Damage to structures is caused directly by the blast wave in a nuclear explosion. When the blast wave is generated it travels faster than the speed of sound and this therefore causes very high jump in pressure at the front of the wave, this iscalled the overpressure. Also, the wave front causes the air behind it to travel at high velocities as well and creates a powerful wind that is termed “dynamic pressure.” It is the combination of the two that causes damage. The velocity of the winds in such a detonation reach 750 miles an hour .
The values that the overpressure and dynamic pressure attain are at their peak immediately and gradually decay as a function of distance. Interestingly, once the blast wave has passed over a certain area, the area is subjected to negative pressure before returning to normal. While the negative pressure can cause damage, its intensity is far less than overpressure or dynamic pressure .
Normally, when talking about structure damage, the overpressure is the value used as opposed to the dynamic pressure or both. This is because below 70psi, the dynamic pressure is significantly less than that of the overpressure. Below are the equations for determining the psi for certain radius as well as a chart that shows the type of structure damage for a given psi value. These equation expand on the destructive radius equations given before by having the constant value known for each psi .
Where the radius is in km, and Y is the yield of the bomb in kT ,
|1 psi||Window glass shatters. Light injuries from fragments occur.|
|3 psi||Residential structures collapse. Serious injuries are common, fatalities may occur.|
|5 psi||Most buildings collapse. Injuries are universal, fatalities are widespread.|
|10 psi||Reinforced concrete buildings are severely damaged or demolished. Most people are killed.|
|20 psi||Heavily built concrete buildings are severally damaged or demolished. Fatalities approach 100%.|
Humans have an amazing resistance to overpressure. Lethal amounts approach 40 psi which is staggering. The only permanent injury sustained is the loss of eardrums. However, an indirect effect does occur to human casualties. Due to the massive winds and pressure created by the blast wave, tons of debris is hurled through the air, not to mention people themselves. The flying debris, (and people) cause massive amounts of casualties such as impalements in the head, chest and abdomen, and broken bones .