Willis Tower: The Engineering Behind Chicago's Sears Tower
October 27, 2026
Willis Tower's outline is not a decorative crown placed on top of a conventional office block. Its stepped groups of dark vertical shafts express the engineering that makes the tower possible. Completed in the 1970s for Sears, Roebuck & Co., the building made a worldwide height record out of a practical problem: how to accommodate a large company's offices while providing other tenants with useful floor sizes. The solution joined structural design, business planning and the image of a growing retailer.
The tower remains a place of work, but it is no longer simply a corporate headquarters with an observation deck attached. Later changes have opened its base to dining, art and public space, while the Skydeck turns the upper reaches into a visitor experience. Its history is best understood at both scales. From far away, it is a dark silhouette in the skyline. Up close, it is an example of how a very large office building has adapted as its original employer, ownership and surrounding city have changed.
Sears needed a new kind of headquarters
Sears commissioned Skidmore, Owings & Merrill to design a headquarters that could consolidate employees and allow for anticipated growth. Its place among the largest retailers gave the project an unusual scale, but size alone does not explain the design. The company needed large office floors for its own operations and smaller floors that other tenants could occupy. A tower with one unchanging floor plate would have answered those two needs less flexibly.
The West Loop location was also practical. The Chicago Architecture Center points to proximity to expressways and commuter rail as advantages for employees. A headquarters does not function only through its internal plan; it depends on people reaching it each day. The site therefore linked the new tower to the larger transportation system, even though the building's eventual fame would make it seem like an isolated vertical object.
The commission gave architect Bruce Graham and structural engineer Fazlur Rahman Khan a demanding brief. Increasing height makes resistance to wind a major design issue, not merely a matter of adding enough material to carry gravity loads. The team needed an economical structure that would permit the required floor area without making the building too rigidly uniform in its usable spaces. The answer was the bundled tube system.
Nine tubes act as one structure
SOM describes the base as nine square structural tubes, each 75 feet by 75 feet. They are arranged in a three-by-three group and connected so they act together. A tube in this context is not a separate circular pipe hidden in the building. It is a structural organization of closely coordinated vertical and horizontal members around a square area. Bundling several together creates a strong system for a much taller building.
The nine tubes rise together through the lower portion and stop at different levels higher up. That produces the setbacks visible from the street and across the city. The form therefore follows changes in the building's structural and spatial needs. Larger lower floors could serve the original corporate operation, while the reduced upper floors provided different sizes of rental space. The silhouette is the visible outcome of those decisions, not an arbitrary stack of shapes.
The Chicago Architecture Center explains that the tubes support one another and that their differing heights also disrupt wind forces. This is a useful way to understand the tower without pretending it can be reduced to a simple engineering diagram. The overall system depends on connections, stiffness and foundations as well as the number of shafts. What is clear is the close relationship between the structure and the exterior profile.
What the dark skin does
Black aluminum and bronze-tinted glass give the tower an appearance very different from Chicago's pale terra-cotta and limestone landmarks. SOM describes the lightweight envelope as helping insulate the interior and exterior structure, limiting temperature-related expansion and contraction. Its surface is therefore part of the building's environmental and structural behavior as well as its visual identity.
The regular window divisions let the large masses read clearly. Unlike a tower whose top is identified by a single ornamental feature, Willis Tower depends on the relationship among several rectangular volumes. The dark skin ties those volumes together. Changing light can make some faces appear almost solid and others reflective, but the repeated grid maintains the sense of a single system beneath those differences.
The foundations extend to bedrock through reinforced-concrete caissons, according to SOM. That unseen support is a reminder that the achievement was not confined to the shafts above the street. A building of this scale requires the ground, frame, skin and vertical transportation to work as a coordinated whole. Its height became the headline, but the coordination of those less photogenic elements made that height usable.
Why two completion years appear
Construction began in 1970. The tower operator's chronology dates completion to 1973, and SOM's narrative also says the headquarters was completed in 1973. Yet SOM's project facts and the Chicago Architecture Center's summary list 1974 as the completion year. The discrepancy appears within authoritative accounts, including between an architect's own narrative and summary data. It should be stated rather than concealed under a falsely exact single date.
For the broad architectural story, the important point is that the tower emerged in the early 1970s and held the world's tallest-building title for nearly a quarter century. The Chicago Architecture Center gives a height of 110 stories; SOM lists 1,450 feet for the building height. Those measurements describe the main building rather than using the antenna tips to inflate the architectural comparison. Height records become misleading when different measuring points are mixed.
