Bomb Blast Design Concept
In order to calculate the blast loading on a structure (whether to design a new structure or to assess the blast effect on an existing structure), two fundamental factors need to be established:
1. Mass and type of explosive charge
2. Distance to the target (stand-off)
Traditionally, the stand-off distance has been defined on the assumption that the detonation will occur at a set distance from the target, e.g. at the site boundary (when typically delineated by a perimeter fence) or at the edge of the kerb in a city centre location.
The loads resulting from a blast are created by the rapid expansion of the energetic material, creating a pressure disturbance or blast wave radiating away from the explosion source, as shown in Figure 6.1. Blast pressure is more properly overpressure, because it is relative to ambient conditions, rather than an absolute pressure. Shock waves are high-pressure blast waves that travel through air (or another medium) at a velocity faster than the speed of sound. Shock waves are characterized by an instantaneous increase in pressure followed by a rapid decay.
Pressure waves are lower amplitude and travel below the speed of sound. Pressure waves are characterized by a more gradual increase in pressure than a
shock wave, with a decay of pressure much slower than a shock wave. In most cases, shock waves have a greater potential for damage and injury than pressure waves.
In order to calculate the blast loading on a structure (whether to design a new structure or to assess the blast effect on an existing structure), two fundamental factors need to be established:
1. Mass and type of explosive charge
2. Distance to the target (stand-off)
Traditionally, the stand-off distance has been defined on the assumption that the detonation will occur at a set distance from the target, e.g. at the site boundary (when typically delineated by a perimeter fence) or at the edge of the kerb in a city centre location.
The loads resulting from a blast are created by the rapid expansion of the energetic material, creating a pressure disturbance or blast wave radiating away from the explosion source, as shown in Figure 6.1. Blast pressure is more properly overpressure, because it is relative to ambient conditions, rather than an absolute pressure. Shock waves are high-pressure blast waves that travel through air (or another medium) at a velocity faster than the speed of sound. Shock waves are characterized by an instantaneous increase in pressure followed by a rapid decay.
Pressure waves are lower amplitude and travel below the speed of sound. Pressure waves are characterized by a more gradual increase in pressure than a
shock wave, with a decay of pressure much slower than a shock wave. In most cases, shock waves have a greater potential for damage and injury than pressure waves.
The performance criteria for the building will be:
- Prevent component failure
- Limit structural collapse
- Maintain building envelope
- Minimise flying debris
- Prevent cascading explosion events
The primary design objective for the exterior envelope of most standard building types, is to mitigate the hazard of flying debris generated by failed exterior walls, windows, and other components, to reduce casualties and business disruption and facilitate rescue and evacuation efforts.
Other objectives are to design the exterior envelope to:
- Fail in a way that does not initiate progressive collapse
- Keep the air blast outside the building to the best of its ability
Extract from DuPont blast resistance technical bulletin:
- Prevent component failure
- Limit structural collapse
- Maintain building envelope
- Minimise flying debris
- Prevent cascading explosion events
The primary design objective for the exterior envelope of most standard building types, is to mitigate the hazard of flying debris generated by failed exterior walls, windows, and other components, to reduce casualties and business disruption and facilitate rescue and evacuation efforts.
Other objectives are to design the exterior envelope to:
- Fail in a way that does not initiate progressive collapse
- Keep the air blast outside the building to the best of its ability
Extract from DuPont blast resistance technical bulletin:
Extract from Simplified Design procedure for blast resistant glazing:
Under air blast pressure loading, the designer should design the framing system and its anchorage to the structural frame to resist the dynamic load that the window glass lite would transfer to it under the air blast pressure loading if the glass did not fracture. The architect or engineer can determine the loading transferred to the frame using dynamic analysis techniques.
For blast resistant glazing, the designer should avoid “dry glazing,” in which gaskets alone hold the blast resistant glazing in its frame. Standard glazing bites with gaskets will not restrain fractured laminated glass under air blast pressure loading and the entire lite could fly from the frame. 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.
Blast resistant glazing should attach to the frame using either structural silicone sealant or adhesive glazing tape. The bite depth should not exceed standard depths any more than necessary to facilitate the width of the structural silicone bead or the glazing tape. When using structural silicone sealant, the width of the bead forming the structural connection should equal the nominal thickness of the blast resistant glazing material with which it is in contact. This thickness will usually be less than the thickness of the entire blast resistant glazing construction.
For example, if the blast resistant glazing construction consists of an insulating glass unit with two nominal 6 mm (1/4 in.) lites and a 12mm (1/2 in.) air space, the authors recommend a 6 mm (1/4 in.) structural silicone bead. In the event of an explosion, this width should result in tearing of the silicone bead before the PVB interlayer tears.
This mode of failure will tend to eliminate flying and falling glass shards while maintaining the blast resistant glazing in its frame, especially insulating
glass units. Glazing tape has more flexibility than structural silicone and the designer should use a width of glazing tape 2 to 3 times the thickness of the blast resistant glazing material with which it is in contact.
Under air blast pressure loading, the designer should design the framing system and its anchorage to the structural frame to resist the dynamic load that the window glass lite would transfer to it under the air blast pressure loading if the glass did not fracture. The architect or engineer can determine the loading transferred to the frame using dynamic analysis techniques.
For blast resistant glazing, the designer should avoid “dry glazing,” in which gaskets alone hold the blast resistant glazing in its frame. Standard glazing bites with gaskets will not restrain fractured laminated glass under air blast pressure loading and the entire lite could fly from the frame. 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.
Blast resistant glazing should attach to the frame using either structural silicone sealant or adhesive glazing tape. The bite depth should not exceed standard depths any more than necessary to facilitate the width of the structural silicone bead or the glazing tape. When using structural silicone sealant, the width of the bead forming the structural connection should equal the nominal thickness of the blast resistant glazing material with which it is in contact. This thickness will usually be less than the thickness of the entire blast resistant glazing construction.
For example, if the blast resistant glazing construction consists of an insulating glass unit with two nominal 6 mm (1/4 in.) lites and a 12mm (1/2 in.) air space, the authors recommend a 6 mm (1/4 in.) structural silicone bead. In the event of an explosion, this width should result in tearing of the silicone bead before the PVB interlayer tears.
This mode of failure will tend to eliminate flying and falling glass shards while maintaining the blast resistant glazing in its frame, especially insulating
glass units. Glazing tape has more flexibility than structural silicone and the designer should use a width of glazing tape 2 to 3 times the thickness of the blast resistant glazing material with which it is in contact.