The Significance of Site Effect Studies for Seismic Design and Assessment of Industrial Facilities

Abstract

Site effects can significantly modify the seismic motion in certain frequency domains, due to the resonance of soft deposits and subsequent amplification of the motion and / or due to the shape of the bedrock surface under soft deposits. Consequently, the shape of an appropriate elastic response spectrum might significantly differ from those proposed in building codes like EC8 based on a few soil classes. A site specific elastic response spectrum can either be lower or higher than the corresponding code spectrum or even both together, depending on the considered frequency band. Especially in the framework of assessment and reinforcement of existing industrial facilities, it might be of great importance to determine a site specific spectrum, much more adapted to account for local site effects. In some cases, such a specific spectrum makes it possible to save millions of unnecessary reinforcements. Some brief methodological aspects are presented, followed by real case examples, showing the importance of specific site effect studies and their consequences in terms of elastic response spectra for a more appropriate assessment or design of industrial facilities. In particular, the soil classification in EC8 is essentially based on Vs30 whereas site specific studies also account for the velocity contrast between the bottom of loose soil deposits and the bedrock, a parameter that can have a great influence on the amplitude of the resulting response spectrum.