Imperfection sensitivity of column instability revisited
Journal of constructional steel research
The buckling of columns is the classic problem in structural stability. It has been studied by many researchers over a large number of years, and it is well known that the severity of the buckling response can be greatly amplified by initial geometric imperfections in the column shape. The current paper presents and discusses the effects of imperfection shape, orientation and magnitude on the buckling behaviour of columns. Analyses are conducted for elastic columns with overall initial
... ions in the form of out-of-straightness and sway displacements, as well as local imperfections that, for instance, model constructional and material defects. Traditionally, the initial imperfections are modelled with the first buckling mode with a size selected according to fabrication tolerances. This approach will not necessarily provide a lower limit to the column prebuckling stiffness and strength. These assertions are supported by numerical results for imperfection-sensitive columns. The influence of end restraint on column strength is also studied since columns in actual frameworks are connected to other structural members such that their ends are restrained. Page 2 of 39 problem . The critical buckling loads are the eigenvalues and the corresponding buckling mode shapes are the eigenvectors of the problem. Practical columns, however, deviate from this ideal model due to the inevitable presence of geometrical, material, structural and load related imperfections that act to reduce the column load-carrying capacity. For modelling purposes, geometrical, material and structural imperfections are usually considered by means of an equivalent initial geometric imperfection, in the form of an initial curvature or out-of-straightness (lateral deflection of the column relative to the undeformed state) and an initial sway imperfection (relative lateral displacement between the column ends)    . The buckling load of a compressed ideal column is also affected by the boundary conditions. For all possible boundary conditions, the critical load can be always related to the basic pin-ended column element through the concept of the effective length, L eff that was first introduced by Jasinsky in 1893  . The effective length is defined as the length of a pin-ended column that has the same critical load as a column with other prescribed end-conditions. This concept allows codes of practice to be simplified considerably -see, for instance, the American specification for structural steel buildings  and the European code of practice for the design of steel structures, EN 1993 , with rules only necessary for pin-ended columns. In fact, much of the research on elastic buckling of columns is based on the behaviour of this simple column. In particular, and among the many analyses of imperfection sensitive columns, the majority is concerned with the buckling of a simple uniform column with equivalent initial geometric imperfections and eccentric axial loads.