Pressure-impulse (P-I) diagrams based on the equivalent single-degree-of-freedom approach (SDOF) have been used during building design in order to assess the effects of blasts on structures. They provide an easy way to describe the likely outcome of the combination of blast pressures and impulses on a particular structural element in a building at the moment an explosion occurs. However, only P-I diagrams of structural members under external blasts have been addressed in current guidelines and previous studies. Due to the complexity of confined scenarios, confined blast pressure-time histories cannot be approximated by simplified representations of pressure-time histories used for external blasts, such as triangular shapes representing linear decay or curves indicating exponential decay. Rather, they should be simplified as bilinear pressure-time histories. Thus, SDOF models which incorporate bilinear blast loads were developed to predict the response of a member with a bilinear, elastic-plastic-hardening, resistance-deflection function. Then using the developed SDOF model, normalised P-I Diagrams for structural members with bilinear resistance-deflection functions under bilinear blasts were generated. These results were then used to undertake a parametric study to investigate the influence of varying blast load shapes and varying bilinear resistance-deflection function shapes on the normalised P-I curves. Also, comparisons against other techniques employed to eliminate pulse load shape effects were also undertaken for bilinear pulse loads and bilinear resistance-deflection function shapes.