Location of Plastic Hinges in Columns of Steel Frames

Abstract

The New Zealand Steel Structures Standard has a seismic provision aiming to ensure that plastic hinges occur at the ends of steel columns rather than along their length. This provision often governs the column sizes in steel frames and concerns have been raised about the need for this requirement and the accuracy of the equation in the provision. This paper describes research carried out to evaluate this equation. Methods were developed from first principles considering second order and residual stress effects. Time history analysis was carried out with RUAUMOKO to examine the likelihood of plastic hinges forming away from the column ends. A computer program was also used to examine the applicability of this equation on different types of steel frame. A new equation is proposed based on the analytical results. It is found that the provision is not applicable to columns in the majority of moment-resisting frames. Columns in eccentrically braced frames, designed in accordance with the capacity design philosophy, often did not yield even at the column base during earthquake excitations. A simple check is proposed to determine whether yielding would occur under a design drift limit which can be used in conjunction with the proposed equation.