Buckling and Post-Buckling Strength of Circular Tubular Sections

ABSTRACT Results of axial load tests carried out on (8 5/8 in.×0.219 in.) seamless and (12 3/4 in.×0.250 in.) electric-weld Grade B line-pipes with slenderness ratios between 40 and 120 agreed reasonably well with predicted load values (using the tangent-modulus expression). Axial deformations prior to buckling were nonlinear. Local plastic buckling caused a drastic reduction of the post buckling strength. Results indicate that an offshore tower under lateral loads can deform under ultimate conditions significantly more than a linear elastic analysis would indicate, thus increasing the earthquake resistance. INTRODUCTION In the design of fixed offshore platforms for ever larger water depths stability of both columns and brace members has become increasingly important. Most structures, built to date are of the common template type and constructed entirely from typical pipe sections. However, for structures located in deeper waters, or in waters with considerable ice-flow, large-diameter thin-walled, stiffened sections have been used as columns. The brace members in general are common pipe sections designed in accordance with the API adopted AISC Specification 1. With increasing emphasis on design refinements, involving environmental loads, steel properties, welding, joint design, cumulative damage, etc. a critical assessment of the design rules for brace members seems necessary. These members, subjected to alternating loads due to waves, currents, winds and earthquakes, are typically designed in accordance with the AISC specifications for compression members. Although the development of these specifications has been based entirely on the results of column tests on rolled wide-flange sections there is no doubt that circular tubular sections - with high proportional stress limits - can be designed safely using these specifications. However, tubular sections with low proportional stresses or so called round-house stress-strain curves cannot be designed with adequate safety. Furthermore, these specifications in general do not define limitations based on the local buckling of tubular members subjected to either compression or bending. The study presented in this paper assesses the buckling and post-buckling strength of a limited number of tubular columns. The column sections are geometrically modelled after the brace members of a deep-water offshore tower. Because the structure is to be located in a highly active seismic region structural ductility was an essential requirement. Consequently, information regarding the potential post-buckling resistance of the braced members was necessary. Although the behavior of the actual pipe sections may be different from the behavior of the selected column test sections the results were considered essential to assess the basic behavior of structural tubes under axial loads. Particularly, since little substantiative test data regarding the compressive strength of tubular members as encountered in offshore structures or other civil engineering structures could be found. BUCKLING The buckling strength of circular tubular sections has been the subject of several studies. However, these investigations have been focussed on either the local instability of large diameter circular sections 3, 4, 5, 6 or on the ultimate strength of small diameter pipes 7, 8, 9, 10, 11.