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Tube (structure)
Tube (structure)
Tube (structure)
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John Hancock Center in Chicago, designed in 1965 and finished in 1969, is an example of the trussed tube structural design

In structural engineering, the tube is a system where, to resist lateral loads (wind, seismic, impact), a building is designed to act like a hollow cylinder, cantilevered perpendicular to the ground. This system was introduced by Fazlur Rahman Khan while at the architectural firm Skidmore, Owings & Merrill (SOM), in their Chicago office.[1] The first example of the tube's use is the 43-story Khan-designed DeWitt-Chestnut Apartment Building, since renamed Plaza on DeWitt, in Chicago, Illinois, finished in 1966.[2]

The system can be built using steel, concrete, or composite construction (the discrete use of both steel and concrete). It can be used for office, apartment, and mixed-use buildings. Most buildings of over 40 stories built since the 1960s are of this structural type.

Concept

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The tube system concept is based on the idea that a building can be made to resist lateral loads by designing it as a hollow cantilever perpendicular to the ground. In the simplest incarnation of the tube, the perimeter of the exterior consists of closely spaced columns that are tied together with deep spandrel beams through moment connections. This assembly of columns and beams forms a rigid frame that amounts to a dense and strong structural wall along the exterior of the building.[3]

The exterior framing is designed sufficiently strong to resist all lateral loads on the building. This allows the interior to be simply framed for gravity loads. Interior columns are comparatively few and located at the core. The distance between the exterior and the core frames is spanned with beams or trusses and can be column-free. This maximizes the effectiveness of the perimeter tube by transferring some of the gravity loads within the structure to it, and increases its ability to resist overturning via lateral loads.

History

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By 1963, a new structural system of framed tubes had appeared in skyscraper design and construction. Fazlur Rahman Khan, a structural engineer from Bangladesh (then called East Pakistan) who worked at Skidmore, Owings & Merrill, defined the framed tube structure as "a three dimensional space structure composed of three, four, or possibly more frames, braced frames, or shear walls, joined at or near their edges to form a vertical tube-like structural system capable of resisting lateral forces in any direction by cantilevering from the foundation."[4] Closely spaced interconnected exterior columns form the tube. Lateral or horizontal loads (wind, seismic, impact) are supported by the structure as a whole. About half the exterior surface is available for windows. Framed tubes require fewer interior columns, and so allow more usable floor space. Where larger openings like garage doors are needed, the tube frame must be interrupted, with transfer girders used to maintain structural integrity.

Khan's tube concept was inspired by his hometown in Dhaka, Bangladesh. His hometown did not have any buildings taller than three stories. He also did not see his first skyscraper in person until the age of 21 years old, and he had not stepped inside a mid-rise building until he moved to the United States for graduate school. Despite this, the environment of his hometown in Dhaka later influenced his tube building concept, which was inspired by the bamboo that sprouted around Dhaka. He found that a hollow tube, like the bamboo in Dhaka, lent a high-rise vertical durability.[5]

The first building to apply the tube-frame construction was the DeWitt-Chestnut Apartment Building which was designed by Khan and finished in Chicago by 1963.[6] This laid the foundations for the tube structural design of many later skyscrapers, including his own John Hancock Center and Sears Tower, and the construction of the World Trade Center, the Petronas Towers, and the Jin Mao Building. This also applies to most other tall skyscrapers since the 1960s, including the world's tallest building as of 2020, the Burj Khalifa.[7]

Variants

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From its conception, the tube has been varied to suit different structural needs.

Framed

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The World Trade Center's Twin Towers were some of the first structures to use a framed tube design. The many columns of the tube can be seen around the exterior of this horizontal cross section. The towers had a core for services, seen in the center. The design was not tube-in-tube since the core had 47 columns spaced relatively evenly, rather than around the edge of the core.

This is the simplest incarnation of the tube. It can appear in a variety of floor plan shapes, including square, rectangular, circular, and freeform. This design was first used in Chicago's DeWitt-Chestnut Apartment Building, designed by Khan and finished in 1965, but the most notable examples are the Aon Center and the original World Trade Center towers.

Trussed or braced

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The trussed tube, also termed braced tube, is similar to the simple tube, but with comparatively fewer and further-spaced exterior columns. Steel bracings or concrete shear walls are introduced along the exterior walls to compensate for the fewer columns by tying them together. The best examples incorporating steel bracing are the John Hancock Center, the Citigroup Center, and the Bank of China Tower.

Hull and core

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These structures have a core tube inside the structure, holding the elevator and other services, and another tube around the exterior. Most of the gravity and lateral loads are normally taken by the outer tube because of its greater strength. The 780 Third Avenue 50-story concrete frame office building in Manhattan uses concrete shear walls for bracing and an off-center core to allow column-free interiors.[8]

Bundled

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Breakdown of the bundled tube structure of the Willis Tower with simplified floor plans.

