桩体长径比对桩基动力特性的影响研究

Research on influence of pile's slenderness ratio on pile-soil-structure system's dynamic interaction

弯曲破坏是地震中桩基失效的一类常见模式,其破坏的一个重要原因就是桩体的抗弯刚度不足。对于一定长度的桩体,桩的抗弯刚度与桩的长径比直接相关,但桩基规范中对摩擦桩的长径比并没有像端承桩那样作出限制。在构建的结构性饱和黄土动力本构模型的基础上,针对黄土地区的单桩基础,结合大型有限元程序建立了摩擦桩-土-结构体系、端承桩-土-结构体系的有限元-无限元模型,探讨了不同桩体长径比对两者动力反应特性的影响,得出如下规律:(1)其他条件相同时,不管是端承桩还是摩擦桩,长径比对桩身截面剪应力和水平加速度的分布形态没有影响。但随着长径比的增大,桩身各截面对应的的剪应力值减小,而加速度值反而增大;(2)随着长径比的增大,桩身变形形态从正弯型逐渐过渡到反弯型,桩体的破坏模式也相应由剪切破坏过渡到弯曲破坏;(3)端承桩体系比摩擦桩体系更容易出现反弯型,但当长径比增大到一定程度时,摩擦桩也会呈现反弯型。从抗震角度来说,对摩擦桩长径比的限制应以桩体变形模式表现为正弯型为宜。所得结论对工程实践具有一定的指导意义。

Bending failure is a normal damage model of pile foundation in earthquake.An important reason of this model is the deficiency of pile＇s flexural stiffness.To a certain length,the flexural stiffness is immediately relate to pile＇s slenderness ratio.But in the pile foundation criterion,there has no limitation for friction pile＇s slenderness ratio like end bearing ones.On the base of structural dynamic model of saturated loess,the finite-infinite element model of single friction pile-soil-structure system and end-bearing pile-soil-structure in loess area are established by finite element program.The influence of slenderness ratio on the system＇s dynamic characteristics was studied;and the regulations were as follows.Firstly,when other conditions are the same,no matter end bearing pile or friction ones,the slenderness ratio has a little influence on the distributions of pile section＇s shearing stress and pile＇s horizontal acceleration.But along with the increase of slenderness ratio,the shearing stress reduces while the horizontal acceleration increase.Secondly,along with the increase of slenderness ratio,the pile＇s deformation type transits from normal flexure model to contraflexure model gradually.Accordingly,the pile＇s damage model transits from shearing failure to bending failure. Thirdly,the end bearing pile＇s deformation curve is more likely to behave as contraflexure model than friction ones.But when the slenderness ratio arrives at a certain value,the friction pile also can behave as contraflexure model.In the aspect of seismic resistance,the appropriate limitation of friction pile＇s slenderness ratio is which can make the pile＇s deformation curve behaves as normal flexure model.The conclusions can give instruction for engineering practice in the future.