Without any protection the corrosion of the steel rod would progress at 0.005 inch per year, eroding the steel rod surface. After 30 years this will amount to 0.150 inch per side, or 0.300 inch of rod thickness. For 5/8" rod (0.625 inch) this amounts to a loss of nearly 50 percent of rod thickness. Therefore the structure will require at least a strength safety factor of 2 to remain structurally sound over 30 years.

In real life the corrosion won't be that bad: the structure will never be exposed to the air; after 8 years an oxide layer will form and inhibit further corrosion; and the use of solar-powered cathodic protection could virtually eliminate corrosion.

A safety factor of at most 2 might still be considered as a baseline from which to further reduce costs.