Research On Criteria For Failure Of The RC Column And The Damage Cumulation Behavior Of Its Concrete Fiber

According to the modern seismic conception, the critical zone of the RC member can be in the state of inelastic deformation after yield under strong motion. The damage cumulation mode of the structure in the process of this random inelastic deformation is different from that of the structure under static loading, i.e., damage cumulating with the increase and cycles of the deformation of the structure. Therefore, a method, which can predict whether structural members will lose its carrying capacity of seismic action and vertical loads in the process of this inelastic deformation, is needed imminently. This method is usually called the criterion for failure of the structural member under the response of strong motion.It has been sought for years that a criterion which can predict the failure of the member in inelastic displacement cycles on the basis of test results of RC specimens under reversed cyclic loading of various modes till failure. In the existing failure criteria, there are various single parameter criteria proposed early, multiparameter criteria proposed subsequently, and the failure criterion based on the damage increment cumulation of single cycle proposed recently. In all these criteria, Park-Ang criterion is the most widely used. On the basis of test results of RC columns with constant axial load ratio under cyclic loading of various modes till failure collected in this thesis, Park-Ang criterion and the failure criterion based on the damage increment cumulation of single cycle have been calibrated. In addition, the 7 low-cycle fatigue tests of RC columns have also been carried out in this thesis in order to get the property of the concrete fiber of the RC column under reversed cyclic loading with symmetric constant displacement amplitude. Among the 7 tests, the data of 3 tests are complete and of other tests are not complete because of mistakes in the test. The conclusions are as following:① Based on the analyses of the low-cycle fatigue test results of RC columns collected in this thesis, the law of damage cumulation is found: the specimens with smaller amplitude of cyclic displacement can undergo much more cycles than that with lager amplitude.② Based on the test results of RC columns with constant axial load under low-cycle reversed cyclic loading, the connotative damage cumulation law of Park-Ang criterion is discovered: in the two main factors, displacement and energy, which result in damage cumulation till failure, the contribution of displacement term decreases withthe increase of the maximum displacement ductility factor that the specimen experienced, while the contribution of the energy term increases. Obviously, this is contrary to the law from tests stated above.③ Based on the test results collected in this thesis, Park-Ang criterion is modified to match the damage cumulation law from tests stated above. The veracity of predicted outcome is improved and the variance decreases.④ Based on the low-fatigue test of RC columns with constant axial load carried out in this thesis, the change of the average strain of the concrete fiber under cyclic loading was firstly measured in the tests, the conclusion of which is as following: when the column with constant axial load is loaded cyclically with symmetric constant displacement amplitude, the maximum compressive strain in each cycle increases and the maximum nominal tensile strain in each cycle decreases with the increase of cycles.⑤ Based on the low-fatigue test of RC columns with constant axial load carried out in this thesis, the relation between the performance of the RC column under cyclic loading and its axial load ratio is discussed. It is found that 1) the descending branch of the envelop of its P —△ curve inclines heavily as the axial load ratio increases; 2) the height of the cover spalling increases with the increase of the axial load ratio; 3) the rupture of the longitudinal bar seems appear when the axial load ratio is small.⑥ Based on the test results collected in this thesis, the failure criterion based on the damage increment cumulation of single cycle, which was proposed by the author's research group, is verified, and the equation of the low-cycle fatigue life of the RC column at various axial load ratios under cyclic loading with symmetric and asymmetric displacement is modified. The validity of the definition for failure with this criterion is improved.⑦ Based on the yield displacement measured in the tests of RC columns finished by the author's research group, the worldly famous equation of the chord rotation at yielding, which is proposed by Panagiotakos and Fardis, is verified. It is found that the value of the coefficient, which considers the slippage of the longitudinal bars from their anchorage beyond the section of maximum moment, need improve.