Location of Plastic Hinges in Axially Loaded Steel Members

New Zealand and Australian steel structure design standards contain equations to encourage yielding at the ends of steel members rather than along their lengths. This paper evaluates the accuracy of these equations using a commercially available computer program as well as an analytical procedure. The analytical procedure considers non-linear geometric effects and material effects of the member stiffness by considering stability functions in conjunction with residual stress effects. New equations to prevent yielding away from the member ends, which are less conservative than the current code equations, are developed. Simplifications of these equations being considered for adoption into the New Zealand steel structure design standard are described

Plastic hinge location in columns of steel frames subjected to seismic actions

Several steel structure standards around the world contain an equation to encourage any column flexural yielding during earthquake shaking to occur at the column ends, rather than along the column length. The accuracy of these equations and their applicability to columns of both moment frames and braced frames are examined in this paper. It is shown, using an analytical procedure developed from first principles considering the reduction in member stiffness from axial force due to geometric and material nonlinearity, that the existing code equations are conservative. Less conservative empirical equations are developed based on the analysis results. It is found that these equations are applicable to frames with a braced connection, rather than a moment connection into the column. Time-history analysis of eccentrically-braced frames with inverted V-bracing, where the active link occurs at the centre of the beam, is carried out. The likely column end moment ratio needed for the new equations is determined. The analysis also shows that yielding often did not occur in the bottom story columns during earthquake excitations. A simple check is proposed to relate the axial force limit and the design drift to flexural yielding of columns which can be used in conjunction with the proposed equation

Evaluation of Lay's equation for plastic hinge location in steel columns

The current New Zealand steel structures standard has a provision that aims to restrict plastic hinges to develop only at the member ends. The equations in this provision were adopted from Lay’s PhD thesis which were developed not only to encourage yielding at the member end, but also to ensure sufficient member rotational capacity can be maintained. The accuracy of the end yielding criteria in Lay’s equations is examined using an analytical technique developed in this study. The analytical procedure incorporates nonlinear geometric and material effects by considering stability functions in conjunction with residual stress effects. It is found that the code equations are more conservative compared with the analysis results. A new equation to prevent yielding away from the member ends is proposed in the research project. This forms the basis for the proposed amendments to Clause 8.4.3.2 in New Zealand steel structures standard

Location of Plastic Hinges in Axially Loaded Steel Members

New Zealand and Australian steel structure design standards contain equations to encourage yielding at the ends of steel members rather than along their lengths. This paper evaluates the accuracy of these equations using a commercially available computer program as well as an analytical procedure. The analytical procedure considers non-linear geometric effects and material effects of the member stiffness by considering stability functions in conjunction with residual stress effects. New equations to prevent yielding away from the member ends, which are less conservative than the current code equations, are developed. Simplifications of these equations being considered for adoption into the New Zealand steel structure design standard are described