This paper deals with the effect of blast loading on R.C. structures. After depicting load and materials characteristics in blast event, different alternative structural models are proposed. The first approach idealizes the beam as a single degree of freedom system, the second models it by means of Euler-Bernoulli’s theory and its elastic-plastic behaviour is expressed through a nonlinear relationship between bending moment and curvature. The effects of strain rate are taken into consideration by introducing time-variable coefficients into the equations of motion derived from the two models. The comparison with some experimental results assesses the reliability of the approach, and a sensibility analysis of the different involved parameters is then developed. Slenderness and span length proved to be the key parameters and let the Authors to find a polynomial relation between them and the maximum displacement/velocity of the beam.