A tube structure treats the entire perimeter of a tall building as a single hollow cantilever, so the outer skin of columns and beams works together to resist wind and seismic loads. Instead of relying mainly on a central core, the building’s outside walls become the main lateral system, which is one of the most efficient ways to build very tall.
The idea transformed high-rise design from the 1960s onward because it let engineers reach far greater heights without the material penalty of older frame systems. In this article I explain how a tube resists load, the shear lag effect that limits it, the main variants in use, and the trade-offs that come with the system.
How a tube structure resists lateral load
The core insight is to imagine the whole building as a hollow box beam stood on its end and fixed at the ground. When wind pushes on the face of the building, this box bends like a cantilever. The windward wall goes into tension, the leeward wall into compression, and the side walls carry shear, exactly as the flanges and webs of a beam would.
To make the perimeter behave like a continuous wall rather than a loose collection of columns, a tube uses closely spaced perimeter columns joined by deep spandrel beams at every floor. The columns might sit only two to four metres apart, and the deep beams tie them tightly together. The result is a perforated tube that is stiff enough to act as a single unit.
Placing the lateral resistance at the extreme perimeter is what makes the system so efficient. The further the resisting material sits from the building’s centre, the greater its leverage against overturning, so a tube controls wind drift with less material than a comparable internal-core scheme.
Shear lag and its consequences
A real tube does not behave as perfectly as an ideal box beam, because of an effect called shear lag. In an ideal beam the flange stress is uniform across its width. In a framed tube, the corner columns attract more force than the columns in the middle of each face, so the flange stress is uneven and “lags” away from the corners.
Shear lag reduces the tube’s efficiency, because the mid-face columns are underused while the corners are overworked. The flexible spandrel beams between columns are the main cause, since they cannot fully transfer shear across the face. Much of the development of tube systems has been about reducing shear lag so more of the perimeter contributes.
Variants of the tube system
Framed tube
The simplest form is the framed tube: closely spaced columns and deep spandrels alone. It is straightforward and efficient up to a point, but shear lag limits how tall it can go before the system becomes inefficient.
Braced or trussed tube
Adding large diagonal braces across the faces of the tube ties the columns together far more effectively, cutting shear lag and stiffening the structure. The John Hancock Center in Chicago, with its prominent exterior X-bracing, is the classic example of a braced tube and shows the diagonals openly on the facade.
Bundled tube
A bundled tube groups several tubes together so they share internal “web” walls, which sharply reduces shear lag and allows great height. The Willis Tower in Chicago, originally the Sears Tower, is the defining example: nine tubes bundled together, with tubes terminating at different heights to create its stepped profile.
Tube-in-tube and diagrid
A tube-in-tube combines a perimeter tube with a stiff internal core, so the two tubes share lateral load. The diagrid takes a different route, replacing vertical columns with a triangulated lattice of diagonals that carries both gravity and lateral load through axial action. The Hearst Tower in New York and 30 St Mary Axe in London, widely known as the Gherkin, are well-known diagrid buildings.
Why the tube is so efficient for height
Tube systems became popular because they offer a strong height-to-material ratio. By engaging the full perimeter, they keep drift within comfortable limits without packing the floors with bracing or wasting material on an oversized core. That frees up valuable interior space and keeps the floor plate flexible.
The system also lends itself to expression. Braced tubes and diagrids put their structure on the outside, where it becomes part of the building’s architectural identity. This close link between structure and facade means the engineer and architect must collaborate closely from the start. If you are weighing structural systems for a tall building, our consultancy team can help you compare the options.
Advantages and limitations
The advantages are significant. A tube gives high lateral stiffness for the material used, makes very tall buildings practical, and can free the interior of obstructing structure. The perimeter location of the lateral system is the most leveraged place to put it.
The limitations need careful handling:
- Constrained facade — closely spaced perimeter columns and deep spandrels limit window sizes and views, which can conflict with architectural goals.
- Shear lag — the framed tube never reaches ideal box-beam efficiency, pushing taller buildings toward braced, bundled or diagrid variants.
- Corner forces — corner columns carry disproportionate load and need careful detailing.
- Fabrication complexity — braced and diagrid tubes rely on demanding nodes and connections that must be precisely made and erected.
Choosing the right variant is a balance of height, plan, wind and seismic demand, architecture and cost, and is a decision a qualified structural engineer should lead.
Frequently asked questions
What makes a tube structure efficient?
A tube places the lateral system at the building’s extreme perimeter, where it has the greatest leverage against overturning. By making the whole outer skin act as a hollow cantilever, it resists wind and seismic load with less material than an internal-core scheme of similar height.
What is shear lag in a tube?
Shear lag is the uneven distribution of force across a tube’s faces, where corner columns attract more load than mid-face columns. It is caused by the flexibility of the spandrel beams and reduces the tube’s efficiency, which is why braced, bundled and diagrid variants exist.
What is the difference between a framed tube and a braced tube?
A framed tube relies only on closely spaced columns and deep spandrel beams. A braced tube adds large diagonal members across the faces, as on Chicago’s John Hancock Center, which ties the columns together more effectively and reduces shear lag.
What is a diagrid?
A diagrid replaces vertical perimeter columns with a triangulated lattice of diagonals that carries both gravity and lateral load through axial force. Well-known examples include the Hearst Tower in New York and 30 St Mary Axe in London.
Why can a tube limit window sizes?
Framed tubes need columns spaced only a few metres apart with deep spandrel beams between floors. This dense perimeter grid restricts how large the windows can be, which is one reason architects and engineers must coordinate the structural choice with the facade design.
Related reading
- Pile Caps Explained
- Caisson Foundations Explained
- Underpinning Foundations Explained
- Foundation Settlement Explained
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