Effect Of The Near-Filed Vertical Ground Motion On The Seismic Response Of R/C Frame

Near-field earthquake has two significant characteristics:First, the horizontal ground motion shows large velocity pulse; second, the vertical ground motion is very significant in near-filed zone. Traditional view is that the horizontal ground motion plays a dominant role. To some extent, the effect of vertical ground motion is ignored. Addressing these issues, combined with vertical earthquake damage statistics, this thesis involves the following works:1. The characteristics of vertical earthquake are introduced by comparing with horizontal ground motion. The effects of the fault distance, magnitude, site condition and mechanism on the ratio of peak acceleration are investigated. The results show that the ratio in the near-field zone is big. In other word the vertical ground motion is very significant. The magnitude is larger, the range of influence is wider. In the soft soil site, the ratio of peak acceleration attenuates much slower. From the scope of the entire fault, the ratio of peak acceleration in strike-slip faults is bigger than the other two mechanisms. In the near field zone and short-period segment, it is found that the spectrum value of vertical ground motion is greater than that of the horizontal ground motion. Moreover, the ratio is much greater than specified coefficient of 0.65 in seismic design code. It is noteworthy that the vertical period of structure is relatively small and it is likely to fall within the short-period segment so that the effect of vertical earthquake would be underestimated according to seismic design code.2. According to the rule of the variable axial force of columns under horizontal ground motion and the combination action of vertical and horizontal ground motion, several axial loading patterns for cyclic loading test were summarized. Using fiber model, cyclic loading test of reinforced concrete columns under variable axial force is simulated. When axial force varies proportionally with the lateral displacement the hysteresis loop becomes asymmetry in each direction and its skeleton curves and hysteretic energy also show different characteristics. When axial force varies nonproportionally with the lateral displacement, the hysteresis loop becomes much more complex. When the frequency of the variation of axial force gets faster, the hysteresis loop becomes more clutter. The variation frequency and the phase of variable axial load has a certain influence on the skeleton curves and hysteretic energy. 3. The simulation of an eight-story reinforced concrete frame structures under vertical and horizontal ground motion shows that vertical ground motion in near-field zone has great influence on the seismic response of R/C frame. The structure amplifies the vertical ground motion and the effect of vertical ground motion on the top displacement and the maximum story drift and the maximum displacement of each floor are small but on the drift of top stories is great. Vertical ground motion increases the vertical displacement and tensile deformation appears in frame columns. With the ratio of peak acceleration getting larger and larger, tensile force appears in the frame columns and it is the largest in the middle of the structure. The appearance of the tensile force makes the state of columns into bending and shearing with tension but not pressure. Significant vertical ground motions make the moment and axial force hysteresis loop of the end section of frame columns much closer to boundary of M-N interaction curve at bending and tension zone without increasing the absolute value of moment. To some extent, the variation of axial force also weakens the shear capacity of reinforced concrete columns.