Sensitivities and prerequisites of the application of the Energy Flux Analysis to high-rise structures excited by wind using in situ measurements

Abstract

As buildings get taller and lighter, structural engineers are increasingly faced with the consequences of the dynamic response of high-rise buildings to wind. Damping is an important property for the dynamic response of high-rise structures, but is a combination of many mechanisms, which makes it a complex phenomenon to account for in the structural design. Empirical damping predictors currently exist, but a large scatter is found among predictors, as well as between predictors and identified damping from measurements. Damping values are prescribed in codes, but are not consistently conservative. Therefore, there is a strong desire from both structural engineers and researchers to obtain further understanding of damping behaviour in high-rise structures.

Damping identification techniques, such as the Half-power Bandwidth method and the Random Decrement technique are commonly used. However, these are not applicable to buildings with closely spaced modes, and require extensive measurements. Besides, they only provide a damping value, and cannot find damping of separate components of a system. A novel technique, the Energy Flux Analysis, approaches damping from an energy point of view, making it more widely applicable, and allowing for damping identification in components of a structure. The Energy Flux Analysis has been verified to lab structures, but its performance when applied to a high-rise structures using in situ measurements is still unknown.

The aim of this research is to investigate the sensitivities of and prerequisites for the application of the Energy Flux Analysis to high-rise buildings excited by wind using spatially limited measurements. The sensitivities were sought for in the uncertainty of required input for the Energy Flux Analysis: structure motion, which includes internal forces, wind load, data acquisition, and structural properties. The research was performed through application of the Energy Flux Analysis to the New Orleans tower in Rotterdam.

While the sensitivity to structural properties and the magnitude of measurements is limited, the Energy Flux Analysis demonstrated to be highly sensitive to the phase of structural motion, internal forces, and wind load. The first two points are relevant for computing the energy flux at the boundary of a system, when one is interested in damping in the superstructure and due to soil-structure interaction separately. The last point is relevant when one is interested in the total or superstructure damping.

The phase differences occurring between structure motion and internal forces are a direct result of damping. Material damping resulted in a phase difference between stress and strain in the numerical model, while a local damper resulted in a phase difference between structure motion at different locations. The many damping mechanisms occurring in a high-rise structure may each affect the phase of structure motion and internal forces differently. When these phase differences are not taken into account in the Energy Flux Analysis, for instance due to extrapolation of measurements, an erroneous result will be obtained. A brief investigation was performed as to whether these effects can be expected in true structures, but additional research is required.

The fluctuating wind load at the natural frequency of the structure is dominant for the flux of energy from wind to the structure, which is obtained by multiplication of the wind load with the structure velocity. Again, the phase of this wind load is highly important. When measured at one location, little is known about the phase of the wind load at other heights. Different approaches of extrapolating the measured wind load demonstrated a large scatter in the Energy Flux Analysis results. In this research, the Energy Flux Analysis found not to be repeatable, which was proven to be a direct result of the phase difference between the measured wind load and structure velocity. Possible causes for this varying phase difference were formulated.

It is essential, but due to the major advantages of the Energy Flux Analysis also profitable, to perform further research into its application to high-rise structures. Therefore, this study provides extensive recommendations mostly focused on simple numerical and lab experiments.