Burj Dubai Wind and Other Studies
Architects Skidmore Owings & Merrill LLC (SOM) retained RWDI to perform wind studies for the design of the 700m+ supertall Burj Dubai in the United Arab Emirates (UAE). SOM selected RWDI for its Wind Engineering expertise, which has been applied to projects such as Taipei 101, Petronas Towers and many other supertall buildings.
Wind Loading and Building Motions
Winds can cause shear forces, overturning moments and torsional moments on high-rise and supertall buildings. Subjecting an instrumented scale model to winds in a boundary layer wind tunnel allows these forces, moments and movements to be measured and eventually translated into structural loads onto the building.
In addition, winds can cause buildings to move and thus create undesirable motions observable by occupants. Wind tunnel results allow the motions to be quantified and then compared with established comfort criteria.
Design Development
A 1:500 scale rigid model of the Burj Dubai.
“We virtually designed [the Tower] in a wind tunnel.” A quote from William F. Baker of Skidmore Owings & Merrill from a McGraw Hill Construction News article.
Using 1:500 scale models, the design of the Burj Dubai was optimized for wind through an iterative sequence of wind tunnel tests and design progressions such that the results from the wind tunnel tests were fed into each step of the design and vice versa.
For these developmental tests, a rigid model on a dynamic base balance was used. As a result of these studies, major reductions in the wind loads and building motions were achieved.
Definitive Wind Loads and Sway Effects
A 1:200 scale aeroelastic model with the construction crane present.
To establish the final design wind loads in the most definitive way, a 1:500 scale aeroelastic model of the tower was constructed and tested in the wind tunnel. Three different heights for the tower were evaluated using alternative aeroelastic spires. The model was also used to determine the building motions that occupants would experience in strong winds to ensure they were within an acceptable range.
An aeroelastic model is flexible in the same manner as the real building, with properly scaled stiffness, mass and damping. The aeroelastic interaction between the structure and wind is fully simulated, including such effects as aerodynamic damping.
Additional tests were undertaken on a 1:200 scale aeroelastic model of the 200m tall spire at the top of the tower to evaluate the response of this lightweight steel portion to wind actions such as vortex excitation. This was done also with the construction crane present so the wind forces on the crane could be evaluated.
Scale Effects
Lin Feng, Peter Irwin, Jiming Xie and William Yakymyk next to the 1:50 scale model in the NRC tunnel.
When testing small-scale modes in a wind tunnel, it is important to ensure the results are representative of the full-scale building.
For sharp edge buildings, the effects of model scale are very small, making extrapolation from the model to the full scale very simple. For curved surface buildings, so called "scale effects" can change the flow patterns around the structure making extrapolation to full scale more complex.
To assess scale effect, RWDI constructed and instrumented a 1:50 scale model of the top portion of the tower and subjected it to wind speeds up to 120 mph or 200 km/h in a 9m x 9m wind tunnel at Canada's National Research Council.
Movements of the Pinnacle
A 1:23 scale model of the Pinnacle with special "spoilers".
At the very top of the tower is a slender pinnacle in the form of a steel tube. To reduce the potential for wind-induced vibrations of the pinnacle due to vortex shedding, special "spoilers" were developed in the wind tunnel using a 1:23 Scale model.
Local Pressures on Exterior Surfaces
The RWDI cladding model of the Burj Dubai with a panel open displaying the pressure taps.
Wind applies positive pressures locally onto a building as it hits the curtain wall on the windward faces and applies negative pressures (suctions) as the wind passes around edges of a building (such as corners). The curtain wall needs to be designed for appropriate positive and negative pressures to ensure safety. The local wind pressures on surfaces are affected strongly by the building shape and can be sensitive to the influence of surrounding buildings.
Local wind loads on the cladding of Burj Dubai were evaluated through studying a 1:500 scale model instrumented with 1142 pressure taps. Two configurations were studied: one with future surrounding and one without surroundings. Three tower heights were tested.
Pedestrian Comfort
Wind Comfort
A close-up of the pavilion at the base of the Burj Dubai at a scale of 1:250.
Special wind tunnel studies were done to assess the wind environment in pedestrian areas for comfort. The model was also utilized to develop and assess the effectiveness of aerodynamic solutions such as landscaping, screens and trellises aimed at reducing wind speeds.
For most tall buildings, winds hitting the windward face are redirected around the building, upwards and downwards. Winds directed downwards tend to accelerate around the base of the building and can potentially create uncomfortable or unsafe conditions for pedestrians.
A 1:500 scale model study of the Burj Dubai was initially used to investigate wind speeds at the entrances, terraces above each setback, and amenity areas at the base of the tower. Subsequent 1:250 scale model studies examined ground level areas, lower-level terraces and higher-level terraces in more detail, and were used to develop detailed mitigation measures.
Thermal Comfort
The contributing factors of wind speed, solar radiation, relative humidity and air temperature were considered in a special Thermal Comfort study. Wind tunnel data obtained from the Wind Speed Comfort studies were used together with other data on solar radiation, shadows, air temperature and humidity to assess thermal comfort. Due to the high solar radiation levels in Dubai, a number of canopies and other types of shade structures were architecturally designed at ground level.
Wind Climate Studies
A wind field simulation of a 1988 Shamal windstorm.
A Comprehensive analysis of wind statistics in the UAE region was undertaken since the probability of various wind load conditions occurring is heavily dependent on these statistics. In addition to ground based anemometer data, balloon data were also analyzed. As well, special computer recreations of some of the large windstorms of the region were executed. The Regional Atmospheric Modelling approach allowed a better understanding to be gained of wind profiles in Shamal windstorms, as well as helping to define wind statistics for the upper levels of the tower.
See Technote 28, A New Role for Weather Simulations to Assist Wind Engineering Projects, for more information.
Stack Effect
This plot shows the impact of wind and stack effect on the internal / external pressure differences. Negative values indicate an inward flow and positive values indicate an outward flow.
Stack effect is a phenomenon that is present in all buildings that are maintained at a different temperature than the outdoor ambient conditions and tends to be most pronounced in very tall buildings. The driving force of stack effect is the difference in indoor-to-outdoor air density, or buoyancy.
In a very hot climate, the stack effect takes the form of the cooled air inside the building being denser than the outside. This causes it to try to slump downwards and out of any leakage points or openings in the lower portions of the building. It also tends to draw air in at the upper portions.
The stack effect can lead to the following:
- increased building energy costs due to uncontrolled airflow from outdoors
- whistling airflow through doors and cracks
- difficulty opening and closing of doors
- banging sounds within elevator doors
- difficulty controlling temperatures on some floors with excessive infiltration
Because of the Burj Dubai's height, the stack effect induced pressure differences are potentially much greater than in shorter buildings. Further, some typical stack effect control strategies used for buildings in cold climates do not work in hot climates.
RWDI built a network flow model of the major inside flow passages of Burj Dubai. This model was used to assist SOM in evaluating the impact of stack effect and mitigating potential adverse flow conditions. The results of the study were used to assess the benefits of door quality, ventilation controls and building operating procedures to moderate the stack effect impact.
See page 8 of the Fall 2007 issue of the CTBUH Journal for more information.
Tuned Sloshing Damper Conceptual Design
A rendering of the
Tuned Sloshing Damper.
At the outset of the Burj Dubai design, it was anticipated that some form of supplementary damping system might be needed to control the building sway motions. Conceptual designs for sloshing water and water column damper systems were developed by Motioneering (a sister company of RWDI).
As the design progressed, it was found that building sway motions could be kept to acceptable levels purely through shape refinements and structural measures. Accordingly, water dampers turned out not to be needed.
