For the analysis of a structure the initial imperfections must be chosen in such a way that the most unfavourable influence on the ultimate capacity is taken into consideration. The choice of the form of the initial imperfection is easy in usual cases: for a member with simple supports subjected to axial forces the form must be similar to a sine wave. For other types of structures, especially continuous beams which are loaded by axial forces and bending moments the choice of the shape of imperfection is not as easy: for a two-span beam loaded by axial forces and transverse loads it is imaginable that an initial imperfection similar to the deflection caused by the transverse loads leads to a smaller ultimate load carrying capacity than an imperfection according to the first eigenmode. Parameter studies were carried out for two-span and three-span beams. For different shapes of imperfections the relation between the magnitude of the axial force and the bending moments was varied as well as the type of cross section and the axis of buckling: weak axis buckling and strong axis buckling. Furthermore different methods of analysis were dealt with, such as ultimate load calculations taking into account the yielding along the beam, plastic hinge theory and elastic second order theory. The results are shown in diagrams.
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