Evaluation of force fluctuations induced by vertical seismic component on reinforced concrete precast structures

In many documented cases, reinforced concrete precast structures collapse during strong earthquakes was ascribed to the combination of horizontal and vertical ground motion components. In fact, the vertical component can increase significantly the axial compression in columns, with a reduction of the section ductility of the columns subject to bending induced by horizontal forces. The vertical components can have also detrimental effect at the beam – column connection level. Starting from these evidence, the effects of the vertical seismic component in reinforced concrete precast buildings are investigated here. In particular, the attention is devoted to the assessment of the fluctuations of the axial force in columns and vertical forces at the beam – column connection level. The paper reports the main outcomes of a series of nonlinear time-history analyses performed on 48 finite element models representative of the behaviour of existing precast structures with hinged connections and single or multi-bay, single or multi-storey, characterized by strong connection-weak column failure mechanisms. Far fault and near fault excitation types have been considered. Pulse-type velocity records were further considered in order to identify if they can cause possible amplification of seismic effects. Numerical outcomes are reported in terms of force response of a synthetic parameter α ratio between vertical force induced by ground motion and that due to gravitational load. For this ratio, an analytical expression is proposed and calibrated based on numerical results. The proposed analytical relation, valid for the class of studied buildings, can be considered for design purpose as a more reliable alternative to the response spectra analysis with the current version of design vertical spectra of the Eurocode to assess the vertical force fluctuations on columns and connections. Lastly, different seismic intensity measures are considered in order to define the best predictor of the seismic vertical force fluctuations expected for this class of buildings.