which may be oriented parallel or transversely respectively to the corrugations of the plate for transverse steel bars suitable apertures are formed in the corrugations of the plates which is traversed by the bar in fig7 the lower plate to further increase the strength and rigidity of a double plate two corrugated plates 2 may be combined into a double plate 38 by placing tubular spacers 40 between aligned peaks and troughs 6 8 respectively of the two plates and bypassing connecting bolts 42 or rivets not shown through the spacers to thereby secure the two plates to each other in a spaced apart relationship the length of the spacers is chosen to suit the particular application as before the hollow interior spaces between the plates may be filled with concrete with or without reinforcing bars not illustrated in fig8 the spacers may comprise simple metallic tubes 44 fig9 which preferably include contoured ends 46 to snugly engage the two corrugated plates between which the spacers are disposed alternatively the spacer may comprise a tubular concrete member 48 fig1 which also has contoured ends 50 the concrete spacer may further be fitted with an insert 52 that has female threads for engaging and securing a pair of bolts threaded into the insert from opposing ends of the spacer to thereby secure the corrugated plates 2 to the spacer and to each other referring now to fig2 corrugated plates.