Nonlinear Modeling and Analysis of R.C. Spatial Frames to Study the Effects of the Vertical Component of Near-Fault Ground Motions

Abstract eng:
Near-fault ground motions are characterized by high values of the ratio PGA between the peak value of the vertical acceleration, PGAV, and the analogous value of the horizontal acceleration, PGAH, which can notably modify the axial force demand in columns (e.g. producing both tension and high compressive forces larger than the balanced force) and the bending moment demand in girders (e.g. plastic hinges are expected along the span of r.c. girders, especially in the upper storeys). At present, the Italian seismic code (NTC08) does not consider the effects of near-fault ground motions in the design of a r.c. framed structure. In order to check the effectiveness of current code provisions, six- and twelve-storey r.c. spatial framed structures are designed according to the provisions of NTC08, considering the horizontal seismic loads acting alone or in combination with the vertical ones. A numerical investigation is carried out considering the nonlinear response of the test structures subjected to horizontal and vertical accelerograms, representative of near-fault ground motions with different values of the acceleration ratio PGA. A lumped plasticity model (LPM) based on the Haar-Kàrmàn principle is proposed to model the inelastic behaviour of the r.c. frame members. Specifically, the lumped plasticity model for a column (LPMC) includes a piecewise linearization of the bounding surface of the axial load-biaxial bending moment elastic domain, at the end sections where inelastic deformations are expected. On the other hand, the lumped plasticity model for a girder (LPMG) takes into account the potential plastic hinges along the span, due to the vertical ground motion, modifying the uniaxial plastic moments of the endsections and so avoiding the computational effort required by the sub-discretization of the frame member.

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