INFLUENCE OF IMPERFECTIONS ON THE STRENGTH OF UNSTIFFENED, FABRICATED, TUBULAR BEAM-COLUMNS

Fabricated tubular steel members are used in offshore structures as pr imary and secondary members; typical applications in fixed structures incorporate large diameter tubes in truss-type assemblies. These unsti ffened tubes are formed by rolling plate into cylindrical components w hich are then seam and girth welded to create the circular tubular sha pe. The members produced in this fashion have characteristic imperfect ions which are different from those found in smaller diameter seamed o r seamless hollow structural and pipe sections. These imperfections an d their influence on the structural behaviour of beam-columns are the subject of the study and results presented here. Earlier experimental studies at the University of Toronto on the influence of fabrication p arameters are the foundation of the results reported here. Extensive m easurements of as-fabricated geometry provided information on actual s hape and these measured geometries are used to establish the analytica l models using a finite element program, ANSYS; this program permits t he modelling of non-linear material and geometric effects. The ultimat e and post-ultimate local buckling behaviour was studied in the analys es to the same extent as observed in experiments. The load-displacemen t behaviour of tubular members depends significantly on the tube geome try and material properties. The discretization of the tube into eleme nts is shown to require careful consideration so that potential deform ation modes are not missed. Bilinear kinematic hardening material base d on coupon tests of the 'as fabricated' steel, in combination with ge ometric modelling techniques yielded a behaviour response with a close resemblance to the experimental observations and measurements. The re lative importance of various parameters is discussed and direct compar isons of experimental and analytical results with current code resista nce formulations are made. The specific effects of residual strains ar e not addressed in this paper.