|It is a well-known fact that all earthquakes have three orthogonal components of acceleration including two horizontal and one vertical acceleration. Current design practice for design of structures in United States only focuses on the impact of the horizontal component of earthquakes. However, according to previous research, vertical peak ground acceleration (PGA) can be higher than the horizontal peak acceleration in the same earthquake, which may contribute to structural collapse. Further research is needed to investigate the impact of vertical ground motion on seismic response of structures. In this paper, four three-story and four six-story steel frame structures are chosen to investigate and represent this problem. For each structure height a special moment frame with reduced beam section (RBS) connections and buckling-restrained brace (BRB) frames are designed by the equivalent lateral force (ELF) method based on ASCE 7-10 and analyzed by using nonlinear dynamic analysis. A suite of 40 strong ground motion records are selected including horizontal and vertical ground motions in this study. The range of the ratio of vertical to horizontal acceleration in this study is from 0.5 to 1.2. All the structural models are analyzed under two different loading cases: 1) Horizontal Only and 2) Horizontal plus Vertical. There is a significant impact of vertical ground motion on the column axial force, vertical acceleration and beam midspan vertical deflection. The demand on rotation of reduced beam sections in upper stories also experience a significant influence from the vertical ground motions.