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PROCEEDINGS OF THE EIGHTH INTERNATIONAL CONFERENCE ON COMPUTATIONAL STRUCTURES TECHNOLOGY
Edited by: B.H.V. Topping, G. Montero and R. Montenegro
Considering Ground Motion Uncertainties in Stochastic Seismic Analysis of Structures
N. Impollonia1, G. Ricciardi2 and M.P. Santisi d'Avila2
1Department ASTRA, University of Siracusa, Italy
N. Impollonia, G. Ricciardi, M.P. Santisi d'Avila, "Considering Ground Motion Uncertainties in Stochastic Seismic Analysis of Structures", in B.H.V. Topping, G. Montero, R. Montenegro, (Editors), "Proceedings of the Eighth International Conference on Computational Structures Technology", Civil-Comp Press, Stirlingshire, UK, Paper 246, 2006. doi:10.4203/ccp.83.246
Keywords: uncertainty modelling, ground motion, stochastic seismic analysis, Kanai-Tajimi filter.
The complicated nature of the earthquake source mechanism, the highly irregular structure of the earth's mantle and crust and the difficulty of making significant measurements make it difficult to elucidate the real influences on the ground motion. Common design rules do not explicitly include the influence of the uncertainties associated with these fundamental features. For example for the Kanai-Tajimi-like filter, the choice of soil natural frequency, damping coefficient and the intensity of the ideal white noise excitation at the bedrock-overburden interface, is not straightforward. These parameters undergo fluctuations and should be modelled as random variables.
The purpose of this study is to investigate and propose methods to incorporate uncertainty into the analytical model of seismic input. Specific objectives are: to propose variation range of uncertain filter parameters to model earthquake ground motion and to study the influence of these uncertainties on structural response. This study considers as uncertain parameters, for each earthquake, the soil overburden effective natural frequency and damping coefficient.
Ground motion records of earthquakes in Southern-Italy are used to develop a procedure that quantifies uncertainties inherent to the ground motion [3,4]. In the proposed model, uncertainty is considered for filter parameters and quantified observing their probability density function. The ground motion accelerogram of each event, the comparison of the recording and the Kanai-Tajimi filter spectral moments provides filter parameters [5,6,7,8].
The proposed ground motion model is used to define uncertain seismic input for use in dynamic analyses of an example frame. The structure is considered as a deterministic system and the relationship between the structural response and the random filter parameters, i.e. the second order moments, are evaluated. The ground motion acceleration maximum peak used in the example is that proposed by current design codes. A structural design considering an uncertain dynamic load, allows an estimate of the real random nature of the earthquake ground motion, because of the uncertain soil characterisation, the statistically limited number of seismic records and the different local geological conditions related to each accelerogram. The resulting structural response maximum peak is compared to that obtained from the time dependent response [9,10,11,12,13].
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