Nobuo Oka’s scientific contributions

The inelastic behavior of cantilever column with circular hollow section subjected to repeated proportional loading of horizontal and torsional loads under a constant axial compression is studied. An accurate numerical method for steel space frames and members with closed thin walled section which was developed by the authors is used to investigate the behavior. It is shown that a nonstationary characteristics, increase of out-of-plane deformation, is observed even in the column without an axial compressive load and a strong axial compressive load causes a significant divergence of out-of-plane deformation

A numerical method for elastoplastic large deformation analysis of steel space frames incorporating the effect of semirigid joints is presented. A node zero length connection element is used to model the joint behavior. The tangent stiffness matrix for a member is obtained by introducing the connection element as the member end compliance. The reliability of the method is examined by available three examples and the influence of the rigidity of gusset plate connections of web members on the inelastic collapse of tubular trussed tower leg is studied.

The inelastic instability of cantilever columns with tapered square hollow section under the combined action of axial force, biaxial bending and torsion is studied. A new elastic-plastic tangent stiffness matrix for members with closed thin walled section is used to investigate the behavior. The stiffness matrix uses a tangent coefficient matrix obtainedd by the numerical integration of the hardening moduli of the fibers about the member section to calculate the generalized plastic strain increments. The effect of torsion is considered in the fiber level using von Mises yield criterion. The ultimate strength of the columns is obtained by using extended Horne's stability criterion, and the results are presented in the form of interaction diagrams