The Influence of Vertical Ground Motion on the Design of Common R/C Frames

In this article, the response of reinforced concrete frames concurrently subjected to both horizontal and vertical seismic motions is assessed. The article is not limited to the variation in response quantities but aims to identify which specific design parameters are affected and how, as well as which specific code provisions could be violated due to the omission of vertical oscillations during the design process. Furthermore, the consequences that a design against vertical ground motion would cause in both technical and economic terms were investigated. For this purpose, six eight-storey 2D frames were designed, neglecting the vertical seismic component in compliance with code provisions. Subsequently, the seismic response of the frames to the concurrent action of horizontal and vertical ground motion was evaluated by applying both modal response spectrum and inelastic dynamic analyses. It was found out that several code violations occurred, mainly due to the fluctuation of the columns’ normalized axial forces and the amplification of up to two times or more of the beam bending moments. Thereafter, the frames were redesigned without neglecting the vertical seismic component, and the changes in the members’ cross-sectional dimensions and reinforcement were determined. Finally, it was estimated that the economic impact of redesigning did not exceed 4% of the initial total construction cost of the frames.