<p>The structure consists of an engine tunnel plate and inner and outer longitudinal members. The inner longitudinal members(4) are formed by integral shaping on the engine tunnel plate and/or on the floor plates connected to it. The outer longitudinal members(13) are formed by integral shaping on the outer edge section of each floor plate(2) and/or on the lower edge section of every two plates(3/1,3/2) forming a side wall(3).</p>