Effects Of Axial-force-ratio Of Column On “Strong Column Weak Beam” Mechanism

In the strong earthquake,the ideal destruction of reinforced concrete frame structure is the plastic hinge forming in end of the beam,column remaining in the elastic state or column’s plastic hinge finally forming.Such the frame structure has better ductility,which can guarantee the fortification target on “it can not fail in the strong earthquake”,names “strong columne weak beam” yield model.According to ensure ductility of reinforced concrete frame,specification limits values for axial-force-ratio of column,structural measures and so on.However,the earthquake damage actually show that part of the frame did not fully achieve the ideal mode of destruction and a lot of frame structure generated lateral displacement,even collapsed.The reason is the effect of strong earthquakes produced larger overturning moment of frame structure,so that the column axial force became large,changed the axial-force-ratio of column,which exceed the limits(especially when used the limit value for axial-force-ratio of column to estimate the size of section of column).By the relation curve of N(axial force)-M(moment),the column axial force increased reduces the capacity of column’s resistance moment.It makes column flexural failure come earlier,leads column plastic hinge happen earlier than beam’s.For column plastic hinge of excessive yield,the floor yield mechanism is forming,so that destructiong of the frame structure is not “strong column weak beam” we expected,it will bring great personal safety and economic property loss.So depend on importance of the influence of axial-force-ratio of column on the ductility of frame,this paper,through the analysis of damage of frame structurd under strong earthquake,researchs the influence of axial-force-ratio of column on “strong column weak beam” yield mechanism by the method of numerical simulation modeling.Including the following parts:From the analysis of the elastic,the influence and basis theory of axial-force-ratio of column on the ductility of frame structure is consolidation analysesd.By adjusting the level of compressive strength of concrete and keeping the column section size unchanged,changing the beam load to realize the control of different axial-force-ratio of column,calculating the structure internal force and reinforcement according to the specification.According to the calculation results to establish the finite element model in SAP2000,using Pushover analysis and nonlinear time historyanalysis to explore the influence of axial compression ratio to "strong column weak beam" mechanism.The collation and results of Pushover analysis and the nonlinear time history analysis shows the relationship between axial-force-ratio of column and the floor displacement,the interstory drift,the order of appearance of plastic hinge.It gets:(1)Along with the invrease of axial load ratio,frame structure ductility decreases,the frame structure is difficult to achieve“strong column weak beam”yield model.(2)Vertical load can improve the frame lateral displacement capacity in the short time,but it is easy to brittle fracture,has ductlle reserve deficiency.