A procedure for combining vertical and horizontal seismic action effects

The vertical component of earthquake ground motion has generally been negle cted in the earthquake-resistant design of structures. This is gradually ch anging due to the increase in near-source records obtained recently, couple d with field observations confirming the possible destructive effect of hig h vertical vibrations. In this paper, simple procedures are suggested for assessing the significan ce of vertical ground motion, indicating when it should be included in the determination of seismic actions on buildings. Proposals are made for the c alculation of elastic and inelastic vertical periods of vibration incorpora ting the effects of vertical and horizontal motion amplitude and the cross- coupling between the two vibration periods. Simplified analysis may then be used to evaluate realistic vertical forces by employing the vertical perio d of vibration with pertinent spectra without resorting to inelastic dynami c analysis. Finally, a procedure is suggested for combining vertical and horizontal sei smic action effects which accounts for the likelihood of coincidence, or ot herwise, of peak response in the two directions.