Space steel frame;  warping;  shear deformation;  Plastic hinges;  ultimate bearing capacity;  Object-oriented programming

The virtual work increment equation of three-dimensional sorghum column element based on UL method is given, the geometric nonlinear tangent stiffness matrix of three-dimensional space beam column element considering the effects of warpage and shear deformation is derived in detail, and the plastic surface of Orbison section is modified to consider the influence of torque and warpage on section strength, and the elastic-plastic tangent stiffness matrix of the element is derived by plastic flow method. According to the object-oriented programming idea, the entire finite element domain is divided into 8 basic classes, a new unit class is derived on the basis of the unit base class, and the object-oriented space steel frame analysis program is compiled in C++ language. The example proves that only one or two units are needed to accurately predict the ultimate bearing capacity and instability mode of the empty steel frame.