Quantcast Ground Motion and Cratering

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APPENDIX D (Continued)
Figure D-1
Blast Pressure Wave
Ground Motion and Cratering. When a bomb explodes at or slightly above the
ground surface, part of the energy forms a crater and sends a shock wave through the ground.
The ground shock wave acts like a short duration earthquake, although the wave mechanisms
have a compressional (vertical) effect on a structure, rather than the shear (lateral) effect that
occurs during an earthquake. This factor may or may not be important, depending on the size of
the explosion and how close it is to the structure.
Fragmentation. Generally, vehicle bombs will break into large pieces during an
explosion. Heavy items, such as axles, engine blocks, and doors, may be thrown long distances.
Initially, all bomb fragments are propelled by the explosion at a high rate of speed that slows
down as they travel through air. Smaller fragments have a higher initial velocity, but lose their
velocity more quickly than heavy fragments as they travel through the air.
Generally, primary fragmentation effects from the bomb itself are not as significant as
blast effects in producing casualties. Fragments and airborne debris, resulting from the blast, are
a more serious factor. These include shattering glass, falling parts of the building faade, and
collapsing components of the structure.
Fire.  Many structures are combustible, and fire can be a significant damage-
producing mechanism, especially after an explosion has occurred. Fires can be started by high-
explosive bombs, but the effect is negligible, compared to the damage caused by the shock wave.


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