creates chamfered "birds' mouths" at the tower's corners at the 14th, 22nd, 30th, and 38th floors. According to The New York Times, the beams and "birds' mouths" run at a 75-degree angle to the horizontal floor slabs, though another author cites the beams as running at a 65-degree angle. The structural system, similar to the Commerzbank Tower in Frankfurt and 30 St Mary Axe in London, was developed in conjunction with Ysrael Seinuk. The triangles comprising the diagrid are prefabricated panels, manufactured by the Cives Steel Company at two plants in New York and Virginia. Each of the triangles are tall. The diagonal beams are typically long by wide. The columns are bolted, rather than welded, to each other at the nodes. The diagrid required of structural steel, twenty percent less than what would have normally been required for a building of similar size. More than ninety percent of the steel in the diagrid is recycled. The exterior curtain wall was constructed by Permasteelisa, which mounted 3,200 glass panels on the facade. The panels are typically tall by wide, although 625 of the panels were built to a custom specification. Because of the facade's intricate design, the tower's window cleaning rig took three years and $3 million to plan. The resulting design incorporates "a rectangular steel box the size of a Smart car" on the roof, which hoists a mast and a hydraulic boom arm. Sixty-seven sensors and switches are housed in the box. A window cleaning deck hangs from the hydraulic boom arm, supported by six wire-rope strands. The device was installed in April 2005 on of elevated steel track looping the roof of the tower. The rig snapped in a 2013 incident that trapped two window cleaners. Features Structural features The Hearst Magazine Building is supported by steel columns at its perimeter. The original framework was intended to support at least seven additional stories. Urban's original plans for the tower no longer exist but, by some accounts, would have been up to 20 stories tall. The Hearst Magazine Building also had six elevator shafts, double or triple the expected number of elevators for a building of its size. As completed, there was a white-brick penthouse above the sixth story for the future expansion of the elevators. The Hearst Magazine Building's entire original framework was removed when the Hearst Tower was constructed in the 2000s. The existing structure was