Influence of vertical earthquake on the variations of axial load ratio of reinforced concrete buildings

Current earthquake engineering field ignores the repeated and vertical ground motion in design and analysis of the structure system even though in actual condition these two phenomena impose the significant effect to the structural system. This gradually changing due to the increase in near source record obtained recently, coupled with field observation confirming the possible destructive effects of high vertical vibration. The aim of this study is to assess the behaviour of regular and irregular reinforced concrete frames due to multiple earthquakes with vertical component. The structural response quantities are expressed in term of variation of axial load. Axial load ratio obtained by dividing axial load in column induced by combined horizontal and vertical component of ground motion (VHGM) to axial load in column induced by horizontal component of ground motion (HGM) load. Obtaining vertical spectral shape by scaling the horizontal ground motion using V IH ratios of 2/3 rule as suggested by many codes can be seriously underestimate action on structures located near earthquake sources and overestimates action in far field regions. The frame models are subjected to the horizontal and vertical ground motions with various peak ground acceleration ratios between horizontal and vertical ground acceleration (V /H) ranging from 0.3 to 1.9 using RUAUMOKO software. This study found out that vertical ground motion showed significant effect to the reinforced concrete building with maximum axial load ratio of 54 for regular and 6 for irregular rc frame. Eight storey regular models showed typical graph with the shape of number three for plotted axial load ratio against height. Axial load ratio values was almost equal to one at base, mid and top floor but increases at one fourth and three fourth of the building height. Irregular model showed typical graphs with higher axial load at lower floor and decreased along the heights.