拉脱法检测预应力的咬合力理论及其模型试验

Theoretical and model experiment study on biting force for detecting prestress using lift⁃off method

拉脱法是一种后张法预应力桥梁锚下预应力的有效检测方法,但是夹片脱开时的受力机理尚不明确。采用该方法检测有效预应力时,如何准确确定夹片咬合力从而获得锚下有效预应力的理论模型还需深入研究。为获得更加准确的咬合力,笔者首先分析了拉脱法检测时锚固体系的受力特点,分析了锚外拉脱曲线的突变原因;其次建立了一种获得夹片与锚板之间咬合力的理论计算方法,分析了咬合力受摩擦系数和张拉力的影响情况;再次,设计了夹片咬合力室内模型试验,进行了足尺预应力梁的室外试验,并将理论计算值和实验值进行了对比分析,结果与理论模型吻合良好。在仿真计算和室内外试验的基础上,提出了拉脱法咬合力与张拉力的理论关系模型,并根据实测结果给出了拉脱法咬合力的计算公式。利用提出的咬合力理论模型和计算公式,能够比较准确地预测拉脱法检测时的咬合力,反映拉脱法检测有效预应力时的实际受力状态,减少了拉脱法预应力检测的系统误差。研究结果可为提高预应力施工质量,增强预应力混凝土桥梁的耐久性,提供实际的工程价值和理论指导。

The lift-off method is an effective way to detect the residual prestress under the anchor of post-tension prestressed bridges,but the mechanism of biting force needs further investigation when the clip is disengaged from the anchor.The biting force should be determined correctly to improve the accuracy of inspection method.Thus,a mathematical model to obtain the biting force of clips was revealed in detail when using the lift-off method to detect prestress.Firstly,the stress distribution characteristic of posttension anchor system was analyzed when the lift-off method was applied.The mechanism of the discontinuity of the lift-off curve was exhibited.Secondly,an indirect theoretical method for calculating the biting force between the clip and anchor plate was established based on the stress balance of the system.At the same time,a solution procedure to gain the biting force from a nonlinear derivative equation was presented.Then the two main influence factors on the biting force,including the friction coefficient and total lift-off force,were discussed extensivaly.Thirdly,to validate the correctness of theoretical model,the laboratory model tests of biting force was designed and carried out.The outdoor experiments were applied by lift-off method to obtain the biting force in full scale in-site posttensioned prestress beams.The theoretical model results and experimental data were carefully compared and analyzed,and the results showed a good consistency.Based on theoretical calculations and series of experimental results,this paper proposed the linear model between the biting force and tensile force of lift-off method and the recommended calculating formula to predict the biting force of lifting-off method.By utilizing the theoretical model and recommended equation proposed in this paper,the biting force of lift-off test can be accurately predicted,which can be applied to reflect the actual remaining prestress of the posttensioned beams and reduce the systematic error of lift-off method.The proposed method to obtain the biting force has practical engineering application value and theoretical significance which could improve the construction quality of anchor system and enhance the durability of the prestressed concrete bridges.