Direct analysis of steel structures allowing for imperfections and without assumption of effective length

The recent codes for design of steel structures like the Eurocode-3 (2005) and The Code of Practice for Structural Uses of Steel Hong Kong (2005) have been based on a new approach of analyzing and designing a steel structure, especially when it is slender. This type of direct analysis, named in the LFRD eliminates the need of assuming an effective length or K-factor for buckling resistance check of columns, which is noted to be arbitrarily taken as the member length by most engineers. This paper details the feature of this code on the use of second-order analysis for design of slender frames which eliminates the need of assuming an effective length for flexural buckling in the design. This alternative new design method in the code was noted to be more reliable and efficient than the present method based on the effective length concept which was found to have limitation in many practical applications. Further, most commercial software is unable to design meeting the code requirement because of use of straight cubic element and does not allow for member and frame imperfections.