Overpressure

At a basic level, the U.S. Department of Defense defines overpressure as the pressure resulting from the blast wave of an explosion, most often nuclear or of a volumetric explosives. It is referred to as “positive” when it exceeds atmospheric pressure and “negative” during the passage of the wave when resulting pressures are less than atmospheric pressure.

From the emergency management standpoint, knowing the level of overpressure to which an area is exposed helps predict casualties and resource needs. It is also predictive of blast injury.

The resistance of various structures to blast is frequently stated in resistance to overpressure.

Amount of overpressure needed for destructive effects

Overpressure	Effect
1 psi	Window glass shatters. Light injury from fragments.
3 psi	Residential structures collapse, with many serious injuries and some blast/fragmentation deaths
5 psi	Most buildings collapse. All within this overpressure are injured and many are killed.
10 psi	Reinforced concrete buildings are severely damaged or demolished. Most die.
20 psi	Heavily built concrete structures are severaly damaged or destroyed, and deaths approach 100%

To put this into perspective with respect to military structures, however, some missile silos are known to have been designed to withstand overpressures into the hundreds or low thousands of PSI.

Overpressures from large explosions, especially nuclear, are determined by the Mach effect, which is a function of burst altitude that, for sufficiently low burst heights, causes reinforcement between the direct shock wave of the burst and of shock waves reflected from the ground.