also and the free blades 22 23 of the steel sheet element 21 contacting them in the state intended for joining enclose an access channel z 2 accessible from outside into which in turn the tool can be introduced which is necessary for pressing the material adjacent to the securing slots 26 27 following the pressing process carried out with the aid of this tool the wall sections 28 29 and the free blades 22 23 are held to one another in a non detachable manner by positive and non positive fit the manner in which the connection is established between the modules 2 3 and 4 is explained here by way of example on the basis of the enlarged sectional representation in fig2 for the connection between the module 2 and the module 4 in the area of the securing slot 17 the connections in the area of the securing slots 18 26 and 27 are produced in a corresponding manner already during prefabrication of the steel sheet element 5 of the module 4 the securing slot 17 is formed into the surfaces of the cropped section 15 allocated to the module 2 the width of the securing slot 17 corresponded in this situation to the thickness d of the peripheral area 19 a of the wall 19 of the module 2 to which the securing slot 17 is allocated in order to provide a non detachable connection with positive and non positive fit of the wall 19 of the module 2 to the steel sheet element 21 of the module 4 after joining the modules 2 4 the material of.