in the 1990s. , the concrete-and-marble facade of the western, southern, and eastern elevations is being removed and replaced with glass panels. Structural features Substructure The foundation of the building extended deep and was excavated to the underlying layer of bedrock. It is surrounded by waterproof retaining walls, which are backfilled with a mixture of loose stone and cement. The foundation itself consists of cast-steel footings, above which rise the building's steel columns. The footings each measure across, and their centers are spaced apart. Each steel footing is placed atop a heavy granite block measuring across and thick, which in turn rests directly on the underlying bedrock. Structural loads from the upper stories are carried down into the footings and then spread across the layer of bedrock, which carries a load of . The retaining walls of the foundation are made of red brick. On the eastern part of the site (where the underlying rock sloped upward), workers built a retaining wall with embedded I-beams, providing additional wind bracing. The building contains three basement levels, the lowest of which is deep. The Times Square subway station encroaches on a portion of the first and second basement levels. The subway station itself is placed below ground and has a ceiling high. The pillars of the subway tunnel were covered in brick and were placed atop sound-dampening sand cushions, minimizing vibrations caused by passing subway trains. Part of the superstructure is cantilevered above the subway tunnel, since the city's Rapid Transit Commission forbade any obstructions in subway tunnel's right-of-way. The northern wall rests on a plate girder above the subway tunnel; at the time of construction, it was the heaviest girder in the world to be installed in an office building. This girder measures long and consists of a group of three I-beams, which collectively measure wide and high. Seven piers in the basement, each measuring high, carry the entire structural load of the upper levels; they are encased in Portland cement. Superstructure The superstructure contains two-story-tall sections of steel columns. At each story, the columns are connected horizontally by a grid of steel girders. On average, each girder measures long, and there are about 150 pieces of steel used on each story. The seven main structural columns are embedded within the walls on each story. Structural engineers Purdy and Henderson designed three systems of wind bracing for the building. The first