GNGTS 2021 - Atti del 39° Convegno Nazionale

GNGTS 2021 S essione 2.2 322 The main geological formations (Fig.2a) outcropping in the North-West and South-East of the urban center, as well as in the “Sassi” area, are represented by Altamura limestone (AL) and Gravina calcarenites (GC). The latter is covered by the Subappennines clay (SC) and, somewhere, a thin layer of sands, with thickness varying from a few meters, near the Sassi area, to 40-50 m inwards. The subsoil was widely investigated by numerous boreholes, down-hole and seismic refraction tests (Gallipoli and Lupo, 2012) highlighting how the thickness of the clay increases from 7m to 40 m moving from East to West, i.e. with increasing the distance from the terrace ridge overlooking the Sassi area. The shear wave velocities of the different soils (Table 1) were obtained from down-hole and seismic refraction tests near the buildings, while the soil mass density was set equal to typical values for soft clay (SC), soft rock (GC), hard rock (AL); the masonry properties were assigned based on the NTC’18 (Tab. C.8.5.1), considering tuff walls with irregular texture, i.e. the most recurrent typology in the historical centre near the “Sassi” area. Subappennine clay (SC) Gravina calcarenites (GC) Altamura limestone (AL) Tuff masonry V S (m/s) 190-450 394-1195 1200 ρ (kg/m 3 ) 1750 1800 2750 1600 G 0 (MPa) 68-361 285-2620 2808 450 Tab. 1 - Mechanical properties of the different materials. Fig. 2b reports the seven analysed soil-foundation-structure (SFS) cross-sections, considering the soil stratigraphy obtained from the subsoil investigations. Also, the main characteristics of the 7 buildings are outlined, i.e. height, h , width, b , fixed-base frequency, f 0 and the experimental frequency, f* EXP , measured on the transverse section of the structure. It is worth noting that for buildings 1-2-5, characterized by the same number of stories, the fundamental frequency reduces with the thickness of the clay layer, highlighting a first influence of the subsoil condition and of SFS interaction. The building height and width were obtained from the survey by Gallipoli et al. (2020), whereas the thickness of the bearing walls was set based on relations with the building height, used in the past for the masonry building design (Augenti and Parisi, 2019). For two-stories buildings it was assumed s =0.70m, for three-stories buildings s =0.81m, for four stories buildings s =0.93m and for five stories buildings s =1.03m. An enlargement of 0.15 m at each side of the bearing wall was considered to determine the foundation width. The value of the foundation embedment, D , was set to 1.5 m for buildings 1, 2, 4, 5, 7, and to 3.50m for the buildings 3, 6, where an underground level is present. The fixed-base period ( T 0 =1/f 0 ) was estimated considering the correlation relevant to masonry buildings on calcarenite obtained by Gallipoli et al. (2020), i.e. T 0 = 0.0137 h , assumed as a first rough estimate of the fixed-base fundamental period of the masonry buildings typical of this area of the city of Matera. Once all the SFS parameters are defined, the weighting coefficients were chosen by comparing the seismic impedance ratios between the actual soil layers below each analysed case, μ , with those pertaining to the different combinations of ground types B, C, D of the parametric study by Piro et al (2020), i.e. μ DD =1.00, μ DB =0.20, μ DC =0.44, μ CB =0.45. The equivalent parameters G eq , ρ eq and V S,eq were computed in order to estimate σ eq and consequently the corresponding value of f*/f 0 according to the approach described in Section 1. Fig. 3 shows the comparison between the experimental periods versus the values obtained from the application of the simplified method, either in its traditional expression (full symbols), i.e. by entering in the equation in Figure 1 through s computed using the shear wave velocity of the soil cover (Veletsos and Meek, 1974), or by the proposed approach (empty symbols), i.e. using s eq . The traditional formulation overestimates the actual values in all cases except one,