Willis Tower contains columns that are spaced apart on their centers. The facade is made of anodized aluminum and black glass. Alcoa manufactured of aluminum sheeting for the building's facade. Black bands appear on the tower around the 29th–32nd, 64th–65th, 88th–89th, and 104th–108th floors. These elements are louvers to ventilate the building's environmental support systems and obscure its belted trusses. The rest of the facade is made of 16,000 rectangular windows. all of which measure and are tinted with bronze. Outside the building, there was originally a plaza made of pink granite. In the late 2010s, a three-level wing was built along the western and southern sides of the tower replacing the tower. The roof garden above the annex spans . The annex contains a facade of black steel and aluminum, similar to in the original building. The Jackson Boulevard facade of the annex contains an artwork by Olafur Eliasson, entitled Atmospheric wave wall. The work, measuring across, comprises almost 2,000 blue-and-green steel tiles, which are decorated with hexagonal motifs. The wall is backlit at night. Structural and mechanical features The interior includes of steel, of aluminum, and of concrete flooring. The building contains diagonal columns only on the two stories immediately below each of the setbacks, thus reducing shear stress. The interior of the building could not contain diagonal beams, since these would have obstructed the connections between each of the "tubes". Therefore, the columns and the horizontal beams on each story are connected by rigid joints. The superstructure was designed to withstand wind gusts of , which on average would occur once every hundred years. According to the Chicago Tribune, the top of the building would be able to bend by as much as , returning to its normal position within 7.2 seconds. The Willis Tower's basement extends deep, resting on a concrete slab. The ground directly beneath the building was largely made of clay; the underlying layer of limestone was as much as beneath ground level. As a result, the foundation was excavated using 201 caissons, of which 114 reached the underlying limestone. The caissons created holes that measured up to across. Some holes at the northwestern and northeastern corners of the site filled up with groundwater and had to be drained. Workers next placed steel tubes into the holes, then poured concrete around the tubes. During the Sears Tower's construction, SOM and Chicago government officials considered