混凝土梁后张预应力损失的概率特征及敏感性评估

Probability distribution and sensitivity evaluation on the post-tensioned prestress loss in concrete beams

为研究混凝土梁中后张预应力损失的分布特征及其对各种不确定因素的敏感性,以概率数值模拟为基本手段,得到各分项预应力损失及预应力总损失的概率分布特征,并对预应力损失进行了区间估计。将各影响因素划分为施工、设计和材料3类,分析了预应力损失对各类因素的敏感性。结果表明:预应力总损失可用对数正态分布描述;95%置信概率下预应力总损失的上、下限偏离均值可达20%以上,各因素不确定性对预应力损失影响不容忽视;各类因素的敏感程度依次为施工条件、材料性能和设计因素,其中张拉控制应力、预应力筋弹性模量和预应力筋面积分别在3类因素中的影响最为显著;各分项预应力损失中,管道局部偏差对摩阻损失的影响强于管道摩擦,预应力筋越长对锚具变形损失影响越弱,而混凝土收缩徐变损失对徐变的敏感程度要强于收缩。

The present paper aims to evaluate the effect of the various unexpected factors on the post-tensioned prestress loss in concrete beam and obtain its probability distribution. For the given research purpose, we have made a discussion of the effects of such factors, as the tensile stress, the elastic modulus, the eccentricity and the tensile strength of the tendons, etc. on the prestress loss of the post-tensioned T-shaped beams through a numerical probability simulation. As is known, the probability distribution of the total and partial loss of prestress is usually caused by the friction, relaxation of tendons, creep, shrinkage, elastic shortening of concrete, which can be worked out by Monte Carlo simulation referring to the code for highway reinforced concrete design and that for the prestressed concrete bridges or culverts （JTG D62＆mdash;2004）. In this paper, the deviation of the prestress loss caused by uncertain factors using the interval estimation is discussed. All the uncertain factors are classified into three classifications, i.e. construction condition, design factors, and material properties, whose effect on prestress loss are analyzed, and they were ranked by the influencing degree on the total prestress loss. The results show that the total loss of prestress does not reject the lognormal distribution, however, the confidence of the upper and lower limits of the total prestress loss may likely deviate about 20% from the mean value under 95% confidence level. Therefore it is necessary to pay enough attention to the effect of unexpected factors on the post-tensioned prestress loss. The construction condition is the most influential factor on the prestress loss, followed by material properties and design features. The controlled tensile stress, elastic modulus, and area of tendons are the dominant influencing factors in three classifications, respectively. For the partial prestress losses, the effect of wobble on the friction loss proves to be much greater than that of curvature, whereas the effect on the anchorage-seating loss turns to be smaller with the increase of the tendon length, however, the concrete creep could be much more sensitive to the creep and shrinkage loss than to the shrinkage itself.