说明：  平面框架有限元程序，可计算线性和非线性（几何、材料非线性）问题，来自《杆系与箱型梁桥结构分析及程序设计》一书，是学习fortran和非线性很好的例子
(Plane finite element program frame, can be used to calculate the linear and nonlinear (geometry, material nonlinear) problems, from the rod and box girder bridge structure analysis and design program, \"a book, is the study FORTRAN and nonlinear good example )