Vertical Earthquake Component Effect on Seismic Demand of Medium Rise Concrete Frames

According to the structural damages observed after the recent near-field earthquakes which are attributed to the vertical component of the ground motion as well as concentration of the damages in column members leading to progressive structural collapse, investigation of ground motion’s vertical component effect has been widely regarded in recent studies. This component is considered less than other component of earthquake and the seismic design codes has been little attention. While the earthquake in near fault zones that has large vertical acceleration comare with horizontal acceleration, caused extensive damage. Damage of concrete columns is an example of the negative effects of the vertical component. vertical component of earthquake is considered in design of spesific members on the recommendation of seismic codes such as the EC-8 and FEMA 356. the design is intended to have with the intended use of the scaled horizontal component , Design this can be done that is unrealistic and will lead to incorrect answers due to lack of stimulation due to the specific characteristics of vertical component of earthquake and structural properties in the vertical direction, also The vertical component of earthquake is less studied in seismic risk analysis. In this study, the effects of vertical earthquake excitations on medium-rise concrete moment frames are investigated in two separate stage including near field and far field records.
In this research, various structural models rep resentative of real structures designed in accordance to seismic codes and under actual gravitational loads have been subjected, simultaneously, to horizontal and vertical components of near- and far-field ground motion records at two stages. Nonlinear time history and progressive dynamic analyses have been performed in this regard. Furthermore, the effect of elevation or reduction of initial gravitational forces as well as columns’ initial axial forces have been investigated by applying differing gravitational loading coefficients. Structural response parameters including tensional and compressional axial loads of the columns as fluctuating forces, columns’ uplift forces at various plan positions and under various gravitational coefficients, the interactive axial-flexural forces of the columns at different gravitational coefficients, shear demand-to-capacity of columns, axial deformation of the columns in presence and absence of vertical component of the earthquake, have been comparatively investigated and the effect of vertical ground motion component has been assessed, separately, for far- and near-field acceleration records and for external and internal columns placed at different stories.
The obtained results reveal that tensional uplift forces are more critical in external columns than the internals. This is mainly true for lower stories while at the upper stories the tensional forces experienced by internal columns are seen to be more critical. Existence of vertical component of the earthquake leads the minimum compression forces to increase and change toward tension range. The amount of this reduction has been shown to reach the value of 84% in the more extreme case. It was also seen that for smaller gravitational coefficients, tensional axial forces are more frequently observed. Presence of earthquake’s vertical component has been shown to amplify the columns’ shear demand by values that reach 31% at the most extreme cases.