Nonlinear Analysis Of Concrete Structure Based On Opensees And Research On Reducing Factor Of Member Stiffness

The second order effect is a common mechanical phenomenon in the structure.In the structural analysis,the second order effect on internal force and deformation cannot be ignored.The second-order effect refers to the additional effect of the load in the post-deformed structure or the axial force in the deflected member in addition to the first-order effect.From the research results of the second-order effect and the design specifications of each country,the current approach iselastic second-order analysis method considering member reduction stiffness.The method considers the nonlinearity of the material under the actual force condition by the stiffness reduction of the component.The geometric nonlinear characteristic is considered by the elastic second order analysis of the structure.This method is also adopted in Chinese 2010 version of "Code for Design of Concrete Structure".The analysis shows that when the reasonable component comprehensive stiffness reduction factor is selected,the calculation accuracy and efficiency of the method are sufficient to meet the needs of engineering design.From 1997 to 2004,Professor Bai Shaoliang led the team(the author's mentor is one of the participants)to study this issue,and put forward thestiffness reduction factor scheme of the frame structure member which has been adopted by our concrete specification.But limited to the conditions and technical means at the time,the proposed stiffness reduction factor needsto be re-examined.In this paper,based on the practical engineering concrete frame structure nonlinear simulation and stiffness reduction factor analysis,the following research work is completed:1 The paper completed the simulation analysis of 4 cantilevered columns and 2 single story and single span frames,showed that the results of nonlinear simulation are in good agreement with the experimental results,verified the effectiveness of nonlinear program OpenSees for simulating the nonlinear behavior of reinforced concrete frame structures.It is also pointed out that the bond-slip effect of steel bar should be considered by zero length element at the end of beam and column to realize the more accurate simulation of structural displacement.2 The paper completed the nonlinear analysis of 3 RC frame structures which derived from practical engineering,and compared with the stiffness reduction coefficient of the current American code and Chinese code,the results show that the second order elastic analysis are in good agreement with the nonlinear results of the second order elastic analysis for the low frame structure,no matter that the American code or the Chinese code member stiffness reduction scheme is adopted;the results of nonlinear displacement and internal force for a high flexible frames are larger than second order elastic analysis of considering stiffness reduction,the contrast difference of internal force are mainly reflected in the control moment of bottom column end,the nonlinearity of the performance is about 20% larger than the two order elastic analysis of considering stiffness reduction.Therefore,the stiffness reduction coefficient of the current code is larger to the high flexible frame.The negative effects of geometric nonlinearity of structures are underestimated.3 The paper designed 2 RC frame-shear structures by using the concepts of integrated frame and composite shear wall,carried out the nonlinear analysis of these 2 frame-shear structures comparison with American code and Chinese code for stiffness reduction coefficient scheme.The results show that,for frame shear structures,the nonlinear analysis of different frames have different laws from the second order of stiffness reduction,when the frame is larger,the results of partial nonlinear analysis of frame are larger than those of elastic second-order analysis in terms of internal force,when the frame is small,the internal force of nonlinear analysis is smaller than elastic second order,so different stiffness reduction schemes should be taken into consideration,but the difference is within 10%,it is considered that the same stiffness reduction scheme can be used for frame-shear wall structures with different frame ratios.The American standard is used when the frame is larger,and the Chinese scheme is used when the frame is small.the American code should be adopted when the frame proportion is large,and the Chinese code should be adopted when the frame occupies a relatively small proportion.The relevant research results can be used to guide the actual engineering design,and provide reference and basis for the revision of relevant provisions and follow-up research work.