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One of the most important forces that civil engineers, architects and urban planners need to take into account in the building design process is wind. For high-rise buildings, the risk of being affected by the wind is particularly critical. If the location deals with high wind speeds—as is the case for coastal cities and other regions—conducting wind engineering studies is important even for urban areas with smaller structures due to complex interactions. Such studies evaluate wind loads and pedestrian wind comfort and are standard in most construction projects. They can predict potential negative effects and can help optimize designs to mitigate these effects early in the development process.

This article focuses on wind loads and how professionals working in the architecture, engineering, and construction (AEC) industry can benefit from simulation to analyze the loads and safety of the built structures.



In most wind engineering studies, there are two main areas of focus:

1. Identifying pressure loads on the structure and facade. This mainly involves steady state simulation to recognize areas of high or low peak pressures that would experience higher wind forces. For simple building designs, basic code methodology is often sufficient for deriving pressure loads, whereas for more complex shapes and dense urban environments, numerical analysis (computational fluid dynamics or CFD) and wind tunnel testing are required.

2. Determining and mitigating the dynamic effects of the wind load. For high-rise slender buildings, unsteady vortex shedding can induce oscillating crosswind forces with a certain frequency. If these oscillations coincide with the natural frequency of the structure, the motion could be enhanced leading to damage or even failure of the structure.

Conducting wind tunnels tests is standard in wind engineering studies but an optimized design process includes numerical analysis with CFD as a complement for an iterative design process. Using CFD simulation, the wind tunnel testing stage is reached once a refined building model is created, with many of the risks already mitigated. This saves time and significant costs.

CFD provides a numerical approach to model a virtual wind tunnel to evaluate pressure loads and dynamic wind loads in a fast and efficient way. Simulation provides both 3D visual contouring and quantitative data for wind pressure, wind forces, and wind speeds. Areas of complex recirculating flow and vortex generation regions can be identified and improved. In-depth results of mean and peak wind conditions can be analyzed and advanced atmospheric boundary layers profiles can be modeled.

The following projects were created with an online simulation platform, focusing on analysis of wind load on two world-renowned buildings, landmarks in both the construction and the wind engineering fields.

Wind Load Analysis: Burj Khalifa, Tallest Building on Earth

The world’s tallest building, Burj Khalifa, was expected to have strong wind loading and wind-induced motions. In fact, the wind engineering team hired for the project prepared for the building to be able to withstand wind gusts of up to 149 miles per hour. At the same time, vortex shedding was a serious risk that needed to be accounted for, due to the structure being so tall (higher than 2,716 feet) and slender.

Based on the results of wind tunnel testing, several design improvements for optimizing the building’s aerodynamics were made, including softening the building's corners, gradually reducing the width of the skyscraper at higher levels, and changing the orientation of the building sections to disturb the wind in our favor.

The result is a stable structure that is renowned not only for its height but also for it being optimized for wind effects.

Confirming Burj Khalifa’s capacity to withstand wind forces, the following numerical simulation results from SimScale give a better overview of this civil engineering and architecture masterpiece.

Video 1: Burj Khalifa

CFD analysis showing slice sections of Burj Khalifa (Source: SimScale)

The animation above shows the wind speeds around the structure with a maximum peak speed of more than 60 miles per hour. The freestream wind is invisible and the animation shows the accelerated (in red) and decelerated areas (in green and blue). The multiple strategies used to optimize the skyscraper—including tapering, corner softening and asymmetric cross-section variation—are very effective in taming the wind. These strategies prevent coherent wind effects and divide them into multiple different non-coherent parts of different frequencies and amplitudes along the building’s height.

Video 2: Burj Khalifa
CFD analysis of Burj Khalifa showing wind speed contours (Source: SimScale)

The main effects of the structure’s optimization efforts can be visualized in the animation above, which illustrate the wind flow physics at three different cross-sections.

Close to the base, the cross-sectional area of the tri-branch is the largest and so is the wake behind the structure, but the corner softening helps to avoid strong effects in different directions.

At mid height, the cross-sectional area not only decreases but also changes between a symmetric and a non-symmetric form. In this animation, a non-symmetric section that shows non-coherent vortex shedding is highlighted. This helps to spread out the frequencies of the cross-wind forces along the height and avoid a single dominant oscillation.

The top of the building is experiencing the highest wind speeds, but as the cross section area is rather small (due to tapering), the wind forces are small as well.

The simulation results have endorsed Burj Khalifa’s reputation to be well-deserved, being optimized with regards to wind loading and effectively taming the wind.

 

Wind Load Analysis: New York’s 432 Park Avenue

At the time of its completion in 2015, 432 Park Avenue was the tallest residential building in the world. As of November 2019, it occupies the third place in the same category, at 1,396 feet high.

Located in a densely built area, on Billionaires' Row in New York, the super skinny skyscraper uses a unique strategy to reduce wind loads; at regular intervals along its height, floors are left completely empty, to allow wind to pass through.

Video 3: 432 Park Avenue

CFD analysis of 432 Park Avenue showing wind speed contours (Source: SimScale)

In this CFD animation, an entire city block is simulated to investigate not only the main structure but also the shielding or shadowing effect from other buildings. The freestream wind is invisible and the simulation shows the accelerated (in red) and decelerated regions (in green and blue). In this case, the wind direction is from the North-East (parallel to Park Avenue).

It can be observed that the first few floors are shielded from the building in front (Four Seasons Hotel tower). For the upper floors, the wind is allowed to flow through the empty floor sections, which helps to reduce the strong wind load and mitigate the vortices. While this strategy is efficient, additional solutions like structure damping would still be employed to reduce crosswind oscillations.

Video 4: 432 Park Avenue

 

 

CFD analysis results showing a slice section of 432 Park Avenue (Source: SimScale)

The animation of the cross-section through the empty floor is shown to understand in detail the physics. The passage allows the airflow to move through along the wind direction and in the cross directions. This not only reduces the crossflow vortices (compared to a bluff body) but also the wake. The flow velocities in the wake are no longer very low, and thus the resulting pressure difference is not as strong. These all add up to significant wind load reduction and mitigating the larger, low frequency vortices that are considered more critical to the structure.

 

Conclusion

In this new age, buildings are not only getting taller to save space, but also becoming complex icons of society that define art and culture. As architects and engineers are building intricate and challenging designs, using innovative simulation solutions for analyzing wind effects is essential.

High fidelity engineering simulation, and computational fluid dynamics in particular, can assist in evaluating and optimizing designs early in the development process, as a complement to legacy wind tunnel testing, for ensuring safety and sustainability of the structure and the environment around it.