From the construction of the first steel framed high rise building in Chicago in 1885 to the Empire State Building in 1931, 10 towers held for some time the distinction as world’s tallest. Then it took 41 years for the World Trade Towers to take the title. The Sears Tower built in 1974 would again hold the title for multiple decades. Over the last 20 years, beginning with the Petronas Towers, we are again in an era of competition.
Are these leaps in tower height the result of new materials, design innovation, or wealth accumulation? Here’s my understanding of the history of high rise design.
William Le Baron Jenny revolutionized building design by introducing the steel framed structure to multi-story commercial construction. This concept, allegedly inspired by a bird cage, freed the exterior facade from providing main building support. Interior steel frames literally provided the heavy lifting. This typology was progressively improved and expanded as steel fabrication become more reliable and steel workers became more skilled, reaching its apex with the construction of the Empire State building.
In the 1970s Fazlur Khan of Skidmore Owings and Merrill competed with Leslie Robertson to find designs that could reach higher. Both realized that there was sound logic to including the historic concept in which the exterior wall contributed to gravity and lateral support. As building get taller and taller, building sway became a big concern. It may have been possible to build a frame building taller, but the building motion would be uncomfortable to people. The Empire State building gets away with it in part because it is so heavy. Mass is a virtue in reducing sway accelerations.
Aside: I worked on a 40-story moment frame building in Indianapolis. We investigated the results of a windstorm that blew 30% of the facade off. The people who worked in the building frequently reported discomfort in building motion and loud creaking in the structural steel. It had a period of around 8 seconds to sway from one extreme to another and back. That’s about double what we might expect in a tube building of that height.
The John Hancock tower in Chicago uses a trussed exterior tube. Leslie Robertson alternately proposed an exterior tube of closely spaced columns with deep spandrels on his project. Allegedly, Robertson did a peer review of the Hancock Tower and recommended his system, but Khan was able to convince the team that the trussed tube was equally viable and more material efficient.
Later, Khan’s Sears Tower eschewed the simple tube and re-applied Jenny’s steel frame concepts, supersized and bundled together. He called the Sears Tower system the bundled tube. Nine tubes start at the base but drop off along the way, until just the one extends to the reported height of 1,451. This is still the highest occupiable floor in the United States, since the Freedom Tower’s famous 1,776 height includes a substantial spire.
Another derivative of Khan and Robertson’s concept is the tube in tube. The Petronas Towers is an example done in concrete. It combines an internal wall with an exterior frame.
The downside of the exterior tube is the interference in the facade by structural members. (Although, leasing agents for Hancock claim that units with the iconic braces actually fetch higher rents.) As for the bundled tube, Sears Tower required a closer column spacing than desired in open plan office space layouts. It also required a lot of expensive labor intensive steel connections.
Designers continued to innovate. Local lore for my Chicago office, which was CBM Engineers before being bought out by Thornton Tomasetti, has it that Eli Cohen was inspired by the design of parabolic nuclear cooling towers to put concrete walls in the core of the building combined with steel columns on the perimeter. Therefore your lateral support is interior where the architects would need to run the elevators and mechanical systems anyway, and the exterior was free for floor to ceiling glass. Most Chicago high rises built from the 80s to today share that structural design. However, New York steel and concrete unions never permitted the overlapping trades until construction of Freedom Tower.
To provide the uninterrupted floor plates of the core wall design at super tall heights engineers had to devise a way to brace the core and shed load out to the perimeter. Taipei 101, which is still the tallest steel tower, has an interior braced frame core but additionally has outriggers every 8 stories which engage massive exterior columns. They work similarly to the stays on sailboat masts.
Burj Khalifa, which is all concrete, has an interior concrete core and buttress walls. The concept of the buttresses goes way back, you can find examples in the Pantheon and most Romanesque Cathedrals. Notre Dame in Paris is famous for the flying buttress, which are a more elegant derivative. The purpose is the same though, to prevent the load bearing walls from buckling or being pushed out of alignment from applied loads. Kingdom Tower, which will be the next tallest building at over 1000m, will share similar design features to Burj Khalifa.
As a general trend, super tall buildings are moving toward concrete design. High strength flowable mixes make the material more viable. The mass of the concrete is important in reducing the accelerations felt by people when the building sways. Concrete also requires much less skilled labor than steel. Because of material demand and labor costs steel prices have increased dramatically in the past decade.
It’s interesting to note that while higher strength steel is more readily available today than in 1974, this has not led to more use of steel in super tall designs. Stiffness is more important than strength. This also proves that we could build much taller yet, designers must contend more with making perception meet the reality of being safe.