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A tubular structural system is used in high-rise buildings to resist lateral loads like wind and seismic forces. These lateral load resisting systems let the building behave like a hollow cylindrical tube cantilevered perpendicular to the ground. Thus the structure exhibits a tubular behavior against the lateral loads.
The tube system was developed by the famous structural designer Fazlur Rahman Khan in 1960. He is considered the father of tubular design in structural engineering. The DeWitt Chestnut Apartment Building in Chicago was the first tube-frame structure designed by Khan in 1963.
Features of Tubular Structural System
The tubular structural system evolved from the concept of the rigid frame system which is a three-dimensional frame capable of resisting all the lateral loads with the façade structure.
The system consists of an internal core arrangement surrounded by exterior columns. The exterior perimeter columns are closely spaced and tied together with deep beams forming a rigid frame. Hence, this arrangement creates a dense and strong structural wall along the exterior of the building.
The exterior framing is designed to be strong enough to resist the lateral loads acting on the building, thereby letting the interior of the building resist gravity loads. The interior columns are few in number and located around the core structure.
The core frame and the perimeter frames are spanned with beams or trusses. These members help to transfer the gravity loads within the structure. This mechanism maximizes the effectiveness of the perimeter tube to resist overturning moments formed due to lateral loads.
Types of Tubular Structural System
The major types of tubular structural systems are:
- Framed tube system
- Trussed tube system
- Tube-in-tube system
- Bundled tube system
- Hybrid tube system
1. Framed Tube Structural System
The framed tube system is Khan’s simplest and first tubular structural system. The framed tube can be used for various floor plan shapes like square, circular, rectangular, and freeform.
Framed tube structural system consists of closely spaced exterior columns that are rigidly connected with deep spandrel beams running continuously along each facade and around the building corners. This arrangement increases the beam and column stiffness by decreasing the clear span dimensions and increasing the member depth. This type is efficient for buildings with a height from 38 to 300 m.
2. Trussed Tube Structural System
Trussed tube structural systems like frame tube systems consist of exterior columns fewer in number compared to framed tube systems that are placed far apart. These columns are connected using steel bracing or concrete shear walls. This interconnection of exterior columns makes a rigid box capable of resisting lateral shear by axial force in members rather than bending or curving (flexure).
The provision of broad column spacing in the trussed tube system allows clear space for windows.
3. Tube-in-Tube Structural System
Tube-in-tube structural system is also known as “hull and core” arrangement. Here, a core tube is surrounded by an exterior tube. The core tube holds the critical elements of a high-rise building like lifts, ducts, stairs, etc. The exterior tube is the usual tube system that takes the majority of gravity and lateral loads.
In this system, the inner and outer tubes interact horizontally as shear and flexural components.
4. Bundled Tube Structural System
In bundled tube structural system, there are several interconnected tubes to form a multi-cell tube. This arrangement together resists lateral loads and overturning moments.
The tube-in-tube system is the most economical and versatile building design that helps to create interesting shapes. Willis Tower in Chicago is the first building to adopt tube-in-tube structural design.
5. Hybrid Tube Structural System
Whenever the building design has slenderness or less strength and stiffness, a hybrid structural system is used. A hybrid tube structural system combines two or more structural forms in different parts of the structure. This system is more common in complex building designs.
A tubular structural system is used in high-rise buildings to resist lateral loads like wind and seismic forces. These lateral load resisting systems let the building behave like a hollow cylindrical tube cantilevered perpendicular to the ground.
A hybrid tube structural system combines two or more structural forms in different parts of the structure. This system is more common in complex building designs.