Imperfection, residual stress and yield slenderness limit of very high strength (VHS) circular steel tubes

This paper investigates the initial geometrical imperfection, residual stresses and yield slenderness limit of very high strength (VHS) circular steel tubes with the yield stress of 1350 MPa. The out-of-straightness is found to be about L/6800 on average, where L is the length of the stub column. The measured residual stress is found to be approximately 4% on average of the yield stress, with the outside surface in compression and the inside surface in tension. Eight stub columns were tested with the element slenderness ratio ranging from 135 to 258. New element slenderness limit (λ_ey) of 258 and non-dimensional local buckling parameter limit of 3.1 are proposed for VHS tubes. The very large value of λ_ey is obtained partly because of the very large yield stress used in the definition of element slenderness and partly because of the very low residual stress in the section. Two stub columns of non-heat-treated tubes were also tested in the same manner. The results were compared with those of the VHS tubes. Both VHS tubes and non-heat-treated tubes have a similar level of geometrical imperfections. The residual stress pattern obtained for non-heat-treated tubes is very different from that of VHS tubes.