Location of plastic hinges in axially loaded steel members

Steel structure design standards for both New Zealand and Australia have equations limiting the maximum axial force which can be applied to a yielding steel column as a function of the member slenderness and end moment ratio. These equations were developed to ensure plastic hinge rotation capacity primarily by encouraging flexural yielding to occur at the member ends rather than along the column length. This paper develops an analytical procedure to determine the axial force associated with plastic hinges occurring away from the column ends. It is shown that current code equations conservatively estimate the axial force to ensure yielding at the column ends when the axial force ratio is greater than 0.15, and they are non-conservative for lower axial force ratios. Empirical equations to prevent yielding away from the member ends are developed, and simplifications of these equations are proposed for code adoption.