Instead of one tube, a building consists of several tubes tied together to resist lateral forces. Such buildings have interior columns along the perimeters of the tubes when they fall within the building envelope. Notable examples include Willis Tower, One Magnificent Mile, and the Newport Tower.

Willis Tower, finished in 1973, introduced the bundled tube structural design and was the world's tallest building until 1998

Beside being efficient structurally and economically, the bundled tube was "innovative in its potential for versatile formulation of architectural space. Efficient towers no longer had to be box-like; the tube-units could take on various shapes and could be bundled together in different sorts of groupings."[9] The bundled tube structure meant that "[buildings] could become sculpture."[10]

Hybrid

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Hybrids include a varied category of structures where the basic concept of tube is used, and supplemented by other structural support(s). This method is used where a building is so thin that one system cannot provide adequate strength or stiffness.

Concrete

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The last major buildings engineered by Khan were the One Magnificent Mile and Onterie Center in Chicago, which employed his bundled tube and trussed tube system designs respectively. In contrast to his earlier buildings, which were mainly steel, his last two buildings were concrete. His earlier DeWitt-Chestnut Apartments building, built in 1963 in Chicago, was also a concrete building with a tube structure.[7] Trump Tower in New York City is also another example that adapted this system.[11]

Lattice towers

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Some lattice towers have steel tube elements, like the guyed Warsaw Radio Mast or free-standing 3803 KM towers.

Diagram

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Evolution of skyscraper structural systems
Evolution of skyscraper structural systems

References

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Revisions and contributorsEdit on WikipediaRead on Wikipedia

Tube (structure)

View on Grokipedia
from Grokipedia
A tube structure, also known as a tubular structural system, is an innovative approach in high-rise that utilizes a perimeter frame of closely spaced columns and deep beams to form a rigid, hollow "tube" capable of efficiently resisting lateral loads such as and seismic forces, functioning like a cantilevered vertical shaft. This system revolutionized construction by minimizing internal structural elements, allowing for larger open floor spaces and more economical use of materials compared to traditional braced or shear walls. The concept of the tube structure was pioneered in the 1960s by structural engineer Fazlur R. Khan, who worked for the architectural firm Skidmore, Owings & Merrill (SOM) and sought to address the escalating costs and inefficiencies of building ever-taller structures under increasing lateral forces. Khan's breakthrough came from analogizing building behavior to that of a or tree, treating the exterior frame as a three-dimensional that distributes loads across its entire surface rather than relying on a central core alone. The first implementation of a pure framed tube system was in Chicago's DeWitt-Chestnut Apartments in 1965, a 43-story residential building. Several variants of the tube system have evolved to suit different building heights, shapes, and load conditions, enhancing its adaptability for supertall and megatall structures. The framed tube relies on moment-resisting connections between perimeter columns and beams to provide stiffness, suitable for buildings around 40 to 100 stories. The trussed or braced tube incorporates diagonal bracing elements within the perimeter frame to increase rigidity and efficiency, allowing heights beyond 100 stories while reducing steel usage compared to framed tubes. The bundled tube arranges multiple interconnected tubes around a central core, optimizing for irregular floor plans and wind resistance, as seen in structures exceeding 400 meters. Other types include the tube-in-tube (or hull-and-core), where an inner core is surrounded by an outer tube connected by floor slabs, ideal for very tall buildings with service cores; and hybrid systems combining tubes with outriggers or belt trusses for additional stability. Iconic examples illustrate the tube system's global impact on modern architecture. The John Hancock Center (now 875 North Michigan Avenue) in , completed in 1969, was the first major braced tube building at 100 stories, using exterior X-bracing to support its mixed-use and withstand Chicago's strong winds. The (formerly Tower), finished in 1973, employed a bundled tube configuration with nine hexagonal and triangular tubes clustered together, reaching 442 meters and setting a record for the world's tallest building at the time while using only 33 pounds of per square foot. More recently, the in (1998) utilized a tube-in-tube system with a core and perimeter columns, achieving twin 452-meter spires that resist tropical seismic activity. These structures highlight the system's advantages in material efficiency, aesthetic flexibility—often featuring exposed framing as a element—and ability to enable unprecedented heights without prohibitive costs.

