Identification Of Shear Overstrength Factors For RC Frame Beams And Columns

Using shear overstrength factors and column moment amplification factors as well as details of seismic design in seismic design all belong to key seismic measures for reinforced concrete(RC)frame members in the system of seismic measures to ensure structural behavior.So far,the research communities in the world have done research from different angles about the plastic zone distribution pattern the frame should form under strong earthquake and the column moment amplification measure to ensure the distribution pattern.Little systematic research has been conducted up to date which focused on the actual effect of shear overstrength factors for different seismic grades.Therefore,it is necessary to make up for this vacancy.The main research work in this paper includes:(1)The test results of seismic shear performance tests and the computational scheme of seismic shear proposed in different countries were summarized.And the preliminary evaluation of these research results and scheme were made.(2)Five regular RC frame structures were designed according to the current code,which respectively located in region 0.10g?0.15 g of Intensity 7?region 0.20g?0.30 g of Intensity 8 and region 0.40 g of Intensity 9.The nonlinear models were established,in which the average strength and nonlinear characteristics of the materials were taken into account.And nonlinear dynamic analyses for each frame under multiple sets of rare earthquakes were finished.(3)Basing on that the shear capacity formulas of frame members given by the code could reasonably estimate shear capacity under the strong ground motion,the ratios of the shear demand under rare earthquakes to the average shear capacity for beams and columns were calculated to identify the effectiveness of shear overstrength factors following the rule that ratios of more than 90% of components were less than 1.0.The factor that did not meet to the rule was adjusted gradually and the above analysis was repeated until that the factor was up to the rule.(4)The amount of stirrups required by details of seismic design and adjusted shear overstrength factors were calculated.The comparison between them was made to make clear that which of the two requirements played a decisive role in design.Based on the above analysis,main conclusions were as follows:(1)It was assumed that the components met the actual flexural strengths or nominal flexural strengths to determine the design shear in the overseas design codes,and shear overstrength factors were adopted to consider the adverse circumstances.The way above was clearly conservative in most cases.(2)Shear overstrength factors for RC frame beams were up to the rule,except Grade 2 frame structure located in region 0.30 g of Intensity 8.It was suggested that the shear overthength factor for Grade 2 frame structure in region 0.30 g of Intensity 8 should be taken as 1.3,which could meet the rule for shear safety.The adjustment of shear overstrength factor above would guarantee the “Strong Shear Weak Bending” to take effect.(3)Shear overstrength factors of RC frame columns failed to guarantee column shear safety at an equal chance under rare earthquake for that column shear force had taken into consideration the influence of column moment amplification factors.That is,shear overstrength factors for columns of Grade 2 and Grade 3 were insufficient,while that for Grade 1 was larger.(4)Two sets of adjustment schemes for shear overstrength factors of column were proposed.Considering the influence of column moment amplified factors being considered in calculation of column shear,shear overstrength factors required for column were taken as: 1.45 for Grade 1,1.35 for Grade 2,1.45 for Grade 3.And if not,shear overstrength factors required for column in different seismic grades were taken 1.80 for grade 3,1.90 for grade 2 in 8(0.20g)zone.And that for Grade 2 in Intersity8(0.30g)and Grade 1 frame structures was taken as 2.0.Therefore,the second adjustment scheme was consistent of shear overstrength factors for beams.According to the second set of adjustment scheme,the values of shear overstrength factors advised was not consistent with the seismic grades,but such values were more reasonable.(5)The amount of actual stirrups required for components at the bottom floor of frame of Grade 2 in region 0.30 g of Intensity 8 were determined by seismic shear requirements,and the same as Grade 1 in region 0.40 g of Intensity 9,and the amount of actual stirrups of in other situation were determined by the requirement of the restraining stirrups.The main innovation of this paper were as follows:(1)Basing on ratios of shear demand of beams and columns under rare earthquake to shear capacity determined by shear stirrups,the effectiveness of shear overstrength factors for different seismic grades were identified.(2)From the angle of initial purpose of "strong shear-weak bending",shear overstrength factors which did not met the rule setted in this paper were adjusted until that the ratio was up to the rule,which was better than the assumption that components met the actual flexural strengths.(3)The amount of stirrups required by details of seismic design and adjusted shear overstrength factors were calculated,and the comparison between them was made to make clear that whether of the two requirements played a decisive role in design and provided the basis to quantify the relationship of the two measures.