【作者】 肖赟；

【导师】 雷俊卿；

【作者基本信息】 北京交通大学 ， 桥梁与隧道工程， 2014， 博士

【摘要】 预应力混凝土桥是我国桥梁的主要桥型之一,高等级公路上的预应力混凝土桥常年承受反复荷载的作用,疲劳和其他因素相互作用引起桥梁结构刚度退化和变形增大的现象日渐普遍,预应力混凝土梁的开裂及过量下挠等病害也变得越来越突出。本文依托国家高技术研究发展计划(863计划)项目“大跨径混凝土桥梁长期变形和开裂控制技术”(2008AA11Z102)以及国家自然科学基金项目“大跨度跨海预应力混凝土梁桥的疲劳损伤与耐久性机理研究”(51178042)的项目,针对预应力混凝土梁在超载疲劳作用下的刚度退化和挠度增大问题开展了研究,主要研究内容和创新点如下：(1)回顾了预应力混凝土梁桥的发展历史,对预应力混凝土梁桥长期过量下挠和开裂病害的成因以及机理进行了简要的分析,阐述了研究预应力混凝土梁桥超载疲劳问题的必要性。总结了疲劳问题研究的一般方法,并对预应力混凝土梁疲劳研究现状进行了综述,说明了本文研究的意义。(2)开展了预应力混凝土试验梁的多级变幅超载疲劳试验,研究了预应力混凝土梁在疲劳荷载循环作用下混凝土和钢筋的应变增长情况、疲劳裂缝的产生和开展情况以及试验梁疲劳挠度的增长情况。试验结果表明：在低于开裂荷载水平或适度超载的疲劳荷载循环作用下,预应力混凝土梁中混凝土和钢筋的应变呈现出两阶段发展的规律,且应变的增长速率随着加载水平和应力比的增大而增大；在低于开裂荷载水平的疲劳荷载循环作用下,预应力混凝土梁也可能会在最大弯矩位置出现疲劳裂缝,并且裂缝的开展与加载水平及应力比有一定关系；疲劳挠度的发展过程与混凝土应变发展过程基本一致,也分为两个阶段,且疲劳挠度增长速率也随着加载水平的提高以及应力比的增大而增大；对于疲劳损伤后的预应力混凝土试验梁,其刚度在弹性阶段内较无疲劳损伤的试验梁降低了20%-30%左右,且经历的疲劳荷载循环水平越高,刚度降低程度越大,到极限荷载附近时,疲劳试验梁刚度平均降低约50%左右,且其极限承载力也有一定程度的降低。(3)从损伤力学的角度,推导了基于不可逆热力学为基础的耗散原理的混凝土材料损伤演化的两阶段模型,并通过已有的混凝土材料疲劳试验数据对模型参数进行了拟合和验证,结果表明该模型能较好的反映运营状态下的混凝土疲劳损伤发展历程。同时,还以断裂力学为基础推导了裂纹尖端附近区域的应力场和位移场,阐述了钢筋疲劳裂纹产生和扩展的三个阶段的特点及裂纹形成的机理,分析了钢筋疲劳裂纹扩展的阶段特性并总结了几种裂纹扩展速率表达式。此外,还总结和推导了五种典型的疲劳损伤累积理论,分析和阐述了各疲劳损伤累积理论的特点,为预应力混凝土梁疲劳刚度退化分析提供了理论上的基础。(4)建立了基于截面刚度损伤的预应力混凝土梁疲劳刚度退化模型,并采用损伤弹性模量来对混凝土进行损伤计量以及采用损伤剩余面积来对钢筋和预应力钢束进行损伤计量,而后进行了预应力混凝土梁疲劳刚度退化的全过程分析,并编制了相应的计算分析程序,结果表明：在考虑了有效刚度修正、疲劳开裂修正和超载修正后的模型计算结果与试验结果吻合程度较好,误差普遍在5%以内,能够较好的反映预应力混凝土梁疲劳刚度退化的演化过程。(5)基于疲劳刚度退化的全过程分析方法,提出了预应力混凝土梁疲劳挠度的预测计算方法,并同时建立了两种人工神经网络模型进行对比分析,结果表明：基于疲劳刚度退化全过程分析的疲劳挠度预测计算方法,其误差随着有效样本的增加而逐渐减小,并且误差较稳定,得到的挠度增长曲线也比较符合试验结果;采用神经网络对预应力混凝土梁疲劳挠度进行预测的方法,其在有效样本数较少时预测误差较大,不过随着样本数目的增多其精度会逐渐提高,但其计算得到的挠度增长曲线受选择的传递函数形式和隐层神经元的数目影响,同时稳定性相对较差,一般要进行多次重复训练才能获取较好的训练结果。(6)对一座4×20m的公路预应力混凝土连续梁桥进行了疲劳以及收缩徐变引起的长期挠度的计算分析,结果表明：在开始运营的一年内,因疲劳引起的挠度增长大于收缩徐变引起的挠度增长。此后,疲劳发展进入第二阶段,由于疲劳引起的挠度增量稳定缓慢增长,而此时收缩徐变仍处于快速发展期,收缩徐变引起的挠度增量仍增长较快且逐渐超过了疲劳引起的挠度增量。由此推断,对于预应力混凝土梁桥长期挠度的增长,在最初的一年左右是疲劳起主导作用,其后由于收缩徐变引起的长期挠度增量增加,并逐渐占据主导地位。而超载会显著的增大疲劳挠度。(7)根据基于截面刚度损伤的预应力混凝土梁疲劳刚度退化分析模型,构建了预应力混凝土梁疲劳挠度的功能函数,并在对各变量进行统计特性分析的基础上,进行了预应力混凝土试验梁疲劳挠度的时变可靠性分析,讨论了疲劳挠度可靠度指标随荷载循环降低的关系。更多还原