Overview

Definition and Core Concept

A tube structure is a structural employed in tall buildings, consisting of a rigid perimeter frame that encloses the building's core and functions as a hollow to primarily resist lateral loads such as and seismic forces. This design innovation, pioneered in the , enables efficient load distribution by treating the building's exterior as a unified vertical shell rather than relying on dispersed internal elements. The core components of a tube structure include closely spaced exterior columns, typically arranged at intervals of 1.5 to 4.5 meters, interconnected by deep beams with depths ranging from 0.5 to 1.2 meters, forming a dense grid along the facade. This perimeter assembly minimizes the need for interior columns, thereby supporting expansive open floor plans while concentrating structural integrity at the building's edges. Unlike traditional skeletal frames, where strength derives from the individual flexural capacities of beams and columns distributed throughout the building, tube structures achieve their rigidity through the monolithic action of the entire perimeter, which acts compositely to enhance overall and . In this system, lateral forces are transferred via shear through the perimeter frame—often incorporating shear walls or braced elements—to the foundation, with the behaving as a fixed at the base. The concept was first implemented in the DeWitt-Chestnut Apartment Building in , completed in 1965.

Structural Principles

Tube structures exhibit monolithic behavior, wherein the closely spaced perimeter columns and stiff beams form a deep, thin-walled tube that resists and shear forces as a unified . In this configuration, the columns primarily endure axial compressive and tensile forces, while the spandrel beams handle flexural moments, collectively mimicking the performance of a continuous box girder and providing exceptional lateral stiffness without extensive interior bracing. This perimeter-dominated approach enhances structural efficiency for high-rise applications, permitting buildings taller than 40 stories by uniformly distributing lateral and gravitational loads across the exterior frame, which minimizes material consumption and foundation demands relative to conventional moment-resisting frames that require more distributed columns. The even load sharing reduces differential settlements and allows for larger open interior spaces, optimizing both economic and functional aspects of tall . A critical phenomenon influencing tube performance is the shear lag effect, which causes nonlinear axial stress distribution along the faces, concentrating higher stresses in corner columns and reducing them toward the mid-face due to differential shear deformations in the spandrel beams. This effect diminishes the structure's effective , potentially increasing deflections and requiring design adjustments. Implications for design include upsizing corner columns, incorporating finite element analysis to compute lag coefficients (typically 0.6-0.8 for framed tubes), and limiting bay widths to mitigate warping, ensuring the structure maintains 80-90% of its theoretical stiffness under or seismic loads. The perimeter tube maintains minimal mechanical to the interior core, which primarily supports vertical loads and houses utilities, allowing the exterior frame to dominate lateral stability while the core contributes supplementary rigidity through floor diaphragm action. This decoupling simplifies sequencing and enables flexible interior layouts.

Historical Development

Origins and Early Concepts

The origins of tube structures in emerged from mid-20th-century engineering research aimed at overcoming the limitations of traditional interior-column-heavy frames for supertall buildings. During the early , Fazlur R. Khan, while working at Skidmore, Owings & Merrill (SOM) in , developed the core ideas of the tube concept, inspired by efficient load-bearing systems in other domains such as the rigid configurations in bridges. These analogies allowed Khan to envision a building's perimeter as a continuous, enclosed structural shell that could act as a , distributing wind and seismic loads more effectively across the entire facade. Khan's first theoretical articulation of the tube concept appeared in his 1964 engineering analyses, where he proposed densely spaced perimeter columns and spandrel beams forming a rigid frame to enable taller, more material-efficient supertall structures without excessive interior supports. This work culminated in the design of the 43-story DeWitt-Chestnut Apartments in , completed in 1965 as the inaugural framed tube building, demonstrating the system's viability for residential high-rises. The approach marked a shift toward treating the as the primary structural element, significantly reducing steel or usage compared to prior methods. Preceding Khan's innovations, the 1950s saw experimental applications of rigid perimeter frames in Chicago-area buildings, such as I.M. Pei's University Apartments (1961), which utilized closely spaced exterior columns for partial load resistance but lacked the full, enclosed tube configuration to maximize behavior. These precursors highlighted the potential of perimeter framing for creating open interior spaces yet fell short in scaling to extreme heights due to insufficient lateral stiffness. The development of tube structures was driven by post-World War II socioeconomic pressures in urban centers like , where rapid population growth, economic expansion, and suburban flight necessitated taller buildings to densify downtown areas affordably. This urban boom demanded designs that minimized construction costs while providing flexible, column-free interiors for offices and residences, aligning perfectly with the tube's emphasis on perimeter efficiency over centralized cores.

