Structural unit with engaging means

Claim 1

IN A STRUCTURAL PANEL ASSEMBLY, AN INTEGRALLY FORMED STRUCTURAL UNIT COMPRISING A PAIR OF SPACED, SUBSTANTIALLY CONTIGUOUSLY DISPOSED WALLS EACH HAVING INNER AND EXTERIOR SURFACES AND EACH BEING PROVIDED ON ONE END THEREOF WITH HOOKED FLANGES, A WEB INTERCONNECTING SAID WALLS IN THE VICINITY OF THE ENDS THEREOF REMOTE FROM SAID HOOKED FLANGES, ABUTMENTS INTEGRAL WITH AND EXTENDING FROM THE ENDS OF SAID WEB, EACH OF SAID ABUTMENTS HAVING OUTER SURFACES DISPOSED SUBSTANTIALLY IN THE PLANE OF THE INNER SURFACE OF THAT WALL CONNECTED TO THE SAME END OF SAID WEB, A GROOVE PROVIDED IN THE OUTER SURFACE OF SAID ABUTMENTS AT THEIR JUNCTION WITH SAID WEB, SAID GROOVE BEING CONTOURED CORRESPONDING TO SAID HOOKED FLANGES TO COACT WITH AND IMMOVABLY SECURE A SIMILARLY HOOKED FLANGE CARRIED BY AN ASSOCIATED STRUCTURAL UNIT, A RIB EXTENDING INWARDLY FROM ONE OF SAID WALLS AT THE JUNCTURE THEREOF WITH SAID WEB, A SECOND RIB EXTENDING INWARDLY FROM THE OTHER OF SAID WALLS AT THE JUNCTURE THEREOF WITH SAID WEB AND BEING SUBSTANTIALLY PARALLEL TO SAID FIRST-MENTIONED RIB, ONE OF SAID RIBS BEING PROVIDED WITH A BEAD AND THE OTHER OF SAID RIBS BEING PROVIDED WITH A GROOVE MATINGLY COMPLEMENTAL TO SAID BEAD TO COACT WITH AND IMMOVABLY SECURE A SIMILARLY FORMED BEAD PROVIDED ON A RIB OF SAID ASSOCIATED STRUCTURAL UNIT.