桩基础极限荷载有限元判定方法研究

摘要

以桩基础极限荷载分析为例，探讨各种极限荷载计算和判定方法，以提高计算精度。有限元极限分析方法分为强度折减法和荷载增量法。强度折减法极限荷载判定方法有：①F⁃Qu曲线判定法，即曲线出现V型尖点，则V型尖点前一点对应的强度折减系数为桩基础的安全系数；②F⁃s曲线判定法，即F⁃s曲线出现拐点，其后曲线近似平行于s轴；③F⁃Δs′ΔP′突变曲线判定法，当曲线发生突变，出现一条近似与F轴垂直的直线，曲线突变前的拐点对应的强度折减系数为安全系数；④数值计算收敛性判定法，即计算恰好不收敛时前一点对应的强度折减系数为安全系数；⑤塑性区贯通判定法，该方法只适用于判断计算的正确性。荷载增量法极限荷载判定方法有：①P⁃s曲线判定法，曲线有明显第2个拐点，其后近似平行于s轴，第2拐点对应荷载为极限荷载；②数值计算收敛性判定法；③塑性区贯通判定法。研究表明上述各种方法判定桩基础的极限荷载几乎相同，只要按规定来操作，各种判定方法都是可行的。强度折减法与荷载增量法所得极限荷载误差很小，表明2类判定方法都是可行的。%Research on the basis of analyzing the ultimate load of pile foundation, computational methods and estimated meth⁃ods are focused on improving the computational accuracy. Finite element limit analysis method fall into two categories-strength re⁃duction method (SRM) and incremental load method (ILM). At present, the estimated criteria of SRM are:①The judgment method of the load⁃pile end resistance curve (F⁃Qu curve). If aV shaped cusp shows up on a curve, the reduction factor of the point before V shaped cusp is the safety factor of pile foundation.②The judgment method of F⁃s curve. If an inflection point shows up on a F⁃s curve and the rest part of curve is almost parallel to s⁃axis, the strength reduction factor of the point before mutation is the safety fac⁃tor.③The judgment method of mutation curve. When the mutation curve of F⁃Δs′ ΔP′generates the inflexion, it is perpendicular to F⁃axis.④The non⁃convergence of finite element numerical solution, which means that if calculation shows non⁃convergence on a point, the strength reduction factor of the point before non⁃convergence point is the safety factor.⑤The judgment method of connec⁃tion of plastic zone, which is to judge the correctness of solutions. For ILM, estimated criteria are:①The judgment method of P⁃s curve. If it has a second distinct inflexion point and parallels to s⁃axis approximately, the load of the second point is the ultimate load.②The non⁃convergence of finite element numerical solution.③The judgment method of the connection of plastic zone. Re⁃sults show that as long as the above⁃mentioned provisions to operate, various judgment methods are feasible. And the errors between SRM and ILM are slight, which indicates that the two methods are feasible.