WILLIS TOWER: TECHNICAL REVIEW AND SPECIFICATIONS

The Willis Tower, formerly known as the Sears Tower, is a seminal structure in the history of high-rise engineering. Standing at 442 meters (1,450 feet) in Chicago, Illinois, it held the title of the world's tallest building for 25 years (1973–1998). It remains a masterclass in structural efficiency, introducing the "bundled tube" structural system that paved the way for modern supertall construction.

This technical review analyzes the revolutionary bundled tube logic developed by Fazlur Khan, the buildings wind-resistant aerodynamic setbacks, and the engineering behind the famous glass balconies known as "The Ledge."

ARCHITECTURAL DESIGN AND THE BUNDLED TUBE CONCEPT

Designed by Bruce Graham and structural engineer Fazlur Khan of Skidmore, Owings & Merrill (SOM), the Willis Tower was the first building to utilize the "bundled tube" structural system. This design was a response to the need for massive open floor plates for the original tenant, Sears Roebuck and Co., while maintaining structural rigidity at extreme heights.

The building is composed of nine square structural tubes, clustered in a 3-by-3 matrix at the base. Each tube measures 75 feet square. As the building rises, individual tubes terminate at varying heights:
- Floors 1-50: 9 tubes
- Floors 51-66: 7 tubes
- Floors 67-90: 5 tubes
- Floors 91-108: 2 tubes

This setback design serves a dual purpose. Architecturally, it creates a distinctive stepped profile. Structurally, it disrupts wind flow, confusing vortices and reducing the lateral swaying forces acting on the upper levels.

STRUCTURAL ENGINEERING AND STEEL FRAMING

The Willis Tower is a steel-frame structure, utilizing significantly less steel per unit area than the Empire State Building due to the efficiency of the tubular design. The frames of the tubes are "welded steel frames," which act like hollow cantilevers jutting out of the ground.

The columns and beams are welded together to form rigid web walls. This rigidity allows the exterior walls to carry the majority of the wind loads, freeing up the interior floor space from the clutter of support columns. The entire structure weighs approximately 222,500 tons. The building leans slightly (a few inches) to the west due to the asymmetric distribution of weight caused by the heavy antenna array added later, though this is structurally accounted for and monitored.

VERTICAL TRANSPORTATION AND MODERNIZATION

The vertical transportation system is a critical component of the towers logistics, moving 25,000 people daily. The building features 104 elevators, including 16 double-deck elevators.

The double-deck units serve the sky lobbies, allowing passengers to transfer to local elevators for specific floors. In recent years, the building underwent a massive modernization of its elevator systems to improve dispatch efficiency and ride quality, utilizing advanced destination control systems (DCS) to manage the flow of tourists to the Skydeck and office tenants to the corporate zones.

THE LEDGE AND GLASS ENGINEERING

In 2009, the Skydeck on the 103rd floor was renovated to include "The Ledge"—four glass boxes that extend 4.3 feet (1.3 meters) out from the façade. This required specialized structural glass engineering.

The boxes are constructed from three layers of half-inch thick glass laminated together. They are designed to hold 5 tons of weight. The panels are hung from a steel carriage that moves on tracks, allowing the boxes to be retracted into the building for cleaning and maintenance. The "low-iron" glass provides near-perfect transparency, offering visitors a vertical view of Wacker Drive 1,353 feet below.

TECHNICAL DATA SHEET

Official Name: Willis Tower
Former Name: Sears Tower
Location: 233 South Wacker Drive, Chicago, Illinois, USA
Architect: Skidmore, Owings & Merrill (Bruce Graham)
Structural Engineer: Fazlur Khan (SOM)
Completion Year: 1974
Architectural Height: 442 meters (1,450 feet)
Tip Height (Antenna): 527 meters (1,729 feet)
Floor Count: 108
Structural System: Bundled Tube (Steel)
Primary Function: Office, Broadcasting, Observation

FREQUENTLY ASKED QUESTIONS

Is Willis Tower taller than One World Trade Center?
By roof height, the Willis Tower (442 meters) is taller than One World Trade Center's roof (417 meters). However, One World Trade Center is officially taller (541 meters) because its spire is counted as an architectural element, whereas the Willis Tower's antennas are functional equipment and do not count toward official height.

Why was the name changed?
In 2009, the global insurance broker Willis Group Holdings leased a significant portion of the building and obtained the naming rights. Despite the official change, many locals still refer to it as the Sears Tower.

Does the building sway?
Yes. Like all supertalls, it is designed to sway in the wind. The average sway is approximately 6 inches from true center, but it can sway up to 3 feet in extreme wind conditions.

Who was Fazlur Khan?
Fazlur Khan was the structural engineer who invented the "tubular" design (including the bundled tube used here). He is considered the "Einstein of structural engineering" because his innovations made modern supertall skyscrapers possible.