| Using column moment amplification factor?_c in seismic design is one of key earthquake resisting measures for reinforced concrete(RC)frame members,which can promote a good yield distribution pattern and good performance and safety of structures under strong earthquakes.So far,although the effectiveness evaluation results of the factor?_c in Chinese design code has been published,little research has been conducted to analysis the composition factor and quantitative evaluation of the factor?_c.To explore the factors to be considered and the impact extent have the essence of finding out the its formation law.This paper attempts to answer this question,accordingly evaluate the effectiveness of the current amplification measure for each earthquake-resistant grades in Chinese seismic design code.This paper mainly carried out the following work:(1)The similarities and differences of "strong column-weak beam" norm among different countries was summarized,then according to related theory and analysis results,a method of quantifying the amplification factor?_c from its composition was proposed.(2)Five regular RC frame structures were strictly designed according to the current Chinese code,which respectively located in earthquake intensity region of 7(0.10g),7(0.15g),8(0.20g),8(0.30g),9(0.40g).Then the control condition of element dimensions and section reinforcement in each example frame was analyzed.Nonlinear dynamic analyses under seven sets of rare earthquakes were finished,after completing the framework of the elastic-plastic modeling.(3)The main quantitative work included: analysising the statistics result of the effective “beam moment overstrength” for edge and middle node in each example frame,and using "refinement analysis" method point by point to identify the "moments proportion growth rate of the upper or lower column ends”,which was the proportion of non-elastic moment in rare level earthquake relative to the proportion of elastic moment in frequent level earthquake;(4)Whether the changing axial force has effect on appear of side column plastic hinge was discussed,and then the paper try to explainned the reason of the discussed results essentially.Based on the above work,the main conclusions were as follows:(1)To achieve the purpose of slowing down the yield of the column end under rare earthquakes,the amplification factor value must taking two kinds of increasing magnitude into account based on the reliability design.One is the overall bending moment increase,which can be equivalent to the element moment overstrength.The other is additional increase of the bending moment of the column end.(2)Maximum elastic-plastic floor drift ratio of each example frame was less than the code limit 2%,and most of them were no more than 1.5%,which indicated that the seismic control of frame structure in China code is generally effective.From the aspect of beam and column yield rate,the frame of earthquake intensity 9 basically achieved the purpose of "strong column-weak beam".And the frame reinforcement of earthquake intensity 7 was generally controlled by the minimum reinforcement ratio,which increased the seismic bending resistance of column,leading to its yield rate within the acceptable range(35% or less).It was worthy of attention that the column yield rate of earthquake intensity 8 was large,and the example frame KJ3 formed instant story-sway mechanism under some seismic waves.All these phenomena indicated that the amplification factor?_c of earthquake intensity 8 should be improved.(3)The yield rate and plastic development degree of side column was smaller than those of middle column.Although the axial force were seriously different between the two side column in same floor at the same time,their yield time was consistent.These phenomena indicated that the significant change of side column axial force in rare level earthquake did not result in excessive yield rate of column.And this paper explained the causes of these phenomena from three aspects.It is further confirmed that the analysis of the amplification factor?_c composition may not take the axial force changes into account.(4)Beam bending “overstrength” in the low intensity region is much serious,especially in high floors,which is the main reason of these beam far from yielding.This paper recommend that "strong column-weak beam" may not be needed in above situation.In other words,this paper argues that the amplification factor?_c can not be divorced from the rare earthquake level.And ?_c should be as much as possible to aviod the column from yielding in rare level,rather than a certain strict guarantee of "strong column-weak beam" for each node.In addition,for the beam end whose bending “overstrength” was not strong enough,using the method of formula 6.2.2-2 in Chinese seismic design can not avoid the column end from yielding.The reason is that "moments proportion growth rate of column ends” is not fully considered.Therefore this paper keeps a reserved attitude about formula 6.2.2-2 in code.(5)Using a limited number of example structures and not taking into account the influence of the structural period on "moments proportion growth rate of the column ends”,this paper took a same representative numerical value for each example frame.The accumulation of analytical results of more research works will provide a more comprehensive reference for judging the rational value of this moments proportion growth.(6)In the Chinese design code,the traditional midset view that the aseismic measures should increase step by step when the earthquake-resistant grade increase.But this paper and the literature of recent research found that if frames of different earthquake-resistant grades,or more accurately different earthquake intensity regions,were able to reach a similar column yield rate,the above-mentioned traditional view may no longer apply.The approximate values obtained from this paper respectively were 2.0,1.7,1.7,1.8 and 1.7,from intensity 7(0.10g)to intensity 9(0.40g).In this paper,proposed values were generally larger than code values,which was based on the premise that frames of different intensity regions had good seismic performance,and the yield rate of the column end could be controlled at about 15%.If the engineering considerations can moderately reduce the structural performance standards,such as relaxing the yield rate of the column end,the recommended value is also reduced accordingly.According to the above research results,the value of amplification factor?_cidentificated by this paper was consistent with that of seismic code specification for earthquake-resistant grade ?,so the value can be kept.It is suggested that the?_c value of earthquake-resistant grade II should be the same as the value of earthquake-resistant grade ?.For earthquake-resistant grade ?,The minimum reinforcement ratio increased the bearing capacity of the column section,which increased the difficulty of giving a reasonable judgment to the?_c value.Because column section reinforcement was still controlled by minimum reinforcement rate if only a small increase in?_c value,then this increase was useless.And the factor?_c should be raised to at least 1.6.to reduce the yield rate of the column.The main innovation of this paper:(1)A method of quantifing the column moment amplification factor?_c from its composition proposed.(2)"Moments proportion growth rate of the upper or lower column ends” of the rare earthquakes was indentified by time point,and this value in the typical frame of each intensity region was given roughly.(3)The situation of the inflection point being out of the column in rare earthquake was identified,basically appeared in the top floor,because its corresponding inflectoin point had been deviated from the farther in frequent earthquake,A little fluctuation in the earthquake would not be in the column.(4)The view that the column moment amplification factor?_c can not be separated from the rare earthquakes was further emphasized.Strict measure of the "strong column-weak beam" and to avoid the column end in a rare level from yield are not exactly the same. |
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