The John Hancock Tower experienced several failures throughout the years, the most well-known of which being the falling windows. Over an acre of the structure was formerly covered in plywood, which replaced the broken glass. The problem was discovered in the design of the windows themselves. They were not strong enough to support their own weight, so they had to be bolted to the wall behind them. The tower is also known for its poor insulation, which caused heating and cooling costs to be high. In an attempt to reduce these expenses, the building was not fully occupied during the summer months. When it came time to heat or cool the building, operators would only supply temperature controls to those floors that needed service at that time. Another issue with the tower is its location: right in the center of Chicago's business district. This made the building highly susceptible to noise pollution. Even today, many people are unaware that this tower even exists! The last major failure occurred in 2002, when a fire broke out on the 54th floor of the building. Although no one was injured, it became clear that the fire protection systems on that level were outdated and insufficient to contain a fire of that size. As a result, the entire 16th floor collapsed into itself. Since then, all occupants above the 47th floor have been required to take elevators down to the ground floor instead of using stairs.