Supplementary energy dissipation is an efficient technique for the seismic retrofitting of a reinforced concrete (r.c.) framed building as long as structural behaviour before failure and sensitivity to damage is evaluated properly. In fact, lack of knowledge on strength and stiffness degradation may not be compensated for in the calculation of a passive control system. On the other hand, the widespread use of supplementary energy dissipation devices is closely related to the availability of simplified yet reliable design procedures along with the new generation of seismic codes based on performance-based design. These observations open up the way for a more realistic estimation of the deformation capacity through a displacement based design procedure in which previous seismic degradation of the multi-degree-of-freedom (MDOF) model of the actual structure is evaluated starting from the initial backbone curve of the corresponding single-degree-of-freedom (SDOF) system. Specifically, the nonlinear static analysis of the MDOF system can generate an idealized force-displacement curve of the equivalent SDOF system, so that an estimation of the seismic capacity of the original structure in terms of the capacity boundary curve is obtained by nonlinear dynamic analysis of the equivalent SDOF system. To evaluate the effectiveness of the proposed design procedure of hysteretic damped braces (HYDBs), the RINTC project, financed by the Italian Department of Civil Protection, has selected an archetype as representative of the Italian residential housing stock constructed during the 1990s. A simulated design of a six-storey reinforced concrete (r.c.) building with rectangular shape, five and three bays along the principal in-plan directions, is carried out in line with the Italian code for a moderate seismic zone. Three configurations of masonry infills (MIs) are considered: Bare Frame (BF, with nonstructural MIs); Infilled Frame (IF, with an uniform in-elevation distribution of structural MIs); Pilotis Frame (PF, with no MIs at the ground floor and structural MIs at the other floors). OpenSees is the computational platform for nonlinear seismic analysis. A lumped plasticity model is adopted for r.c. frame members, with moment-chord rotation at critical end sections defined by the modified peak-oriented Ibarra-Medina-Krawinkler deterioration model, also accounting for shear failure mode. The shear behaviour of the beam-column joints is modelled by means of rigid offsets, reflecting the joint panel dimensions, and a zero-length rotational spring defined through a quadri-linear moment-rotation relationship. A simplified diagonal pin-jointed strut model takes into account the in-plane failure modes that can occur in the infill panels when subjected to seismic loading (i.e. diagonal compression, crushing the corners in contact with the frame, sliding shear along horizontal joints and diagonal tension), while a bilinear model describes the nonlinear response of the HYDBs. Finally, nonlinear bi-directional dynamic analyses of the unbraced and damped braced BF, IF and PF structures are carried out, with or without seismic damage, with reference to records scaled in line with the hypotheses adopted.

Influence of previous seismic degradation of r.c. buildings in the case of retrofitting by damped braces

Fabio Mazza;
2020

Abstract

Supplementary energy dissipation is an efficient technique for the seismic retrofitting of a reinforced concrete (r.c.) framed building as long as structural behaviour before failure and sensitivity to damage is evaluated properly. In fact, lack of knowledge on strength and stiffness degradation may not be compensated for in the calculation of a passive control system. On the other hand, the widespread use of supplementary energy dissipation devices is closely related to the availability of simplified yet reliable design procedures along with the new generation of seismic codes based on performance-based design. These observations open up the way for a more realistic estimation of the deformation capacity through a displacement based design procedure in which previous seismic degradation of the multi-degree-of-freedom (MDOF) model of the actual structure is evaluated starting from the initial backbone curve of the corresponding single-degree-of-freedom (SDOF) system. Specifically, the nonlinear static analysis of the MDOF system can generate an idealized force-displacement curve of the equivalent SDOF system, so that an estimation of the seismic capacity of the original structure in terms of the capacity boundary curve is obtained by nonlinear dynamic analysis of the equivalent SDOF system. To evaluate the effectiveness of the proposed design procedure of hysteretic damped braces (HYDBs), the RINTC project, financed by the Italian Department of Civil Protection, has selected an archetype as representative of the Italian residential housing stock constructed during the 1990s. A simulated design of a six-storey reinforced concrete (r.c.) building with rectangular shape, five and three bays along the principal in-plan directions, is carried out in line with the Italian code for a moderate seismic zone. Three configurations of masonry infills (MIs) are considered: Bare Frame (BF, with nonstructural MIs); Infilled Frame (IF, with an uniform in-elevation distribution of structural MIs); Pilotis Frame (PF, with no MIs at the ground floor and structural MIs at the other floors). OpenSees is the computational platform for nonlinear seismic analysis. A lumped plasticity model is adopted for r.c. frame members, with moment-chord rotation at critical end sections defined by the modified peak-oriented Ibarra-Medina-Krawinkler deterioration model, also accounting for shear failure mode. The shear behaviour of the beam-column joints is modelled by means of rigid offsets, reflecting the joint panel dimensions, and a zero-length rotational spring defined through a quadri-linear moment-rotation relationship. A simplified diagonal pin-jointed strut model takes into account the in-plane failure modes that can occur in the infill panels when subjected to seismic loading (i.e. diagonal compression, crushing the corners in contact with the frame, sliding shear along horizontal joints and diagonal tension), while a bilinear model describes the nonlinear response of the HYDBs. Finally, nonlinear bi-directional dynamic analyses of the unbraced and damped braced BF, IF and PF structures are carried out, with or without seismic damage, with reference to records scaled in line with the hypotheses adopted.
r.c. framed structures; strength and stiffness degradation; hysteretic damped braces; displacement-based design procedure; seismic retrofitting.
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Utilizza questo identificativo per citare o creare un link a questo documento: http://hdl.handle.net/20.500.11770/310471
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