The principal driver towards the construction of ‘tall’ structures stems from the desire to maximize returns from efficient use of land and a recognition of the growing population problems. This is the 21st century whereby most cities in the developed world’s appear to be running out of ‘usable land’. By adding extra floors or by simply building tall on any given footprint, it becomes possible to substantially increase the number of people that can be accommodated in offices, residential apartments and other dwellings. In fact, this seeming necessity is not the only motivation for tall buildings. Constructing tall buildings confers an identity on cities, they modernize the city skyline and improve the value of their landed properties. Also cities with skyscrapers tend to become a natural tourist destination.
The development of modern construction technology, and computer methods of analysis, has also aided the design and construction of high-rise buildings. The demand from the global construction market today requires that a civil/structural engineer has the skillset required to design and analyze these very challenging structures. To this end, this article is set out to describe the fundamentals of tall buildings and how its design differs from those of medium and low-rise buildings.
What is a Tall Building?
There is no consensus on a universal definition for tall buildings, even in design and practice, the definition of what should be classified as high-rise, medium and low-rise is ambiguous. For example, in Nigeria, buildings less than 5 stories are generally accepted as low-rise buildings, while those between 5-15 stories classified as medium rise building and those greater than 15 regarded to be high-rise buildings. However, in a city like Hong Kong, which is arguably the world’s center of skyscrapers, a 20-storey building would never be regarded as a tall building. Hence the classification of tall buildings is subject to the environment under consideration.
However, from the structural engineer’s perspective, the first consideration in the design of almost all high-rise buildings is the lateral stability system. If the significance of the lateral loads such as wind and earthquake is so huge such that it becomes the critical design consideration, it would radically impact on the lateral stability system. Thus, we can classify a building as high-rise, when its lateral stability system is the first consideration in its analysis and design. One definition that readily satisfy this requirement is that, any buildings whose aspect ratio is more than 5:1 should be classified as a tall building.
A Tall building is classified as any building with an height to width ratio more than 5:1Concrete Centre, 2014
Component of High-rise Building
The floor design of high-rise building is almost not different from those of low-rise buildings, similar flooring systems are used – solid slabs, ribbed and waffle slabs, flat slabs post-tensioned slabs etc.
However, for high-rise buildings certain floor types are favored than others. Firstly, the weight of the floor needs to be minimized, due to the number of stories, providing light weight floors will result in a more economical foundation. Secondly, the floor design needs to consider the fire resistance, especially for composite floor system; thirdly, the floor should also resist the load during the construction; finally, due to the large column spacing inside most tall buildings, the slabs also need to have long distance span.
Vertical Support System
The floor system is supported on vertical load carrying elements such as columns, walls, hangers, they are intended to perform the primary function of transferring the gravity load safely to the foundations. The vertical load support system can be done in structural steel, reinforced concrete or even composites. An important vertical support feature of many high-rise buildings is the transfer structures. Transfer structures are required where vertical load carrying elements can not be taken directly to the foundations due to geometry constraints. Most high-rise transfer structures are either in the form of a truss or a beam.
Lateral Stability System
Of utmost importance and a key feature of almost all tall buildings is the lateral stability structural system. The lateral stability system consists of a series of vertical and horizontal elements (diaphragms) used to resist the lateral loads. In a typical tall building, the flooring system described above also form part of the lateral stability system. The first point of contact of all lateral load is the cladding, the cladding elements then passes same to the floor system which delivers the loads to the foundations through the vertical elements.
Various structural options available depending on the magnitude of the lateral forces to be designed against. This can be sub-divided into four main categories and is well covered in a previous article, please see: Stability System in High-rise Buildings.
Actions on Tall Buildings
The actions on tall buildings are well defined in most structural codes of practice. This can be classified into five distinct categories according to EN 1991-1-1.
Permanent Actions are loads that permanently stay in a building. This includes the self-weights of the flooring system and other structural members such as beams, columns, and core walls. It also includes the self-weights of any objects that are attached to the structure such as finishes, ceilings, insulation materials, and partitions.
Live Variable Action
Live variable actions are loads that varies in magnitude and location with time. An example of live loads can be people, furniture, and equipment and general loads caused by occupancy. Depending on the function of the building, either commercial, residential or even industrial, the value of the live load varies. These loadings are generally tabulated in design codes. EN 1990 gives the value of the live loads for different use of building and with different occupancy.
Snow Variable Action
For buildings located in places susceptible to snow, the magnitude of the snow load on the building must be calculated. Snow load is actually a variable action i.e., it also varies in magnitude and location with time. However due to its unique feature, it becomes an independent load category in most of the design codes. In design code such as BS EN 1991-3 snow loads are determined from zone map reporting 50-year recurrence interval.
Wind Variable Action
Effects of wind on buildings depend on density and flow of air, angle of incidence, shape and stiffness of the structure, and roughness of surface. Its load value also depends on roof geometry, wind exposure, location, and its importance. In design, wind loadings can be treated as static or dynamic, approach will depends on the height of the buildings. EN 1991-1-4 gives guidance on how to determine natural wind actions for the structural design of building. However, EN1991-1-4 is only applicable to buildings with maximum height of 200 m. For building over 200 m, dynamic approach would become prevalent, and wind tunnel test is required for most of the projects.
Where a building is to be sited in a zone susceptible to seismic events, earthquake loading must be applied on the building. This is well detailed in Eurocode 8.
The design of a tall building consists of all the basic procedures involved in the design of low and medium-rise buildings. However, there are some additional issues that designers must consider. These are explained below
Stability and Dynamics
The main design issue of all tall buildings is how to resist the lateral loads and safely deliver them to the foundation. The target is to provide sufficient stiffness to tall buildings in other to be able to resist the lateral loading . Thus choosing the lateral structural system is central to the design of a high-rise building and must be taken into account at the very outset of the project development stage. The effect of lateral loading is a major influence in the design of tall buildings, this is the major factor distinguishing a high-rise building design from a medium/low-rise building.
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 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 offices.
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. Many innovative methods are now being used in the construction of tall buildings. One of them is the ‘Top down construction‘ employed in the construction of the Shard-the tallest building in western Europe. A top down construction is a method that allows the construction of the superstructure, excavation and basements to be carried out simultaneously (See figure 1).
Generally, 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