Computer aided design of steel structures covers two main directions of software developing, one of them is the computer design and draft of steel constructions, the other is the computer verification of the structural models according to the relevant design standards and using mostly finite element analysis. Sufficient softwares have been developed for these problems. Some of them are devoted only to one of these aims but recently great investigation started into development of integrated programs which do constructional design, verification and draft as well. In the latter case one may meet considerable difficulty when the verification includes the stability limit states. For instance the design formula of a beam-column member is based on the slenderness ratio as the main geometric parameter of the structural model. The slenderness ratio may be determined using stability analysis of the structural model or it can be calculated from the effective length given by the user as an input data. However, these methods use the theorem of elastic extrapolation, which interrupts the automatic process of the stress analysis. Theoretically, this problem may be eliminated using imperfect model and second order stress analysis. The additional stresses on the imperfect model caused by the relevant initial curvatures as geometric imperfections should be so determined that the strength limit state were obtained at the same time when the stability limit state (column buckling, lateral-torsional buckling or interactive buckling) is achieved. Eurocode 3 gives these imperfections for the case of column buckling but the relevant imperfections for lateral-torsional buckling of beam and beam-column is an open question. This paper presents a general formula of the relevant overall imperfection of beam-column, which satisfies the specifications of the Hungarian Standard for overall stability and illustrates the "errors" of the proposed algorithm based on overall imperfection and second-order finite element analysis. This algorithm extends the theorem of elastic extrapolation to the computer aided verification of steel beam-column structures.

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