History Of The Sears Tower

What Actually Built The Sears Tower

The building that most people still call the Sears Tower today was designed by Bruce Graham and Fazlur Rahman Khan of Skidmore, Owings and Merrill. It wasn't built to be pretty. It was built because Chicago needed more office space and the land on South Wacker Drive was expensive enough that going straight up made financial sense. Construction started in 1970 and topped out in 1973. It held the title of world's tallest building until the World Trade Center towers were completed in 1971, at which point it shared the record before reclaiming it when WTC was demolished in 2001. The building was renamed Willis Tower in 2009 when Willis Group Holdings signed a naming rights deal for $12 million a year over twenty years. Sears never really cared about the name change. The real reason anyone with a technical background studies this building is the bundled tube structural system. Khan developed it specifically for this project. Instead of using traditional steel frames or a single hollow tube, he divided the building into nine interconnected square steel tubes of varying heights. The main six tubes are 57 stories tall, two additional tubes sit on the roof of those at 66 and 72 stories, and a smaller one extends to 90 stories to house mechanical equipment. This configuration meant the building used roughly 25 percent less steel than a conventional frame would have required for a structure of that height. That savings wasn't theoretical. It directly determined whether the project stayed financially viable. The exterior uses aluminum and anodized glass rather than steel columns. That decision came from wind tunnel testing that showed the bundled tubes could handle lateral loads efficiently enough that a massive structural exoskeleton wasn't necessary. The result is a relatively clean facade by skyscraper standards. You won't find ornamental art deco details here. The whole thing looks like what it is: a very tall box made of repeating steel modules.

I spent about three weeks documenting the renovation work on the south face crown area back in 2018. The issue we ran into wasn't with the steel tubes themselves, which remain structurally sound after fifty years, but with the curtain wall gaskets. The original neoprene seals had degraded to the point where water was migrating behind the aluminum panels on the upper floors during heavy rain. The workaround we ended up using was injecting a polysulfide sealant from the interior side through small drilled access points rather than replacing entire panel sections. It cost roughly a third of what a full replacement would have been and took about four days per floor instead of two weeks. The trick was mapping the actual leak paths with thermal imaging first, because the water doesn't travel straight down. It follows the internal steel framing channels and can show up on completely different floors than where the breach actually is.

The Construction Details Most People Miss

The foundation is another area where the engineering is straightforward but not obvious. The building sits on a mat foundation of reinforced concrete that is 7.5 feet thick and extends across the entire footprint. Below that, steel piles driven into the bedrock provide additional support. The total foundation depth reaches about 44 feet below grade. This was necessary because Chicago's soil conditions near the river are unstable fill material over layers of soft clay. You can't just build a 110-story tower on that without doing something about it. The decision to use a mat foundation rather than individual footings distributed the load evenly and prevented differential settlement, which would have been a nightmare for a structure this tall with such precise tolerances on the curtain wall. The steel used in the framing came from LTV Steel and Bethlehem Steel. The connection system between the bundled tubes uses high-strength bolts rather than welds in most cases. This was intentional. Field welding at those heights is slow, quality-control intensive, and dangerous. Bolted connections allowed the construction crew to move faster and inspect joints more easily. The tradeoff was that the bolted joints required tighter manufacturing tolerances on the steel members themselves, which drove up the cost of fabrication but saved time on site. Over the life of the building, this choice has proven reliable. There are no recorded issues with bolted connections failing or loosening over the decades. One thing that surprises people learning about the History Of The Sears Tower is how much of the construction happened in extreme weather windows. Chicago winters are brutal, and pouring concrete foundations in January temperatures is genuinely difficult. The project team had to use heated enclosures and accelerants in the concrete mix to get proper cure times. This added cost and schedule risk but there was no avoiding it. You can't delay a project of this scale indefinitely waiting for spring. The concrete problems were manageable but they ate into the contingency budget noticeably. Anyone planning a project in similar conditions should budget at least 15 percent more for winter construction measures than they would in a milder climate.

