| In building constructions the overturning moment caused by horizontal earthquake is the main source for the additional axial force of the bottom frame columns. Generally, the additional axial force is changing all the time and is not proportional to the horizontal seismic shear when the structure is in nonlinear stage. The variable axial force not only changes the bending capacity, shear strength and deformability of members, but also the hysteretic behaviors. Therefore, how to consider the effects of the variable axial force reasonably is one of the concerned issues in the nonlinear dynamic seismic analysis.Taking the frame-supported masonry buildings as examples, the effects of variable axial force in frame columns were studied in this paper.(1) The multi-spring model was introduced and a dynamic analysis program was coded using the model. Comparisons to some literature experimental results showed the multi-spring model was accurate relatively and the program was reliable and effective.(2) Dynamic analysis for a six-storey frame-supported masonry building was performed in four different levels of horizontal earthquake intensity. It showed the peak axial force at all bottom frame columns had a certain increase according to the increase of the ground motion, and the increase rate improved with the ground motion’s increasing. The increase at side columns was obviously larger than that at the middle column, the increase of variable axial force at middle frame column was about37%-55%to the increase at side frame columns in different seismic intensity. The yielding strength of middle frame column at yielding stage was close in both positive and negative direction, but the strength of side columns were obviously different in the opposite directions because of the variable axial force effect, the difference was nearly30%. Therefore, the influence of variable axial force could not be ignored.(3) The maximum displacement and maximum shear at bottom story of frame supported building were smaller than the storey-model’s results due to the effect of the variable axial force. The difference increased with the horizontal seismic intensity increasing.(4) The dynamic analysis was conducted for frame supported buildings with four-storey, six-storey, and eight-storey. It indicated the changing rate of the variable axial force of each column had an increasing trend to the increase of building stories. The middle column’s changing rate was smaller relatively than the side columns’. After the structure’s yielding, the difference of3d elding strength of side columns in positive direction to negative direction increased with the story’s increasing.(5) Comparison of dynamic response of six-storey structure was done between in horizontal shock and that with vertical shock (av=2/3ah). It showed the peak axial force at all bottom frame columns under horizontal and vertical excitations increased according to the increase of the ground motion, and the increase rate improved with the ground motion’s increasing. But the increase rate at middle column was greater than the side columns relatively to horizontal excitations only. That is, the effect of vertical shock was greater on middle column than on side columns. The relative changing rate of peak axial force of middle column was15.4%. So the vertical excitation could not be ignored for the structure’s internal force response. However, the relative difference of peak displacement at bottom storey was only3.0%in the large intensity (ah=0.45g, av=0.29g). Therefore, the effect of vertical excitation could be ignored for the lateral displacement. |
