Energy flux method for identification of damping in high-rise buildings subject to wind

Research output: ThesisDissertation (TU Delft)

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Abstract

Buildings are becoming taller, lighter, slenderer. These changing characteristics make tall buildings more sensitive to environmental loads, including wind gusts. A building is considered "tall" when its height and slenderness influence the design. Given the demand of improving building performance, the serviceability limit state (SLS) has become the most important design criterion of tall buildings. The structural serviceability is directly related to the building motions generated by wind gusts. These motions can influence the well-being of the building occupants. Whereas the human perception of movement is related to the jerk sensation, acceleration is the widely accepted parameter for measuring comfort level. In literature, a few well-established criteria for determining human perception to building vibrations can be found. In this work, the van Koten criteria are used to study human perception of building vibrations, using data collected from full-scale measurements of several high-rise buildings in The Netherlands. Whereas results clearly show that acceleration levels are barely perceptible, people still often feel insecure in the interior of high-rise buildings, meaning that human perception is extremely subjective.

Dynamic systems are governed by their mass, damping, and stiffness. Damping can be understood as the energy dissipation in a system. Therefore, it determines the maximum acceleration that can be felt. Given its physical complexity, damping is the most uncertain parameter to be predicted. Presently, there are several damping predictors to determine damping in high-rise buildings. The resultant damping obtained by means of damping predictors is the result of the contribution of two main energy dissipation sources: the soil foundation interaction and the internal damping in the structure. Using these predictors, damping related to soil-foundation is a constant value, whereas structural damping increases with respect to the amplitude of vibration. Unfortunately, the use of these predictors result in large scatter compared to the experimentally identified damping values of buildings located in The Netherlands. Given that the parameters of these predictors are tuned based on full-scale experimental values, the discrepancy between experimentally identified damping of the buildings and the resultant values obtained by means of damping predictors is not easy to explain. In this work, a predictor based on the same principles, and tuned to fit the data collected from the full-scale measurements is presented and applied. Unfortunately, this predictor does not give enough insight to understand the behaviour of the dissipation mechanisms in a tall building.

It is therefore the aim of this work to develop a tool for better assessing the energy dissipation in high-rise buildings to improve damping prediction. In a tall building, there are three types of energy dissipation (i.e the structural energy dissipation; soil energy dissipation and energy dissipation caused by the wind around the building). In this work, the aerodynamic damping caused by the wind around a building is considered negligible. To get a better overall damping prediction, an attempt to identify the contribution of the different damping sources to the overall damping is carried out. However, given the fact that wind loads cannot excite higher frequency modes in a tall building, the energy dissipation of specific areas of the structure cannot be adequately identified by using modal based techniques. Therefore, a different approach is needed to identify the energy dissipated in local areas without a modal description of the structure. In this work, the energy-flux analysis is proposed as a damping identification tool. This approach isolates a certain area of the structure to formulate an energy balance around it. The connection between this local area and the rest of the structure is made via the energy flux, which accounts for the energy coming in and going out of the local area. By doing this analysis, the energy dissipation of a local area can be identified. In Chapters 4 and 5, an energy-flux analysis is used to identify the energy dissipation in local areas of the structure. Then, a damping operator can be quantified. Another advantage of this approach is the added possibility of studying the behaviours of different damping operators by computing their energy dissipation. To validate the method two lab-scale structures, a lab-scaled beam, a lab-scaled steel-frame building and a full-scale high-rise building are used. This is done in the following manner. First, the structures are instrumented using accelerometers in the case of the lab-scale beam and accelerometers and strain gauges in the case of the lab-scale steel frame and high-rise building. Then, equivalent viscous damping is experimentally identified by means of the collected data. Second, a model representative of the structure to be analysed is developed. The model is made with continuous and discrete structural elements (e.g. beams, springs, dashpots). These models are used in order to interpret energy change, energy flux and dissipation energy. The energy balance can be formulated around a specific area of the model. Then, by making use of experimental data, the energy enclosed in this specific area can be computed, and energy dissipation can be identified. To compare percentages of critical damping, the energy dissipation is formulated in terms of a damping operator. This operator can be used to compute equivalent viscous damping, which makes use of the energy-flux analysis by comparing it to the experimentally identified equivalent damping values. Based on the results presented in this work, it is proven that this approach is a consistent framework for damping identification.

In Chapter 6, a basic model for tall-building damping assessment during the design phase is presented. The model combines different models. The cone model describes the soil-foundation interaction and a Euler-Bernoulli beam model represents the building. Assuming a small vibration field, the mechanism responsible for the energy dissipation in the building is presumed to be directly related to the building's deformation. Therefore, the influence of building damping is studied based on the bending of the beam model used to describe the building. This influence varies with the change in the building deformation caused by different foundation stiffnesses. Likewise, the influence of soil-building interaction damping varies when changing the soil-foundation stiffness. Results provide evidence that the soil-foundation interaction of tall buildings may play an important role in the overall damping identification for certain soil characteristics, like the ones present in The Netherlands.
Original languageEnglish
Awarding Institution
  • Delft University of Technology
Supervisors/Advisors
  • Metrikine, A., Supervisor
Award date21 Mar 2019
DOIs
Publication statusPublished - 2019

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