锚索桩设计计算的几个力学问题及有关概念探讨

Study of several mechanics problems and related concepts in engineering design of anchored slide-resistant piles

分析探讨了锚索桩设计计算的几个力学问题及有关概念。传统上对于桩身嵌入段的计算,须根据桩身与桩周岩土的相对刚度划分为刚性桩和弹性桩两种不同的计算模型,并各自对应不同的算法及计算公式,通过对两种模型计算式的数学极限分析表明,刚性桩模型算式仅为弹性桩计算法中将嵌入段的桩身刚度取很大值下的特例,两种模型算法在力学本质上是统一的。由锚索桩嵌入段分别按弹、刚性桩模型算式的位移、转角的对比分析可知,基于弹性桩模型计算出的锚索受力大于按刚性桩模型,以弹性桩模型进行锚索设计偏于安全,而嵌入段的换算长度取1.3("m法")或0.8("常数法"),从工程设计角度方可作为刚/弹性桩模型的划分判据,此时两种模型的最大位移或转角差别才小于5%。桩上锚索倾角的力学效应除矢量分解作用影响水平锚拉力大小外,更重要的是将影响锚拉点处的水平约束刚度,进而改变锚索桩超静定结构的受力分担,因此锚索最优倾角的选取可使较好地控制桩身位移且降低锚索初拉力得以兼顾。力图令桩身正负弯矩峰值大致相等的所谓"平衡设计原则"实际上具有较强条件性,须依赖于锚索既要初拉力合适又能在正常工作状态下具备足够大的锚拉力。

Several mechanics problems and related concepts in engineering design of anchored slide-resistant piles were analyzed and discussed. There are two mechanics models and corresponding calculation methods of embedding section of anchored slide-resistant piles, which were classfied into rigid pile model and elastic pile model according to the relative rigidity of pile-soil system. By means of mathematical operation of limit, the mechanics consistency of the two calculation methods was illustrated. That is, calculation results based on rigid pile model is just only a special case of elastic pile model. Comparition results of displacement and bending angle of embedding section between the two mechanics models showed that, by using elastic pile model is more safe for determining tensile loads of anchor cable than by using rigid pile model in design of anchored slide-resistant piles. The criterion of converted length of embedding section is 1.3 （＂m method＂） or 0.8 （＂K method＂）, for if calculating depth are less than them, the difference between two mechanics models are less than 5 %. Because the dip angle of pile＇s anchor cable effect on horizontal restraining force and horizontal restraining rigidity as well, the optimal dip angle can give consideration to both diminish initial prestress loads of cable and decrease horizontal displacement of pile. The principle of bending moment equilibrating in engineering designs was conditional actually, because the target of making approximate equality of peak value of pile＇s positive and negative bending moment extremely depends on sufficiently large tensile loads can be supplied by anchor cables and appropriate initial prestress loads were applied.