Horizontal and Vertical Seismic Acceleration Demands in Multi-Storey Buildings

Accurate prediction of peak floor accelerations is a crucial step in developing simplified methods for the seismic analysis and vulnerability estimation of acceleration-sensitive non-structural components (NSCs) attached to buildings subjected to earthquakes. A particular characteristic of seismic floor accelerations is that they increase along the building height, with a maximum amplification at the rooftop level. While the amount of horizontal acceleration amplification currently suggested in most modern building codes is empirical, the amount of vertical acceleration amplification is not fully addressed yet in most building codes. This paper discusses the amplification of horizontal and vertical rooftop accelerations based on the analysis of recorded accelerations in 7 buildings during the 1999 Chi Chi earthquake in Taiwan. The ratio between vertical and horizontal rooftop accelerations and spectra is also discussed. Evaluated parameters affecting the amplification of vertical accelerations include: the building vertical period of vibration, the number of stories, and the location of the building relative to the fault. Findings are discussed and compared to provisions currently proposed in the ASCE 7, the NBCC 2015, the CSA S832 and the Eurocode 8. The most significant trends are highlighted.