Wind effects on a tall building with recessed cavities

Abstract

In Hong Kong and other Asian metropolitan cites, high-rise residential buildings become very common. They are highly wind-sensitive, subjected to enormous wind loads at high wind speeds and may exhibit aeroelastic behavior at normal wind speeds. Many residential buildings often have an irregular shape with apartments arranged as wing sections extending from a central core so that all apartments on the floor can enjoy views. Between adjacent building wings are deeply recessed cavities. Most wind loading codes and guidelines assume stagnant flow inside the recessed cavities and wind loads are calculated based on the simplified enveloping building shape. This research studies how the static and dynamic wind loads on a tall building are modified by the presence of recessed cavities. The H-section tall building, with square enveloping shape, is selected to represent a building with two recessed cavities. A number of these building models with a systematic variation of breadths and depths of the recessed cavities are tested in the wind tunnel. Fluctuating wind forces and moments on the building models are measured and the dynamic building responses are investigated by the HFFB technique. The results show significant reductions in across-wind load fluctuations on the H-section buildings at wind incidence normal to building face with a cavity. Greater reductions are found on the fluctuation levels and the spectral energies at the vortex excitation frequency when the width/depth of the recessed cavities becomes larger. A resonant across-wind response modification factor (RMF) is adopted to quantify the effect on wind-induced dynamic building responses. For the across-wind response at critical wind incidence, the presence of recessed cavities can lead the value of RMF to as low as 0.67, that is reduction of building responses by 33%. To understand the mechanism of wind load modification caused by the presence of recessed cavities, wind pressure on all faces of the H-section tall buildings including the cavity faces are measured. Correlations of across-wind forces contributed by different building faces are analyzed. A clear quasi-periodic fluctuating component is found on the forces from the two building side faces and they act in phase in the across-wind direction. The across-wind force contributions from side faces of the windward or leeward recessed cavity are generally out-of-phase. As a result, the overall excitation levels of the total across-wind force on the building are reduced due to the presence of recessed cavities. To complement the wind tunnel study, computational fluid dynamics modeling using large-eddy simulation (LES) is carried out to study the unsteady wind flow around and wind loads on the H-section tall building with the widest and deepest recessed cavities. Furthermore, the two-dimensional (2D) case of smooth flow past 2D H-section cylinders is investigated. Both the LES and 2D experimental results give consistent observations and mechanisms of the effects of recessed cavities on the modification of dynamic wind loads on the tall building as the wind tunnel results. The thesis also reports studies on the effects of upstream terrain types, building heights and building shapes on the wind effects of H-section tall buildings.