Structural load calculations. For new, remounted or relocated signs, a statement by a licensed structural engineer or an Illinois licensed architect, that all sign structures, including, without limitation, the sign foundations, support structures, secondary framing, anchorages, fastenings and facing materials, will be: (i) structurally sound; (ii) designed to resist wind pressures as required by the Building Code; and (iii) designed at a pressure not less than 20 pounds per square foot of sign surface and supporting structure, which pressure shall be considered as acting as both positive and negative pressure, but not acting concurrently.