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The improvement of structural stiffness is crucial because the trend of building construction is toward taller, wider floor, and longer span structures. So, in order to reduce deflections and improve resistance of such structures against lateral loads such as wind and seismic, suitable method shall be considered to ensure satisfactory structural stiffness.
The methods of increasing structural stiffness of tall buildings are by providing central core, shear walls, tubes, braced frame and double tube. The use of one or combination of these techniques can increase structural stiffness considerably. The stiffness of a structure is the ability of a bulding to resist deformation. Structural stiffness describes the capacity of a structure to resist deformations induced by applied loads
1. Central Core
The stiffness of buildings can be increased substantially through the construction of central core. Commonly, central core structure used to contain lifts, stairs, and building services.
2. Shear Walls
Shear wall is a structural member used to resist lateral forces. They are constructed at opposite ends of a building to provide stiffness in a particular direction. Shear walls are particularly useful in non-square buildings, where the wind forces predominantly come from one direction.
The shape and plan position of the shear wall influences the behavior of the structure considerably. Structurally, the best position for the shear walls is in the center of each half of the building. This is rarely practical, since it also utilizes the space a lot, so they are positioned at the ends. It is better to use walls with no openings in them.
3. Braced Frame
Braced frame is the simple structure with bracing that help to increase the structural stiffness. A braced frame is similar to floor bracing, but it does not depend on the stiffness provided by the floor system, rather on the addition of another diagonal cross members. Compared to the system with floor bracing, this system is less easy to construct.
4. Tube system
A tube system is essentially two sets of shear walls. The tube system allows the building to be stiff in all directions of loading. The building would have a high-torsional resistance. The interior of the building is kept clear.
In the case where making opening in tube system is a must, the openings need to be as minimum as possible. It acts as hollow cylinder, cantilevered perpendicular to the ground which enables the structure to resist lateral loads from wind, seismic, and so on.
5. Double Tube System
A double tube system is a combination of the central core and tube system as shown in figure below. This combination of both systems allows the building to be extremely stiff. The building has high torsional resistance. For the tube system, apertures for windows must be kept to minimum. The central core takes much valuable space. This type of system is used for very tall building.