EFFICIENT NONLINEAR BEAM MODEL FOR THE ANALYSIS OF RC SPATIAL FRAMES UNDER VERTICAL COMPONENT OF NEAR-FAULT GROUND MOTIONS

Strong near-fault ground motions are characterized by high values of the ratio alpha(PGA) between the peak value of the vertical acceleration, PGA(v), and the analogous value of the horizontal acceleration, PGA(H). High values of the acceleration ratio alpha(PGA) can notably modify the axial load in reinforced concrete (r.c.) columns, with high compressive forces, larger than the balanced force, and even tension. Moreover, plastic hinges are expected along the span of r.c. girders due to the vertical ground motion, especially in the upper storeys of framed structures. A lumped plasticity model based on the Haar-Karman principle is proposed for modeling the inelastic behaviour of r.c. spatial frames. In particular, a multi-surface piecewise linearization of the axial load-biaxial bending moment elastic domain is considered for the column. Moreover, to take into account the potential plastic hinges along the span of the girder, the uniaxial plastic moments of the end-sections are modified according to the plastic moments of other selected critical sections, so avoiding the computational effort required by the sub-discretization of the frame member.