2 comprises a c shaped extruded profile on which the u shaped bar 3 is positioned the u201c u u201d being upside down with the result that the opening of the u shaped profile is oriented downward the two load bearing profiles 2 and the two bars 3 are of identical configuration the bar 3 and the load bearing profile 2 being screw connected via hammer head screws 13 which are inserted into t grooves of the aluminum load bearing profile 2 and pass through bores in the legs of the u shaped bar 3 a cross section through the load bearing profile 2 is illustrated on an enlarged scale in fig3 it can clearly be seen here that the load bearing profile 2 is of cross sectionally c shaped configuration with an opening 14 which is oriented to the right and that the load bearing profile 2 is provided with a plurality of t grooves 15 which run in the longitudinal direction and serve for fastening add on parts furthermore as can be seen on the right hand side of the c shaped load bearing profile the opening region 14 is provided with set back portions 16 in the load bearing profile 2 these forming a rest for a closure plate not illustrated which can be used to close the opening 14 of the load bearing profile 2 the cavity 17 which is formed in the load bearing profile 2 is suitable for accommodating cable harnesses 18 control parts initiators or the like with the result that these parts can be installed in an easily reachable manner and if required are easily accessible possibly with the covering having been removed beforehand a shoulder 19 is formed in the top region.