[en] The P-I characterization scheme greatly facilitates specification of critical loads for structures subjected to air blast loading and is generally useful for structures subjected to pulse loads. A general feature of the P-I characterization scheme is that critical load curves are represented reasonably well by a rectangular hyperbola [(P/P₀)-1][(I/I₀)-1]=1, where P is the peak load, P₀ the step load required to produce the critical displacement, I the impulse, area under the load-time curve, I₀ the ideal (zero duration) impulse required to produce the critical displacement. This representation is useful for common structural elements such as beams, plates and shells. Some increase in accuracy is obtained by replacing the right-hand side with a constant C, determined by fitting to experimental results or to a theoretical critical load curve. A key feature of the P-I characterization scheme is that the critical load curves are not sensitive to pulse shape. The P-I characterization scheme contains the same information as the dynamic load factor (DLF). Although they contain the same information, these two characterizations emphasize different aspects. The DLF characterization displays the relation of dynamic loads to an equivalent static load. The P-I characterization displays equivalent dynamic loads in terms of peak load and impulse, the load parameters that are important at the long-duration and short-duration extremes. (Auth.)