GNGTS 2021 - Atti del 39° Convegno Nazionale

373 GNGTS 2021 S essione 2.2 Beams and columns were designed considering a C25/30 according to [NTC] with characteristic compressive strength f ck equal to 25MPa, and a reinforcing steel B450Cwith characteristic yielding tensile strength f yk equal to 450 MPa. To account for the non-linear material behavior, suitable models are adopted: in particular, Mander et al. (1998) model Concrete04 and Menegotto and Pinto (1973) model Steel02 materials for core/cover concrete and reinforcement rebars are used, whereas single-strut truss elements with a non-linear behavior characterized by Di Trapani et al. (2018) model are adopted to capture the stiffening effect caused by masonry infills. Masonry compressive strength f m and the elastic modulus E m along the two orthogonal directions are assumed equal to 2.4 MPa and 4408 MPa for the direction, and about to 7.28 MPa and 7400 MPa for the other one. Masonry infills are characterized by a thickness of 25 cm and distributed over the entire external perimeter of the buildings, whereas the contribution of the staircase to the stiffness of the building was neglected. Regarding the loading actions, 5.5 kN/m 2 and 0.5 kN/ m 2 are considered as the dead and live loads for the roof, and a 6.5 kN/m 2 dead load and a 2 kN/ m 2 live load are considered for the remaining floors. Both high ductility class (DCH) and medium ductility class (DCM) are considered, thus leading to a total of 12 different archetypes resulting from the combination of the different number of stories, ductility class, and presence/absence of masonry infills. Results The seismic hazard curves computed for each Italian municipality with reference to its main soil class are coupled with the appropriate fragility curves representative of the code-compliant archetype that a designer may have sized in that location to get the seismic failure rates associated with a code-compliant design, and thus obtain the respective seismic reliability maps for bare and infilled code-compliant RC frames. The following relevant damage state (DS) are defined: • Low Damage ( ds 1 ), corresponding to the achievement of the yielding point in the SDOF’s behavior curve; • Near Collapse ( ds 2 ), placed at the beginning of the backbone’s descending branch; • Collapse ( ds 3 ), identified when base shear is approximately equal to the 80% of the maximum shear capacity. The results show how infilled configurations are generally characterized by significantly higher seismic failure rates than bare frames, and such difference is magnified in low-seismic hazard regions. The choice of DCM or DCH design ductility class leads to similar results in terms of seismic safety: however, DCM seems to imply the design of slightly safer code-compliant buildings, although a designer is led to think otherwise, i.e. that DCHmay allow safer designs than DCM. Results show also how 6- and 9- stories archetypes are characterized by similar intervals, with values higher than the 3-stories configurations. As sake of example, Fig. 2 shows results for ds 2 . Here for bare RC frames, 6-stories layouts display the worst performance, with the worst seismic failure rates equal to 4.41∙10- 4 for DCH designs. Overall, it is observed how values are between 1.13∙10- 7 and 4.41∙10- 4 for the entire subset of bare frame archetypes. As regards the companion infilled configurations, more severe performances are observed, with worst effects for the 9-stories layouts, whereas in low seismicity regions the presence of infills appears to be beneficial for 3-stories archetypes, with reaching minimum values around 5.68∙10-8. In summary, all the analyzed infilled configurations have as overall interval values between 5.68∙10- 8 and 4.24∙10- 3 . Lastly, Fig. 3 illustrates a graphical comparison between intervals derived for the abovementioned design layouts.