Study On The Bending Capacity Of Beam And Column Of RC Frame Joints

Investigation on seismic damage of reinforced concrete frames in Wenchuanearthquake showed that columns were more vulnerable than beams, which was indisagreement with the philosophy of strong column weak beam of the current seismiccode (2001). It causes people pay much attention to the issue of strong column weak beamagain. According to the influnce of strong column weak beam, some studies areconducted as bellows:(1) the influnce of the floor to the beam’s bending capacity;(2) thedifferences of the bending capacity of the same secation between calculating accordingto double–side reinforced rectangular secation and calculating according to single–sidereinforced rectangular secation;(3) the differences of the bending capacity due tocalculating position from the place of the beam central axis intersects the column centralaxis to the place of the beam intersects the column;(4) the study of the momentmagnifying factors of columns;(5) the study of the limit vavlues of axial compressionratio.The main conclusions are listed as follows:1) When the story drift of the RC frame acheives1/50,the effective width of slabparticipating in beam strength is simply suggested as follows:0.20times of transversebeam span for exterior joints;0.35times of transverse beam span for interior joints.2) Put forward a formula for calculating the ratio of the over-strength of slabpaticipating in the beam’s bend capacity; the ratio of the over-strength is greatly changedamong RC frames; for a RC frame,if the beam span and slab reinforcement are the samethe ratio of over-strength decreases as the ratio of steel of beam increases; for the varietisof span of beam and the ratio of steel,it is difficult to define a factor to symbolize the ratioof the over-strength in a frame.3) The bending capacity of the support section of beam has improved whencalculating according to double-side reinforced section in comparison with single-sidereinforced section,the improvement is proportionate to the ratio of the compressive steel’sarea to the tensive steel’s area and the ratio of the tensive steel.4) If the support section of the beam was designed according to the moment of theinterface between the column axis and the beam axis,the bending capacity of beam would improve. The improvement increases with the ratio of the width of column to the clearspan of beam. As to RC multistory frames, the proximate value of the improvement cantake12.7%.5) Because of the insufficient estimation of the bending capacity of beam,thevalues of the moment magnifying factors of columns in the current seismic code (2001)are low in general, causing RC frames to occur strong beam-weak column failure inearthquake.The values of the moment magnifying factors of columns should beimproved,and this paper suggests that the values should be no less than2.0.6) The limit values of axial compression ratio given in the current seismic code(2001) are high in general. On one hand,the values do not guarantee the column occourlarge eccentric compression failure; on the other hand, the yield displacement of columndecreases as the values of axial compression ratio increase,causing the frame easier tooccur strong beam weak column failure. So the limit values of axial compression ratioshould be reduced.This paper suggests lower the values of the first,second and thirddegree of seismic of RC frames to0.55,0.65and0.75respectively.