Evolution and Optimisation of Braced Tube Structures (35-story office tower at 100 Mount St. in North Sydney, Australia). Source: Structures Magazine
In 1968, the John Hancock Center's construction reached its zenith, marking a milestone that transformed Chicago's skyline and the field of tall building design and engineering. The "braced tube" structural system, conceived by SOM engineer Fazlur Khan and architect Bruce Graham, introduced a new era of efficient skyscraper design. This innovation, however, had roots in earlier developments and continues to evolve even today.
John Hancock Center Under Construction, 1968. Source: Pinterest
The Birth of the Braced Tube Structural System
The journey of the braced tube system began with Khan's Tube Frame structural system, first implemented in the Chestnut-Dewitt apartment building in Chicago, completed in 1966. Prior to this, rigid frame structures comprised two-dimensional frames aligned in two orthogonal directions. The spacing of columns was limited by practical considerations to avoid compromising interior space functionality. Khan's breakthrough came by placing closely-spaced columns around the building perimeter, allowing the structure to act like a thin-walled tube cantilevering from the ground. This increased stiffness at the perimeter and allowed interior columns to focus solely on gravity loads, enhancing the building's functionality and flexibility.
Types of Braced Tube Structures. Source: Structures Magazine
The development of the braced tube system for the John Hancock Center, now known as 875 North Michigan Avenue, marked a significant leap. Khan envisioned replacing the perimeter frame tube with a diagonalized steel braced truss on the building's exterior. This design resisted lateral deformations more efficiently through axial strains in the truss members, setting a new standard for structural material usage in tall buildings.
Advancements in Truss Geometry Optimization
In the late 2000s, SOM structural engineer William F. Baker led a research team to explore optimal truss geometries. Building on earlier studies, the team found that the iconic X-brace configuration of the John Hancock Center could be improved. By adjusting the central intersection node's height, they increased material efficiency by approximately 10%. This optimization reflected advances in structural engineering analysis made possible by modern computing.
SOM quickly applied these findings to new projects. The 35-story 100 Mount Street office tower in North Sydney, completed in 2019, incorporated these optimal truss geometries. The design aimed to maximize daylight and open space while ensuring structural efficiency and stability against lateral loads.
Modern Innovations and Challenges
The construction industry has shifted from steel to reinforced concrete as the preferred material for tall buildings. This transition poses challenges, as steel's isotropic characteristics make it ideal for braces, but integrating steel braces with concrete columns is complex due to differences in long-term behavior. For instance, concrete columns shorten over time due to creep, whereas steel braces do not, leading to potential inefficiencies.
To address this, SOM implemented a sliding detail at the central node of the brace in 100 Mount Street, isolating lateral and gravity load paths. This innovation was further refined in the design of 800 Fulton Market, completed in Chicago in 2021. Here, the team replaced the complex sliding mechanism with a simpler geometric solution, ensuring efficient load transfer and stability.
The legacy of the braced tube structural system, pioneered by Fazlur Khan, continues to influence modern skyscraper design. Through continuous research and innovation, SOM has enhanced the efficiency and functionality of tall buildings. Projects like 100 Mount Street and 800 Fulton Market exemplify how integrating cutting-edge structural solutions with architectural design can create iconic, efficient, and sustainable skyscrapers. This ongoing evolution underscores the vital role of innovative engineering in shaping the future of urban landscapes.
At 800 Fulton Market in Chicago, the design and engineering team replaced the complex sliding mechanism used at 100 Mount Street with a more straightforward geometric solution. Source: Structures Magazine
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