The drive towards the construction of tall or high-rise buildings stems from the desire to maximize returns from efficient of use lands. Thus, by building tall or adding extra floors on any giving footprint, it is possible to substantially increase the number of people that can be accommodated in offices, residential apartments and other settings.
The definition of a high rise building has been a major debate amongst several building regulatory councils, each of them giving a distinct definition. In this post, a highrise building is understood as any building whose geometry impacts significantly on the structural design aspects of the building.
One definition is if the building aspect ratio, height divided by the lowest overall lateral dimension, is more than 5:1, then the building may be considered tall (concrete centre, 2014).
The design of a tall building consists of all the basic procedures observed in the design of low and medium-rise buildings. However, there are some additional issues that designers must consider.
Stability and Dynamics
Choosing the structural system is central to the design of a high-rise building and must be taken into account at the very outset. The effect of lateral loading is one of the major influences in the design of tall buildings and the biggest distinction in the design of low-rise buildings.
It is important to understand in depth the dynamic performance of tall buildings. Loading from wind and seismic behaviour takes place over a wide range of frequencies, and the building’s response would be determined by its natural frequency and the degree of intrinsic damping. Where the building’s natural frequencies are similar to the frequencies of the loads applied, there is a possibility that the reaction may be amplified, resulting in increased loads and motion. This function involves thorough analysis by the structural engineer to examine the efficiency of the system through the entire frequency range of the loads applied. If accelerations associated with any movement are severe, building users could experience motion sickness
Specific regions of the world are prone to earthquakes and seismic activities. Thus the response of high-rise buildings during such an event must be considered during design.
Consideration of the essential proportions of the system is recommended at the initial planning stage. The slenderness ratio will provide a clear initial indicator of how hard the chosen structural system would have to work.
The slenderness ratio is calculated by dividing the overall height of the building (h) by the smaller base width (b). The structural system will typically comfortably handle the lateral loads at a slenderness ratio of 5 or less, while the structural system must operate harder with 8 or above and the dynamic behaviour is likely to prevail in the structural solution.
At the beginning of the construction of the tall building, the structural designer must consider how the principal structural elements are to be arranged. The vertical elements are typically arranged to give clear floor spans. The mechanism by which the lateral loads can be resisted and passed to the ground is a crucial factor. The possible structural system are going to be described in the next post.
The structural core of the building, the area that holds the lifts, usually comprises much or more of the lateral strength and stability of the structure. The layout and design of the core is perhaps the most critical factor of the final design and the total performance and effectiveness of the structure. The layout of the core is primarily dictated by the number of lifts needed, which, in effect, is dictated by the use of the building and the vertical transport strategy. Residential users usually have a much lower need for lifts relative to more densely populated buildings, such as schools.
Tall buildings will move laterally by significant amounts, typically in excess of half a metre. This lateral displacement or ‘drift’ must be calculated and may need to be limited. Excessive lateral displacement could potentially affect finishes, internal partitions and external cladding, particularly if the inter-storey drift (lateral displacement over one storey) is too high. Thus excessive movement in tall buildings must be given due considerations.
High rise buildings exert tremendous forces on the supporting ground and hence it is important that the prevailing ground conditions are thoroughly investigated.
A tall building’s impact will penetrate vast heights beyond the immediate footprints, and affect the earth. Therefore, Investigations must extend to depth and plan that captures the full effect. It is important to conduct an analysis of the foundations for any nearby structures and the impact on the surrounding facilities, both above and below ground. Generally, the site investigations for a high-rise building must be carried out by a geotechnical specialist.
Finally, the speed of construction is often paramount to the viability of tall buildings. Designers must work closely with the contractors to ensure that the building can be constructed efficiently.
An effective collaboration between parties, will result in a design that have:
- Typical floor layouts
- Core arrangements that are suitable for slip- or jump-forming
- Slab structures that can be made off-site or cast and struck easily and that can bear the weight of the floors below.
The Concrete Centre Publication (2014)-Tall Buildings-Structural Design of Tall Buildings up to 300m -A cement and concrete industry publication.
Feng F (2018)– Design and Analysis of Tall and Complex Structures (1Ed)- Elsevier Ltd.
The Concrete Centre Publication (2018)-Guidance on the design and construction of building tall in concrete – Concrete Tall Buildings