Evolution and Key Innovations

The late 1960s and 1970s marked pivotal advancements in tube structure design, beginning with the introduction of the braced tube system in the John Hancock Center (completed 1969), which incorporated diagonal bracing for greater efficiency, followed by the bundled tube system by structural engineer Fazlur Rahman Khan, first implemented in the Sears Tower (now Willis Tower) completed in 1973. This innovation clustered multiple individual tubes into a composite perimeter frame, enabling tapered architectural forms and efficient load sharing across the interconnected units, which enhanced overall stability and material economy for super-tall buildings. By the 1980s, tube structures transitioned from predominant steel framing to incorporate concrete-filled steel tubes, leveraging the composite action of concrete's and steel's tensile capacity to achieve greater and fire resistance. This shift was exemplified in early applications like the 1983 One Magnificent Mile in , the first concrete bundled tube structure, which demonstrated improved performance under lateral loads compared to all-steel systems. Advancements in concrete pumping technology during this decade further facilitated the widespread adoption of these hybrid elements in high-rise construction. The global adoption of tube structures accelerated in the 1990s, particularly in , where they were adapted for regions with moderate to high , such as Hong Kong's Central Plaza completed in , which employed a tube-in-tube system to provide and dissipation under dynamic loads. This era saw tube designs refined to integrate with local codes emphasizing seismic resilience, allowing taller buildings in earthquake-prone areas without excessive material use. Up to 2025, innovations in tube structures for supertall have focused on integrating advanced perimeter framing with core enhancements to tackle mega-scale challenges like extreme wind and slenderness ratios. This evolution addresses unprecedented height demands by minimizing internal obstructions and maximizing perimeter efficiency.

and

Load-Bearing Mechanisms

In tube structures, the perimeter frame serves as the primary system for resisting lateral loads such as and seismic forces, functioning as a hollow beam that efficiently distributes these loads across closely spaced exterior columns and deep beams. The overturning moments induced by or earthquakes are countered by axial tension in the windward facade columns and axial compression in the leeward columns, minimizing stresses in individual members and enhancing overall stability. This mechanism allows tube structures to achieve high lateral , with the perimeter tube providing significant resistance to shear in some designs, reducing inter-story drift under . For seismic resistance, the tube system's ductility is enhanced through moment-resisting frames at the perimeter and core, where energy dissipation occurs via inelastic deformations in beams while columns remain elastic, preventing progressive collapse. The perimeter tube acts in tandem with an interior core to share lateral forces, with the core providing additional rigidity against torsion and the perimeter handling primary shear, resulting in substantial drift reductions compared to conventional frames. Gravity loads from the building's weight and live loads are integrated through floor diaphragms that transfer vertical forces to both the perimeter columns and the interior core, ensuring balanced load paths and minimizing differential settlement. These diaphragms, typically composed of slabs or composite decks, exhibit high shear rigidity that uniformly distributes lateral loads to the vertical elements, preventing localized stress concentrations and promoting synchronous deformation across the . This rigidity is critical for maintaining the tube's , as it couples the floor planes to act as a continuous shear panel, thereby optimizing load transfer without excessive twisting or racking. Potential failure modes in tube structures primarily involve buckling of perimeter columns under combined axial compression and bending from lateral loads, particularly in unbraced segments where slenderness can amplify instability. Design practices limit the slenderness ratio λ=KLr<200\lambda = \frac{KL}{r} < 200 to ensure elastic buckling does not govern, with KK as the effective length factor, LL as the unbraced length, and rr as the radius of gyration, thereby maintaining compressive capacity and preventing premature failure.

Analysis Methods

Analysis of tube structures relies on advanced engineering tools to predict their response to lateral loads, such as wind and seismic forces, ensuring structural integrity in tall buildings. Finite element modeling (FEM) is a primary technique for simulating the complex behavior of these systems, particularly the perimeter frame's interaction under shear lag effects, where axial stresses in corner columns are higher than in intermediate ones due to uneven load sharing. Three-dimensional FEM software, such as ETABS, models the closely spaced columns, beams, and floor diaphragms as interconnected elements to capture this phenomenon accurately, enabling designers to optimize member sizes and assess overall stiffness. For preliminary design stages, the equivalent static method provides a simplified approach to estimate lateral load effects on tube structures by converting dynamic forces into static equivalents. This involves calculating the base shear V based on building mass, site seismicity, and code provisions, then applying lateral forces totaling the base shear V distributed vertically over the height, typically in a linear or parabolic profile to approximate the structure's fundamental mode shape. This method is particularly useful for initial of the tube's perimeter elements in framed-tube systems, where the distribution accounts for the cantilever-like against overturning moments. Dynamic analysis is essential for capturing the time-varying nature of seismic and excitations in tube structures, with methods being widely adopted for seismic evaluation. These methods superimpose modal responses from a spectrum of accelerations corresponding to the structure's frequencies, providing maximum displacements and forces without full time-history . The frequency ff of a tube system is determined by f=12πkmf = \frac{1}{2\pi} \sqrt{\frac{k}{m}}
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