桥梁在役桩的竖向动力响应特性及损伤识别研究

Study on Vertical Dynamic Response Characteristics and Damage Identification of Existing Bridge Pile

基于ANSYS LS-DYNA建立桥梁的墩-承台-桩-土有限元显式动力学模型,模拟桥梁的桩基础在承台上表面施加冲击荷载后完整桩和有断裂缺陷桩的竖向速度响应,分六桩-承台和八桩-承台两种桩基础进行数值计算。结果表明:在所要检测的基桩对应的承台上表面施加冲击力,产生的应力波通过承台到达下方的基桩后沿桩身向下传播,类似于低应变反射波法测桩的原理,应力波在到达桩底桩土交界面或者断裂面等阻抗变化较大处会发生应力波反射,在桩头处的竖向速度响应波形曲线中能识别出反射回的应力波,进而判别桩是完整还是存在断裂损伤;数值计算同时记录承台表面的竖向速度响应,发现承台表面的竖向速度响应波形比桩头处的竖向速度响应波形由于应力波在桩承台界面的多次反射而更加复杂,难以准确判断反射波。

Integrity detection of the pile foundations of existing structures is a difficult problem worldwide.Integrity testing of newly constructed pile foundations includes detection methods such as mining with the observation method,drilling core method,low-strain test method,and ultrasonic method.However,for damage detection of pile foundations in service,the existence of platforms and bridge structures above pile foundations makes the observation method difficult because the mining depth of the soil around the pile is very limited.Drilling core methods cannot be achieved because drill core must be drilled from the pile top.For ultrasonic detection,there is no channel available for installing ultrasonic transducers.For the low-strain test method,velocity sensors cannot be placed at the top of the pile;therefore,vertical vibration cannot be recorded.In this paper,vertical impact load is applied on the upper cap surface above the detected pile.According to the vertical velocity response of the cap surface and pile head,we analyze the integrity of the bridge pile foundation in-service.Bridge pier-cap-pile-soil explicit dynamic finite element models are based on ANSYS LS-DYNA.In the finite element models,impact load is applied on the cap surface,and the vertical velocity responses of the complete pile and the cracked pile are both recorded.Two types of pier-cap-pile-soil numerical models are used in this study with six and eight piles,respectively.The results show that when the impact load is applied at the cap surface on top of the pile to be detected,the stress wave goes through the cap and propagates downward along the pile.Similar to the principle of sonic echo in pile integrity testing,the stress wave reflects at the pile bottom or fracture surface where impedance changes significantly.From the vertical velocity response curve of the pile head,the reflected stress wave from the pile bottom or the fracture surface can be identified to determine whether the pile is complete or cracked.This paper also presents the vertical velocity response of the cap surface;the response curve is more complicated than the pile head because of multiple reflections of stress wave at the pile-cap interface.Therefore,accurate evaluation of the reflected wave is difficult.Loading the vertical impact at the pile head produces a simpler waveform of vertical velocity response of the pile head than that when using the cap surface.The reflected wave response of the pile head is more obvious,and it is easy to judge the pile bottom or the fracture surface by the reflected wave.Moreover,the waveform of vertical velocity response of the pile head can be used to accurately evaluate the pile integrity.However,arrangement of velocity sensors at the pile head is difficult;that at the cap surface is more practical.The waveform of vertical velocity response of the cap surface is very sensitive to the positions of velocity sensors and changes often.Thus,when recording the vertical velocity response of the cap surface,there is a strong need for a plural arrangement of velocity sensors to measure the response waveform.Such an arrangement is also important for filtering and analysis aided by signal processing technology.