Effects of the Vertical Component of Near-Fault Earthquakes on the Nonlinear Response of R.C. Structures Retrofitted By Different Base-Isolation Systems

Abstract eng:
Reinforced concrete (r.c.) framed buildings designed for vertical loads only or in compliance with inadequate seismic classifications and/or code provisions present in many cases a high seismic vulnerability and need retrofitting. To this end, the insertion of a base-isolation system allows a considerable reduction of the horizontal loads transmitted to the superstructure. However, strong near-fault ground motions, which are characterized by long-duration horizontal pulses and high values of the ratio between the peak value of the vertical acceleration (PGA V ) and the analogous value of the horizontal acceleration (PGA H ), can become critical for a base-isolated structure. More specifically, the horizontal deformability of a base-isolated structure may amplify the inelastic response of the superstructure and induce failure of the isolation system. Moreover, high values of the acceleration ratio α PGA (PGA V /PGA H ) can notably modify the axial load in r.c. columns, while yielding is expected along the span of the beams, especially at the upper storeys; in addition, elastomeric and sliding bearings can undergo tensile loads and uplifts, respectively. To check the effectiveness of different baseisolation systems for retrofitting a r.c. framed structure located in a near-fault area, a numerical investigation is carried out analyzing the nonlinear dynamic response of the fixed-base and isolated structures. For this purpose, a six-storey r.c. framed building primarily designed (as to be a fixed-base one) in compliance with an old Italian seismic code (1996) for a mediumrisk zone, is supposed to be retrofitted by insertion of an isolation system at the base for attaining performance levels imposed by the current Italian code (NTC 2008) in a high-risk seismic zone. In detail, elastomeric (HDLRBs) and friction (steel-PTFE sliding bearings, SBs, or friction pendulum bearings, FPBs) bearings are considered. Besides the (fixed-base) primary structure, three cases of base isolation are studied: HDLRBs acting alone (i.e. EBI structure); in-parallel combination of HDLRBs and SBs (i.e. EFBI structure); FPBs acting alone (i.e. FPBI structure). The EBI, EFBI and FPBI structures are designed assuming the same values of the fundamental vibration period and equivalent viscous damping ratio in the horizontal direction. Then, the nonlinear response of the fixed-base structure is compared with those of the differently isolated structures, considering horizontal and vertical components of seven near-fault ground motions available in the Pacific Earthquake Engineering Research Center database. These motions are selected and scaled on the basis of the design hypotheses adopted for the test structure. The dynamic analyses are carried out using a step-by-step procedure based on a two-parameter implicit integration scheme and an initial stress-like iterative procedure. R.c. frame members are idealized by a two-component model, assuming a bilinear moment-curvature law; the effect of the axial load on the ultimate bending moment of columns is taken into account. The response of an elastomeric bearing is simulated by a model with variable stiffness properties in the horizontal and vertical directions, depending on the axial load and lateral deformation; while the response of a friction bearing is described by a nonlinear force-displacement law, with friction variability depending on sliding velocity and axial load.

• Export as: BibTeX | MARC | MARCXML | DC | EndNote | NLM | RefWorks
• Add to your basket