基于振动台实验的结构损伤识别研究

Structural Damage Identification Based on Shaking Table Tests

选择美国加州大学圣地亚哥分校7层钢筋混凝土剪力墙足尺结构振动台实验,开展结构损伤识别研究,实验采用白噪声、环境振动和不同强度的地震动交替激发,记录地震动激发实验前后的结构反应。基于该记录计算和对比自振频率和振型曲率的变化、剪切波走时及其变化和结构层间位移角,分析发现一层和二层振型曲率较大,走时较长,走时变化也较大,现场检查发现一层和二层的破坏也较为严重,这些参数可用于识别结构损伤程度和定位损伤位置,而自振频率和层间位移角变化仅可反映出结构损伤程度,难以揭示结构损伤位置。

In this study,we chose high-performance shaking-table tests of a full-scale seven-story reinforced-concrete shear wall structure at the University of California,San Diego to test this method with respect to damage identification.We alternately tested the structure under the excitations of white noise,the environment,and nine earthquakes.For each case,we scaled the amplitudes of the input ground motions to various levels.We recorded the acceleration responses before and after the earthquake excitations with seismometers located on the seven floors.We determined the vibration characteristics for each earthquake excitation by analyzing the acceleration responses mentioned above.These characteristics include the modal information,the shear-wave propagation characteristics,and the inter-story drift ratio.We estimated the modal frequencies of the first three modes from the recordings when white noise was first applied to the building,and considered these as the criterion.The subsequent modal frequencies were then normalized and compared with this criterion.The normalized frequencies diminished gradually with the load case tests and the normalized frequency reduced by 51 percent for the first mode.The reduction in the modal characteristics indicates that crevices develop as the amplitude of the input ground motions increase,and thereby decrease the rigidity.Lower rigidity suggests that damage throughout the building has been aggravated.However,changes in modal frequencies cannot be used to locate damage.Mode shape curvatures of the building were similarly applied to identify the building damage.Test results demonstrate that the mode shape curvatures increase significantly with the test process and the main changes were concentrated on the second floor.After the excitations of earthquakes 1,2,3,and 4,the curvature values were 0.214,1.214,7.101,and 9.641,respectively.Therefore,we conclude that the damage on the second floor was more severe.Subsequently,we used a one-dimensional shear-wave propagation model to form the virtual waveform by deconvolving the recordings on each floor with the signal on the seventh floor.This waveform has a wave equation that is identical with that of a physical waveform and reflects the propagation characteristics of the shear wave in the building.Upward traveling and downward traveling waves are recognized in the virtual waveform.The travel time of the shear waves is inferred from the upward and downward traveling waves.At the same time,we obtained the changes in the travel time.The travel time and its changes both increase with the amplitude of the input ground motions.The travel change after the earthquake-4excitation rose by 44.5percent on the first floor.The travel time and its changes suggest that the lower two floors were more damaged than the upper floors,and are appropriate for damage identification as well.Finally,we computed the inter-story drift ratio and compared the results with the response after the excitations of the four earthquakes.The inter-story drift ratio increases after the input ground motions and breaks through the limit values of immediate occupancy of 0.5percent and life safety of 1.0percent for a reinforced concrete building.After excitation by earthquake 4,the drift ratio approaches the limit value of collapse prevention of 2.0percent.Our analysis indicates that the parameters described above are sufficient to identify the damage.