Seismic Performance Of Reinforced Concrete Columns Under Near-field Earthquake

In frame structures,reinforced concrete columns are important members to resist lateral loads. The damage of the RC columns, which is difficult to be repaired after earthquake, is a major cause of the collapse of the RC frame. According to the features of the near-field earthquake, low cyclic loading experiments were carried out on 4 large scale RC column models. Based on the results, the anti-seismic property of RC column and its damage are analyzed. The research work of this paper could be summarized as the following:(1) Devised the entire test schedule and deployment, designed and assembled the test equipments. In accordance with the present national code, designed and constructed 4 large scale RC column models, conducted the material property test of the steel bar and concrete used, and finished the nonlinear time-history analysis of a high rise frame which is designed to resist the 8th degree earthquake intensity. In the analysis, there are in total 20 near-field earthquake wave records used.12 among them have their PGV/PGA>0.2, and the other 8 have their PGV/PGA<0.2. The result shows that the near-field earthquake poses a stricter demand on the ductility of the frame structures. In the 12 tests using the earthquake wave with their PGV/PGA>0.2,92% of the frame failed to meet the standard in the 8th degree rare earthquake elastic-plastic verification. Based on the features of the time-history curve of the RC columns in near-field earthquake, devised the loading policy and measuring policy in detail accordingly.(2) Low cyclic repeated loading tests were conducted on 4 RC columns in total. P-1 model column was used to confirm the reliability of the test equipments. L-1 model column was loaded following the standard loading policy. It was used to examine the ductility of the RC column designed according to the current code for seismic design of buildings GB 50011-2001. L-2 and L-3 model columns were loaded respectively according to the loading policy taking into consideration of the near-field earthquake features and the traditional loading policy. They were used to distinguish the influence of loading policy on the anti-seismic property of RC columns. The paper exhaustively analyzed the hysteresis properties of the 3 L series RC columns, such as their skeleton curve, deterioration of stiffness and energy dissipation. The paper also investigated the influence of loading policy on the anti-seismic property of the columns and on the length of the plastic hinge in the members, considering the strain distribution of the stirrup and the lateral pressure. It is concluded that:(1) The ultimate inter-story displacement angle is 3% in the L-1 test, and the displacement ductility is 4.47. Therefore, in view of the current code, the frame column does not have enough ductility storage in the near-field earthquake; (2) The average secant stiffness of the L-3 column corresponding to the small displacement after the peak displacement approximate that of the peak displacement. With the decrease of the displacement loading amplitude, the average secant stiffness rose a little. Compared with the average secant stiffness of the L-2 column, the non-peak displacement average secant stiffness is remarkably smaller. (3) Accumulative energy dissipation ability has nothing to do with the sequence of the peak displacement. (4) The maximum strain of the stirrup in the plastic zone generally appears half the cross section height near the pedestal. (5) Because of the eccentricity of the cross sectional internal force, the stress of the stirrup on each side of the cross section can not reach yield stress at the same time. Therefore, the cross sectional restrictive force calculated according to the J.B.Mander formula may have been overestimated. (6) The length of the plastic hinge depends on the ultimate displacement. Hence, the lengths of the plastic hinge in L-2 and L-3 columns loaded according to the loading policy stipulated in this paper are significantly less than that calculated from the experience-based formula.(3) Researched the accuracy of the Park-Ang concrete Damage Model with respect to the assessment of the test damage discussed in this paper. The comparison shows that though the parameters in the damage model were obtained from the test result of the L-1 model column, it is still difficult to make an accurate assessment of the damage development in the L-2 and L-3 RC column models. The fundamental reason is the Park-Ang damage model parameters is difficult to be accurately determined, but it dose not account for the loading policy. This paper also investigated the relationship between the lateral deformation of the key cross sections of the RC columns and the structural damage, the result shows that lateral deformation is a good indicator of RC column shear damage, buckling damage of longitudinal steel bar and concrete collapsing damage.