| Concrete rigid frame bridge with long spans and high piers has been widely used both home and abroad due to its several advantages, such as:legerity structure, good look, large span, and convenient construction. However, the seismic resistance capacity cannot be understand by the designers because of difficulty to run dynamic analysis and lack of earthquake damages. This study is aimed to enhance the understanding of the seismic resistance capacity of this kind of bridge based on Niu lanjiang bridge in Yunnan Province through some research methods, such as Pseudo-dynamic test, on-site model test and numerical simulation. The following work has been done:Firstly, on-site scientific investigation is conducted on a mount of bridges in seismic region of Wen chuan Earthquake. The result shows that the most fragile part of a bridge which under seismic load is the support system, then the pier and the base follow. The last part to damage is the superstructure. The bridges which have a novel appearance and complicated structure show a poor behavior under seismic actions.Secondly, to obtain modal parameters of Niu lanjiang bridge, On-site model test is carried out through ambient vibration test. The data indicate that the first order frequency in the transverse direction is that of this bridge, so this direction is chosen for Pseudo-dynamic test. The test result shows that the ratio of the transverse first order frequency to the second order frequency of this bridge is quite close to that ratio of elastic homogeneous straight bar. So the transverse frequency of this kind of bridge can be calculated approximately according to the elastic homogeneous straight bar.Thirdly, the model analysis is accomplished by FEM software for this bridge, the result illuminate that the most part of mode parameters can be obtained by the method ignoring the soil structure interaction.Fourthly, Due to the PSD test only be used to lump mass system, the reduction of degree of freedom is finished based on the mode test result. The contributions of first two modes to displacement on the top of pier, the moment and shear force at the bottom of the pier are calculated by mode superposition method, the result show that the contribution of the second mode to shear force is most, then moment follows. The impact to the displacement can be ignored.Fifthly, Base on the theory of similarity, a scaled down model is designed, the scaling factor equal to 12 is chosen. According to the comparison of strengths of the three materials used to construction of the model obtained by the method of mortar echo and the standard method in the code, the reliability of the method of mortar echo is verified.Sixthly, the result of the SDOF Pseudo-dynamic test of this bridge indicates that the bottom of piers shows nonlinear behavior under the El Centro earthquake motion which of PGA is 1000gal. One side of the pier shows horizontal circular cracks because of tension. This belongs to flexural failure mode. The result demonstrates that this kind of bridge possesses a high level of seismic resistance capacity. Furthermore, the weak point is at the bottom of the piers. |
