GNGTS 2019 - Atti del 38° Convegno Nazionale

GNGTS 2019 S essione 2.2 379 Table 1 - Soil categories according to NTC18. Category Topographic surface characteristics Outcropping rock masses or very rigid soils characterized by shear wave velocity A larger than 800 m/s, possibly including poorer mechanical soils with a maximum thickness of 3 m on the surface. Soft rocks and deposits of very thick coarse-grained soils or very substantial B fine-grained soils , characterized by improved mechanical properties with depth and equivalent velocity values between 360 m/s and 800 m/s. Deposits of medium thick coarse-grained soils or medium substantial fine-grained C soils with substrate depths greater than 30 m , characterized by improved mechanical properties with depth and equivalent velocity values between 180 m/s and 360 m/s. Deposits of poorly thickened coarse-grained soils or thin-grained soils with substrate D depths of more than 30 m , characterized by improved mechanical properties with depth and equivalent velocity values of between 100 and 180 m/s. E Soils with characteristics and equivalent velocity values as those defined for categories C or D , with depth of bedrock not exceeding 30 m. increased. Indeed, local geological and soil conditions play a fundamental role in the feature of seismic motion changing its characteristics in terms of amplitude, frequency content and duration, with respect to what would be observed on rock site. Therefore, the definition of seismic actions must consider both the seismic hazard of the area under consideration and the local effects. The European regulation on seismic design and verification, named Eurocode 8 (EC8; CEN 2004) and the old Italian one, Norme Tecniche per le Costruzioni “D.M. 14 gennaio 2008” (NTC08), adopt the average shear wave velocity in the first 30 m of depth from the ground surface, V S ,30 , as parameter to classify soils and evaluate the seismic response. In absence of direct measurement of V S ,30 , the standards allow to define the soil categories in accordance to the soil resistance parameters, N SPT ,30 and C u ,30 . Five soil categories are introduced based on fixed ranges of V S ,30 values, in order to consider soils with increasingly poorer mechanical properties, from class A, corresponding to rock sites with V S ,30 > 800 m/s, to class E. The standards provide simplified methods to define the amplification factors for each class to be used for modifying the elastic design response spectrum on rock. The recent Italian regulation, Norme Tecniche per le Costruzioni “D.M. 17 gennaio 2018” (NTC18) updates the NTC08 and mainly modifies the parameter to be considered for site classification to the equivalent shear velocity, V S , eq , which is calculated as: where: h i thickness (in meters) of i-th layer; V S , i shear wave velocity in i-th layer; N number of layers; H depth of the bedrock characterized by V S > 800 m/s. The sites are classified according to Tab. 1. Other modifications are: i) soil class E (i.e. maximum depth of bedrock changed from 20 m to 30 m); ii) the resistance parameters are no longer be considered for site classification. The amplification factors in NTC18 are the same of NTC08 and they are defined as a function of site class, topographic class, and seismic hazard parameters on rock, according to Tab. 2.