In this paper, two loosely coupled single degree of freedom (SDOF) systems are used to generate pressure-impulse diagrams(P-I) with the flexural and direct shear responses of one-way reinforced concrete slabs subjected to blast loading. The effect of span length to height ratio in P-I diagrams is investigated. Based on numerical results, different failure modes are got with different the span length to height ratio on the P-I diagrams.