As an ideal blast wave encounters the front face, it is reflected to some higher value that is a function of the peak incident overpressure and the orientation of the surface with respect to the direction of propagation of the shock front (see Fig. 4.1). The blast wave then bends or diffracts around the structure subjecting first the sides and roof and finally the rear face to pressures equal to the incident overpressure. At.the same time, these surfaces are also subjected to drag pressures that are a function of the dynamic pressure and drag coefficient for the surface. Presented in Fig. 4.3 are surface drag coefficients at overpressures less than about 100 psi (700 kPa) for a rectangular structure resting on a planar surface.