【Abstract】 The prestressed concrete bridges are widespread and most constructed in China. As most prestressed concrete bridges on highways are under cycle loading of vehicle, stiffness degradation and deformation growth induced by fatigue and other factors is much more common than before. And problems such as excessive deflection and cracking are prominent. This paper is mainly studied on stiffness degradation and deflection evolution of prestressed concrete beam under overload fatigue, which is funded by the National High Technology Research and Development Program "Long term deformation and cracking control technology of large span concrete bridges"(Grant No.2008AA11Z102) and the National Natural Science Foundation "Research on durability and fatigue damage mechanism of large span prestressed concrete bridges over sea"(Grant No.51178042). The detailed contents and highlights are listed as following:(1) The development of prestressed concrete bridges is reviewed. Then, the causes and mechanism of excessive deflection and cracking is briefly analyzed, and the significance of study on fatigue of prestressed concrete bridges is introduced. The general analysis methods of fatigue as well as latest research on fatigue of prestressed concrete beams are summarized, and the significance and research basis of this paper is introduced.(2) Fatigue test of prestressed concrete beams under multilevel amplitude cycle loading of overload was carried out. The strain increasement of concrete or steel rebar, cracking development and deflection evolution of prestressed concrete test beam under fatigue loading was obtained. Results indicate that the fatigue strain of concrete or steel rebar presents a two-stage evolution under the fatigue loading level below cracking level or just overloading temperately, and the strain grows more rapidly with the increasing of the loading level and stress ratio. Even while the loading level is below cracking level, crack appears at the position of maximum bending moment on the prestressed concrete test beams, and the crack development indicates that it’s related to the fatigue loading level and stress ratio. The fatigue deflection evolution is somewhat like the development of the concrete strain, as it also performs a two-stage evolution and related to the loading level and stress ratio. The stiffness of the fatigue damaged test beams is about20%-30%lower in the elastic stage than those with no fatigue damage, and50%lower while near the ultimate load. And the ultimate bearing capacity is decreased as well.(3) Based on damage mechanics theories, a two-stage damage evolution model of concrete is derived, and the parameters in the model are fitted and verified by existing fatigue experiments. Results indicate that the model could well present the concrete damage evolution process in normal operating condition. Meanwhile, with regard to steel, the displacement field and stress field, which is near the crack tip region, is derived based on fracture mechanics theories. The mechanism of crack developing and the characteristics of the three stage of fatigue crack development are elaborated, and several crack propagation rate expressions are summarized. Besides, five typical fatigue cumulative damage theories and characteristics of which are discussed. And those are the theoretical basis of stiffness degradation analysis for prestressed concrete beam.(4) A fatigue stiffness degradation model for prestressed concrete beam is established based on section stiffness damage analysis, and damaged modulus of elasticity is applied to count concrete damage while damaged effective remaining area is applied to count steel damage. Then, a whole process analysis is carried out based on this model, and a program script is generated. The analysis results exhibit good agreements with the fatigue test results. The errors are generally within5%. And the model could well describe the stiffness degradation evolution of prestressed concrete beam under fatigue loading.(5) Based on the whole process analysis method of the fatigue stiffness degradation model, a method for fatigue deflection predicting analysis is presented. And two artificial neural network models are also established to gain an analysis compare. Results indicate that the results of the predicting method based on whole process analysis of fatigue stiffness degradation present good agreements with the test results, and the errors are generally stable and appropriately decrease with the increasing of sample size. The errors of artificial neural network models are exceeded when the sample size is not large enough, but errors would reduce while the sample size enlarges. However, the predicted fatigue deflection curve is influenced by the form of transfer functions and the number of hidden neurons. And, the stability of the predicting results is relatively poor. It is usually need a vast number of repeated simulating calculations to gain a preferable result.(6) Long term deflection leading by fatigue or concrete shrinkage and creep of a real prestressed concrete bridge of4x20m span was analyzed. Results indicate that during not a long time after opening to traffic, the deflection increasement leading by fatigue is larger than which leading by concrete shrinkage and creep. After then, the deflection increasement leading by fatigue grows steadly and slowly. However, the deflection growth leading by shrinkage and creep is still developing fast, and the shrinkage and creep deflection exceeds the fatigue deflection. As a result, it could be inferred that fatigue is a primary factor of deflection growth in the early time after opening to traffic, but deflection increasement leading by concrete shrinkage and creep is larger and larger and take the place of fatigue to be a primary factor. And overload lead to significant increasement on fatigue deflection.(7) Based on the fatigue stiffness degradation model, the performance function of fatigue deflection for prestressed concrete beam is obtained. The statistical properties of the variables are analyzed, and the time-variant reliability analysis of fatigue deflection for prestressed concrete beams is conducted. The relationship between reliability index decreasing and fatigue cycles is discussed.更多还原