blast energy. Because of this successful energy dissipation less load is transferred into the structural frame.

In order for a blast-mitigating window to fail properly, the glazing must be held in place long enough to develop sufficient stress to cause glass failure. Fundamental to the behavior of the window or curtainwall is the ability of the laminated or filmed glass to remain attached to the frame or mullions. The silicone sealant, essential for retaining the glazing, is first compressed by the glass bearing against the frame or mullions, and then subjected to a complex triaxial state of superimposing bending, shear, and tensile stresses as the glaz­ing is crazed and deformed. Since the deformation process occurs within a few milliseconds, giving rise to stresses in the silicone sealant close to its perfor­mance limits, it is of importance when selecting products in the design of blast – mitigating glazing systems to know the response of silicone sealants to high strain rates and high stress loads.

Some of the deformation conditions that the silicone sealant experiences can be approximately derived from actual blast tests on laminated or filmed glass. In a simplistic (quasi-static) approach, the maximum force acting on the sealant at the center of the long edge of the glass pane can be estimated by replacing the three-dimensional membrane with a two-dimensional cross sec­tion represented by a flexible “rope” fixed between two points under constant load (see Fig. 5). In this model, the load is acting perpendicular on the rope at any point along its length. Simple static considerations allow derivation of the line load and its perpendicular components at the fixation points and at the center of the membrane, as follows:

Obviously, a more sophisticated approach should be based on finite-element (FE) or computational fluid dynamics (hydro-codes) modeling considering the silicone sealant as a hyper-elastic material attached to the glass plate edges. However, for the purpose of deriving the key experimental parameters for sealant testing, only the simplistic approach is considered here.

Kranzer and colleagues report pressure and displacement versus time histories measured on laminated glass panes (two panes of 3-mm thick float glass laminated with 1.52-mm thick PVB film) exposed to blasts generated by high explosive field charges or pressurized air releases in shock tubes [29]. The glass area loaded by the blasts was 1.0 X 0.8 m2 and the blast impulses were designed to take the laminated glass to the point when the glass pane just crazes (referred to as the Break Safely/No Hazard level). Under these conditions, center pane displacements of around 15 mm and maximum center pane velocities of 4.9 to 7.5 m/s were measured using a noncontact, laser-optical displacement measurement technique.

Laminated glass can be made from preprocessed glass panes (annealed, heat strengthened, or fully tempered) of different structural strength (and post-breakage behavior). Furthermore, many secondary effects, such as the age of glass, scratches at the glass surface, the adherence of the laminate to the glass surface, the stiffness of mounting, the moment on the support, dry or wet glazing, ambient temperature and humidity, etc., result in changes of the window’s resistance to explosive blast [29,30]. In general, however, for laminated glass with a size of 1.0 X 0.8 m2 made from two float glass panes with an individual thickness of up to 6 mm and laminated with PVB film of up to 2.28-mm thickness, the Break Safely/No Hazard Level is achieved for a displacement ratio h/b < 0.03. However, much higher displacement ratios, in the range of h /b ~ 0.2, may be observed for high impact implosions, taking the laminated glass to GSA/ISC performance levels of 3 or 4.

Considering the above experimental blast testing information, and using Eqs 4 and 5, typical deflection angles of 4° to 40° and typical line loads in the range of 60-160 N/mm were estimated for a laminated glass of 1.0 X 0.8 m2 size. Using a range of 5 to 20 milliseconds for the positive pressure phase and the typical displacement ratios given above (h/b ratios of 0.03 to 0.2), maximum center pane velocities for successful blast tests, i. e., without the pane dislodging, are estimated to fall in the 4to30m/s range. Considering the post-breakage ductility of the laminated glass at high strain rates [31], an estimate for maximum movement rates on the sealant ranged between 1 and 15 m/s. However, it should be emphasized that the cornerstones of this range represent an attempt at a conservative “educated guess;” a better defined range should be derived from direct experimental measurements.

Behavior of Silicone Sealants at High Strain Rates Relevant for Bomb Blast Mitigating Window Designs