Identification of Vibration Mode Shapes for a Bridge Deck under Ambient Vibration for the Health Monitoring

振动测量是结构健康监测的有力手段。为了检测结构的局部损伤，需要识别高阶振动特征，特别是振型。然而，在民用基础设施中，空间振动和输入激励测量的困难降低了这些高次模态的识别精度。激光多普勒测振仪可用于大空间结构在环境振动等微观振动作用下振动的可靠测量。本研究建立了一套空间振动测量系统，通过对感兴趣区域进行扫频和对记录的时间历程进行反复叠加，消除了测量环境振动中的非平稳分量。利用该激光振动测量系统识别了某桥梁桥面的振动模态。然而，在这种情况下，由于激光反射量的减少，大量的光噪声包含在记录的混凝土的时间历程。为了克服这一点，对于不可能测量振动的测量点，在其周围设置许多子测量点，并且分别测量每个子测量点的激光反射量。通过计算平均值，选择给出最大激光反射的最佳测量点。基于该功能，识别了环境振动下桥面的振型，验证了激光多普勒测振仪非接触振动测量应用于桥梁健康监测的可行性。

Vibration measurement is a powerful technique for structural health monitoring. Identification of high order vibration characteristics, especially vibration mode shapes, is needed to detect local damage in structures. However, in civil infrastructures, the difficulties of measuring spatial vibration and input excitation decrease the identification accuracy of these higher modes. Laser Doppler Vibrometer can be applied to reliable measurement of vibration of spatially large structures under microscopic vibrations such as ambient vibration. In this study, a spatial vibration measurement system is established, and non-stationary component in measured ambient vibrations are eliminated by sweeping the area of interest and repeatedly stacking the time histories recorded. In this way, mode shapes can be identified from these vibration measurement results without knowledge of input excitation.Vibration mode shapes of a bridge deck are identified using this laser vibration measurement system. However, in this case, lots of optical noise is contained in the time histories recorded for that made of concrete, because of the reduction of laser reflection quantity. To overcome this, for measurement points which are impossible to measure vibration, many sub-measurement points around that are set, and laser reflection quantity of each sub-measurement point is measured, respectively. By calculating the mean values, optimum measurement points that give maximum laser reflection are selected. Based on this function, the vibration mode shapes of the bridge deck are identified under ambient vibration, and it is verified experimentally that non-contact vibration measurement using Laser Doppler Vibrometer is possible to apply for bridge health monitoring.