Blast loads are defined by their “time history,” which is the trace of over-pressure throughout the load duration.  An idealized blast time history is shown in the figure below (after UFC 3-340-01):

The pressure (relative to ambient) is on the vertical axis and time is on the horizontal axis.  The blast duration is generally taken to be the amount of time it takes for the blast load to initially decay to ambient pressure with respect to its time of onset (typically assumed to be the time of peak pressure).  Another term that you will hear is ‘impulse’.   The impulse can be thought of as the amount of energy delivered during the blast load, and is calculated as the area under the pressure vs. time curve.  When performing a simplified dynamic analysis, there are two common simplifications made when using blast time histories.

• Only positive phase of history taken.
• Use equivalent right triangular shock load shape preserving impulse.

When speaking about the magnitude of blast forces that a building is subjected to, that a design should resist, or that a product needs to protect against, both the peak pressure and duration (or impulse) must be used.  This is because a load of 10 psi (for example) that lasts for 2 msecs can result in significantly less damage than the same 10 psi load that lasts for 20 msecs.

Time matters.