Concrete has evolved into a universal building material with new structural forms such as shear walls and tube structures. The height of a concrete building has little bearing on the high dead load characteristics. Otherwise, the concrete’s dead load is more important in reducing sway deflection and floor vibration, as well as the problem of instability.
Tall buildings cannot be characterised solely in terms of height or floor count. When the structural calculations and design are influenced by lateral loads, the building is called tall. When building heights rise, lateral loads begin to dominate the structural system and become increasingly important in the total building system.
The effects of vertical and lateral loads on a building are extremely diverse, and they increase rapidly as the building’s height rises. The design of such structures had to take into account three primary factors: strength, stiffness, and stability. Basically, there are two ways to meet these requirements: either increase the size of the member beyond what is required to meet the strength requirement, or transform the structure’s form into something more rigid and stable.
Table of Contents
Tall Building System Classification
The main purpose of the structural system in a tall building is to resist lateral loads. In terms of relative effectiveness in resisting lateral loads, the hierarchy of system creation can be loosely classified. In any case, the approach for documenting tall buildings in terms of their structural systems was rigorously developed in 1984. The classification approach was then implemented, with four separate degrees of framing-oriented division (Falconer and Beedle, 1984). Tall building systems include braced framed and moment resisting frame systems, shear wall systems, core and outrigger systems, and tubular systems.
The Tube Tall Building is a development of the tube. Shear wall and framed tube with closely spaced columns and large spandrels make up a tube in a tube building. This system is widely regarded as a high-performance structural system for tall structures. Dr. Fazlur Khan of the architectural and engineering firm Skidmore, Owings and Merrill was the first to introduce the tube system’s simplicity. The tube system simply needs to use the most basic structural elements, such as beams and carefully placing column sites. What’s more, it doesn’t necessitate a new way of analysis. Because the bending and transverse shears are supported three-dimensionally at the flange and web surface in the structure, the tube structure in a high-rise construction is an effective system.
Behavior of Tube
The development of the tube system ushered in a new era in high-rise building architecture. The efficiency of this technique in terms of lateral strength and stiffness is due to the use of the external wall alone as a wind-resisting element, causing the entire structure to operate as a hollow tube cantilevering out of the ground. In essence, the method aims to generate a tube-like form structure through the building’s outside wall. Inside the building, the service core for purpose house elevators, emergency stairways, and electrical and mechanical equipment is normally installed. The core’s walls were then used as extra rigidity for the construction, operating as a second tube within the exterior tube.
The rigidity of a hollow tube system is greatly improved by utilising the core to withstand lateral stresses as well as gravity loads. The floor construction connects the outside and inner tubes, allowing them to respond to lateral forces as a single unit. Wind has a similar effect on a tube in tube system as it does on a frame and shear wall structure. The framed external tube, on the other hand, is much stiffer than a rigid frame. The external tube resists the majority of the wind in the top half of the building, whilst the core carries the majority of the stresses in the bottom portion, as shown in Tube Behaviour in a Tube Tall Building Deform the frame’s shape.
Advantages of Tube in Tube Tall Building
The usual deflection, moments, and shears curve of a wall-frame structure are shown in . The deflection curve and the wall moment curve both show a reversal in curvature at a point of inflexion, above which the wall moment is in the opposite direction as in a free cantilever depicts the shear as being nearly consistent throughout the frame’s height, except near the base, where it decreases to a minimal amount. The frame is subjected to a considerable positive shear at the top, where the external shear is zero, which is balanced by an equal negative shear at the top of the wall, with a corresponding concentrated contact force operating between the frame and the wall.
The tube concept has a number of distinct advantages over alternative framing systems, not just in terms of cost and efficiency, but also in terms of structural considerations, such as: Because the wind-resisting system was installed on the building’s perimeter, the entire width of the structure was maximised to resist overturning moments.
Because the wind-resisting system is concentrated on the periphery, the internal framework may usually be designed solely for gravity loads. As a result, there is more flexibility in where columns and beams are placed within the core, and their size is reduced significantly. As a result, the core frame can be designed to best fit the core’s many non-structural requirements, resulting in a significant increase in efficiency.
Reaction of Vertical and Horizontal Component
Because the floor members are not susceptible to the variable internal forces caused by lateral loads, the tube system results in similar framing for all levels. The load distribution capacity of a framed tube with close column spacing and deep spandrels is considerable, resulting in virtually uniform column loading, allowing many columns in each tube wall to be similar.
From a practical standpoint, the lengthy process of addressing detail layout and service needs in the core area has no bearing on the tube’s final analysis and design. Some Important Factors in Tube Concepts Construction The tube concept alone does not provide frame rigidity sufficient to meet deflection and vibration limits. Fortunately, this is not the case in.
Unless the columns are located inside the building facade, major issues with temperature differences in the columns of the building may develop. The floor diaphragm becomes a critical component, not only for distributing wind forces to the tube’s side walls, but also for providing lateral support to all columns. Wind Load Reaction of Vertical and Horizontal Components The necessity to resist large lateral pressures due to wind or earthquake is one of the most distinctive characteristics of a tall building.
Design Concept
The wind load resisting system must accomplish this while also preventing excessive deflections or accelerations and aiding in stability. In general, a lateral system is thought to be efficient.
The use of big columns with deep spandrel beams clearly results in a surprisingly strong outer tube that interacts with the core to produce a substantially lesser deflection when subjected to wind loads. Then there’s the necessity for rigidity in very tall buildings to counteract lateral wind stresses.
The overall lateral load transfer approach is to gather all lateral loads acting on the facade and transfer them horizontally along planes through floor levels to the main lateral load resisting element in the building’s centre. Before being transported to the foundation, the load is moved vertically via the shortest way. The technique for overcoming the toppling effect is to rely on the core’s dead load.
Skyscrapers are created using a steel skeleton structure. Giant girder grids are formed by riveting metal beams.
To qualify as a true “skyscraper” a structure must be self-supporting and not require tension cables or supports in order to remain upright.
Outrigger systems functions by tying together two structural systems (core system and a perimeter system).
Also Check
http://Modern Methods of Construction -Details and Applications