Master of Engineering Science
Civil and Environmental Engineering
Bitsuamlak, Girma T.
As city develops there will be changes in the aerodynamic characteristics due to varying building surrounding and topology. For example, the city growth around a tall building can cause an increase in wind load due to venturi or wake effects or decrease due to sheltering effects depending on the type of surrounding. The densification of cities will also increase the turbulence level due to wake effects. In the present study, the implication of these changes on the aerodynamic forces on cladding connections is investigated. With increased fluctuations in the wind loads, the cladding connections will also experience stress fluctuations, which may result in fatigue problems in its lifetime. The growth of an urban area is represented by placing surrounding buildings with different height around the study building, and testing them in a boundary layer wind tunnel. Five surrounding conditions were investigated including isolated, surrounded with 1/4 H,1/2 H, 3/4 H, and H, where H is the height of the study building. The wind tunnel measured cladding pressure was then used to estimate the stress in the connections. Additional CFD simulations were carried out to explain aerodynamic mechanisms and results. Finally, fatigue damages were evaluated for different city development stages using two methods (i) direct wind time history method developed as part of this study and (ii) Weibull wind distribution methods adopted from literature. The highest fatigue loads are found to occur in the following order 1/4H,1/2 H, 3/4 H, and H.
Summary for Lay Audience
In the process of urban development, a building is subjected to change of surrounding conditions, which most of the time imply to the increase of surrounding structures’ height and density. As a result, wind load acting on the target building may also be affected. For instance, the wake of upstream structures can disturb the air flow coming into the building, and therefore it may experience an increase of wind pressure fluctuation. On the other hand, the surrounding group may act as a “shelter” to the target building such that the amount of wind load received by the building is relaxed. When wind force is distributed on the building’s surface, i.e. the cladding panels, connections underneath these panels will carry the load and transfer it to the main structural system. Therefore, it is not hard to imagine that an increase or decrease of wind load fluctuation will also be experienced by cladding connections, and this brings a structural problem: fatigue damage. Fatigue refers to the weakening of a material that is initiated by repetitions of load, and these repetitions will cause cracks and damages gradually, instead of a sudden failure when the material experiences an one-time critical load. This type of damages are often associated with metallic materials, and since the connections used to connect cladding panel and walls are made out of metal, fatigue damages can be seen on these components. As discussed earlier, the change of surrounding environment can make airflow more turbulent and introduce more load fluctuations, and this will lead to more stress repetitions in the connections, causing a concern on their fatigue life. As a result, it is important that real city growth can be simulated so that wind load received by target building, as well as stress inside the cladding connections, can be evaluated. After that, fatigue damage can be estimated using the connections’ material property. In this study, the growth of an urban area is represented by placing surrounding buildings with different height around the study building, and testing them in wind tunnel. Five surrounding conditions were investigated, including: isolated, surrounded with 1/4H,1/2H, 3/4 H, and H, where H is the height of the study building. The wind tunnel measured cladding pressure was then used to estimate the stress in the connections. Eventually, fatigue damages were evaluated for different city development stages, and it is found that the highest fatigue loads occur in the cases where 1/4H, and 1/2 H is surrounded.
You, Hang, "Effects of City Growth on Tall Building Cladding Fatigue" (2020). Electronic Thesis and Dissertation Repository. 7550.
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