4.5] Design of high rise buildings

4.5 Design of High rise buildings

In recent decades, high-rise buildings have become increasingly popular. High-rise buildings or tall buildings are complex buildings and your design choices are affected by many factors. Poorly considered initial design choices can lead to poor and wasteful planning, making it necessary to fully consider and understand all aspects at the initial stage of conceptual design. Due to immigrants from rural areas leading to expanding demand for urban housing, rising land costs, etc., Indian cities are experiencing huge population expansion. Industry, commerce and commercial activities, as well as some educational centres in the city, attract migrants. This allowed the city to expand in all directions, which in turn increased the complexity of urban areas. Urban development can be divided into four categories:

1] High Rise High Density, 2] High Rise Low Density, 3] Low Rise High Density, 4] Low Rise Low Density.

Factors affecting the design of high-rise buildings vary from place to place, such as local climate, zoning regulations, cultural conditions, technological opportunities etc. Generally the more simple and regular the floor shape is, the easier it is to respond to user requirements in terms of space planning and furnishing. Square, circular, hexagonal, octagonal and similar plan forms are more space efficient than the rectangular plans with high aspect ratios and irregular shapes. Buildings with symmetrical plan shapes are also less susceptible to wind and seismic loads.

Figure 13) – Plans of High-rise buildings Source -

https://www.cpwd.gov.in/Publication/Compendium_of_High_Rise_Buildings_December_2019 .pdf

Planning of service cores in high rise office buildings – Service cores are an increasingly important aspect of building design, architecturally especially in High Rise buildings. In high tech building the size of service cores tends to increase thereby affecting the building’s net to gross area ratio. Simply stated, a service core is defined as those parts of a building that consist of elevators, the elevator shafts, the elevator lobby, staircases, toilets, E&M services, riser ducts. Its structure can also contribute to the stability of the building.

Figure 14) – Smart core design

Source -

https://www.cpwd.gov.in/Publication/Compendium_of_High_Rise_Buildings_December_2019 .pdf

Service cores typically contain the following elements –

1] Elevator shafts 2] Elevator lobby 3] Staircases – both main & fire escape

4] Toilets 5] Ancillary rooms such as pantry

6] Mechanical vertical service riser – ducts e.g. for electrical power & lighting, water, sewerage pipes, rainwater down take pipes, fire fighting, exhaust ducts etc.

7] Mechanical vertical fire protection risers for sprinklers, hose reels, wet & dry risers.

Based on the location on the floor plate the cores can be classified as under –1] Central core 2] Split core 3] End core 4] Atrium core

Figure 15) – Core design ideas

Figure 16) – Core design ideas

Source - https://www.cpwd.gov.in/Publication/Compendium_of_High_Rise_Buildings_December_2019.pdf

Leasing Depth or lease span is the distance of the usable area between the exterior wall and the fixed interior element, such as the core or the multi-tenant corridor. Smaller core-to-exterior window dimensions allow the users to maintain a relationship with the outside, thus benefitting from the natural light. According to Ali and Armstrong (1995) the depth of lease span must be between 10 and 14m for office functions, except where very large single tenant groups are to be accommodated.

Figure 17) – Leasing depth diagram

Source - https://www.cpwd.gov.in/Publication/Compendium_of_High_Rise_Buildings_December_2019.pdf

The Floor to floor height of a building is a function of the required ceiling height, the depth of the raised floor (if used), and the depth of the structural floor system and material and the depth of the space required for mechanical distribution. The floor-to-floor heights range between 3.73m and 4.20m with an average of 3.98m.

Depending on the distance between the farthest point of the floor and the stairs, there must be at least two escape stairs in the building, so that in the event of a fire, one of the stairs cannot be used and the occupants can use the other staircase. Stairs must be fireproof or fire compartments must be related to other parts of the building and be used as a safe space. As a necessary way of exit, location is usually one of the decisive factors for any large building. If any, the escape stairs are separate from the ceremonial internal stairs. For high-rise buildings, elevators are not regarded as "legal exits" in the event of a fire emergency. Normally, firefighters will immediately take all elevators to the first floor and use the designated fire-fighter elevators to extinguish the fire. Building stairs is a key part of yourlife safety system. Therefore, their number and location are crucial in the design.

The toilets are very often located within the service core especially in high rise buildings for ease of plumbing and accessibility to the vertical risers. The extent and number of male, female, executive and disable friendly facilities are calculated, following local codes and occupancy load.

Shafts are very important from ventilation point of view as well as maintenance point of view. Minimum size of shaft, placement of pipes, access/entry to shaft for maintenance needs to be taken into account.

Wind Loads

Tall building structural systems must handle vertical gravity loads, but lateral stresses, such as those caused by wind and earthquakes, must also be considered. As theimpact of ground friction reduces, wind forces rise with building height to a constant or gradient value. Wind forces have two effects on tall structures. A tall structure may be conceived of as a cantilever beam with one fixed end at the ground that bends with the highest deflection at the top due to wind pressure. Furthermore, the wind flow through the structure creates vortices around the corners on the leeward side; these vortices are unstable, and they break away downwind every minute or so, switching from one side to the other.

As a result, a high-rise building must fulfil numerous performance criteria when subjected to wind forces. The first is stability—the building must not topple over; the second is that the deflection, or sideway at the top, must not exceed a maximum value (usually 1/500 of the height) to avoid damage to brittle building elements like partitions; and the third is that the swaying motion caused by vortex shedding must not be readily perceptible to the building occupants in the form of acceleration.

Earthquake forces

Unlike wind forces, earthquakes or seismic forces are restricted to very small regions, usually at the margins of the Earth's crust's slowly moving continental plates. When the boundaries of these plates shift abruptly, the energy produced propagates waves through the crust; the Earth's wave motion is transmitted to buildings sitting on it. Masonry structures are heavy and brittle, and are prone to significant damage. Timber frame buildings are light and flexible, and are typically minimally affected by earthquakes. Continuous steel or reinforced concrete frames fall in the middle of the seismic reaction spectrum, and thus may be constructed to withstand very minimal damage.