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A building that is much higher than the surrounding structures or has a percentage narrow enough to give the impression of a towering structure is recognized as a high-rise.

Figure 1 high-rise buildings (India block , 2019)

Stabilization

Knowledge, technology, and building materials as well as the competition in building high-rises are continually developing over time. However, this involves many challenges and drawbacks. To begin with, vertical loads grow in proportion to the height of buildings. Moreover, the building is also exposed to a substantial horizontal wind-load impact.

The performance of the building under literal loading will represent an overhang fixed to the ground. If the wind has an equal distribution, the base moment grows nonlinear with its height. However, the real wind pressure increases with the height, resulting in an even bigger base moment.

One primary issue in high-rise structure designs is the capacity to absorb horizontal stresses and send the subsequent moment into the foundation. Linked load-bearing vertical walls are one great way of achieving this. However, this may result in maximum loads in the concrete walls on the overloaded side. To reduce this risk, approaches such as the use of self-weight of slabs are put on walls to generate compressive pressures.

The choice of cross-sections, materials, structures and the demand on operating becomes increasingly vital as the structure rises in height. Factors such as unexpected deflections and wind cause deflections and accelerations. It is important to handle these issues strategically. Moreover, unexpected deflections occur when the fundamentals are of low quality or when the foundation is unevenly owned to an inhomogeneous location.

Concrete high-rise buildings

Concrete structures are cast in situ, made of precast pieces, or a mix of both. As in a cast-in-situ building, the operations can begin on-site as soon as the foundation is laid. Scaffolding and molding preparation can start as soon as the contracts are handed over to the contractors. However, when using precast elements, this will not be feasible as decisions on forms and dimensions are made strategically before the operations on-site start.

It is vital to industrialize manufacturing operations when employing precast elements. It entails production in enclosed facilities, as well as the utilization of automated equipment and strategic planning. Cans-in-place and precast structure are two distinct aspects. As per cast-in-situ structures, the elements are manufactured in on-site molds and are regularly tested for altitude.  However, in the case of precast components, the elements are made in factories where the employees are completely reliant on the drawing.

Figure 2 Wind-load, moment and stiffness diagram for a high-rise building (HALLEBRAND & JAKOBSSON, 2016)

Connecting two or more elements

It is vital to consider the connections between elements when designing a precast building. The use of concrete or mortar defines the difference between wet and dry connections. Dry connections such as free supports welded connections and quick assembly of cold joins are less fire-resistant and more sensitive to tolerant standards and less ductile than wet connections such as mortar joints.

 Loads

Vertical loads from self-weight, imposed loads, horizontal loads and more, from both wind unplanned slopes are considered when planning a structure. Generally, the design load for a tall building is horizontal wind loading. Vertical loads, which include self-weights, finishing loads and live loads on the other hand are conveyed to the foundation through columns, towers, or load-bearing walls.

Horizontal wind loads act as a distributed load on the façade, transferring the loads on the slabs. These act as diaphragms, transferring sheer loads to the vertical beams while simultaneously acting as a stability unit for the compression site of the steel beam underneath. As per its in-plane stiffness, frictional force in diaphragms occur mainly in concrete. Horizontal loads are carried from the slabs to the beams through welded studs. Sheer zones in slabs may change depending on how they are attached to the walls.

The structures

There are many structural methods when a building is horizontal load stabilized. All these systems have developed from classically tightly joined structural frames. The key design principle for any of these structural systems is to position as much load-carrying material as possible around the high-rise building’s external border to increase flexural stiffness. Some of these systems are framed tube systems, bundled tube systems, tube in tube systems, diagonalized systems and more. Advantages are gained for all structural systems by situating the major vertical elements and reducing the applied load’s sheer force with compressive stresses from measured quantity. This task takes place to avoid net tension in the vertical members and uplifting of the foundation. However, some structural systems involve self-weight at the external vertical elements.

Figure 3 Different structural systems, where A represents a framed tube system, B represents a bundled tube system, C represents a tube in tube system, D represents a diagonalized system, E represents a core and outrigger system, and F represents a hybrid system (HALLEBRAND & JAKOBSSON, 2016)

Framed tube system  

The horizontal resistance in framed tube building is given by very rigid tension resisting frames that form a tube around the perimeter of the building. These frames are made up of columns with a length of 2-4 meters apart and they are joined by support beams. While the self-weight is balanced between the external tube and in internal walls, the horizontal load is carried entirely by tubes.

Bundled tube system

Four parallel stiff frames in each orthogonal direction are linked together to produce nine bundled tubes to make the bundled tube structure. However, the idea is the same as in the case of single tube construction, with frames in horizontal load direction acting as webs and parallel frames acting as outlets.

Tube-in-tube system

This system varies from the above-mentioned structural methods in that an external framed tube is connected to an internal tube at the core, frequently elevator shafts and stairs, to sustain both horizontal and vertical loads. This improves the horizontal stiffness.

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