A technique of idealising frame models using substructures is outlined. All joints are modelled in 3D but, using substructures, each 3D joint is converted to an equivalent stiffness element. The slender regions between the joints are modelled using beam elements. Coupling between the 3D substructures and the beam elements is carried out in a manner that conforms with the theory of elasticity. Post-elastic stressing is evaluated using yield loci of the joint substructures. The elastic response of complete frames can therefore be predicted in an accurate and efficient manner.
Frames are major structural components in many engineering structures. The design and assessment of such components require consideration in strength, fatigue and fracture. Accurate predictions of static collapse behaviour for aging oil platforms is essential so that reserve strength and likelihood of damage can be assessed. Bolt et al (1994), Grenda et al (1988), and Mortazavi and Bea (1997), studied the collapse behaviour of planar frames with X and K joints, by experiment, where the collapse load and failure locations and modes were of primary concern. Hyde and Fessler (1996), showed that the static strength of tubular joints and of plane frames and three dimensional structures can be determined from tests using lead-tin models. The alloy's stress-strain curve is similar to that of structural steel and therefore the normalised static strength obtained from the model joint (or frame) tests are applicable to actual steel joints and frames, but the models are expensive to make and test. Hyde, (1999), showed that the finite element method can be used to simulate the behaviour of joints of complex geometry. Using full 3D elastic-plastic analysis, they used finite element analysis to predict the responses of framed structures to the point of collapse.
