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Plastic design of columns and its inherent uncertainties Christoffersen, Per Trond

Abstract

The two well-known methods for the plastic design of steel columns subjected to combined bending and normal force were reviewed and compared. The validity of some of the basic design assumptions was examined, and the influence of unknown variables affecting practical applications was investigated. It was assumed that the existing column criteria provide a true prediction of failure within the possible range of yield stress values of mild structural steel. It was found that bending moments in columns vary greatly depending on the assumed conditions of load application, the method of analysis, and the actual values of the yield stresses in the different structural members. Some of the recommended methods of analysis seem inadequate and in many cases unsafe. Elastic analysis appears to represent a necessary part of plastic design of rigid frames. A procedure based on an elastic analysis with subsequent redistribution of bending moments after the formation of plastic hinges was used to illustrate the possible variations in column end moments. This procedure takes into account the effect of the individual structural members having different yield stresses. It is shown that only through careful analysis and critical evaluation of unknowns can the main object of plastic design be achieved: The design of structures with consistent factors of safety.

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