Characteristics of a pressure-impulse diagram for blast loads are studied based on a dimensional analysis and a single-degree-of-freedom model. Structural behavior is dominated by the fundamental elastic response mode and the structural damage is controlled by the maximum structural deflection. The blast loading is simplified into a descending pressure pulse. A characteristic curve in nondimensional loading parameter space is used to define an isodamage critical diagram, pressure-impulse diagram, to distinguish damaged and undamaged ranges in the loading parameter space. Three damage regimes on a pressure-impulse diagram, i.e., (I) impulse-controlled damage, (II) peak load and impulse-controlled damage, and (III) peak load-controlled damage, exist respectively for impulsive, dynamic and quasistatic structural response regimes determined by the ratio between loading time and the response period of a structure. It is observed that there is a noticeable loading shape influence on the pressure-impulse diagram in Regime-II when both peak load and impulse are important for dynamic structural response. A unique effective pressure-impulse diagram is proposed to eliminate the pulse loading shape effect on a pressure-impulse diagram.