### COMPARATIVE STUDY OF THE NONLINEAR RESPONSE OF R.C. SPATIAL FRAMES SUBJECTED TO NEAR-FAULT AND FAR-FAULT GROUND MOTIONS

*Fabio Mazza, Mirko Mazza*

Ultima modifica: 2011-07-19

#### Sommario

Severe and quite unforeseen structural damage of reinforced concrete (r.c.) framed buildings, designed according to recent seismic codes and built within a few kilometres of a fault rupture zone, has been observed during strong near-fault ground motions. These motions are characterized by a forward-directivity effect, typically occuring in the fault-normal direction and generating long-duration horizontal pulses, and high values of the ratio between the peak value of the vertical acceleration and the analogous value of the horizontal acceleration. In particular, the pulse-type nature of a (horizontal) near-fault ground motion can induce unexpected ductility demands at the end sections of both girders and columns, depending on the ratio between the pulse period and the fundamental vibration period of the structure. On the other hand, high values of the acceleration ratio can notably modify the axial load in r.c. columns, producing even tension and high compressive forces (larger than the balanced force); moreover, plastic hinges are expected along the span of r.c. girders, especially in the upper storeys.

Generally, the design provisions of current seismic codes are not very accurate for assessing near-fault effects, because only far-fault ground motions are considered. 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. Therefore, in order to establish if suitable additional code guidelines are needed, it is very important to compare the nonlinear response of r.c. spatial frames subjected to near-fault ground motions or far-fault ones. To this end, six- and twelve-storey r.c. spatial framed structures are designed according to the provisions of NTC08 considering (besides the gravity loads) the horizontal seismic loads acting alone or in combination with the vertical ones and assuming: high ductility class, high-risk seismic region and medium subsoil class.

The nonlinear seismic analysis of the test structures is performed using a step-by-step procedure based on a two-parameter implicit integration scheme and an initial-stress-like iterative procedure. A lumped plasticity model (LPM) based on the Haar-Kàrmàn principle is proposed to model the inelastic behaviour of r.c. spatial frames under strong near-fault and far-fault ground motions. The lumped plasticity model for a r.c. column (LPMC) includes a piecewise linearization of the axial load-biaxial bending moment elastic domain, at the end sections where inelastic deformations are expected. Each flat surface corresponds to a plastic strain mechanism for the section, defined by axial strains and curvatures. Moreover, the lumped plasticity model for a r.c. girder (LPMG) takes into account the potential plastic hinges along the span of the girders, due to the vertical ground motion, modifying the uniaxial plastic moments of the end-sections and so avoiding the computational effort required by the sub-discretization of the frame member. The LPMC and LPMG allow a satisfactory compromise between accuracy and computational efficiency.

A comparative study of the nonlinear response of the test structures subjected to near-fault and far-fault ground motions is carried out. Specifically, horizontal and vertical accelerograms, representative of near-fault ground motions with different values of the acceleration ratio, are considered. Following recent seismological studies which allow the extraction of the largest (horizontal) velocity pulse from a near-fault ground motion, the corresponding far-fault ground motions is represented by the residual motion, after the pulse extraction. Finally, the occurrence of directivity effect at arbitrary orientations is checked rotating the horizontal components of the selected near-fault ground motions, rather than considering only fault-normal and fault-parallel orientations.

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