Figure 1. Top 10 tallest buildings in the world, measured
by architectural height (Image CTBUH).

Buildings are becoming taller. According to the Council on Tall Buildings and Urban Habitat, a tall building is one up to 300 meters high. A supertall building is over 300 meters and a megatall building is more than 600 meters. For example, the Burj Khalifa in Dubai stands at 828 meters. There are currently 173 supertall buildings and only three megatalls globally. Figure 1 illustrates the relative height of the top 10 tallest buildings. A list of the top 100 can be found here.

Unique Challenges

When designing tall buildings, special consideration must be given to the wind-induced aerodynamic, structural and harmonic properties of the building. Very tall buildings are reaching scales where even traditional types of analysis, such as wind tunnel testing, is becoming difficult. A wind tunnel will typically try to replicate a real wind speed and turbulence profile that varies with height and surface properties including roughness, obstacles such as neighboring buildings and trees, and atmospheric conditions.

The higher a building reaches in the sky, the harder it is becoming to create an analog representation of the complex wind, pressure, turbulence and air properties using a wind tunnel approach. Using a scale model in a room simply does not allow designers to capture enough of the physics. Computational Fluid Dynamics is the mathematical modeling of the behavior of fluids using software. CFD allows a designer to create a digital model of a building and simulate how it will respond against atmospheric and environmental variables such as wind speed and direction. A user can run multiple simulations of the design, testing against various wind speeds, atmospheric conditions such as gusts and storms, and import CAD models of neighboring buildings to evaluate how an existing site will be impacted by a proposed development.

Tall buildings might be disproportionately impacted by severe weather. Due to their very slender design, they tend to be sensitive to small changes in atmospheric conditions, such as wind pressure. Designing tall buildings is a trade-off between its function, serviceability, occupant wellbeing, structural integrity and cost. Novel technologies have been developed to enable tall buildings to withstand severe wind forces, wind-induced motion and facade degradation. These include aerodynamically sculpting or shaping the design of the building, adding damping devices, structural columns, braces and various types of trusses. Complaints about strong gusts outside the 20 Fenchurch Street Building in London (Figure 2), better known as the Walkie Talkie, have been common and contributed to several independent studies commissioned by the City of London to better predict microclimate impacts of tall buildings. This eventually led to the new City of London Wind Microclimate Guidelines, the first of their kind in the world. Since then, many other cities and municipalities have, or are, developing their own guidelines.

Figure 2. CFD simulation of the Walkie Talkie
building (London), showing the wind-induced pressure
from a westerly wind. High pressures
are shown in red.

A more extreme example was when Hurricane Ike hit Houston, Tex. in 2008. Localized downdraft flows were generated due to the interference effects between two adjacent structures, causing extensive damage to cladding and facades. This was found to be a consequence of urban aerodynamics that added significant wind loads due to a unique combination of wind orientation and the building’s layout that caused these adverse effects. These were not anticipated in the design of these structures as there was no way to correctly model this behavior.

Simulating a Tall Building

Figure 3. The downdraft effect:
wind hitting a building head-on
has no place to go except up or
down. The latter causes severe
discomfort and high winds as
it reaches the ground and
spreads at street level.
(Image: BBC Thinkstock)

A common cause of the windy conditions around the Walkie Talkie and other tall buildings is called the downdraft effect, when wind hitting a tall building is directed downwards to street level, as illustrated in Figure 3. Other common aerodynamic effects include channeling and corner acceleration. Channeling is when wind passes through two buildings with a relatively narrow distance between them and accelerates, causing very high wind speeds compared to its surroundings. The corner effect is a result of flow separation around sharp building corners, which again, causes a part of the air stream to accelerate. All three effects are very common in city centers, especially where tall buildings dominate the urban space. These buildings tend to be concentrated in specific areas of a city, usually where high prestige office space is reserved for larger companies. In older cities, where the street layout is much denser, this high building density further compounds the problem.

Figure 4. CFD simulation of a tall
building (plan view). Wind enters from
the left and, downstream of the building (right).
Note the flow separation around the
corners (corner acceleration) and a long
turbulent wake region.

A CFD simulation of a tall building can yield exceptional results. Figures 4 and 5 illustrate high fidelity engineering simulation capabilities for modeling a tall building and the complex aerodynamic phenomenon it is subjected to. In both figures, the wind is flowing from left to right. Figure 4 depicts a plan view of the flow behavior. Visible is the flow separation and corner acceleration around the corners of the building. This leads to high wind speeds at the corners and, at a pedestrian level, can cause discomfort and danger. At this stage, various mitigation strategies might be employed to avoid this, such as windscreens, planting trees or even changing the shape of the building to have smoother corners.

Figure 5. CFD simulation of a tall
building (side view). Wind enters
from the left and, downstream of the
building (right). Note the flow causing
a long turbulent wake region.
In reality, this wake will cause high
wind speeds and discomfort for other
buildings and pedestrians downstream.
Only CFD can quantify this
phenomenon.

The side view in Figure 5 illustrates the high levels of turbulence downstream of the building. In practice, this wake region might extend to several times the height of the building, causing wind-related issues for many other buildings and pedestrians. A city center location would give rise to multiple instances of this wind-induced turbulence, with each building generating a downstream disturbance. The result is a complex and overlapping turbulent microclimate that is hard to predict and thus, manage. The number of super tall buildings and higher, is growing. Development plans show an increasing number of ambitious building designs in all the major cities globally. Current thinking on very tall buildings will need more advances in testing designs to withstand the atmospheric forces as our buildings reach the 1 kilometer (.62 mile) height mark. Future designers are already thinking about adaptive buildings that change shape to counter extreme forces.