The loading on the rear face of an aboveground, closed rectangular structure is shown in Fig. 4.4(b). No pressure is transmitted to the rear face until the shock front reaches that face. Using the same time reference as for the front face, average pressure begins to build up on the back face at a time equal to the length of the structure parallel to the direction of shock propagation divided by the velocity of shock propagation. At some later time, after the rear face has become completely engulfed in the blast, the pressure reaches a maximum value equal to the side-on overpressure reduced by an amount equal to the drag pressure, which build up linearly in a time equal to 4S/U, in which Sand U are defined in the same manner as for the front face.