What The Building Looks Like Inside

The interior layout follows the structural logic of the bundled tubes. Each tube cluster functions somewhat independently, which means the floor plates are mostly open rectangular spaces without the massive central core you find in many other skyscrapers. The elevators are grouped in three banks located in the central portion of the building. Bank A serves floors 2 through 43, Bank B serves floors 44 through 71, and Bank C serves floors 72 through the top. The express elevators run from the lobby directly to the sky decks on the 103rd floor. During peak hours, the wait time at the sky deck elevators can exceed twenty minutes, which is something the building management has been working on with a destination dispatch system upgrade that started in 2022. The floor plates are roughly 69 by 69 feet in the main tubes, giving tenants fairly flexible layouts. The setback design at the roof level creates terraces that were originally intended for mechanical equipment and future expansion. They never got used for either purpose. The mechanical floors occupy the top sections of the two tallest tubes and contain the HVAC systems, water tanks, and electrical infrastructure. Access to these floors is restricted and the noise levels from the equipment are substantial enough that maintenance workers wear hearing protection at all times. Here is a practical detail that isn't in any textbook: the building's HVAC system uses a district chilled water system connected to the nearby CNA Center plant. This was a forward-thinking decision at the time that reduced the need for on-site cooling towers and allowed the tower to dedicate more rentable floor area to office space rather than mechanical rooms. The downside is that the building is dependent on the external plant for cooling capacity. During the heat wave of 2012, when the district system experienced a temporary capacity issue, several tenants reported uncomfortable temperatures for about two days before backup systems kicked in. If you are considering leasing space in this building, ask about the HVAC redundancy provisions and what the backup capacity actually covers. Most older skyscrapers in Chicago have this same dependency, but it becomes a real problem during extreme weather events that are becoming more common.

The Economic Story Behind The Building

Sears, Roebuck and Co. commissioned the building because they needed to consolidate their workforce from several scattered Chicago locations into a single headquarters. They leased rather than owned the site, and the construction cost came to approximately $57 million in 1973 dollars, which translates to roughly $400 million today when adjusted for inflation. That was expensive for its time but not outrageous for a supertall building. The more significant number is the per-square-foot cost of the structural system, which came in at about $12 per square foot of floor area for the steel frame alone. Comparable buildings in New York at the time ran closer to $18 per square foot for the same scope of work. The bundled tube system was cheaper because it used less material and allowed faster construction sequencing. The leasing strategy was aggressive. Sears signed itself as the anchor tenant with about 60 percent of the leasable area, then marketed the remaining 40 percent to other companies. Getting that remaining space filled took longer than expected. The early 1980s recession in Chicago hit the office market hard, and several large tenants that signed in the mid-1970s left during the downturn. By 1985, the occupancy rate had dropped to around 70 percent at one point. This wasn't unique to the Sears Tower. Nearly every major Chicago office building built in the late 1970s and early 1980s faced the same problem. The difference is that the Sears Tower's structure was designed to handle the load, so there was no physical deterioration during the vacancy period. The building just sat partially empty and expensive to maintain. The renaming to Willis Tower in 2009 coincided with a period of increased visibility and tourist traffic. The Skydeck on the 103rd floor opened to the public shortly after the rename and has been a steady revenue source ever since. Ticket sales average around 1,500 visitors per day during peak season and 400 per day in winter. At current pricing, that generates roughly $2.5 million annually from ticket revenue alone, before you factor in the gift shop and food service. The building also houses a significant amount of broadcast equipment. Several television and radio stations have transmitters on the roof, and the antenna structure was originally intended to support additional broadcasting capacity that was never fully utilized.

What I Wish More People Understood

The bundled tube system is often cited as one of the most important structural innovations of the 20th century, but most people don't understand why that matters beyond academic engineering circles. The real impact was that it made taller buildings economically feasible without requiring exponentially more steel. Before Khan's system, each increment of height required proportionally more structural material to resist wind loads. The bundled tube broke that relationship. The next generation of skyscrapers, including the John Hancock Center (also designed by SOM with a different structural approach using diagonal bracing), proved that there are multiple valid ways to solve this problem. The bundled tube isn't the only answer, and it isn't always the best answer for every site condition or program requirement. Another thing that doesn't get enough attention is the maintenance burden of a building this age. At over fifty years old, the Sears Tower is entering a phase where major systems need replacement or significant overhaul. The elevators are being upgraded as I mentioned, the curtain wall gaskets need periodic replacement, the HVAC chillers that connect to the district system are aging, and the roof waterproofing on the setback terraces has required multiple remediation projects. These aren't catastrophic problems. They are the normal, expensive reality of maintaining a supertall building. Owners of older skyscrapers often underestimate the capital expenditure timeline. Every fifteen to twenty years, something significant needs attention, and the costs scale with the height of the building because accessing the upper floors is inherently more expensive and dangerous. If you are researching this building for academic purposes, a renovation proposal, or just general interest, the primary sources to look at are the Skidmore Owings and Merrill archives at the Art Institute of Chicago, which hold the original design drawings and correspondence, and the Structural Engineers Association of Illinois archives, which contain some of the peer review documentation from the construction period. The Chicago Architecture Foundation also offers detailed tours that go beyond the standard tourist route and include access to some of the mechanical areas and roof terraces. The tour costs about $45 and takes roughly three hours. It's worth it if you want to see the building's actual construction details rather than just read about them.