Last week's answer

Question: Completed in 1977, the massive New River Gorge Bridge soars 876 feet above the riverbed, cutting a 40-minute mountain detour down to a 45-second drive. To eliminate the multi-million dollar ongoing cost of painting a 44,000-ton steel bridge nearly 900 feet in the air, what specialized material choice did engineers make for its structural frame?

Answer: Constructing the entire arch using weathering steel (Cor-Ten), which forms a stable, dense rust-like patina that protects the underlying metal from deep atmospheric corrosion.

Cor-Ten steel creates a rust-like appearance that never needs painting and prevents corrosion. The material was first developed in the 1930s for use on railroad coal cars. Its special blend of steel alloys develops a thin, protective rust layer that coats and protects the metal. The use of Cor-Ten saved $300,000 in costruction costs, and is said to save an estimated $1,000 every time the bridge would have required repainting. 

Sources: Bridge Day