Estimation of Wind-Induced Dynamic Responses of Tall Buildings of Non-Ideal Mode Shapes by Time Domain Analysis

This dissertation, "Estimation of Wind-induced Dynamic Responses of Tall Buildings of Non-ideal Mode Shapes by Time Domain Analysis" by Ai, Li, 李愛, was obtained from The University of Hong Kong (Pokfulam, Hong Kong) and is being sold pursuant to Creative Commons: Attribution 3.0 Hong Kong License. The content of this dissertation has not been altered in any way. We have altered the formatting in order to facilitate the ease of printing and reading of the dissertation. All rights not granted by the above license are retained by the author. Abstract: Abstract of thesis entitled ESTIMATION OF WIND-INDUCED DYNAMIC RESPONSES OF TALL BUILDINGS OF NON-IDEAL MODE SHAPES BY TIME DOMAIN ANALYSIS submitted by LI Ai for the degree of Master of Philosophy at The University of Hong Kong in September 2007 Modern architectural building design is becoming increasingly complex, and must take into account aesthetic considerations or land availability all over the world. Many buildings possess extraordinary shapes and spectacular dynamic characteristics which make them vulnerable to extreme wind events. Therefore, wind-induced response assessment exerts an important influence on the structural specifications of tall buildings during the design processes. Ad-hoc wind tunnel modelling is widely used for tall building design. In particular, the high frequency force balance (HFFB) technique has gained wide popularity. This technique assumes ideal building mode shapes, which are linear in translational directions and constant in torsion. However, this assumption does not hold good for most realistic structures of complex properties. Furthermore, the force balance technique does not give information on the vertical distribution pattern of wind loads along the building height. The present study aims to develop a wind tunnel technique to assess wind-induced responses of a tall building with complex dynamic characteristics. Time histories of the wind loads on various levels along the building height are obtained from simultaneous surface pressure measurement in the wind tunnel. Subsequent wind-induced responses are calculated by direct time integration and modal analysis. This technique inherently accounts for any nonlinear mode shapes and readily handles the wind-induced response analysis of complex structures. Verification of the techniques is made by performing a comparison HFFB measurement. It is shown that for a building of ideal mode shapes, the surface pressure measurement and the time domain response computation result in similar spectra of the base moments and building responses as those from the HFFB technique. The developed time domain technique is then used to investigate the mode shape correction factors for force balance technique on buildings of non-ideal mode shapes. A parametric study is carried out to integrate the effect of non-ideality of mode shapes, vertical distribution of wind loads and masses along the building height on the mode shape correction factors used for HFFB technique. The results agree well with those from the literature. Tall buildings of complex shapes and structural systems, or uneven mass distribution patterns are becoming increasingly popular, which greatly complicates the performance assessment under extreme wind events. A number of building models are analyzed by a structural dynamics analysis software ETABS to investigate the influence of building mass distribution pattern and mass-rigidity- center eccentricity on the dynamic properties and the wind-induced responses. Results show that uneven mass distribution considerably influences structure stiffness and subsequently the natural frequencies of vibration. Response computation reveals the fact that lighter upper storey masses induce increasing excessive building vibration. At the same time, the mass-rigidity-center offset induces extra coupled building translational-torsional modes of vibration,