Study On The Inelastic Stability Checking Method Of Frame Column Members In Reinforced Concrete Building Structures

For any reliable structural system,they all need to meet the basic requirements of strength,stiffness(or deformation)and stability.In steel structures,steel has the characteristics of high strength,good toughness,uniform materiality and near elastic-perfectly-plastic mechanical properties,so when meeting the requirements of strength and stiffness,structural members are generally designed to be slender or thin,and the stability of members will control the design of members.Therefore,the steel structure design codes around the world have made specific provisions on overall and local stability checking of single members in structures,in addition to lateral stability checking of the whole structure.In reinforced concrete structures,the strength of concrete materials is significantly less than that of steel,and the slenderness ratio of members are usually not too large to cause instability,thus the concrete structure design codes in various countries have made clear provisions on considering the second-order effect of column members,but there is still no specification or a reference method for the stability checking of column members.Nevertheless,when faced with some special column members with relatively large slenderness and axial force(e.g.cross layer column),structural designers still need to answer the question that whether they are at risk of instability.So far there is no widely accepted effective method for inelastic stability checking of RC frame column columns in published research results at home and abroad.Therefore,due to the actual needs of structural design,and based on the results of static inelastic analysis or inelastic dynamic response analysis of structure,this paper attempts to give a stability checking method for single column member in structures under inelastic stress when the whole structural system satisfies the lateral stability requirements under gravity load.The main conclusions of this paper are as follows:? So far some researchers still misused the concepts of effective length in ?-l0 method and instability buckling mode during the stability checking of column members,thus this paper illustrated the basic concepts of column stability and paid special attention to the concepts of effective length and instability buckling mode,meanwhile briefly explained the difference and connection between the effective length of second-order effect(e.g.the effective length l0 used in ?-lo method)and the effective length of stability.Both types of effective length indicate some equivalent relationship,but the equivalent content of the two is different in concept,the effective length of second-order effect principally reflect the equivalent relationship between moment amplification factor of control section of“standard column”corresponding to the actual stress state of frame column and moment amplification factor of column end control section when considering P-? effect;the effective length of stability is used to replace the critical load of column which does not meet the condition of hinged support at both ends with a hinged column which has same cross section and material,so the effective length of stability principally reflects the equivalent relationship of the critical load of columns.On the other hand,for the sake of simplifying the calculation method,some codes represented by the ACI 318 Code[1]"borrowed" the effective length of stability derived from the "member isolation method" to replace the effective length of second-order effect when approximatively calculating the P-? effect of sway frame columns by ?s-l0 method which considers layer effect,but due to the complexity of second-order effect distribution regularity in actual structures,this“borrowing”does not guarantee that the P-? effect regularity of frame columns can be accurately reflected under various conditions.Therefore,?-l0 method can only be regarded as a simplified method for approximately calculating P-? effect of frame columns before the elastic second-order analysis method which can directly considers the second-order effects of structures is not yet mature,as the nonlinear analysis of building structures(such as finite element method based on geometric stiffness)is relatively mature at present,?-l0 method seems to be no longer suitable for considering the P-? effect of frame columns,let alone apply l0 used in this method to column stability analysis.?This paper briefly introduced the relevant methods proposed by current research community for the stability checking of some special column members in structural design,and pointed out the main problems may exist in these methods.For example,in the absence of sufficient understanding of the preconditions and scopes of buckling mode,applying the high-order buckling modes obtained from structural buckling analysis to check the stability of column members should be regarded as an undesirable method with incorrect theoretical concept.? This paper proposed a stability checking method based on the actual inelastic deformation characteristics of frame column-type members,according to the main idea of this method,a method for deriving the inelastic effective length of frame columns from their actual inelastic deformation and stress states was given,on this basis,a series of simulation analysis of inelastic column model corresponding to frame column is completed by general finite element analysis software ABAQUS which is widely accepted,and the validity of finite element models was verified,eventually,a mathematical expression for calculating the critical load of frame columns was fitted.?Based on the results of inelastic dynamic response analysis of a high-rise building structure example with a slender cross layer column,this paper applied the stability checking method proposed in this paper to check the stability of the slender cross layer column at the probably most unfavorable time point,and the specific operation process of the stability checking method proposed in this paper was illustrated.Checking results showed that,under the seven rare level one-way ground motion inputs given in this paper,the slender cross layer column in structure example at the time point to be investigated(including the maximum displacement time point and the maximum axial compression force time point)was not yet occurred instability.The main characteristics of the column members stability checking method proposed in this paper are as follows:? The method conforms to the basic principle of stability theory conceptually;? The method considers the inelastic behavior of reinforced concrete materials as much as possible;?The checking process is relatively simple and convenient for structural design.Based on the results of static inelastic analysis or inelastic dynamic response analysis of structure,and according to the stress and deformation characteristics of frame column members in different parts of the structure at different states or different points during the analysis process,designers can select frame column members with relatively larger slenderness and axial pressure,which may have higher risk of instability,and check them one by one according to the proposed method in this paper.