The tower's long reign matters historically, but it is not the only reason to care about it now. Later buildings can exceed its height without making its structural idea irrelevant. The bundled tube system helped show how a supertall could achieve strength and flexibility together. That contribution remains important even for a visitor who has no interest in ranking observation decks by the last few feet.
The Sears name and the Willis name
For many people, Sears Tower remains the familiar name. The official change to Willis Tower took place in 2009 after the insurance broker leased office space and obtained naming rights. The Chicago Architecture Center gives July 16 as the change date. The older name persists because it connects the building to the retailer that commissioned it and to the period when the tower first became part of Chicago's identity.
Accounts of Sears's departure require care. Skydeck's history says the company sold and moved out in 1988, while SOM says its headquarters remained in the building until a suburban relocation in 1995. Because those are materially different descriptions, the earlier date should not be repeated as a settled move-out date. The reliable larger sequence is clear: the tower began as Sears's headquarters and later became a multi-tenant property with a different official name.
A name change does not alter the structure that made the building significant. It does change how an address is sold and remembered. The tension between Sears and Willis is therefore more than local stubbornness. It shows how a corporate name can become embedded in public geography, surviving after the company no longer occupies the same role. The building's architecture provides continuity while its commercial identity shifts.
A visitor steps beyond the wall
The Ledge, added in 2009 and renovated again in 2021, changed the way visitors experience the upper tower. Glass-enclosed boxes project from the 103rd-floor Skydeck, allowing people to look down through a transparent floor as well as outward through the windows. SOM describes laminated-glass enclosures supported by steel frames that can retract for cleaning and maintenance. The apparent absence of support is carefully engineered, not literal.
That experience differs from looking through a conventional observation window. It brings the exterior face and the distance to the street into the visitor's immediate field of view. Yet the attraction depends on the existing tower, whose structural clarity remains the larger story. A photograph on a glass floor records a moment; the building behind it records decades of engineering, occupation and adaptation.
The visitor route now includes lower-level exhibitions about Chicago before the elevator journey upward. This connects the tower to the neighborhoods and city history visible from above. It is useful to keep the experiences distinct when planning: the ticketed Skydeck, the public retail areas at the base and private tenant amenities do not share one universal access rule or opening schedule.
Opening the base to the city
Blackstone purchased the tower in 2015, according to the building's chronology. A major renovation began in 2017, with completion recorded in 2022. The project reworked the building's relationship with the street through Catalog, a multi-level retail and dining environment whose name recalls the Sears catalog. The Chicago Architecture Center credits Gensler, SkB Architects and OLIN with the broader transformation.
A roof terrace, public art and places to eat changed what a visitor could do without going to the observation deck. That is a significant shift in a building originally conceived around a dominant office use. The operator distinguishes these public areas from tenant-only fitness and lounge facilities and from the private Metropolitan club. Opening part of the building does not mean every amenity pictured in its marketing is available to everyone.
Willis Tower's later life makes the original design more interesting rather than less. A structure built to serve one of the largest retailers has outlasted that particular headquarters role, kept its place in the skyline and acquired new activities at its base. Its lasting achievement is the ability to connect very large ambitions with usable space. The skyline image and the ordinary work of keeping a building occupied remain inseparable.
Visiting
Willis Tower is at 233 South Wacker Drive. Skydeck's entrance is on Jackson Boulevard. For October 1-February 28, its current dedicated visitor page lists 9 a.m.-8 p.m., with Saturdays to 9 p.m., and last entry 30 minutes before closing. It also lists a special 1 p.m. opening on November 8. These attraction-specific hours differ from the general tower website's broader hours. Reserve a dated entry and check its terms rather than relying on a general lobby schedule. Details were read on the official pages on October 27, 2026; confirm before visiting.
Where can you eat or drink at Willis Tower?
Catalog has public dining separate from the Skydeck ticket. Kindling, with an entrance at Franklin and Adams, 202 South Franklin Street, is an on-site restaurant and bar. Its dedicated reservations page lists Monday-Thursday, 11 a.m.-9 p.m.; Friday, 11 a.m.-10 p.m.; Saturday, 4-10 p.m.; Sunday closed. Happy hour is listed Monday-Friday, 4-6 p.m. The restaurant's site contains inconsistent older FAQ hours, so use the reservations page and confirm directly at 312-789-5992 before making a special trip. Hours were read on October 27, 2026. The tower's private clubs and tenant lounges are not substitutes for a public restaurant reservation.