With its 17 stories (16 rentable plus an attic), its load-bearing walls were the tallest of any commercial structure in the world. To support the towering structure and reinforce against wind, the masonry walls were braced with an interior frame of cast and wrought iron. Root devised for this frame the first attempt at a portal system of wind bracing in America, in which iron struts were riveted between the columns of the frame for reinforcement. The narrow lot allowed only a single, double-loaded corridor, which was appointed with a wainscot of white Carrara marble, red oak trim, and feather-chipped glass that allowed outside light to filter from the offices on each side into the hallways. Floors were covered with hand-carved marble mosaic tiles. Skylit open staircases were made of bronze-plated cast iron on upper floors. On the ground floor, they were crafted in cast aluminum—an exotic and expensive material at the time—representing the first use of aluminum in building construction.