GNGTS 2016 - Atti del 35° Convegno Nazionale

394 GNGTS 2016 S essione 2.2 displacement spectrum, and magnitude dependency of the corner period TD, separating the constant velocity and the constant displacement branches of the spectrum. Since one of the objectives is to move the different definitions of seismic input recommended in different parts of the Eurocodes into a uniform presentation within the single Part 1 of EC8, other issues are dealt with, such as empirical formulas of peak values of ground motion (PGV, PGD) or the simplified evaluation of the earthquakemagnitude to be associated to either selection of input accelerograms for seismic analyses or for soil liquefaction potential assessment. One of the key and most debated problems for the seismic actions to be introduced into the norms is the definition of site amplification factors and its connection with classification of different soil types. The question is wide, and reflects the experiences and interests of different European regions with different prevailing site conditions, such as Scandinavia, where the dominance of thin soft clay layers may induce large amplification at high frequencies, or middle Europe, where at the opposite, geological conditions are dominated by thick soil sediments. The constrains are different: - the code should be simple and easy-to-use; - soil classification should be easily understandable by practicioners and directly related to well calibrated site amplification factors; - the classification should be quantitative and based on a limited number of parameters, possibly a single one; - in spite of its simplicity, the classification should be detailed enough to cover the variety of stratigraphic profile that may be present in whatever country of Europe; - the number of nationally determined parameters within the norms should be reduced, in order for the norm to be as widely as possible recognizable as a European norm; - there is a strong request that new norms do not involve an unnecessary increase of construction costs. As obvious, such requirements are very hard to be managed simultaneously. The present EC8 approach, based on the definition of different site classes, each associated to different site amplification factors and corner periods and low vs high seismicity levels, was questioned in various countries, which in some instances, such as Italy, Spain, Germany, proposed independently evaluated site amplification factors and different classification criteria with respect to the standard EC8 ones. The following issues are probably the most debated ones: - are additional parameters rather than Vs,30 alone needed to provide a more reliable soil classification, such as the fundamental frequency of the soil from microtremor measurements? – are available studies sufficient to associate to such additional parameters a reliable site amplification factor? – is it advisable to propose a continuous variation of site amplification factors as a function of Vs,30, in order to avoid strong jumps of seismic action when moving from one class to the other one? – are the present approaches to relate geotechnical parameters, such as results of SPT or CPT in situ tests, to site classification reliable enough? – what, if any, penalty to be prescribed if only a limited amount of information is available for site classification? – are the current site amplification factors suitable to account for non-linear soil response, or they tend to overestimate site effects for large levels of input motion, and how to account in a simple way such non-linear effects? – in which conditions should site-specific response analyses be prescribed as mandatory? And, last but not least, if and how results of microzonation studies can be explicitly included as a possible alternative approach for the definition of soil subdivision and site amplification factors for specific areas, in order to let the quantitative work made in the microzonation process be applicable in the seismic norms.