Seismic assessment of existing RC buildings under alternative ground motion ensembles compatible to EC8 and NTC 2008

This work investigates the effects of the choice of different ensembles of ground motions on the seismic assessment of existing RC buildings through nonlinear dynamic analysis. Nowadays indeed, all the main International Seismic Codes provide a soil classification which is based on the shear wave velocity, the soil morphology and the assumed distance from the fault source. Depending on the soil properties, a suitable elastic spectrum is provided as target, defined on the basis of average properties assumed for the soil. An ensemble of ground motions, compatible to the target one, must be selected to perform a nonlinear dynamic analysis. The ensemble can be made by artificial or natural ground motions, compatible with the Code spectrum for the assumed soil-type. Alternatively, the set of ground motions can be assumed as compatible with the bedrock Code spectrum and, subsequently, subjected to site response analysis, i.e. filtered through the specific stratigraphy of the site soil. In this work a comparison among these different approaches, all compatible to the European (Eurocode 8, EC8) and Italian (NTC 2008) Code provisions, has been made on a case-study, i.e. a real RC Italian building. The seismic response of the case-study under the assumed seismic inputs, expressed in terms of chord rotation and shear force, has been found by performing a nonlinear dynamic analysis under the different assumed seismic excitations. The comparison has been made in terms of seismic performance, expressed as the ratio between the seismic response found for each structural element and the corresponding capacity.