Figure 1 shows the idealised profile of the pressure in relation to time for the case of a freeair
blast wave, which reaches a point at a certain distance from the detonation. The
pressure surrounding the element is initially equal to the ambient pressure Po, and it
undergoes an instantaneous increase to a peak pressure Pso at the arrival time tA, when the
shock front reaches that point. The time needed for the pressure to reach its peak value is
very small and for design purposes it is assumed to be equal to zero. The peak pressure Pso
is also known as side-on overpressure or peak overpressure. The value of the peak
overpressure as well as the velocity of propagation of the shock wave decrease with
increasing distance from the detonation center. After its peak value, the pressure decreases
with an exponential rate until it reaches the ambient pressure at tA+to, to being called the
positive phase duration. After the positive phase of the pressure-time diagram, the pressure
becomes smaller (referred to as negative) than the ambient value, and finally returns to it.
The negative phase is longer than the positive one, its minimum pressure value is denoted
as Pso
- and its duration as to
-. During this phase the structures are subjected to suction
forces, which is the reason why sometimes during blast loading glass fragments from
failures of facades are found outside a building instead in its interior.