Additional structural damage from ground shock as a result of a bombing

The term bombing refers to an uncontrolled demolition (implosion) where a terrorist employs an explosive of a charge weight and detonates it at range and height (air or ground blast) suitable to cause the maximum damage. For a designer trying to design a structure in advance of such a situation happening is extremely difficult thus requiring the consideration of past bombings for the pertinent design parameters. Ground shock is a small earthquake that impacts the structure via energy released from the detonation of the bomb into the ground being converted to wave energy with compression (P) waves, shear (S) waves and the most damaging surface Raleigh (R) waves moving out in all directions from the point of detonation whether it is on or above ground. The common measurement for quantifying ground shock from blast loadings is the peak particle velocity (PPV) as it correlates reasonably well with both building damage and the annoyance levels that people can tolerate under normal circumstances. The Oklahoma City VIED bombing has been used as an example using CONWEP software for a 2200kg VIED detonated at 4.75m from the building. Ground shock results showed that a PPV of 11.5m/s (11500mm/s) was achieved well in excess of guidelines set by AS 2187 and DIN 4150. As well the structure was subjected to radial displacements of 100mm to 350mm adding to the overall damage. Considering the ground shock results along with a reflected overpressure of 51.17MPa it is unsurprising that the building collapsed and many were killed and injured. This is the recommended design overpressure.