the main structural core, though this idea was not feasible. Bunshaft instead decided to use curved facades on the north and south elevations, avoiding the need for rectangular setbacks. One author compared the curves to the face of a ski slope. The north and south walls are made of gray-tinted glass. Immediately above the ground level, the north and south elevations end in a large gutter, which collects rainwater. Custom rails were installed on the facade for the window-washing scaffolds. The glass panels consist of windows as well as spandrels in between stories. The panels are attached to the superstructure by black gaskets made of neoprene. The western and eastern facades are clad in tan bands of travertine limestone. On either side is a narrow window opening spanning several stories. The building's structural steel bracing crosses diagonally in front of these windows. The sections of the steel bracing in front of the windows are coated with black aluminum. Plaza The building has a setback of from 57th Street and from 58th Street. A plaza extends in front of the building on both 57th and 58th Streets. The plaza is clad in travertine and extends the width of the block from 57th to 58th Streets. A red sculpture of the digit "9" on the 57th Street side of the building was designed by Ivan Chermayeff. The sculpture weighs and measures about high by wide. It faces east toward Fifth Avenue and is supported by a three-story column in the basement. The sculpture was installed because Solow thought the plaza as designed was excessively large, and also because Solow wanted to draw attention away from the bare walls of other nearby buildings. The idea for the sculpture had come after Chermayeff had joked that the facade could be a "launching pad" for the "9". After the sculpture was first installed in 1972, Solow had removed the sculpture for a year due to a dispute over where it should be placed. He ultimately agreed to install the sculpture on the sidewalk, initially paying the city $1,000 annually in rent. By the 1990s, the sculpture was described in The New York Times as "a cultural artifact as well as the building's signature". The 58th Street side of the plaza contains Moonbird, a sculpture by Joan Miró. The sculpture, originally commissioned in 1966, is tall and made of bronze. Solow installed Moonbird in 1994, saying