At 64 stories and 841 feet, the tallest structure in downtown Pittsburgh, the USX Tower is symbolic of the city both in its triangular shape and its structural innovations in steel. The exterior features eighteen exposed vertical steel columns, each set three feet outside the curtain wall, such that columns and curtain wall connect at every third floor. The columns of Cor-Ten steel are self-oxidizing, hence unable to rust further. Every column circulates coolant inside, so should the tower ever be engulfed in flames, it would keep cool for four hours before surrendering to the heat.