Willis Tower

Microclimate/Climate

- Chicago’s topography and density immediately surrounding the tower offer some interesting climatic benefits during the summertime. During the day, taller buildings to the east and south help to block the sun on the lower portions of the building; however, these factors lower the amount of sun that can be used to passively heat the building during the winter. Additionally, the southern branch of the Chicago River allows for some evaporative cooling immediately surrounding the structure, reducing the heat-island effect and allowing for a more comfortable lobby space.

- The overall climate of Chicago is cold and windy, which typically requires that a building have a more massive skin that will not easily let heat be transferred out of the building. However, that was not a major design consideration for the architects. The building does respond to its climate through the vertical disruption of cores that give the building its setback form. Broad building faces generally cause a lot of turbulence on the ground level in front of the building where plazas and entrances are generally placed, but the Willis Tower avoids this and disrupts any uniform turbulence pattern.

Mechanical Systems

- Because of the building’s age, there have been a number of changes to its HVAC, plumbing and electrical services throughout its life, the most important of which is currently underway. With the purchase of Sears Tower by the Willis Group and the subsequent name change, the current owners have hired architects Adrian Smith and Gordon Gill to investigate how the tower might lower its overall energy consumption.

- For heating the tower, cogeneration plants have been installed which make use of natural gas fuel to simultaneously generate heat and electricity. Cooling the building uses the same process as before, vapor compression and evaporative cooling via chilling towers, but the new system in place uses less energy than the previous models. Finally, the distribution system in the floor section has been reworked to appropriately heat and cool the interiors in a manner more consistent with modern heating/cooling practice: heating generally occurs at the floor near windows to rise and help prevent heat loss immediately surrounding the thermal break, and cooling is spread throughout the ceiling to keep the whole floor space comfortable. The mechanical equipment is housed on various floors throughout the building which are visible from the street as the black bands which encircle the building.

- Small renovations to the plumbing include the use of more efficient toilets and faucets that use more water pressure and less actually water to perform their tasks. The expected difference in usage from the last major renovation (1989) is 34 million gallons of water saved.

- The electrical systems will see an upgrade as well, with testing of wind turbines on various roof surfaces and photovoltaic panels on the southern roofs.

Solar Profile

- The design of the building had to make use of materials that could absorb heat well during the winter, but reject solar gain during the summer. The solution makes some sacrifices for both of these ideas: black aluminum is used to fit out the plenum space between windows and bronze tinted glass is used for the windows (these can absorb heat during the winter); the bronze tinted glass reflects solar radiation during the summer, reducing cooling costs. For the renovation, Adrian Smith considered painting the aluminum silver and replacing the glass with more reflective silver tinted glass, but this idea was thrown out in favor of maintaining the iconic look of the building and the need for more solar gain during the winter.

Superstructure

- Willis Tower consists of nine cores of steel framing tied together with additional steel creating what is often referred to as a “bundled tube structure.”

- When vertically loaded, each of the nine individual tubes that make up the body of the tower can carry their loads successfully to the ground with the use of a relatively standard steel structure which uses steel trusses attached to columns to form deep rooms in each extruded square prism. These trusses also create plenum space for running the mechanical systems.

- This system of incorporating nine 75’x75’ tubes truncated at various heights offers multiple benefits for the tower’s structure. As mentioned above, when the face of a skyscraper is broken up into terraces and setbacks, wind turbulence is reduced by a fraction of the amount that would be present if the entire tower had a flat face. Additionally, when the tubes are connected at various floors with cross-bracing, lateral loading has a much greater chance to transfer to the vertical members. Where one of these tubes would be too small to resist Chicago’s winds, nine are well-apt to resist these loads and to act in concert.

Skydeck

- These four boxes on the 103^rd floor of the tower offer amazing views of Chicago and offer visitors a chance to experience something truly unique – the use of structural glass in a skyscraper.

- With six layers of chemically tempered glass on the bottoms and sides of these boxes, and with large steel tubing running along the perimeter of the top of the boxes, these elements are actually very secure. The key to designing such death-defying structures out of glass is the realization that glass can be layered and worked just like sheet metal or concrete – the more material used, the greater the forces an architectural element can handle. The glass floors are connected to the walls through the use of stainless steel ties with silicon-based rubber expansion joints along the bottom.

- When the boxes need to be cleaned, they can be retracted into the building on the 103^rd floor and safely navigated for maintenance.

Foundation

- The foundation is a typical pier foundation with about 200 concrete piles running from the bottom of the basement floor (about 100 feet below the ground) to the bedrock below.