Seminar - Alan Moore - Predicting the Natural Frequencies of Tall Buildings
Wednesday, March 15, 2006, 1.10 - 1.50 pm
Civil Engineering Lecture Theatre 3
Structural engineers regularly use finite element analysis to facilitate the design of tall buildings. The finite element models are used to predict the dynamic characteristics of the entire structure, as well as to determine loads on the individual structural elements. Accurate predictions of the dynamic characteristics, including the natural frequencies and mode shapes, are important parameters for use in wind tunnel testing to determine design wind loads and the wind-induced responses.
Despite advances in finite element analysis of tall buildings, discrepancies between the predicted and actual dynamic characteristics are still encountered. Since the actual dynamic characteristics cannot be accurately measured until the structure is almost completed, there is a reliance on finite element models to predict dynamic characteristics to avoid costly alterations to the structure, such as the installation of damping devices. By comparing finite element models and full-scale measurements, it is possible to improve understanding of the mechanisms that affect the dynamic behaviour of tall buildings and to build a useful database of knowledge regarding finite element modelling of tall buildings.
This presentation includes a comparison between measured and predicted natural frequencies for a 190 metre tall office building. The measured natural frequencies are determined by a full-scale forced vibration test using a mechanical shaker. The predicted natural natural frequencies are attained using finite element analysis of the structure. A number of finite element model iterations are compared, with each iteration focusing on a particular aspect of the structure to attain more knowledge regarding the structures behaviour. Particular attention is directed toward the loading rate of concrete and its effect on the elastic modulus of concrete. It is suggested that structures with shorter periods of vibration that ignore the loading rate effects may consistently underestimate the natural frequencies when using finite element models.