Preliminary Study On Requirement Of RC Frame Structures To Achieve "Strong Column-weak Beam"

Most reinforced concrete frame structures did not achieve the "strongcolumn-weak beam" failure modes under the excitation of strong ground motions inrecent earthquakes. On the contrary, the column ends of some structures formed plastichinge firstly before beam ends. This kind of failure is likely to cause severe damage ofsome weak stories or even the whole collapse of structures, resulting in a large numberof casualties and significant property loss.To solve this serious problem, a strong column-weak beam factor is presented anddiscussed to characterize the relative yield situation of column ends and beam ends. Thereinforced concrete frame structures could achieve the "strong column-weak beam"damage mechanism under strong ground motions by limiting the strong column-weakbeam factor. Furthermore, the critical value of the strong column weak-beam factors,which is defined as the value of the strong column-weak beam factor when the damagejust transits from the "strong column-weak beam" mode to the "strong beam-weakcolumn" mode, is defined as the limit value of strong column-weak beam factor. Finally,the limit values of strong column-weak beam factor are calculated, analyzed andverified by using ABAQUS software. The main contents and conclusions aresummarized as follows:1. The damage characteristics of reinforced concrete frame structures under strongground motions are summarized firstly. Then the strong column-weak beam factor andthe limit value of strong column-weak beam factor are presented and defined. Thesimplest model is selected to demonstrate the calculating procedure for the strongcolumn-weak beam factor and the limit value of strong column-weak beam factor byusing the finite element software ABAQUS.2. The characteristics of limit values of strong column-weak beam factor underdifferent conditions are discussed according to a large number of simulated structuresby using the finite element software ABAQUS. It is found that:(1) the limit value ofstrong column-weak beam factor increases smoothly at first and then fast with theincrease of the axial compression ratio;(2) the limit values of strong column-weakbeam factor of structures under bi-directional strong motion input are smaller than thoseof uni-directional seismic input;(3) the limit value of strong column-weak beam factoris effected little by the same ground motion with different PGA (Peak GroundAcceleration);(4) the limit value of strong column-weak beam is effected greatly bydifferent strong ground motions even adjusted to same peak acceleration, but theinfluence is smaller when the axial compression ratio is small and greater when the axialcompression ratio is large. The limit value of strong column-weak beam factor and themoment magnifying factor of columns in the code for seismic design of buildings arecompared to examine the rationality of the regulations in the code. After analysis, it isshown that the “strong column-weak beam” may not be realized by using current seismic design code, considering the requirement of relative strength of column endsand beam ends only.3. A5-story reinforced concrete frame structure is designed to show the applicationof the presented factor. Verification of the appearance and the law based on limit valueof strong column-weak beam factor is given with two representative strong groundmotions. Also, possible failure modes in certain strong ground motions of structuresmeeting the requirements of codes are verified. In some cases, the plastic hinges formedat neither beam ends nor column ends because of high strength of the members ofstructures, and the structures are safe enough in these cases.