least one inch and more preferably five inces short of the bottom of the area wall sections 12 and 14 as shown in fig2 the strengthening rib 18 comprises a portion of the sidewall 24 of the second area wall 14 curled into a tube 26 although the strengthening rib 18 may be constructed of any desired material in the referred embodiment the strengthening rib 18 is constructed of galvanized steel the sidewall 24 and strengthening rib 18 are constructed of galvanized steel and are integrally formed with one another the tube 26 is preferably provided with a diameter between 0 1 and 5 inches more preferably between 0 25 and 2 inches and most preferably approximately one half inch as shown in fig1 and 2 the sidewall 24 is provided with a generally sinusoidal cross section forming valleys 28 and ribs 30 to increase the strength of the area wall 14 as shown in fig2 the connection plate 16 includes a portion of the sidewall 24 extending below the attachment fins 20 and 22 including a rib 32 and a portion of a valley 34 as shown in fig1 and 2 the sidewall 24 is provided with a plurality of holes 36 preferably the first hole 38 is centered 1 inch from the end 40 of the second area wall section 14 the second hole 42 is preferably.