# Experimental Assessment of the Effect of Vertical Earthquake Motion on Underground Metro Station
## 4 Conclusions  
An atrium-style metro station is a favorite choice owing to its excellence in providing a larger and clearer space for the passengers and commercial clients at the station hall floor. However, concern for the seismic safety of such a station is growing since its hall slabs are replaced with the flat-beams. Tests are needed for the evaluation of its seismic safety. A series of 1 g shaking table tests were conducted to investigate the seismic responses of an atrium-style metro station.  
In this study, extensive experimental results were presented to investigate the influence of the RVH (ratio of vertical to horizontal peak ground acceleration) on the seismic responses of both the soil and the underground metro station. Results show that the acceleration amplification factors (AAFs) on the ground surface are about 2.3-3.1, and that the soil presents significant amplification under several considered conditions. Soil horizontal acceleration is more sensitive to the higher RVH or higher vertical acceleration. Significant differences in horizontal acceleration amplitude between the side wall and the adjacent soil at the depth of the ceiling slab are found, and attention should be paid to these kind of differences for the underground structures with zero or near-zero buried depth, such as atrium-style metro stations. With increasing RVH, the differences in the horizontal acceleration amplitude between the side wall and the adjacent soil also rise. The RVH has a significant influence on the dynamic interaction, especially for higher RVH. Under the input motion of Loma Prieta, the distribution of the peak DSNS (dynamic soil normal stress) along the left and right side walls represents significant difference. The peak DSNS along the left side wall follows an approximate L-shaped distribution, while it is different for the right side wall. With increasing RVH, there is no significant change in the amplitude of the peak DSNS along the left side wall on the whole, while it increases a lot (the maximum increase reach about 148%, 159%, and 199% under RVH = 0.5, 1, and 1.5 when comparing with that under RVH = 0) for the right side wall. The soil and structure behave differently in the seismic responses with increasing RVH. The stresses in the cross-section of every structural element are uneven under horizontal-vertical earthquake excitation.  
In conclusion, from the perspective of the soil acceleration, structural acceleration, and dynamic soil normal stresses along the side wall of station, experimental results have proven that ratio of vertical to horizontal peak ground acceleration (RVH) has significant influence on the dynamic soil-structure interaction. It is believed that under extreme earthquake loading, such as near fault zones, RVH is a parameter of paramount importance and should be accounted for in the seismic analyses and seismic performance assessments of the underground structures, especially for those with zero or near-zero buried depth, such as atrium-style metro stations.  
It is worth mentioning that further investigation of the effect of the vertical earthquake motion is essential through a series of numerical analyses using a minimum number of the earthquake records specified by a certain code. In this way, a cross-check of the findings from the multiple records could be expected to accomplished and this is a work in progress.  
Test scheme, Z.Z., Y.Y., H.Z., and H.Y.; experimental works, Z.Z.; discussion and analysis, E.B. and Y.Y.; writing--original draft preparation, Z.Z.; writing--review and editing, E.B. and Y.Y.; funding acquisition, Y.Y. and H.Z.  
**Funding:** This research was funded by the National Key Research and Development Plan of China (Grants Nos. 2018 YFC1504305, 2018 YFC0809600, 2018 YFC0809602, and 2017 YFC1500703), the State Key Laboratory of Disaster Reduction in Civil Engineering, Tongji University (Grant No. SLDRCE15-02), and the China Scholarship Council (Grant No. 201706260143).  
**Conflicts of Interest:** The authors declare no conflict of interest.