Frames are major structural components in many engineering structures. The design and assessment of such components require consideration in strength, fatigue and fracture. The finite element method is used to simulate the behaviour of joints of complex geometry and welded joints with material heterogeneity. Analysis of complete frame structures is desirable so that load paths and magnitudes are computed correctly. Local stress-strain history, and failure modes and their locations can then be evaluated precisely. Accurate predictions of static collapse behaviour for aging oil platforms is essential so that reserve strength and likelihood of damage can be assessed.

Full 3D elastic-plastic analyses are currently used to predict the responses of framed structures to the point of collapse. Due to the structural complexity, these impractical analyses result in long computing times. A substantial reduction in analysis time is achieved if idealisation techniques such as reduced element types and substructuring are used in the model. Between frame joints, the structures are long and slender, and can be replaced by beam elements. In order to achieve a state of compatibility of displacements and stress equilibrium at the junction or interface between the differing element types, it is important to integrate into the analysis some scheme for coupling the element types in a way that concurs with the theories of elasticity. Substructures can be employed to model the joints but are only valid in the linear elastic range. Where the linear elastic range is exceeded, full 3D analysis of the region is necessary. Therefore, a scheme to determine the maximum stress levels in each substructure is also required.

A technique of adaptively substructuring frame models is outlined. Initially, all joints are modeled in 3D but, using substructures, the 3D model is converted to an equivalent joint stiffness element. The slender regions between the joints are modeled using beam elements. Coupling between the substructures and beam elements is carried out in a manner that conforms to the theories of elasticity. As the loads are increased, stressing beyond the linear elastic range will occur at certain joints. This post-elastic stressing is evaluated using yield loci of the joint substructures, and the substructure is then replaced with 3D continuum elements. As loads are increased further, more of the substructures are replaced. The elastic-plastic response of complete frames can therefore be predicted in an accurate manner and since high computational efficiency is achieved, the cost of the analysis is substantially reduced.
  So that the complex framework can be represented by a fairly simple framework.