Willis Tower: Chicago's iconic 108‑story skyscraper

Design and structural characteristics

The tower is widely cited as a signature example of the bundled‑tube structural system developed in part by engineer and architect Fazlur Khan. In this approach multiple tubes are arranged and connected to form a stiff, efficient frame that resists wind loads while permitting tall, open office floors. The exterior is clad in dark glass and aluminum, giving the building its distinctive appearance. Dedicated levels house plant and support equipment; owners and technical descriptions often distinguish principal office floors from mechanical floors and penthouses (mechanical, penthouse).

Construction, records and history

When it was completed in the early 1970s, the structure surpassed earlier tall towers in New York such as the World Trade Center in New York City to become the tallest building in the world. It held that distinction for a quarter of a century until superseded by the Petronas Towers in Malaysia. For many years afterward it remained the tallest building in the Western Hemisphere until taller projects, including One World Trade Center in New York, were completed. Different sources and owner statements sometimes count roof and mechanical levels differently, which accounts for minor variations in reported floor counts.

Naming and ownership

Originally known as the Sears Tower because it served as the headquarters of the retail company Sears, the building was renamed in 2009 after a long‑term lease of naming rights to an international brokerage. Both the original and the newer name are used in public discourse and media, and the tower's ownership and management have evolved with successive commercial real estate arrangements.

Observation deck and public access

The tower's observation facilities attract large numbers of visitors annually. The Skydeck offers sweeping views across the city and Lake Michigan and includes projecting glass ledges that allow visitors to look straight down from a high vantage point. These public features are a significant part of the building's appeal for tourists and local visitors; for planning details and tickets consult official visitor resources such as the building's published guides and portals (official visitor information).

Tenants, use and major occupants

Although built for a single corporate client, the tower now accommodates a broad mix of commercial tenants. Among the better‑known occupants are national airlines and major financial firms; for example, a leading carrier relocated its corporate offices to the tower in the 2010s (United Airlines). Investment and banking firms with a presence include companies such as Morgan Stanley, and well‑known law firms and professional services companies also lease space in the complex.

Architectural and cultural significance

Architecturally, the building is frequently cited in discussions of late‑20th‑century skyscraper design because its bundled‑tube concept enabled both great height and efficient floor plates. It remains a widely recognized symbol of Chicago's modern skyline and appears often in cultural references, guidebooks and architectural histories. It is also included in tall building listings and U.S. rankings that document comparative height, floor count and other measures (tall building listings, U.S. rankings).

Renovations, sustainability and systems

Over time the building has seen upgrades to elevators, mechanical systems and public areas to meet contemporary code, energy and tenant expectations. Large, older office towers commonly undergo phased renovations to improve energy performance, accessibility and safety while preserving defining architectural elements.

Access, visitor guidance and further reading

Visitors planning a trip to the observation level or to meet tenants should consult current information about hours, tickets and access procedures from official sources linked through the tower's site and local tourism offices. Additional material on the tower's engineering, history and tenancy is available from architectural journals, municipal archives and commercial real estate reports (comparative towers, international context).

Quick facts

• Commonly reported height: about 1,450 feet; commonly reported number of stories: 108.
• Completed in the early 1970s and for decades noted among the tallest buildings worldwide.
• Known both as the Willis Tower and historically as the Sears Tower; both names remain in use.

For authoritative and up‑to‑date details on floor counts, ownership, tenancy and visitor arrangements, consult the building management and primary archival sources, or authoritative tall‑building databases and municipal records (architect profile, technical references, service floor descriptions).