The tube structure is a recent development in structural engineering that is a technique to design a high-rise structure with having better resistance to lateral loads such as wind, seismic and impact load.
It is a structural system such that the building acts like a hollow cylinder perpendicular to the ground. The tube system was developed by Fazlur Rahman Khan while at the architectural firm Skidmore, Owings and Merrill (SOM) when he was at Chicago office. Khan is known as father of tubular design in structural engineering because of this development.
The first tubular framed structure is a 43 storey Dewitt-Chestnut apartment building designed by Fazlur Rahman Khan which is now known as Plaza on Dewitt located in Chicago Illinois and was completed in 1966.
The tubular structure is made using concrete, steel or composite materials. This type of structure is idea for office, apartments and mix used buildings. Today high-rise constructions of more than 40 stories are being constructed using tubular structural system.
Table of Contents
- Tube system Concept
- Types of Tubular Structural system
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Tube system Concept
The tube system is based on the concept that a hollow cylinder system perpendicular to the ground is good in resisting lateral forces. The same technique is used for high rise buildings in which a large number of columns are placed along the perimeter of building with less space between them and then they are tied together with deep beams such as spandrel beams with moment connections.
The framework of columns and beams form a strong rigid frame structure which act as a strong and dense structural wall along the exterior of building. In comparison with much other structural system, tubular system is better as it has less story shear, story displacement, and drift. You can further enhance the efficiency of the system by using X bracings.
In tube system the exterior structural framing is designed in such a way that it is strong enough to resist all types of lateral loads acting on a building.
While the interior of building is only designed to resist gravity loads. There are only few columns located at the core while a large number of columns along the exterior perimeter to form a dense framing structure.
By connecting all the columns as a tube, we’re increasing the lateral strength and stiffness of a structure maximizing the efficiency of the entire structure.
Another relevant technique for constructing tall buildings is rigid frame system. It is a three-dimensional frame that is capable of resisting all the loads. A rigid frame structure acts like a long-resisting skeleton structure with straight or curved members having rigid connections. These members are able to resist movements at the joints taking all bending moment, shear, and axial loads.
Also Read: Load Bearing Structure – Types, advantages Vs Frame Structure
Types of Tubular Structural system
The types of tubular structural systems are:
- Framed tube system
- Trussed tube system
- Tube-in-tubular system
- Bundled tube system
- Hybrid tube system
Framed tubular Structural System
Framed tubular structural system is the first and simple tubular system. This type of framed structure is used for floor plan of square, rectangular, freeform and circular.
This type of tube structural system consists of exterior columns closely spaced and rigidly connected with a spandrel beam of sufficient depth running continuously along each façade and around building corners.
The closely spaced columns and spandrel beams of large depth increases the stiffness of beams and columns. This type of framed structure is suitable for building height of 38 to 300m.
The initial system designed by khan was framed tube structural system. According to him framed tubular structure is a system consists of three-dimensional space structure made of three, four or possibly more frames, braced frames or shear walls joined at or near their edges to form a tube-like structural system which is capable of resisting lateral forces such as wind, seismic and impact loads by cantilever form from foundation of building.
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Trussed tube Structural System
Trussed structural system also called braced tube system is similar to framed tube structure except few changes. In Trussed tube system there are less columns which are located at large distance which create more space along columns.
To form a rigid structure and to compensate for fewer columns steel bracing along diagonals or concrete shear walls are provided which form a strong bond between the large space columns.
A strong interconnection by concrete shear walls between columns helps in resisting lateral shear forces which in case of framed tube structure is resisted by bending or flexure.
The large columns spacing is beneficial because it provides a large space for windows.
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Tube in tube Structural System
Tube in tube structural system is also called hull and core arrangement. In this system the core tube is located inside the exterior tube.
This type of system is suitable when in architectural design the critical elements of buildings such as lifts, ducts, stairs etc. are designed at the center of building. The core tube helps in holding the critical elements of the buildings while exterior tube is the usual tube which resist lateral and gravity loads.
The inner and outer tubes are connected horizontally as shear and flexural elements of framed structure. The major advantage of this type tube structure is that it increases the lateral stiffness.
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Bundled Tube Structural System
In bundled tube structural system instead of exterior one tube there are large numbers of tubes interconnected with each other to form multi cell tube. The interconnected bundled tubes together resist overturning moments and lateral loads.
This technique allows designer to minimize the amount of steel required for high towers. That’s because there’s no wind braces, or any other supporting members – just the perimeter columns bearing all the weight.
The tube in tube system is not only an affordable but also allows for more versatile design of buildings which forms a variety of different shapes and bundled in dynamic groupings rather than forming box like structure. The first tube in tube structural system was adopted for Willis tower in Chicago.
Hybrid tube structural System
A hybrid tube structural system is also one of the types of tube structure which is used for building where building has slenderness and single structure cannot provide required stiffness and strength.
The required strength and stiffness are achieved by joining two or more structural forms.it can be either by direct combination or by adopting different forms in different parts of structure. Hybrid tube structural design is more common for high rise building which have complex design.
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The main feature of tubular structural system is the strong exterior frame. In any building, the exterior framing plays a critical role in resisting the lateral loads acting on the building like earthquake, wind, or rain.
So, because the exterior façade is well protected, there’re no major structural requirements in the interior of the building. However, still there are few interior columns throughout the structure.
Closely spaced columns along the exterior framework will result in dense and strong structural wall along the perimeter of the building. These columns encircle a central arrangement making a monolithic structure. This concept is in line with the rigid frame concept.